BK Precision 2120 trace issues - Page 1
Forum section: electronics / repair

author: vexatag
author_rank: Regular Contributor
timestamp: July 11, 2020, 10:13:14 am
content: In quite possibly the most foolish decision I'll make all year, I bought a broken BK 2120. There are some dents on the chassis that the seller didn't do a good job of pointing out, one of the feet on the back is cracked off, and several screws are missing from the bottom of the chassis.The main issue, however, is the trace. Both channels behave identically. When hooked up to the calibration point, they show two shimmering ~7mm lines ~3mm apart from each other. The x position knob turned all the way left is off screen, but turned all the way right is dead center. Shifting this rapidly back and forth does reveal some amount of waveform.Hereis a link to the pictures I took this afternoon after unpacking it. If there are any other diagnostic steps I should take before poking with a multimeter please let me know.

author: med6753
author_rank: Super Contributor
timestamp: July 11, 2020, 03:07:12 pm
content: Quote from: vexatag on July 11, 2020, 10:13:14 amI have a 2120 in my collection of scopes which obviously is in much better condition than that one. That's really been beat up.Anyway, if you don't have the service manual download one. Make sure it's the service manual and not the user manual. There are plenty of on line sources.Start with the basics. All power supply voltages must be present. Then take it from that point.

author: bdunham7
author_rank: Super Contributor
timestamp: July 11, 2020, 05:50:58 pm
content: Has anyone pushed that XY button yet? Did you ask the seller to do that and if so, what happened? If pushing the button and setting the trigger to AUTO doesn't give you a trace, you know the horizonatal sweep isn't working--and that's a bad thing.Also, do you look at eBay auctions on your phone or on a computer. I saw that damage in the eBay photos, at least the cracked back part.You're going to have a tough time. the BK2120 was made in several iterations (not including the explicitly different A, B, versions, etc) and it is likely that what you have in front of you will not exactly match the manuals and schematics that are available.The first thing would be to take the covers off and examine it for physical damage. Beware that although it is not an accelerated CRT, there is still a good -1500VDC at the gun and many hundreds of volts elsewhere, and then there's mains voltages in certain places.

author: vexatag
author_rank: Regular Contributor
timestamp: July 11, 2020, 11:03:23 pm
content: Quote from: med6753 on July 11, 2020, 03:07:12 pmThe first thing I did after buying this was download the service manual from one of your posts here. I'll check the voltages this evening (Pacific time) and report back.Quote from: bdunham7 on July 11, 2020, 05:50:58 pmPressing the XY button moves what's displayed over a centimeter or two, no actual change in the trace generation.Quote from: bdunham7 on July 11, 2020, 05:50:58 pmI look at auctions on my computer, there's no excuse for my lack of diligence hereQuote from: bdunham7 on July 11, 2020, 05:50:58 pmIs there any easy way you know of to check which version it is?Quote from: bdunham7 on July 11, 2020, 05:50:58 pmI understand and appreciate the advice.

author: vexatag
author_rank: Regular Contributor
timestamp: July 12, 2020, 04:04:20 am
content: Update: Took off the casing, no damage to the PCB amazingly enough. 140v test point is reading 141 on my DMM, 260v test point is only reading 79-83v. I can't test the 2kv test point unfortunately. I also went ahead and tapped the bottom of the case more flat, I want to sand it to bare metal and repaint it, question is what color is closest.

author: bdunham7
author_rank: Super Contributor
timestamp: July 12, 2020, 05:20:19 am
content: Well, that could be somewhat good. The -2kV supply is not an issue for now because you have a beam, so don't kill yourself trying to measure it. The 260V supply is sort of 'on top' of the 140V, so the fact that is measures less indicates it is not working at all. The horizontal amp section runs off of this supply, so that might be the whole problem. You can hope.Start by checking for AC (bridge input) and DC (bridge ouput) for the +260V branch.

author: vexatag
author_rank: Regular Contributor
timestamp: July 12, 2020, 06:54:33 am
content: For DC voltages I measured -130v on R504 and 38v on the positive leg of C503. I'm not sure which part you're pointing at for the AC voltage measurements, could you clarify?

author: bdunham7
author_rank: Super Contributor
timestamp: July 12, 2020, 01:20:21 pm
content: I'm pointing at the two 120V winding wires marked ORG on the diagram--they should be orange--or the AC inputs to bridge rectifier BD101.What exactly are you using to measure the voltages? I'm assuming that you have been leaving the negative end on ground. You will need to start moving both leads, but I want to make sure you have a proper DMM, preferably with test hooks so you can do this hands off.OK, so you have +140V at TP140V but -130V at the junction of R509, R504 and BD501(-). Now (only if you have a proper DMM--let us know) measure with the negative probe at the R509/R504 junction and the positive at C503(+) and TP260V (check both to make sure they are the same).

author: vexatag
author_rank: Regular Contributor
timestamp: July 13, 2020, 12:09:14 am
content: I've been measuring with negative clipped to the chassis. My multimeter is a UEI DM 383 with some aftermarket leads (they've got screw-on alligator clips). I've ordered an Aneng AN860B+ but it won't be here for a while unfortunately.So, negative probe at R504/R509 junction and positive at both C503 positive leg and TP260V? I'm unable to find BD101, are you referring to BD501?

author: bdunham7
author_rank: Super Contributor
timestamp: July 13, 2020, 01:29:32 am
content: Quote from: vexatag on July 13, 2020, 12:09:14 amYes, BD501. The meter you have with alligators is actually good, hopefully you can do this hands-off with a minute of power-off time in between for capacitor discharge. And still be careful even with that!

author: vexatag
author_rank: Regular Contributor
timestamp: July 13, 2020, 01:46:51 am
content: To be absolutely sure: hook up when powered off, power on, record the data, power off again, wait a few minutes, carefully relocate the probes, rinse repeat (metaphorically)?

author: bdunham7
author_rank: Super Contributor
timestamp: July 13, 2020, 02:25:27 am
content: Yes. No rinsing.

author: vexatag
author_rank: Regular Contributor
timestamp: July 13, 2020, 05:48:07 am
content: Alright, measuring from R509 to both the test point and the positive leg of C503 got 170v±1, measuring the AC lines of BD501 got 129v.

author: bdunham7
author_rank: Super Contributor
timestamp: July 13, 2020, 02:56:36 pm
content: Very good. You just might fix this thing yet!If you look at the diagram, you'll see that the only place you can be dropping all that voltage is across either R504 or Q501. This doesn't mean that the problem is one of the two components, but that's where to start. So the last two places I need you to test voltages is between TP+140V and both sides of R504.After you do that, let it sit for a few minutes and then use the ohms mode to check each resistor in the 260V circuit--R503 through R509. In-circuit they could read lower then their spec, but they can't read higher. Also do a diode check both ways on D502. If you are very lucky, you will find that R504 has gone open or high resistance.

author: vexatag
author_rank: Regular Contributor
timestamp: July 13, 2020, 08:00:38 pm
content: Some very interesting developments have occurred. Going from either side of R504 to TP104 revealed no current, but that's not the most unusual thing. R503 and R506, R507, R508, and R509 completely failed a continuity check. On R504 I measured 92Ω and on R505 I measured about 500Ω. On D502 I got 0.560, not sure what that means.

author: bdunham7
author_rank: Super Contributor
timestamp: July 13, 2020, 08:07:52 pm
content: What do you mean by 'no current'?? There should have been a voltage reading of some sort on each.The D502 reading is as expected--about a half a volt forward drop. R504 and R505 are close enough to not worry about yet. The others you probably have selected the wrong resistance range on your meter. 100K means 100,000 ohms, 47K means 47,000 ohms, 4K7 means 4,700 ohms, etc. 100KF means 100,000 ohms with a 1% tolerance.

author: vexatag
author_rank: Regular Contributor
timestamp: July 13, 2020, 08:10:00 pm
content: By "no current" I mean I measured 0 volts of current. As for the resistors, I selected the continuity tester after failing to get any resistance in any range. As far as my meter could tell, the resistors are open.

author: bdunham7
author_rank: Super Contributor
timestamp: July 13, 2020, 08:22:08 pm
content: OK. Just say 'volts' not 'volts of current' because the latter gives me visions of sparks flying due to putting the test leads in the wrong jacks on your meter.You need to check your meter. Given what we have so far, I don't think you would have zero volts there. Just to be clear, one lead on TP+140V test point, the other on one side or the other of R504? Try putting that second lead to ground and verify you get 140 volts again. And add in a third measurement from TP+140V to the lower side of D502 (or both sides if isn't clear which is which.As for the resistors, it seems unlikely that they would all be open, user error or a malfunction your meter is more likely. So try measuring some resistors in the 140V supply , lke R516, R517, etc. Since we know that supply works, the resistors will probably be OK.

author: vexatag
author_rank: Regular Contributor
timestamp: July 14, 2020, 12:27:36 am
content: TP+140v to the chassis gets 140v, TP+140v to either side of R504 gets a 1 on the far left of my multimeter's display no matter what range I'm on. All of this is measured in DC. In AC mode some sporadic measurements happen but nothing more substantial than random noise. I connected in continuity mode very briefly but I started hearing a rapid clicking so I shut it down. TP+104 to the negative side of D502 got nothing, the positive side got 102v. After checking a couple known-good resistors on another PCB I have (little 1khz signal generator kit I have), I admit that I'm probably measuring them wrong.

author: bdunham7
author_rank: Super Contributor
timestamp: July 14, 2020, 12:51:26 am
content: OK, there's still work to do.First, please only use the DC 1000V or AC 750 volt ranges when doing power-on measurements on the oscilloscope--and definitely never ohms or continuity. There's no need for any resolution better than 1 volt at this point. The '1' on the left side may indicate overrange, meaning the voltage has exceeded the range. If this happened on the 1000V range, there's something very wrong, hopefully with your meter because I have no idea where 1000V could come from. Do you have a 9 volt battery or some other low voltage source handy? Try setting your meter on 2V DC and giving it 9 volts (it won't hurt it) just to see if that '1' does indicate overrange. And does your meter should indicate zero when there's no voltage, so when you measured from TP+140V to both sides of D502, you said it was 102V on one side and 'nothing' on the other. If that 'nothing' was a zero reading that's lot different than an overrange reading.Do figure out how to get the ohms function working and verify it. Getting your test instruments to work properly is all part of the struggle!

author: vexatag
author_rank: Regular Contributor
timestamp: July 14, 2020, 04:15:42 am
content: Quote from: bdunham7 on July 14, 2020, 12:51:26 amThe 1 indicates overrange, I'm getting 274 volts from TP+140v to either side of R504.Quote from: bdunham7 on July 14, 2020, 12:51:26 amMy meter indicates 0.0 when there's no voltage, TP+140V to D502 resulted in 102V on one side and 0.0V on the other. I will be more precise in the future, my apologies.As for the ohms I'll get it figured out,

author: vexatag
author_rank: Regular Contributor
timestamp: July 14, 2020, 05:00:21 am
content: Update, I've figured out the ohms, none of the resistors were open after all.R503: 4.68k OhmR504: 99 OhmR505: 500 OhmR506: 48k OhmR507: 180k OhmR508: 1.68 mega Ohm97.5k Ohm

author: bdunham7
author_rank: Super Contributor
timestamp: July 14, 2020, 05:17:03 am
content: If those readings are accurate, R507 and R508 are both bad and that might be the whole problem. They are the voltage divider that sets the output of the supply. It looks like they are a different type than the others? They seem to be specifying 1/4 watt metal film 1% tolerance, which are readily available in 100K and 115K values. I wouldn't do any more testing until you have replaced those.

author: vexatag
author_rank: Regular Contributor
timestamp: July 14, 2020, 05:56:59 am
content: I'll round up a source for them (annoying that Mouser and Digikey shipping costs can easily exceed the cost of parts in situations like this), it'll be a bit before I'm able to replace them as something in my soldering setup is super janked, all my joints are cold.1/2 watt metal film resistors are probably also fine, right?

author: bdunham7
author_rank: Super Contributor
timestamp: July 14, 2020, 03:50:22 pm
content: !/2 watt are OK if they fit OK, but unnecessary. Try eBay for very small orders, for something like resistors they'll be fine. You might want to just get a big set of resistors if you plan on making stuff in the future. You will have to buy the 115K separately, or else use a 113K or 120K, depending on the set you get.https://www.ebay.com/itm/US-1460Pcs-1-4W-Metal-Film-Resistor-Kit-Assortment-Set-Labelled-1-Precision-Set/133464890909?hash=item1f13209e1d:g:B1MAAOSwDO9fDSDIAnd yes, figure out your soldering before you start any repair work.

author: vexatag
author_rank: Regular Contributor
timestamp: July 22, 2020, 11:31:47 pm
content: Update: the resistors came in and I figured out the problem with my soldering (cheapo 63/37 solder doesn't leave a nice shiny joint ;__;), now I have the casing off but I don't know how to proceed with removing the pcb in question. Is there a "best" order for removing the connectors? How do I disconnect the switches from the front face?

author: bdunham7
author_rank: Super Contributor
timestamp: July 23, 2020, 02:00:40 am
content: Quote from: vexatag on July 22, 2020, 11:31:47 pmFake solder seems to be a thing now too. I have some that came 'free' with some junk or another and it is like stirring mud. As far as disassembly, take pictures for putting it back together, post them if you want. I don't remember how the knobs and switches came off and the one I fixed was a little different than yours. It can be tricky. Can you do it without disassembling the front?

author: vexatag
author_rank: Regular Contributor
timestamp: July 23, 2020, 02:14:34 am
content: Practically speaking, no. Between needing to desolder and solder the resistors from a very small vantage point without damaging any other components and the cleanup of the fresh component leads, it's a bit beyond my comfort zone. The square buttons are easy to remove however (pull on the metal shaft) and I know I somehow got the x position knob removed from the pot by accident, so it does seem possible.

author: vexatag
author_rank: Regular Contributor
timestamp: July 23, 2020, 08:18:22 am
content: Ok, swapped the resistors out. TP260V is reading 265.5v, however the trace is still just a point. The trace is also even further to the left, I can get it about 1.5 cm away from the left edge in either X-Y mode or in x10 mode.

author: bdunham7
author_rank: Super Contributor
timestamp: July 23, 2020, 05:18:27 pm
content: That's too bad--it was looking hopeful for a bit once you found those bad resistors.So if you feed a signal (perhaps the probe calibrator) into the X-channel (in XY mode) can you get at least a small horizontal line that varies with the input voltage range setting? And if you feed the same signal into the Y channel, can you get a vertical signal at all? What else do you have that you can use to input a signal?If you get a vertical signal at all and the X-axis responds even slightly, my first guess is that your next problem is within the horizontal final amp circuit. You can start testing that circuit by verifying the +24 and +12V points are correct, and then depowering and measuring all of the resistors, starting with everything to the right of Q807. Remember, high resistance = broken, low resistance = needs further thought.

author: vexatag
author_rank: Regular Contributor
timestamp: July 23, 2020, 08:55:03 pm
content: I uploaded a video of the channel testing toYoutube, testing on a cheap 1khz triangle wave generator in addition to the calibration point.12v and 24v test points are reading ~12v and ~24v respectively. I'm about to test the resistors.

author: bdunham7
author_rank: Super Contributor
timestamp: July 23, 2020, 09:31:26 pm
content: What happens at about 1 minute of your video is not clear to me--the dot splits up at an angle and I don't have a good explanation for that. When you do channel 1, it appears to have a proper vertical response. I'm going to have to think about that and I'm away for a bit, so test those resistors and don't give up yet.If you don't find a problem, you should try turning XY mode off and look at it in the normal mode. Figure out what moves the dot and what doesn't.

author: vexatag
author_rank: Regular Contributor
timestamp: July 23, 2020, 10:00:15 pm
content: Tested all of the resistors to the right of Q807 in the horizontal final amp circuit (aside from R828 which I can't for the life of me find on the PCB). All of them are within an ohm or so of spec, with the exception of R838 and R839 (measuring at 15.92k and 14.97k respectively).When I replaced the resistors, for some reason the normal mode wasn't able to come on the display at all. I fiddled with... something and got it displaying, the results are the same as in XY mode. Additionally, the x position knob is now functioning similarly in XY mode and normal mode, giving ~5mm of horizontal movement at the center of display either direction. The directions of movement are different, however. Channel 1 XY mode clockwise is left, normal mode clockwise is right. Channel 2 clockwise is left for both modes.

author: bdunham7
author_rank: Super Contributor
timestamp: July 23, 2020, 10:35:58 pm
content: OK, you need to stick with the normal mode for a while. The normal mode will not normally show on the screen at all unless it is triggered or in AUTO mode. So let's set this up for normal mode operation:All six of the square pushbuttons in the 'out' position. Coupling to DC/LF. Source to CH1. Connect the calibrator output to CH1 and set for 0.5 V/div for now. Set the TIM/DIV to 1 ms/div, push in the center knob and make sure the XPOS/MAG knob button is pushed in. Turn the TRIG LVL button to about the center and push it in. Tell me what you have at that point.

author: vexatag
author_rank: Regular Contributor
timestamp: July 23, 2020, 10:52:52 pm
content: I wasn't able move the center of the time/div knob in or out, but running all the settings as you said gives me two points stacked vertically, about 8 mm away from each other.

author: bdunham7
author_rank: Super Contributor
timestamp: July 23, 2020, 11:00:19 pm
content: OK, the TIM/DIV center knob turns instead of pushing, no big deal.If you change the V/DIV one click each way does that 8mm become 4 mm one way and 15 the other (or something like that--precision is unimportant).And then, if you change the SOURCE to EXT, pull the TRIG LVL out and center it--do the does stay or go away?

author: vexatag
author_rank: Regular Contributor
timestamp: July 23, 2020, 11:04:18 pm
content: Quote from: bdunham7 on July 23, 2020, 11:00:19 pmChanging to 1 v/div halves the distance, 0.2 v/div roughly doubles it.Quote from: bdunham7 on July 23, 2020, 11:00:19 pmThis makes absolutely zero difference.

author: bdunham7
author_rank: Super Contributor
timestamp: July 23, 2020, 11:08:18 pm
content: I'm out of ideas for the moment, but don't throw it out the window yet. Test the rest of the resistors in the horizontal final amp and maybe give it a break for a day or two...One more thing--is the dot in the same area that is was before you replaced the defective resistors?

author: vexatag
author_rank: Regular Contributor
timestamp: July 23, 2020, 11:14:55 pm
content: Quote from: bdunham7 on July 23, 2020, 11:08:18 pmNo, when I first got the scope the dot's position was strongly on the left, the x-pos knob moved the dot from the left (off screen) to a little over 1 cm away from the center. Now the entire range of the x-pos knob is within 1 cm to the left of the center line (the entire range fits inside one measured box) but it's much closer at the closest setting.I'll check the rest of the resistors and report back.

author: vexatag
author_rank: Regular Contributor
timestamp: July 24, 2020, 04:14:27 am
content: Checked the remaining resistors in the horizontal amp, here are the ones that were out of spec enough for me to notice:R823: 1.58kR825: 86.kR827: 9.2kR831: 4.57kR840: 1.55k

author: bdunham7
author_rank: Super Contributor
timestamp: July 24, 2020, 02:37:28 pm
content: R825--is that a typo--is is 8.6K or 86K?You're getting to the point where one would typically use another oscilloscope to repair the oscilloscope, but since that's not possible, we'll keep trying with what you have.If you don't already know how this circuit works, it helps to understand the theory a bit. The horizontal final amp moves the beam by varying the voltage on two plates on either side, those are the outputs D1 and D2. In normal mode, the horizontal sweep generator (elsewhere) would send a sawtooth-shaped sweep signal to the input marked FROM X-Y MODE IN. When you switch to XY mode, the horizontal final amp just follows the X input. The controls to the left of Q807 all control the overall fixed position of the beam. Since you don't have another scope, we can try using very slow inputs and trying to see if they show up on your meter.Take the following measurements using your 1000VDC range, and do this again all hands-off by turning off, waiting, connecting and turning on. Set XPOS to center, XMAGX10 off (pushed in) and XY MODE on (pushed in).1. DMM negative lead to ground, positive to the TO CRT D1 point.2. DMM neg to grd, pos to TO CRT D2 point.3. DMM neg to TO CRT D1, pos to TO CRT D2 (reading across the two plates)Now, with the DMM still connected across the plates in the 3. configuration, adjust the XPOS control all the way counterclockwise, record the voltage, then all the way clockwise and record again. Pull the knob out (turn on XMAGX10) and do it all again.Next we can try generating the slowest sweep possible, by turning off XY MODE and XMAGX10 and setting the sweep speed to the slowest setting, which I think is .2S/div. You can turn the center knob counterclockwise to slow it down a bit more. See if there's any change on the plate voltage this way, and then try again with XMAGX10 on (out).Lastly, do the same as the last step, except connect the DMM neg to ground, pos to the left side of R834 or where it says FROM XYMODE IN. You may have to turn the voltage setting down on the DMM, but start at 1000VDC and go down one range at a time (always a good practice on unknowns) If you can see the sweep voltage ramping up over 2-3 seconds here but not on the plates, then we know you are in the right area.

author: vexatag
author_rank: Regular Contributor
timestamp: July 24, 2020, 09:45:47 pm
content: I don't see anything labeled TO CRT D1 or D2 on the pcb, but there is a plug, PG106, that seems to be where the D1 and D2 would be, is this a correct understanding?Also I can technically use my computer's sound card as an oscilloscope but the cabling I'd be using is very sketchy (test probe into a bnc to rca adapter into an rca switcher into an rca to 3.5 mm stereo adapter into a very cheap 3.5 mm audio cable into my sound card).

author: bdunham7
author_rank: Super Contributor
timestamp: July 24, 2020, 09:55:32 pm
content: I'm not sure. Look at the schematic and see if you can relate that plug to R838 and R839. The top of R838 and the bottom of R839 are alternate points to measure from.

author: vexatag
author_rank: Regular Contributor
timestamp: July 24, 2020, 10:05:18 pm
content: I'm definitely kicking myself for not taking high-quality photos when I had the pcb underside easily accessiblePG106 has four wires, tan, tan, orange, and black. R838 has continuity with the left tan pin, R389 has continuity with the right tan pin.

author: bdunham7
author_rank: Super Contributor
timestamp: July 24, 2020, 10:10:36 pm
content: There you go. Measure away.

author: vexatag
author_rank: Regular Contributor
timestamp: July 24, 2020, 10:17:23 pm
content: Oh and R825 was 8.6k

author: vexatag
author_rank: Regular Contributor
timestamp: July 24, 2020, 11:59:48 pm
content: Measurements!Checking to ground:Left pin: 825 mVRight pin: 38.2 VNeg on left, pos on right:X-pos neutral: 146 VX-pos clockwise: 73 VX-pos counterclockwise: 220 VX10 mode neutral: 115 VX10 mode clockwise: 1.3 VX10 mode counterclockwise: 260 VSlowing the sweep:X-pos neutral: 141 VX-pos clockwise: 145 VX-pos counterclockwise: range between 136 V and 138 VX10 mode neutral: 141 VX10 mode clockwise: 1.3 VX10 mode counterclockwise: range between 136 V and 138 VMeasuring the left leg of R834 (when viewed from the front of the scope) at 0.5mS/div in XY mode gave about 1.6 V through the entire x range and X10 mode, but at 0.2S/div in normal mode gave -1.42 V at neutral and -2.32 V when the xpos knob was rotated fully clockwise.

author: bdunham7
author_rank: Super Contributor
timestamp: July 25, 2020, 01:54:36 am
content: Quote from: vexatag on July 24, 2020, 11:59:48 pmI meant the left side on the schematic, IOW the side that is not connected to the base of Q808. It would help if you had a signal generator that could feed in a very slow ramp or triangle wave of about 1 volt and 0.1 Hz, but the sweep generator in the 0.2S/div mode will maybe do. What I'm looking for is a smooth change from low to high that repeats itself. This may be difficult to spot depending on how fast your meter updates.

author: vexatag
author_rank: Regular Contributor
timestamp: July 25, 2020, 05:20:55 am
content: Oops, my bad. Doing the proper test (checked the leg labeled B for continuity with either side of R834, then hooked the positive lead to the leg without continuity and the negative lead to the chassis), it started at about -15 mV but has been falling over about three or four minutes. I wish I could have a visual of this data, I understand now why they say you need a scope to fix a scope.

author: bdunham7
author_rank: Super Contributor
timestamp: July 25, 2020, 05:36:46 pm
content: Quote from: vexatag on July 25, 2020, 05:20:55 amYes, another scope is the best way! What did you measure R834 at? It is only a 68R resistor so I would not expect a continuity meter to show open.So what we are trying to do is trace signals. If you look up at the Trig & Sweep section, you'll see that S605 is your XY MODE switch. In normal mode, the sweep generator sends a ramp signal to the horizonal final amp and that is amplified and sent to the horizontal deflection plates causing the beam to sweep from left to right. In XY mode, the CH2/X signal is fed instead, causing the beam to move according to the signal. Since neither of these functions work and they are at least partly independent of each other, I was going on the assumption that your problem lies somewhere in the horizontal final amp. Now we should take a step back and verify as many of those assumptions as we can, and tracing a signal into R834 is on very important point. The other would be verifying as best we can that the CRT is responding to the horizontal deflection voltage. So, let's try a few things.First, unplug PG106 and power up the scope. Where is the dot?Second, tell me what equipment you do have, other than your DMM. Power supplies, electronic parts such as potentiometers, batteries, wire, solder, breadboards, etc etc. We need to rig up a signal we can trace through the scope. Even a battery and a 10K potentiometer might work.

author: vexatag
author_rank: Regular Contributor
timestamp: July 25, 2020, 06:28:53 pm
content: R834 measured 68.4 ohms.Unplugging PG106 leaves no dot on the screen. As for other equipment, I have some 100k pots, a 10k pot, a custom 16k pot, assorted capacitors, aVelleman MK 105 1khz signal generator, the ability to use my computer's soundcard or a usb soundcard as a somewhat limited scope (the wiring setup is admittedly janky and I don't want to risk super high voltage through it), a small breadboard, wire, solder, a soldering iron, and two multimeters (the DM 383 and a recently arrived AN860B+).

author: bdunham7
author_rank: Super Contributor
timestamp: July 25, 2020, 06:46:00 pm
content: OK, let's not burn up your computer just yet, there's enough useful stuff that looks helpful. I'll try to think of a way to make it all work.In the meantime, no dot with the connector unplugged seriously worries me, although I'm very unsure what to think at this point--it's bit of uncharted territory for me to. What I was expecting was that the beam would just continue right down the center without being deflected. You said there were other wires on PG106, can you tell where they go to?

author: vexatag
author_rank: Regular Contributor
timestamp: July 25, 2020, 06:55:55 pm
content: All four wires on PG106 go to the base (I believe that's what it's called) on the back of the CRT. Re-testing, I saw a square faintly illuminate and quickly drift to the lower left corner while growing in size.

author: bdunham7
author_rank: Super Contributor
timestamp: July 25, 2020, 07:19:11 pm
content: OK, I see that the other two are the FOCUS (orange) and ASTIGMATISM (black). Be careful, theres about -2000 volts on the focus wire (but not much current). That's why you need a few minutes of power off before handling anything.Is there any practical way to disconnect just the two tan wires? I don't know what the connector looks like.

author: vexatag
author_rank: Regular Contributor
timestamp: July 25, 2020, 07:27:32 pm
content: I believe it's a JST connector?

author: vexatag
author_rank: Regular Contributor
timestamp: July 25, 2020, 07:44:23 pm
content: Oh, forgot to mention that I have some MPSA13 transistors from an impulse addition to an ebay purchase.

author: bdunham7
author_rank: Super Contributor
timestamp: July 26, 2020, 01:40:32 am
content: So here's a simple test you can try since you have two multimeters now. Put the first DMM back on the measurement between R834 and chassis ground. Now go into XY mode and hook up that BNC-to-clips adapter I saw on the CH1/Y input. Use the diode check or ohms function--it will generate a small voltage--and input it into the scope via the clips. You will have to turn the V/DIV setting down on that channel to 1V/DIV or less--do that until the dot moves about halfway (up and down) across the screen. You may have to reverse polarity to get it to go the right way. Once that works, move the BNC connection to the CH2/X input and set the V/DIV to the same setting as you just arrive at for CH1/Y. Now repeatedly connect and disconnect the signal-producing DMM (presumably then new one) and observe if there is any change on the other meter that is hooked up to R834. Turn the range down one step at a time until you see some change--it should be clearly evident.

author: vexatag
author_rank: Regular Contributor
timestamp: July 26, 2020, 02:09:05 am
content: On CH1, I read ~1.558 V when the test load was connected and ~1.553 V when the test load was disconnected. On CH2 I read -4.34 V when the test load was connected and 1.45 V when the test load was disconnected.

author: bdunham7
author_rank: Super Contributor
timestamp: July 26, 2020, 02:21:18 am
content: OK, does the dot move when you are connected to CH1/Y?

author: vexatag
author_rank: Regular Contributor
timestamp: July 26, 2020, 02:27:41 am
content: I can get the dot to form a vertical line, yeah.

author: bdunham7
author_rank: Super Contributor
timestamp: July 26, 2020, 02:34:06 am
content: OK, very good. Now we have a way to trace the signal through the amplifier. We might actually be close, I've been suspecting that Q808 may be bad, but haven't had a way to prove it. So now repeat the test on CH2/X but hook the measuring DMM to the other side of R834 (the base of Q808) and write down the voltages each way, then at the emitter of Q808 (the side with the outward pointing arrow on the schematic.EDIT: If you have any difficulty getting at the base or emitter of Q808, you can use the something else connected to it like R835 or R837.

author: vexatag
author_rank: Regular Contributor
timestamp: July 26, 2020, 02:55:30 am
content: At the base, voltage when the test load was connected was -4.37 V, voltage when the test load disconnected was 0.92 V. At the emitter, voltage when the test load was connected was -5.01 V, voltage when the test load disconnected was 0.28 V.

author: bdunham7
author_rank: Super Contributor
timestamp: July 26, 2020, 03:06:22 am
content: That actually looks good--the emitter follows the base with about a 0.6 volt consistent difference. Can you do the same tests on the base and emitter of Q807 and Q809?EDIT: And while we're at it, can you run the same tests measuring at the TO D1 and TO D2 terminals of the plug?EDIT2: Also at the junction of 835 and 836, which is the collector of Q807.

author: vexatag
author_rank: Regular Contributor
timestamp: July 26, 2020, 05:30:11 am
content: Here's the result of my testingLoad appliedNo loadQ807 base-8.63 V-8.63 VQ807 emitter-9.36 V-9.36 VQ809 base1.04 V1.65 VQ809 emitter0.24 V0.88 VLeft tan pin0.28 V0.91 VRight tan pin0.20 V0.65 V835/836 junction-5.37 V-2.40 V

author: bdunham7
author_rank: Super Contributor
timestamp: July 26, 2020, 06:02:53 am
content: OK, we're missing something here. Where's the power? You had some voltage on TO D1 and TO D2 before.Can you check the voltage at the junction of L801 and L802 (this is the+260V supply we fixed before) and then power down and measure the resistance from that junction to TO D1 and TO D2.

author: vexatag
author_rank: Regular Contributor
timestamp: July 26, 2020, 07:48:23 am
content: Note for clarification: The left tan pin is D2, the right tan pin is D1.Voltage to D1, -116 V. To D2, -121 V. I tried measuring the resistance but got either overload or around 5 megaOhms on both my meters.EDIT: I also remeasured the voltage between D2 and ground, got a result of around 3 volts. Maybe my testing methodology is borked somehow?

author: bdunham7
author_rank: Super Contributor
timestamp: July 26, 2020, 04:21:17 pm
content: Yes, there seems to be some issue measuring, so let's back up. Is the scope partially disassembled so that any of the grounds are disconnected?So first, let's go back and measure the +260V supply that you fixed earlier, make sure it still has power. DMM neg lead to ground and pos to +260V. Then move just the pos lead to the L801-L802 junction, the same 260 volts or so should be there as it is a direct connection.Next, power down and look at L801, L802, R838 and R839. You can see how they are connected, L801 and L802 are inductors so they should have a very low DC resistance. If you measure from the L801-802 junction to either TO D1 or TO D2, you should get approximately 15K ohms--and if not, you need to measure each component separately.Also look at all the connections and wiring to make sure everything is firmly connected and you haven't missed anything.

author: vexatag
author_rank: Regular Contributor
timestamp: July 26, 2020, 10:41:05 pm
content: Scope is fully assembled, just out of the case.TP260V and the L801-L802 junction both have 265V. As for the megaohm resistance I was measuring on the wrong side of the inductorBoth are around 15K to D1 and D2.Ground to D1, 37.4 V. Ground to D2, -3.3 V.

author: bdunham7
author_rank: Super Contributor
timestamp: July 26, 2020, 11:40:35 pm
content: Alright, just to be clear, you measure from the L801-802 junction to D1 and D2 and each time you get 15K? I don't see why using the wrong side of the inductor would matter. Can you just check L801 and L802 to see what their DC resistance is?I'm trying to figure out how D2 can be at -3.3 volts. Can you also measure again voltage from D1 to D2?

author: vexatag
author_rank: Regular Contributor
timestamp: July 26, 2020, 11:54:40 pm
content: Quote from: bdunham7 on July 26, 2020, 11:40:35 pmMeasuring from what I believe the junction is, I get 15k ohms. Measuring from the other side, I get 5.3 mega ohms.L801 measures 7.3 ohms, L802 measures 7.1 ohms. Each leg has continuity with every other leg as well, don't know if that's important to mention or not.Checking D1 to D2 gives 73 V.

author: bdunham7
author_rank: Super Contributor
timestamp: July 27, 2020, 12:31:16 am
content: OK, let's try this. On the picture I've attached, can you confirm that your board looks like this, more or less?I've illustrated where the +260V point is, that is also the L801-802 junction. The A side of L802 is connected to that, then the PCB trace on the B side of L802 goes under and around it back to the other B, which is the B side of R839. Then the C side of R839 is connected to D1. So when you measure from A to C, you should get about 15K, which is the combined resistance of L802 and R839. Apply the same logic to the other side. So where are you seeing the 5.3M ohms?

author: vexatag
author_rank: Regular Contributor
timestamp: July 27, 2020, 12:47:30 am
content: That's about what my board looks like, yes.I'm seeing 5.3M ohms from L801 A to R838 B and C. (L802 A to R389 C gives the normal value of ~15k, I was measuring from L801 because it's more convenient to clip to)

author: bdunham7
author_rank: Super Contributor
timestamp: July 27, 2020, 12:56:33 am
content: Well, there's your problem. Or at least 'a' problem--I can't promise you there won't be more!So if L801 measures 7-ish ohms and R838-B measures 15K all the way to D2, then measure the trace (from B to B) between L801 and R838. There must be an open circuit there somewhere. If you can't easily see the problem but you can confirm that there is not continuity between the two B points, you can run a jumper wire as I've drawn it. Just tack on a wire to the component leads--it's called a bodge wire.

author: vexatag
author_rank: Regular Contributor
timestamp: July 27, 2020, 01:44:10 am
content: Hmm, this might be the problemI'll try cleaning these joints up a bit first, see if that helps.

author: vexatag
author_rank: Regular Contributor
timestamp: July 27, 2020, 02:26:48 am
content: It did not. What am I connecting with the bodge wire? L801 B directly to R838 B?

author: bdunham7
author_rank: Super Contributor
timestamp: July 27, 2020, 02:34:21 am
content: This certainly explains some of the inconsistencies I was seeing in the measurements.That looks ugly. It will take some scraping! And at some point perhaps you should replace both of those resistor, but for now you need to see if anything else is causing this or the scope was just used/abused a lot.

author: bdunham7
author_rank: Super Contributor
timestamp: July 27, 2020, 02:37:44 am
content: You tried resoldering? I see at least two solder joints that look toasted. The left side of R839 is not looking good. If you can't get a connection and just want a bodge for testing now, then yes just replace the trace with a wire, from that burnt side of R838 to the left side (near the resistor symbol) connection of L801.

author: bdunham7
author_rank: Super Contributor
timestamp: July 27, 2020, 02:38:19 am
content: Quote from: vexatag on July 27, 2020, 02:26:48 amYes.

author: vexatag
author_rank: Regular Contributor
timestamp: July 27, 2020, 04:14:40 am
content: The pads for R838 have completely lifted off the PCB, explains why resoldering didn't fix the problem. I guess my next step is to wire in two bodges, one to L801 and one to D2?

author: bdunham7
author_rank: Super Contributor
timestamp: July 27, 2020, 04:31:43 am
content: Yes, and if you eventually replace the resistor, you can just use the long leads to go through the holes and over to the point that the trace used to go to. If there's any danger of touching something else, the small clear teflon tubing can be put over the wires.

author: vexatag
author_rank: Regular Contributor
timestamp: July 27, 2020, 05:20:51 am
content: Bodge installed, not pretty but functional. The horizontal deflection still isn't happening but L801 to D2 is reading at 15.9k ohm (same value as R838 in isolation). What should I test next?

author: bdunham7
author_rank: Super Contributor
timestamp: July 27, 2020, 04:09:10 pm
content: I'm a little worried that the resistor burning up may have been caused by another problem, so don't leave the scope powered on too long at a time.I want you to go back to the step where you used one multimeter as an input to move the dot vertically and then changed to CH2/X to stimulate the horizontal circuit. I had you give me a bunch of voltages measured at the transistors. I didn't explain much, but you should understand what I was looking for to help you go forward here. What I was looking for was both for the signal to change in response to the X input from your DMM--which it did in the appropriate areas--and for the B to E voltage difference to stay at approximately 0.6 volts--which it did. The first thing to look for in transistor troubleshooting is for the base-to-emitter voltage to be something different than about 0.6V, like 0V or over 1V. So you can rerun those tests at some point.But first, lets make sure that there isn't a short somewhere dragging down that D2 line and burning up the resistor. Check the voltages at D1 and D2, then disconnect PG106 (after a power-down, of course) and check them again. If there is a big difference with PG106 disconnected, especially if D2 is near zero when connected but pops up a lot after disconnecting PG106, then stop and turn the scope off. The CRT might be damaged.If there isn't a big change and you have significant positive voltage on D1 and D2 with PG106 connected (say at least 50 volts on each) then you want to start testing at each point both with and without the input from the DMM. Test this way at each point on each component on the schematic going all the way around from L801/802 to Q807--that's L801, R838, Q808, R835, Q807 on the D2 side and you can figure it out for the D1 side.

author: vexatag
author_rank: Regular Contributor
timestamp: July 27, 2020, 08:26:26 pm
content: Right-o, D1 connected is 192 V, D1 disconnected is 193 V. D2 connected is 101 V, D2 disconnected is 102 V. I'll test the other points in a bit but wanted to give that little update.

author: vexatag
author_rank: Regular Contributor
timestamp: July 27, 2020, 11:32:39 pm
content: Here's the underside of the PCB by the way, I took this just before I soldered the bodge wires in.

author: vexatag
author_rank: Regular Contributor
timestamp: July 28, 2020, 12:16:06 am
content: Ok, measurements have been made. All measurements were made with the negative lead on either D1 or D2 and the positive lead on the component being tested.From D1:L802: 71 V in either stateR839: 71 V in either stateQ809 base: -193 V in either stateQ809 emitter: -256 V in either stateR836: -193 V in either stateFrom D2:L801: 162 V in either stateR838: 162 V in either stateQ808 base: -190 V in either stateQ808 emitter: -104 V in either stateR835: -104 V in either state.The only component I wasn't able to test properly was Q807, it's small and sits too close to the pcb for me to clip a test lead to it. I suppose the next step is to test at the resistors directly adjacent to the base and emitter?

author: bdunham7
author_rank: Super Contributor
timestamp: July 28, 2020, 12:41:50 am
content: Sorry, I think I wasn't clear. That helps, but it is confusing for me to try and calculate what I need. Can you do this again with the negative DMM lead to chassis ground and the positive at the test point? Are you in XY mode and have verified that you can 'move the dot' in the Y channel again before switching to the X channel at the same V/DIV? Something isn't working that was working before. Don't worry about Q807 at this point if it is too hard to get a safe measurement there.EDIT: Check at the base of Q808 first and verify that it changes with and without an input. If it does not, stop right there.EDIT2: Check both sides of R835 and R836. One side of each is connected to the other but I'm not sure it is obvious which just by looking at the board.

author: vexatag
author_rank: Regular Contributor
timestamp: July 28, 2020, 05:08:34 am
content: Having bnc leads connected to both channels is tripping the system up, but that's an issue to troubleshoot later.I wasn't testing in XY mode, there's a lot of new concepts for me and things are escaping my notice. Y and X channels both have full motion from the position knobs. Additionally, the X position knob is now going off the screen both left and right.Testing in XY mode with the BNC lead plugged into CH1/Y, DMM1 negative lead on chassis ground:Q808 base goes from 1.527 V without a load and 1.532 V with a load. (it's hard to be specific, the exact reading can change just by moving the bnc cable around (I guess this is why you don't get the cheap cables?))Q808 emitter: .961 V to .955 VL801: 265 V either wayL802: 265 V either wayR835 (side not connected to R836): .960 V to .969 VR835 (side connected to R836): -2.30 V to -2.29 VR836 (side connected to R835): -2.30 V to -2.30 VR836 (side not connected to R835): .488 to .488Q809 emitter: .514 V to .514 VQ809 base: 1.08 to 1.08

author: bdunham7
author_rank: Super Contributor
timestamp: July 28, 2020, 05:22:27 am
content: So go back to normal mode and see if you get a sweep with the trigger level knob centered and pushed in. If not, try switching the COUPLING to 'LINE' and you may have to switch source to EXT as well--but try both ways. Make sure all 4 buttons below there are out. Set the timebase to one of the slowest settings.

author: bdunham7
author_rank: Super Contributor
timestamp: July 28, 2020, 05:42:29 am
content: Quote from: vexatag on July 28, 2020, 05:08:34 amThat's very good news.QuoteOops! You were supposed to check that you could deflect the dot vertically with CH1/Y but do the measurements on CH2/X. But you don't need to redo everything. The scope may be working at this point. If not, just redo this test and look at the base and emitter of Q808. Everything else looks good and the fact that you can get full control with the XPOS knob proves out most of the rest of the horizontal final amp.

author: vexatag
author_rank: Regular Contributor
timestamp: July 28, 2020, 06:25:18 am
content: Q808 emitter: 0.94 V to -5.59 VQ808 base: 5.61 V to 4.25 V

author: bdunham7
author_rank: Super Contributor
timestamp: July 28, 2020, 03:29:34 pm
content: If you go back a few pages to your reply #61, you were able to get about a 5 volt swing on Q808.QuoteThose numbers looked OK for Q808, but the latest ones don't. So either there's an issue with your input method or something else is going on. So check everything as best you can--make sure you can move the Y channel about half a screen like before, then move the BNC to the X channel. I'm not sure what you meant by two BNCs 'tripping the system', but that doesn't sound good!Next, the fact that you have full X-position control is very good because it shows the CRT and a lot of other stuff is working. So we are down to Q808 and its input signal. I'd like you to see if you can get a proper signal change on Q808 like we had before. Try on both sides of R834 to make sure you aren't losing it across that resistor. Also, perhaps first, give me the voltages on D1 and D2 (neg-to-ground, pos to test point) three different ways--with the XPOS set so the dot is on the left edge of the screen, centered, and on the right edge.This scope has obviously been worked on before and one possibility that we have to consider is that a previous unsuccessful repair attempt may have included some adjustments to the trim pots that are now going to need to be corrected.

author: vexatag
author_rank: Regular Contributor
timestamp: July 28, 2020, 07:15:36 pm
content: Here's a Youtube videoto what I mean by "tripping the system up". I neededsomethingto test with and these were the cheapest leads that shipped from the USA (since international shipping is a bit of a mess right now). I'm assuming there's very little shielding, if any, and the actual probe clips have either bent out of shape or snapped off (it's hard to tell which), so I'll definitely be replacing them once the scope is able to generate a solid line across the screen when unplugged.Testing without a test load in XY mode on channel 2 at 0.5 V/div (so I know where the center is, it's a 45 degree line at lower levels)D1, dot left: 198 VD1, dot center: 142 VD1, dot right: 89 VD2, dot left: 98 VD2, dot center: 155 VD2, dot right: 210 VTesting on channel 2 at 10 mV/div, using my second multimeter in ohms mode as a test loadR834 (side not connected to Q808): 1.4 V to -4.6 VR834 (side connected to Q808 base): 1.6 V to -4.3 VQ808 B: 1.58 V to -4.16 VQ808 E: 0.98 V to -4.40 V

author: bdunham7
author_rank: Super Contributor
timestamp: July 28, 2020, 07:57:17 pm
content: Which leads did you order? Link? They don't look like they are necessarily junk, but maybe that is an issue for later. It looks like you have an actual XY pattern going on there, but something is off. What are those cables connected to on the other end?The good news is you will know a lot more about oscilloscopes by the time we're done here. The bad news is I think your scope is a beaten, cobbled wreck that is going to fight us all the way to the end. No matter, I've fixed worse. Sometimes I think the most important quality in a repair tech is sheer stubbornness.So, if you start with the dot on the left, nothing connected, put it in the non-XY normal mode (button out, right?) and set the timebase to 0.1S/Div, all 4 pushbuttons out, CH1 to 10mv/DIV with DC coupling, and the trigger centered and pushed in, what does it do? And if you give it an input with your DMM in ohms just like the test before, does it trigger? (Does the dot move horizontally at all?) If not, how about if you select trigger coupling "LINE"?If not, then there's two separate things needed to track this down.First, the inputs. Something is not right. It looks like the polarity is inverted from what I was expecting, but that alone shouldn't be a serious problem. Still, take the one you're using as the source (the Aneng I presume) and connect it directly to the other DMM, pos-to-pos, neg-to-neg and test the output voltage of the ohms function, including whether or not it is a negative reading. Then continuity-check your BNC cable to make sure you know which clip is connected to the center pin of the BNC connector. I want you to make sure in future tests that the positive source voltage is connected to the center pin--and if your voltage was negative in the first step, that means the negative lead of the DMM. Then, can you verify that when connected like this to the scope, the DMM reads approximately 1Megohm when connected to either channel?Second, it looks like we are getting some signal through to Q808, but I've no idea what the actual signal level should be. So go back to XY mode, set the dot in the center to start and do the same measurements as the last step of last time, only on Q809 and at the base, emitter and the other side of R836. Don't worry about R832 unless it is super easy to get to. Then we'll compare the voltage swing at the base that it takes for Q809 to deflect the dot all the way from one side to the other to see if the 5-volt or so swing we had at Q808 should be enough to see(in which case Q808 is bad) or if the signal needs to be a lot more (in which case the horizontal final amp is fixed and we move on...).

author: vexatag
author_rank: Regular Contributor
timestamp: July 28, 2020, 10:06:53 pm
content: Quote from: bdunham7 on July 28, 2020, 07:57:17 pmThese bad boysare hooked up to... nothing at all. All the changes in the display are from me moving the cables around.Quote from: bdunham7 on July 28, 2020, 07:57:17 pmWith settings dialed as specified and nothing connected (no lead hooked up) it's just a dot. With a lead hooked up to CH1/Y, it's a ~20mm vertical line. With one lead of the multimeter hooked up to the test lead, the line goes from top to bottom, growing thinner and dimmer. With both leads of the multimeter hooked up, it is impossible to notice. Trigger coupling "LINE" mode has the same results.Quote from: bdunham7 on July 28, 2020, 07:57:17 pmMy source is the Aneng, hooking it to the other meter pos-pos and neg-neg gives 0.499 V. Checking the bnc, the red jack is the center and the black jack is the outset shield. I've been connecting neg-pos on my measurements with the bnc so far, should I connect pos-pos in the future? And yes, both channels are around 1 Megaohm.Quote from: bdunham7 on July 28, 2020, 07:57:17 pmTest of changes when adding a load:Q809 base: 1.06 V to 1.08 VQ809 emitter: 0.508 V to 3.34 VR836 (Q808 side): -2.27 V to 3.46 VDeflection test at base of Q809:Left: 3.85 VCenter: 1.09 VRight: -1.80 V

author: bdunham7
author_rank: Super Contributor
timestamp: July 29, 2020, 12:46:44 am
content: Yes, use the pos lead of the Aneng to the red clip on your BNC. In your video, I think you are just picking up noise, you have it set at 10mV/div. So now that we know that 5 volts is enough of change at the base of Q809 to deflect from one side to the other, lets see what we get when we use a proper input. You have about 0.5 volts, so if we set the CH2/X input to 100mV/div (I think that is 0.1V/div on your scope) then your meter's output should deflect the dot 5 divisions (if it were working).So do that. Set it to 0.1V/div on CH2/X, use XY mode as before and test at Q808 base, emitter, D2, D1 and the side of R835 that is not connected to Q808's emitter. Then we can compare the base voltage change and the D1-D2 change with what Q809 produces.

author: vexatag
author_rank: Regular Contributor
timestamp: July 29, 2020, 03:42:15 am
content: I figured they're probably just poorly shielded cables, I'll pick some decent ones up eventually.As for the measurements (set as you outlined, connecting pos-pos on the test load):Q808 emitter: 0.96 V to 1.55 VQ808 base: 1.53 V to 2.14 VD1: 143 V to 135 VD2: 154 V to 161 VR835 (Q809 side): 0.96 V to 1.55 V

author: bdunham7
author_rank: Super Contributor
timestamp: July 29, 2020, 04:31:16 am
content: OK, does the dot move when you do this? How far?If it does move, the go down to 50mV/div and do the same test again, including recording all the same voltages, then go down to 20mv/div and do it again, then 10, etc until you get either get a difference between load and no load of at least 5 volts on the base of Q808 or you get the dot to move at least halfway, preferably all the way, across the screen (you can recenter as needed to give it room to move).

author: vexatag
author_rank: Regular Contributor
timestamp: July 29, 2020, 04:49:41 am
content: In XY mode, the dot is not a dot but a 45 degree angle line as I mentioned hereQuote from: vexatag on July 28, 2020, 07:15:36 pmWhen I apply the voltage, the line shifts position up and to the right about 1 CM, in addition to getting larger and fainter. I don't know if this changes anything in what I should be looking for.

author: bdunham7
author_rank: Super Contributor
timestamp: July 29, 2020, 04:55:35 am
content: The 45 degree line is what happens when you feed the same signal to both channels in XY mode. There are noise and signal issues here that we haven't gotten to yet--I saw your videos and something is....wierd. But for now I want to see whether the horizontal final amp is fixed before moving on. So just to confirm, if you move the 'dot' or whatever it is with the XPOS knob, you apparently can get it all the way to the left or right. Does it change shape or anything when you are moving it with the XPOS knob?

author: vexatag
author_rank: Regular Contributor
timestamp: July 29, 2020, 05:06:17 am
content: Yes, if I move it from left to right quickly the trace distorts, I wasn't able to capture it on camera but it almost looks like a sine wave or a poorly ramped square wave.Here's the video.

author: bdunham7
author_rank: Super Contributor
timestamp: July 29, 2020, 05:42:51 am
content: OK, that isn't a problem. The reason that it distorts is that you don't really have a diagonal line there, you have a rapidly moving dot and persistence of phosphor (and vision) makes it look like a line. When you move it that quickly it can look like that. If you were able to move it quickly and evenly enough, you would even see a coherent waveform. I was concerned with whether there was a stable sort of distortion (bad CRT) or if it moved other than straight back and forth (somethingreallywierd). Since it doesn't do those things, that part is fine and you can proceed with further testing. If it helps, as you are turning the CH2/X volts/div down, you can leave the CH1/Y setting much higher, like 1V/div, if that keeps the line under control.That diagonal line is noise in the input amplifiers somewhere and it is correlated (in sync) between the two channels. First suspect would be bad power supply caps, so after you do the horizontal deflection tests, you can check the power supply ripple by measuring each power supply voltage on DCV and then ACV. ACV should be really low. I would try both of your meters just to see if you get different results.

author: vexatag
author_rank: Regular Contributor
timestamp: July 29, 2020, 06:30:58 am
content: Results of testing Q808 base with different volts per division:0.1 v/div: 1.57 to 2.1650 mv/div: 1.56 to 2.7520 mv/div: 1.56 to 4.5210 mv/div: 1.58 to 6.80Results of testing power supply voltages with the DM383:5V: 4.93 V DC, 0 V AC+12V: +12.23 V DC , 0 V AC-12V: -12.05 V DC, 0 V AC24V: 24.4 V DC, 0 V AC140V: 141.8 V DC, 0 V AC260V: 265 V DC, 0 V ACPower supply voltages testing, Aneng AN860B+:5V: 4.935 V DC, 0.000 V AC+12V: +12.23 V DC, 0.001 or 0.002 V AC-12V: -12.03 V DC, +0.002 or +0.003 V AC24V: 24.49 V DC, 0.000 V AC140V: 142.0 V DC, 0.000 V AC260V: 265.2 V DC, 0.000 V AC

author: bdunham7
author_rank: Super Contributor
timestamp: July 29, 2020, 02:55:49 pm
content: OK, but we also need to know what happens to D1 and D2 when the base voltage changes on Q808. Just the 20mv/DIV and 10mv/DIV will do. If it changes like it did on the Q809 test the horizontal amp is good.I thought about your issue with the line getting long and as you are doing the above test, try this. Set X and Y to the same (high) setting so you get that small 45-degree angle line. Then when doing the above test center that line so that it is goes through the center point on the screen with about half above and half below. Then when you do the readings, turn both down together and just let the line get long but only pay attention to the point where it goes thru the horizontal center line. That intersection should move left and right just like the dot would if it weren't all stretched out. Does it do that or does it do something else? If it gets faint or blurry, just turn up the intensity, dim the lights and make your best guess.

author: vexatag
author_rank: Regular Contributor
timestamp: July 29, 2020, 11:56:54 pm
content: Right, my bad.D1 testing:20 mv/div: 144 V to 96 V10 mv/div: 144 V to 65 VD2 testing:20 mv/div: 155 V to 186 V10 mv/div: 152 V to 241 VAs for the line, when the test load is applied the entire segment moves up and to the right, even at 5 mv/div.When checking CH1 in XY mode with a bnc lead in CH2, it's displaying a horizontal line of about the same length as the diagonal line from CH2. Normal mode is still just a trace, but I thought I should point that out.

author: bdunham7
author_rank: Super Contributor
timestamp: July 30, 2020, 12:55:06 am
content: OK, those numbers look good. As you can see, the plate voltage difference can be driven quite significantly by Q808, so that proves out the horizontal final amp, at least roughly. There are still some gain issues somewhere, but I think we are done in this area for now, as long as your bodge wires hold up and the resistors don't fry entirely. You'll need new ones eventually.QuoteI'm not sure yet what is going on here, and I'm not sure what you mean by 'normal mode is just a trace'. You have a trace in normal mode??? Or is it noise? What happens when you slow down the timebase to 0.1S/div?So lets look at the input circuits. It is the 'Attenuators and Pre-Amplifier' schematic, followed by 'Vertical Amp and Control Logic'. Something strange appears to be happening here. Try setting the inputs to GRD, XY-mode on, all 4 of the center pushbuttons out, 5mv/div and try to center the dot or whatever is showing. Then reduce the input sensitivity to 10mv/div, and so on. Tell me what you see and make a video if you want--those are helpful--but make sure you get the whole scope face in it so I can see every control.

author: vexatag
author_rank: Regular Contributor
timestamp: July 30, 2020, 06:16:31 am
content: Quote from: bdunham7 on July 30, 2020, 12:55:06 amSorry, I used the wrong wording. In non-xy mode, channel 1 is still just showing a single dot.Here'sthe video of me testing the inputs. The thing I had the second bnc leads plugged into was just a breadboard with some component leads and pin headers I've been using as a breakout board for slightly more convenient access to the hooks ever since the ends of the hooks got bent out of shape (hook onto the component lead, press the multimeter lead against the pin header).

author: bdunham7
author_rank: Super Contributor
timestamp: July 30, 2020, 02:38:30 pm
content: OK, a couple of things. First, the video did help, but there are two misunderstandings about scope theory that are apparent.First, I didn't elaborate, but when you switch to the GND, you are disconnecting the inputs internally and shunting the preamplifier to ground. Once you verify that the inputs are unresponsive, you know that that feature is OK and there's no point in supplying any inputs.Second, in XY mode, those 4 pushbuttons don't do anything. They don't change the channel selection, for example, which I saw you pushing.You can and should look at the schematics that I referenced and look at the diagrams of at least the switches to see if you can understand how that works.In any case, we've determined that the problem seems to be on the input side. I also noticed the strange behavior as you were changing the CH2/X attentuator level. It's not uncommon to see a brief jump in the dot as you change ranges, but on CH2/X, it breaks up or jumps at a 45-degree angle--but that channel should only affect horizontal position. So there's some leakage or crossover between channels and it looks roughly equal.We can also try some testing in the normal, non-XY mode. If you leave the inputs grounded, all 4 pushbutton out and turn XY off, what happens?Do you have a 9-volt battery, or preferably, several of them? I'd like to try using a larger input signal through the attenuators to try and track down the area where the noise is generated.

author: vexatag
author_rank: Regular Contributor
timestamp: July 30, 2020, 08:49:25 pm
content: Very quick comment, I just wanted to say that when the input is set to DC or AC, the channel selection does switch between the two channels. I don't know if they're supposed to or not after what you've just told me but it's the functionality I've been experiencing so far. Also, I do have several 9v batteries lying around.EDIT: Switching to normal mode with the inputs grounded shifts the dot to the far left of the display, additionally it limits the rightward motion to one square right of center.

author: bdunham7
author_rank: Super Contributor
timestamp: July 30, 2020, 10:07:45 pm
content: Quote from: vexatag on July 30, 2020, 08:49:25 pmI actually don't know what happens if you push the channel select button in XY-mode, I'd have to look at the schematic. It is possible that it does something like reverse it into YX mode or something weird, but that would be an undocumented feature--or a bug.So, in normal with the dot to the left try switching one channel at a time from GRD to DC, then one at a time to AC, then both to DC and last both to AC. What happens?Then, go back to both set to GRD and connect a BNC-to-clip cable to your EXT TRIGGER connection, trigger level centered and knob pulled out, all six pushbuttons out, COUPLING set to DC/LF, SOURCE set to EXT, timebase to the slowest setting (0.2S/div?) and then briefly connect the 9 volt battery to the cable clips, red to positive (the smaller terminal on the battery). If nothing happens, try turning the trigger knob a few degrees clockwise at a time and repeating. If still nothing, try every position of the trigger knob (5 or 6 places).Next, let's test the vertical amplifiers directly. Turn both channel attenuators counterclockwise to the 5V/div setting. Unground one at a time to the DC setting and give it the same 9 volt input you just did with the trigger. Then do the same in the AC setting. Don't change any other settings the first time, but then push in the second vertical mode button to CH2 and repeat the test on both channels. Lastly, repeat all that except set BOTH channels to DC (you don't need to do AC right now) but run the test one channel at a time, again doing both with the 2nd vertical mode button out, then both with it in.That seems like a big rigmarole, but we're looking for crosstalk between channels as well as vertical amplifier response. The 9-volt battery should give you a little less than 2 divisions of movement where it moves anything, so a lot more or less than that should be noted.

author: vexatag
author_rank: Regular Contributor
timestamp: July 30, 2020, 10:39:22 pm
content: Quote from: bdunham7 on July 30, 2020, 10:07:45 pmThere is no visual difference between any of these settings in normal mode.Quote from: bdunham7 on July 30, 2020, 10:07:45 pmNothing at any of the positions on the trigger knob.Quote from: bdunham7 on July 30, 2020, 10:07:45 pmI could only get the dot to jump upwards when directly connected to the channel in question (only on channel 1 when the bnc was plugged into CH1, CH1 was set to DC, and the CH1/CH2 switch was not pressed in).Also, I checked the manual and the four vertical mode switches are in fact supposed to be disengaged when XY mode is engaged. So yes, something is wrong on my end

author: bdunham7
author_rank: Super Contributor
timestamp: July 31, 2020, 04:15:50 am
content: OK, now we are getting into a tricky area. Give me some time and when I have a chance, I'll look at the schematics and try to figure out how much you can test with what you have. You can have a look at the Trigger and Sweep Generator schematic, and then look at the board for those same parts and just visually check everything and see if there are any more burned parts or physical damage. The circuit in question is very near the previous area that you repaired, so maybe you'll get lucky and spot something.Basically what I'm looking at is the Trigger and Sweep Generator, along with the Timebase Generator, are pretty much self contained. The LINE and EXT trigger options don't go through the regular channels and they feed the sweep ramp directly to your old friend R834, and we know things are good from there. The operation of the vertical amps in the previous test make me optimistic that those are functioning well enough for now, even though the XY-mode seems to be screwed up.So, have a look. See if you can find R602, upper left hand corner of the Trig/Sweep schematic, near the trigger source switch. See if you can verify that there is -12V on both sides of it.

author: vexatag
author_rank: Regular Contributor
timestamp: July 31, 2020, 05:08:32 am
content: Found R602, it's either near or in the repair job someone else had done. There's no obvious charring or anything to make me think it was damaged. The side connected to ground measured -12V, but the other side measured -9.36V. Checking the resistance, it's 4.68k ohms, so the drop is probably coming from somewhere else in the circuit?

author: bdunham7
author_rank: Super Contributor
timestamp: July 31, 2020, 02:54:55 pm
content: That seems OK, although if you switch the SOURCE to ALT, the drop should go away and both sides would be -12V.So we're headed into a tricky section here, especially since there could be some variance between the schematic and your circuit board due to the different versions I alluded to much earlier. It hasn't happened yet, but it is one more thing to worry about. I'm still pondering how to test this without another scope, but there are a few things you should check first just so we don't miss something simple.The upper left portion (on the schematic, not the board) of the Trig & Sweep Generator schematic is the basic input switching part. You can follow a 9V input into the EXT input through the switches all the way to the SLOPE switch S603, and you can do that with the power off. See if you can figure out how to do that just looking at the schematic. On the BK2120 I had, the COUPLING and SOURCE switches were damaged and bent from some unknown abuse, but straightening them out seemed to fix them, so that is one of the first things to check.Next, those bodge wires near this area. It occurred to me that they might be some sort of unsuccessful repair or modification or patch, instead of an actual repair that worked. Can you look carefully at those wires and tell me exactly where they are attached in the schematic? Is there any other damage to the board, or missing parts orLast, let's have a quick look at the LINE trigger option. If you see the input on the left edge marked FROM BD503, LINE 60HZ, find that point on the board and R610. This is power on--you should have an AC voltage on both sides of R610 and then the same voltage at the upper center terminal (on the schematic--you have to find where this is on the switch) of S603. The voltage will only be at S603 when the COUPLING switch is in LINE, so you can differentiate it from noise by turning the switch from LINE to some other setting and back (slowly--AC readings take time to settle) and so on. The LINE signal will be attenuated at each step through the system, so don't worry if it is lower as you go along--just see if it is there and comes and goes in response to the switch.

author: vexatag
author_rank: Regular Contributor
timestamp: July 31, 2020, 11:39:30 pm
content: Quote from: bdunham7 on July 31, 2020, 02:54:55 pmSomewhat confused as to what you mean by this, hook the battery up to the ext port and check for voltages at each component in the circuit? It's been a hot day and my motivation has been low, but I'll remove the circuit board and try to figure out the bodge job. I can't find R610 on either the Trig and Sweep Generator schematic or anywhere on the circuit board.

author: bdunham7
author_rank: Super Contributor
timestamp: August 01, 2020, 12:18:38 am
content: Yes, just connect the battery to the EXT trigger BNC with your handy cables, switch the mode to EXT and then follow the trail as far as the slope switch with your DMM in the usual way. If the board is out, you'll have to find a ground point, but the BNC or battery negative should do fine. And you can change the switch back and forth from EXT to ALT to make sure your battery signal comes and goes.As far as motivation goes, you've been working on that thing for a while. I'll still be here later if you want to give it a break for now!

author: bdunham7
author_rank: Super Contributor
timestamp: August 01, 2020, 12:40:27 am
content: Quote from: vexatag on July 31, 2020, 11:39:30 pmR610 on the schematic is on the left side right next to the "FROM BD503 LINE 60 HZ". It is all run together as R61027K. I don't see in on your board either, nor on the board diagram in the manual, but it should be connected to R611 and might be off-board or connected by a wire. It should be near a wire coming from the power supply---if you look at that schematic you'll see it coming off of the top of bridge rectifier BD603--'to R610'.I've noticed the first version discrepancy--the trigger op amp U601 in the manual is U615 on your board. There might be more, so if you see anything clearly different let me know.EDIT: R610 is on the low voltage power supply board.

author: vexatag
author_rank: Regular Contributor
timestamp: August 01, 2020, 07:42:22 am
content: Re: I've been working on this a while, it's the sort of thing where if I set it down I won't take care of it for many many months, this thing is big enough that I want it to be workingQuote from: bdunham7 on August 01, 2020, 12:40:27 amWeirdly, U615 is also called U601 on the opposite side of the PCB.Voltages for each part I could find in the circuit, listed in the order I tested them. Some are probably only used for other triggering modes but I tested them anyway. I tested with the coupling switch set to DC/LF and the source set to ext, I was able to have the board out with everything except the CRT inputs plugged in. All numbers are in volts, none of that mv rubbish.Battery voltage: 9.39Voltage at the switch: 9.16R601: 9.16 to 9.37C601: 9.16 to 9.37R602: 0.00 and 0.00R605: 9.16 to 9.08R603: 9.16 to 0.00R606: 9.09 to 4.745C603: 9.16 to 0.482R604: 0.199 to 0.00R607: 0.200 to 0.200C630: 0.415 to 0.417R609: 4.024 to 4.804C604: 9.08 to 4.787C606: 4.097 and 0.00I went ahead and checked the resistance of the resistors that had an input voltage of ~9V and an output voltage of 0V, all were within spec.The other testing will happen tomorrow.

author: vexatag
author_rank: Regular Contributor
timestamp: August 01, 2020, 10:40:31 pm
content: R610 measured at 7.3 V and 1.67 V AC, I also found two points on S603 that measured 1.67 V AC only when the coupling was set to LINE.

author: bdunham7
author_rank: Super Contributor
timestamp: August 02, 2020, 03:56:15 pm
content: OK, remember those two points, that is the input side of the switch.So I think the next thing to do is make a test tool. You'll need a 9 volt battery, a clip or something to reliably wire to the battery (I snip them off of dead or obsolete devices) and the 10K potentiometer you said you had earlier. Wire up the 9 volt battery to the outside (fixed) terminals of the potentiometer, then attach one wire to the center (variable) tap and one to the side where the negative battery lead went. You should now have a short-circuit proof variable voltage source, which you can confirm with your voltmeter. It might be worthwhile to take the time to make the whole thing neat and durable.Next, connect it to the EXT trigger in with DC/LF/TV-V and try to find a signal right where you found the AC signal on S603. If everything works, you should be able to turn your pot up and down and see the voltage change in sync at the center input terminal of S603.Last for now, move your voltmeter lead to the switch terminal exactly opposite of wherever you find your signal in the last step. By 'opposite', I mean in the other row--there's two rows of 4 or 5 terminals, same postion/opposite row. Then, with the scope turned on and the trigger level knob not it AUTO (pulled out), you should be able to turn it up and down and see a similar change in voltage at that point. These are the inputs to U601 (or U615??) which is the trigger comparator--it turns on when the voltage on the input side exceeds the voltage on the trigger level side--or vice versa if SLOPE (-) is selected.The rest of the trigger logic circuit is more complicated and I'm still trying to figure out a way to have you test it with what you have on hand.

author: vexatag
author_rank: Regular Contributor
timestamp: August 02, 2020, 09:06:48 pm
content: I've also got an eight-AA holder if that'd maybe work better? The only 9v clip I have to sacrifice is on the 1khz signal generator and I'd hate to sacrifice it.

author: bdunham7
author_rank: Super Contributor
timestamp: August 02, 2020, 09:18:45 pm
content: Yes that should work. 10 x 1.5V =12V. 12V thru 10K pot = 1.2mA, 14.4mW dissipation. But you need 8 AA cells, I suppose the Sunbeam ones from a dollar store will work.Tell me more about this signal generator, like output specs and so forth. It might be handy.

author: vexatag
author_rank: Regular Contributor
timestamp: August 02, 2020, 09:31:14 pm
content: The holder actually has eight rechargeable AAs in it, I've been using it to power a 140mm PC fan I've been using to extract rosin fumes when soldering. It has a standard 5.5mm DC barrel jack, so hooking it to the pot should be easy enough. (and yes, I've got 5.5mm barrel jacks for the receiving end, I purchased a twenty pack with ten of each for an earlier project)The signal generator is aVelleman MK105, I believe I mentioned it earlier in the thread as one of my resources.EDIT: I hooked the positive line to pin 1 and the negative line to pin 3, next I hook up a wire connecting pin 2 to pin 3?

author: bdunham7
author_rank: Super Contributor
timestamp: August 04, 2020, 12:16:13 am
content: Your output wires will be pin 2 (+) and pin 3 (-). Try it on a voltmeter.

author: vexatag
author_rank: Regular Contributor
timestamp: August 04, 2020, 06:43:50 am
content: I totally misunderstood how potentiometers work, whoops. The only thing I added was component leads on 2 and 3 to give my garbage scope probes something to hook onto.Quote from: bdunham7 on August 02, 2020, 03:56:15 pmI did the tests you asked me to and was able to get AC voltage changes on both pins.Side note, I did find four three digit voltmeter modules from a previous project (rated at 2.6-30V according to the packaging), so that's a cool thing (or would be if they cooperated with the 10k pot, I hooked one up to pin 2 and 3 but it didn't respond correctly, the display dimmed at 2.35 v with the pot mostly closed but cut out at 3.5 with the pot mostly open. It's something I'll need to look at further later I think).

author: bdunham7
author_rank: Super Contributor
timestamp: August 04, 2020, 04:05:06 pm
content: The voltmeter modules draw their power from the circuit and the 10K pot can't supply much current. They aren't a good idea for this application.So on those tests, what ranges of voltages did you see on the input side and on the trigger level side? You should do this again and make a chart showing what the input voltage side measure at with the pot all the way up, 75%, 50% and so on. Then do the same for the trigger level pot. You will need to be able to set certain voltages without too much measuring because the next step will be to send those voltages to the comparator U601 and observe its output.

author: vexatag
author_rank: Regular Contributor
timestamp: August 04, 2020, 10:33:48 pm
content: The changes were very brief unfortunately, generally ranging between 1.25 V and ~3 V. I don't think I saw anything higher than 5.5 V. Of course, it might have just been noise since it also settled down to 0 V. This definitely seems like the sort of situation where another scope would be useful, is it time to try using a soundcard scope?

author: bdunham7
author_rank: Super Contributor
timestamp: August 05, 2020, 12:24:32 am
content: The one on the trigger level side should be steady. The other one, I think, should also be steady if the coupling is set to TV-V/DC, but let me review the schematic. Are you saying they both settle down to 0 volts no matter where you put them?Yes, we are getting to the point where another scope would be extremely useful. I don't want you to damage your computer, but I suppose as long as you used high value resistors for the input maybe it works. Do you have soundcard scope software and an adapter cable? Your sig-gen might be useful too, but the output is low.

author: vexatag
author_rank: Regular Contributor
timestamp: August 05, 2020, 12:46:48 am
content: I'm not actually sure which side is which, but both sides settle at 0 V when changing the pot on the battery pack, one side settles at 0 V with minimum trigger level and 0.031 V at maximum trigger level, the other side settles at 0 V with minimum trigger and 0.015 V with maximum trigger.Quote from: bdunham7 on August 05, 2020, 12:24:32 amI've got both of those things, it's somewhat janky because I bought the wrong connectors but seems better than nothing.

author: bdunham7
author_rank: Super Contributor
timestamp: August 05, 2020, 03:57:35 pm
content: OK, why don't you see if you can get your soundcard scope to work with your signal generator and show some waveforms. Then you can try feeding them in to see if you can trip the comparator. I don't actually think the problem is in this area, but it needs to be confirmed before moving on to the trigger logic circuit. Use a resistor on the input of the soundcard scope, I'd recommend at least 10K, to minimize the possibility of computer damage.

author: vexatag
author_rank: Regular Contributor
timestamp: August 06, 2020, 03:06:00 am
content: Quote from: bdunham7 on August 05, 2020, 03:57:35 pmDone, waveform screenshot is attached.Quote from: bdunham7 on August 05, 2020, 03:57:35 pmWhat do you mean by that? Use the signal generator as the EXT input and run the previously mentioned tests?

author: bdunham7
author_rank: Super Contributor
timestamp: August 06, 2020, 04:33:05 am
content: Yes, do the same tests to prove out the system. Make sure you have the ground side of the signal generator and the soundcard scope both together at the same point (probably the black lead of your BNC clip cables). You may want to pretest each new test point with your DMM in AC mode to make sure there isn't more than a few volts present before hooking up the computer.Then, one additional step. If you have a signal at S603, connect the scopecard to the #9 pin of U601 (you may want to either look up IC pinout rules--or post a photo) and see if you can get any signal by adjusting the trigger or even pushing it in and using auto.You can see on the schematic that if finding and connecting to pin 9 is too hard, then the correct sides of R470 or R632 will do.

author: vexatag
author_rank: Regular Contributor
timestamp: August 06, 2020, 06:12:32 am
content: So, BNC 1: red lead to signal gen out, black lead to signal gen gnd, connector to EXT. BNC 2: red lead to test point, black lead to signal gen gnd, connector to soundcard scope. Is this a correct understanding?

author: bdunham7
author_rank: Super Contributor
timestamp: August 06, 2020, 01:19:40 pm
content: Yes, that's it.

author: vexatag
author_rank: Regular Contributor
timestamp: August 07, 2020, 05:37:35 am
content: The ends of my bnc probes are bent out of shape and refuse to stay clipped to the gnd/out pins of the signal generator or the test points of the switch.

author: bdunham7
author_rank: Super Contributor
timestamp: August 07, 2020, 02:09:17 pm
content: Crappy test leads are worse than crappy instruments.These are not high-end, but they should be acceptable.https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2047675.m570.l1313&_nkw=401929974978&_sacat=0EDIT: The 10X feature of the probe will also protect your computer soundcard in case you make a mistake.

author: vexatag
author_rank: Regular Contributor
timestamp: August 07, 2020, 07:54:32 pm
content: Alright, guess this is on hold until I can get the money in one place.

author: vexatag
author_rank: Regular Contributor
timestamp: March 09, 2021, 04:42:54 am
content: Well that took a bit longer than expected. New probes have been ordered and I should be able to resume testing this next week.I've also got the funds to get a cheap but somewhat proper oscilloscope like a Miniware DS212 and a slightly more proper function generator (some ~$80 Aliexpress special), would those be good purchases towards getting this thing running?

author: bdunham7
author_rank: Super Contributor
timestamp: March 09, 2021, 08:35:01 pm
content: There's another recent BK 2120 repair thread in this forum with a problem sort of similar to yours. Have a look.

author: vexatag
author_rank: Regular Contributor
timestamp: March 09, 2021, 09:55:54 pm
content: I saw that, I'm keeping an eye on the thread to see where it goes.

author: vexatag
author_rank: Regular Contributor
timestamp: March 14, 2021, 12:32:13 am
content: Probes arrived, how should I use them? Set them to 10x mode and set them up like in this post?Quote from: vexatag on August 06, 2020, 06:12:32 am

author: bdunham7
author_rank: Super Contributor
timestamp: March 14, 2021, 07:39:05 pm
content: It's been a while, remind me where you are at with your scope. I seem to recall that you have a dot on the screen but no horizontal sweep, right?As for your new probes and sig gen, can you just post a photo of everything so I can see it? Far easier then explaining. The first thing to accomplish would be to make sure you can generate a signal and view that signal on your computer/soundcard scope setup without connecting anything to the BK scope.

author: vexatag
author_rank: Regular Contributor
timestamp: March 14, 2021, 07:47:31 pm
content: Yes, a dot but no horizontal sweep. I'll set things up and take a picture in a little while, need to clear things out.Edit: I also checked the probes with just the signal generator last night, got a good waveform with 1x but not 10x.Edit 2: Sorry if I misunderstood things, is this what you wanted to see?

author: vexatag
author_rank: Regular Contributor
timestamp: March 14, 2021, 11:38:14 pm
content: A double post, for which I'm sorry, but I'm trying to figure out what I was trying to test back in August. Was it the waveforms on a few of the pins in S603 followed by one of the pins on U601?

author: bdunham7
author_rank: Super Contributor
timestamp: March 15, 2021, 03:13:14 am
content: OK, what does the input to the soundcard scope look like?And what is wrong with your signal at 10X? We really want to use the 10X setting if possible.You can use the signal generator or we can just use the line trigger if you can get a good picture of it on your scope setup. Switch S603, then pin 9 of U601 and pins 3, 5 and 5 of U603. Screenshots would help.

author: vexatag
author_rank: Regular Contributor
timestamp: March 15, 2021, 03:39:51 am
content: Quote from: bdunham7 on March 15, 2021, 03:13:14 amAnswering these questions with two images, first is the scope with the lead at the 1x setting, second with the lead at the 10x setting. I zoomed in and compared it to the unplugged second channel, it's just background noise. I can most likely test the other components on Monday, the school quarter is almost done and I have a few more assignments to take care of.One more thing, when checking the board over I noticed that all the variable resistors on the board have some adhesive goop on them, but the application on the sweep length resistor is nearly non-existent, almost like someone has adjusted it. I'm not checking it out just yet but I've attached a pic of what I mean for reference.

author: bdunham7
author_rank: Super Contributor
timestamp: March 15, 2021, 04:05:29 am
content: OK, soundcard 'oscilloscope' apparently has a low input impedance, so no 10X won't work.How do the probes connect to the soundcard? There needs to be some additional resistance put in there somewhere.

author: vexatag
author_rank: Regular Contributor
timestamp: March 15, 2021, 04:11:39 am
content: Guess I should delay the testing process until it can be done safely, eh?The signal chain is probe > BNC to RCA adapter > RCA Switcher > RCA to 3.5mm jack > 3.5mm cable > USB soundcard. Would it be a better idea to bite the bullet and get a different oscilloscope for testing? In my first 2021 update I mentioned that I was thinking of getting a DS212, so it would be within my budget.

author: vexatag
author_rank: Regular Contributor
timestamp: March 15, 2021, 06:51:03 pm
content: Update, I bought a DS212 on Ebay, it should be here by next week.

author: vexatag
author_rank: Regular Contributor
timestamp: March 18, 2021, 10:49:27 pm
content: DS212 arrived, it has built-in signal generation (square, sine, saw, from 10hz to 1mhz) and proper 10x attenuation support. I'm guessing I should hook the output channel of the 212 to the external trigger of the 2120 (dangerous potential for typos here) using a 10khz square wave, then connect the ground of channel A to the output channel ground and probe the points? Additionally, should testing be done with the 2120 on or off? It's been long enough that I don't remember how I was testing it.

author: bdunham7
author_rank: Super Contributor
timestamp: March 18, 2021, 10:53:07 pm
content: Quote from: vexatag on March 18, 2021, 10:49:27 pmYes, that sounds correct, 2120 on, but be careful and use 10X all the time. Also, I don't think you need to worry about connecting grounds since all the BNC shells on the 2120 are grounded.

author: vexatag
author_rank: Regular Contributor
timestamp: March 18, 2021, 11:27:29 pm
content: In that case I should connect the 212 input probe to the chassis, yeah?

author: bdunham7
author_rank: Super Contributor
timestamp: March 18, 2021, 11:38:29 pm
content: I'm not familiar with the DS212. I had a quick look and I'm not sure how it connects. Can you use your BNC scope probes with it? Does it need/have an adapter for those? Or are those connectors on the DS212 actually BNC somehow?

author: vexatag
author_rank: Regular Contributor
timestamp: March 18, 2021, 11:44:58 pm
content: The connectors aren't BNC, but they're easily adapted. I bought one adapter (should have bought two) and it came with one compatible lead.

author: bdunham7
author_rank: Super Contributor
timestamp: March 18, 2021, 11:49:47 pm
content: Quote from: vexatag on March 18, 2021, 11:44:58 pmOK, then you should be good to go. The signal to the EXT trigger can be the compatible lead and you can probe with the BNC adapted lead set to 10X. Just don't go near the high voltage like the other guy...

author: vexatag
author_rank: Regular Contributor
timestamp: March 18, 2021, 11:54:14 pm
content: Yeah haha, gonna check each point with my multimeter first.

author: vexatag
author_rank: Regular Contributor
timestamp: March 19, 2021, 02:46:27 am
content: Quote from: bdunham7 on March 15, 2021, 03:13:14 amWhich other pin did you mean here?EDIT: Judging by the other BK owner's thread I'm assuming you want me to test pins 3, 5, and 6 on U603.Extra edit: tested S601, the two end pins have triangle waves depending on the switch position. I have screenshots if you would like. As for U601 pin 9, there's not enough signal to pick up even at a low voltage scale (is that the correct term?). Testing U603 now, will update with my findings.Third edit: U603 pin 3 had a triangle wave, pins 5 and 6 just had noise.

author: vexatag
author_rank: Regular Contributor
timestamp: March 22, 2021, 02:01:33 am
content: Here are the screenshots, the first two are the switch pins and the third is U603 pin 3. I also ordered two more BNC adapters.Side note, the 212 has a math channel with a-b mode, so I could easily use it as a differential scope if that would be useful or relevant at all.Secondary side note, I tested the calibration point on the 2120 and it seems to be working.

author: bdunham7
author_rank: Super Contributor
timestamp: March 22, 2021, 02:25:39 am
content: Screenshots are good. I need to figure out what those translate to as far a scale.Can you show pins 5 and 6 of U603? How much noise?Edit: Just for reference, can you show pins 7 and 14 of U603? Should be ground and 5V, but I just want to see where they are.

author: vexatag
author_rank: Regular Contributor
timestamp: March 22, 2021, 02:33:33 am
content: I'll retest things later this evening with measurements onscreen (forgot to set them up last time).

author: vexatag
author_rank: Regular Contributor
timestamp: March 22, 2021, 04:45:55 am
content: Ok, here are screenshots of the pins being tested.Screenshot order:Switch pins when activeU601 pin 9U603 pin 3U603 pins 5 and 6U603 pins 7 and 14If needed, I can check the pins again in 1x mode because there's something fishy going on here.

author: bdunham7
author_rank: Super Contributor
timestamp: March 22, 2021, 06:05:24 am
content: Is the scope turned on???

author: vexatag
author_rank: Regular Contributor
timestamp: March 22, 2021, 06:06:45 am
content: Yeah, scope is on.

author: bdunham7
author_rank: Super Contributor
timestamp: March 22, 2021, 06:14:33 am
content: OK, on U603, pin 7 is ground, pin 14 is 5 volts. Use your multimeter and see if that is so. If so, then you have to figure out why your scope doesn't show you that in those measurements--and the 10X setting is not likely to be the cause, so leave that at 10X for now to avoid any calamities. If you don't have proper power supply to U603, then we have a different issue somewhere. You checked all the power supplies way, way back, right?

author: vexatag
author_rank: Regular Contributor
timestamp: March 22, 2021, 06:26:47 am
content: Quote from: bdunham7 on March 22, 2021, 06:14:33 amPin 7 goes to ground, pin 14 measured around 4.3 volts.Quote from: bdunham7 on March 22, 2021, 06:14:33 amI checked the power supplies at the beginning of this process, yeah.

author: bdunham7
author_rank: Super Contributor
timestamp: March 22, 2021, 06:03:18 pm
content: OK, let's back up (again) and make sure your new scope can give you a proper reading.Are you not using a ground clip on the scope probe? I believe I may have told you not to, but I'm not familiar with your little scope and it may have isolated inputs, so lets try this:1) Don't use the signal generator at all--disconnect it and set the leads aside2) Use the LINE trigger setting on the BK21203) Use one probe on the scope and connect the ground clip securely, preferably to one of the BNC shells or alternatively to the frame of the instrument.4) Try again, starting with pins 7 and 14 of U603. You need to be able to see 0 (or so) volts and 5 volts (4.3 is probably OK) just as your multimeter shows before proceeding.5) If that works, start looking for the 60Hz line trigger signal, starting again at the switch pins, then pin 9/U601 and pin3/U603. Screenshots are needed here.6) If those look good, then pins 5 and 6 of U603.

author: vexatag
author_rank: Regular Contributor
timestamp: March 22, 2021, 07:56:06 pm
content: I have been using the ground clip on the new scope, yes. Additionally, I've stopped using the external signal generator and have instead been using the new scope's built-in function generator.The first two screenshots are from the slope switch with coupling set to LINE. When the ground cable was connected to the chassis, I got 4.3 volts to show up on pin 14 once in like 20 minutes of testing at various timebases, however when I checked without the ground cable connected I instantly got a reading. That reading is in the third screenshot. Going to hold off on testing the rest of U603 and pin 9 of U601 until you've had a chance to analyze this information.

author: bdunham7
author_rank: Super Contributor
timestamp: March 22, 2021, 08:20:49 pm
content: Is there an option in that scope for AC and DC coupling? Pin 7 of U603 should be a more or less flat line at about the level of the A channel marker and pin 14 should be a flat line about 5 volts (2.5 divisions) higher than that.

author: vexatag
author_rank: Regular Contributor
timestamp: March 22, 2021, 08:25:51 pm
content: Quote from: bdunham7 on March 22, 2021, 08:20:49 pmThe DS212 has AC/DC coupling, yes. I've been using AC coupling in all the readings so far.Quote from: bdunham7 on March 22, 2021, 08:20:49 pmJust checked this in DC coupling and you're correct. Going to go ahead and check the remaining pins in DC coupling.

author: bdunham7
author_rank: Super Contributor
timestamp: March 22, 2021, 08:35:34 pm
content: Change to DC coupling and try all that again!

author: vexatag
author_rank: Regular Contributor
timestamp: March 22, 2021, 09:37:46 pm
content: Order of the screenshots: Slope switch pins (two images), U601 pin 9, U603 pins 3, 5, and 6.

author: bdunham7
author_rank: Super Contributor
timestamp: March 24, 2021, 02:37:43 am
content: I don't know why you don't have a signal at U601 pin 9, but have it at U603, pin 3. That seems strange.Nonetheless, the problem appears to be that U603 isn't switching, even though it has an input that should switch it. I'm probably not going to be able to think about this for a day or two, but in the meantime, you can try scoping every pin of U603 and posting all the screenshots. I think you are getting close to the problem area.

author: vexatag
author_rank: Regular Contributor
timestamp: March 26, 2021, 05:07:49 am
content: Sorry this took a few days, I just got a vr headset so that's been using a lot of my time. Here's the screenshots of U603's pins, I hope a zip file isn't too suspicious for you.

author: bdunham7
author_rank: Super Contributor
timestamp: April 14, 2021, 03:49:14 am
content: I may have forgotten about you! Where were we on this?

author: vexatag
author_rank: Regular Contributor
timestamp: April 19, 2021, 12:51:28 am
content: We were looking at U601 and U603, I scoped every pin of U603 and put the screenshots in a zip folder

author: bdunham7
author_rank: Super Contributor
timestamp: April 24, 2021, 10:27:02 pm
content: OK then, lets dig in to the mess with the 7474 logic ICs. Prior to this I had no specific idea how this circuit works, so I'm figuring it out with you right here.The 7474 is a dual D-type flip-flop, each half works independently but they share power and ground. The general idea of the trigger circuit is that when it sees a trigger pulse coming from U601, it turns on the beam (UNBLK PULSE to R521) and starts the timebase generator sweep. It then ignores any additional trigger pulses until the timebase sweep is done, then the circuit resets and waits for another trigger.Looking at this circuit, we can start with the 1/2 of the 7474 that gets the trigger signal. By your scope shots we can see that power (14) and ground (7) are good, so we only need to worry about pins 1 to 6. Since it is TTL logic, the signal should be mostly either around 0 volts or 5 volts--LO and HI. Pins 1,2,4 and 5 are HI, pin 6 is LO and pin 3 is the only one that moves--it is seeing the trigger signal from U601. We then have to look at the function table to see what that all means and what state the flip-flop is in. Pins 5 and 6 are complementary outputs Q and Qnot, which means they will generally be the opposite of one another, with the transient exception shown.Pins 1,2 and 4 are inputs CL(ea)R, D and PR(eset), so all three being HI correspond to the 4th line in the function table. If the D line were to revert to low, you would expect the outputs to flip on the next trigger. However, by the wiring diagram, you can see that D is connected right to +5V, so it will never be low. As you can see by the function table, the only other way to get the flip-flop to transition would be for the CLR to go LO while the PR stays or becomes high. Then the CLR would have to revert to HI and the flip-flop would be 'armed' and ready to transition back to the fourth state when the CLK transitions from LO to HI.It is hard to say where the problem so far. It looks like CL (pin 1) is pulled low by pin 8, which is the Q output of the second half--we need to call these CL1 and Q2 now--so now we have to look at the second D-flip-flop. Pins 8,9,10,11 and 12 are all high, pin 13 is low. So the only way both outputs can be high is line 3, which requires PR2 and CLR2 to both be low--which they aren't. So something is wrong here. I had a closer look at the scope shots and I now see that Q2 pin 5 is not all the way high--3.40 volts indicates a problem or struggle with excessive source current. Q2notis also not all the way at 5 volts, so it may be trying to go low but something is exceeding its sink current limit. Or maybe the 7474 is bad--that would be too obvious!Here are some things to try next:Use your scope and check all the pins of Q601, Q602, Q603, Q604 and Q605. I'm not sure how they are marked on the board, so photos may help. Then check C618 and C629 to make sure they aren't shorted to ground. Then fidn a way to disconnect the wires that go to UNBLK PULSE and ALT (at the top of the diagram) and then recheck al the pins of U603 to see if anything changes.

author: vexatag
author_rank: Regular Contributor
timestamp: April 26, 2021, 08:26:02 am
content: Testing on this is most likely going to wait until the weekend, I closed up the scope and the place I had it has quickly grown cluttered.Quote from: bdunham7 on April 24, 2021, 10:27:02 pmAm I correctly assuming that you're saying to only disconnect the UNBLK PULSE and ALT wires? From what reference photos I have on my computer I think those two wires are part of a multi-wire plug.

author: slbender
author_rank: Regular Contributor
timestamp: July 03, 2021, 03:22:10 pm
content: Some ten or so years back, I opened up an undamaged on the outside (read - minty looking) B&K-2190 that had one channel not working, to have a look. I found near the input on the dead channel - an open NEC dual JFet of some part number (unobtainium) that could not be found. Also, many bad internal soldering problems, it was like… how did this set ever work?Every plug-in connector I touched was either loose or cold-solder jointed to its board. What a mess! It’s still unassembled like ten years later. I thought about using an LSK-389(a)(b)(c) dual JFet, but have no idea if it’s even a close match. Never got around to curve tracking the NEC dual JFet from the working channel, due too many other projects not needing a hundred hours of solder-sucking and re-soldering. So beware and consider that multiple cold solder joints may also be a possible “failure mode”. In any B&K 21xx scope that is not working.Steven

author: ccendo
author_rank: Newbie
timestamp: June 22, 2024, 03:21:55 pm
content: Did you ever get this oscilloscope working? I hate to see such a long repair end without a satisfying conclusion.