Tektronix 2246 - No beam! - Page 1
Forum section: electronics / testgear

author: cs.dk
author_rank: Unknown
timestamp: October 19, 2013, 05:12:04 pm
content: Hi,Just got a Tektronix 2246 for free - But there is no beam. The CRT "backlight" is on, and if i switch the power quickly, is gives a short trace/beam. I think maybe the powersupply could be a culprit? I just don't like the HV, how to measure something there?.I don't know what the screen should look like, when powered up, so i attached a couple of pics.All clues are welcome, thanks in advance

author: PaulAm
author_rank: Frequent Contributor
timestamp: October 19, 2013, 06:08:28 pm
content: That's a nice scope.If you see a flash of trace, it's almost definately not in the HV supply.Get a service manual and check the LVPS test points. There's probably good odds that a tant has shorted someplace and is holding down a PS rail. If you can't find a manual, there may be a board on the side of the scope with test points and voltages labeled. I'm not sure of that - I haven't had one of those open, but it was done in other 24xx series.If you find a dead rail, check the resistance to ground and see if it's shorted.

author: David_AVD
author_rank: Super Contributor
timestamp: October 19, 2013, 10:30:05 pm
content: Is the trigger set up correctly? If not, it won't display a trace.

author: sync
author_rank: Frequent Contributor
timestamp: October 20, 2013, 01:08:20 am
content: Yes it's a nice scope. I have a 2246A.Is the front panel responsive? Can you select the different channels? DC or AC coupling? The 2246 is almost completely computer controlled. Including the brightness, etc.Does the beam find button do anything?Get the service manual (http://elektrotanya.com/tektronix_2246_sm.pdf/download.html). Read the maintenance and troubleshooting section. Check the voltages of the main power supply first.A few tips:Be careful with these ribbon cables. Especially when sliding the scope back in it's case.Don't touch the ceramic hybrids.Don't adjust anything unless you know what you are doing. Consult the service manual. Mark the position before adjustment so you can reverse it.

author: cs.dk
author_rank: Unknown
timestamp: October 20, 2013, 06:45:08 am
content: Thanks. I'll take a closer look on the PSU. A little side question. Are there some caps that are charged to a high voltage, or is it safe to touch and probe the board?All controls are responsive, and if i use the testsignal in front, i can get i to trigger. I tried with the beamfind button, and the vertical/horizontal controls, but no luck.I took a little video. At 0:33 you can see the beam when powering the thing up.EDIT: Guess the 2247 PSU is much the same as my 2246?This video also give some tips,

author: Bryan
author_rank: Frequent Contributor
timestamp: October 20, 2013, 10:04:33 am
content: First, get a service manual,easily downloaded from various sites. test the power supply as a start. Test the voltages on Jumper 119 on the mainboard. The expected voltages are all in the service manual. I suspect it is not booting fully due to a fault in the power supply. Let us know what the power supply voltages test at and we can go from there.

author: sync
author_rank: Frequent Contributor
timestamp: October 20, 2013, 03:55:12 pm
content: Quote from: cs.dk on October 20, 2013, 06:45:08 amYes, there are caps at high voltage. It's a switch mode PSU. It has mains voltage and also high voltage secondaries. But for testing the "low voltage" section you don't need to disassemble the scope like in the video. It can and should be check on the mainboard. It's the bottom PCB. There is a big connector from the PSU. Use this as test point. See the service manual.A warning: The "low voltage" section has 130V supply rails. And it's also present on the mainborad circuit.

author: cs.dk
author_rank: Unknown
timestamp: October 21, 2013, 05:26:20 pm
content: I've got a couple of measurements now - They are taken from the white 13-pin PSU-connector on the mainboard.The VAC measurement is from two different meteres. The steady 128 VAC reading is from a Fluke T5-600. My other meter claims 300 VAC at start, and climbing up to roughly 360 VAC. It is an (old) Elma/rebranded Brymen i think.

author: Bryan
author_rank: Frequent Contributor
timestamp: October 21, 2013, 07:24:00 pm
content: You really need to take the measurements from J119 on the mainboard to verify the operation of the power supply.

author: sync
author_rank: Frequent Contributor
timestamp: October 21, 2013, 07:27:27 pm
content: Is the -15.07V on the +15V rail a typo?The 130VAC comes from the SMPS transformer. Don't expect that DMMs show a correct value.

author: sync
author_rank: Frequent Contributor
timestamp: October 21, 2013, 07:28:58 pm
content: Quote from: Bryan on October 21, 2013, 07:24:00 pmWrong scope. It's a2246.

author: cs.dk
author_rank: Unknown
timestamp: October 21, 2013, 07:50:29 pm
content: Quote from: sync on October 21, 2013, 07:27:27 pmSorry, yes it's a typo. I don't think the T5 is a RMS-meter. Got another scope, it is not home now. I'll verify which one of the meters are right.I only see a 13-pin long white connector, the black one showed in the picture is not on mine.Btw. Is it normal, that these relative "mid-end" things have a lot of budge caps, resistors, etc.? My board is allmost full of them.

author: Bryan
author_rank: Frequent Contributor
timestamp: October 22, 2013, 06:20:47 am
content: Quote from: sync on October 21, 2013, 07:28:58 pmOops...

author: Stonent
author_rank: Super Contributor
timestamp: October 22, 2013, 07:30:32 am
content: I had almost the same exact issue on my Tenma scope. I had the position way off to the side and the time base was like 1/sec/div.Now everything was fine.

author: cs.dk
author_rank: Unknown
timestamp: October 22, 2013, 03:46:07 pm
content: Any clues, on going further?

author: sync
author_rank: Frequent Contributor
timestamp: October 22, 2013, 03:53:48 pm
content: Quote from: cs.dk on October 21, 2013, 07:50:29 pmOk, then the low voltage supply should be ok. The 130VAC should fine too. The 130VDC is generated from it.Maybe it has no high voltage. Do you have access to a high voltage (DMM) probe? To measure the -2.7kV and perhaps the 13.7kV. Unfortunately I have no experience in fixing the high voltage circuits of oscilloscopes. Maybe others can help.Do a visually inspection. Especially the high voltage CRT circuit. It's under a plastic shield on the main board. There are 4 neon bulbs. These are voltage clamps for protecting the CRT. They must not glow when the scope is running. When it's switch on/off they may lit. And look at the high voltage PSU. Maybe it has the same fault like in the 2247 video.QuoteI don't think it's normal. Can you make photos?Edit:Measure the deflection plate voltages. Perhaps the vertical or horizontal amplifier is broken and deflects the beam off screen. There are test points on the main board:2K +40V2L +40V6H +60.6V6I +78.2V

author: cs.dk
author_rank: Unknown
timestamp: October 22, 2013, 04:37:11 pm
content: Quote from: sync on October 22, 2013, 03:53:48 pmI don't have access to HV-probes, and I really don't like to play around with 13 kV anyway. I might take the PSU-board out for visual inspection. But i think the the on/off "beam" shown in the video is HV-feeded? Sorry, don't know anything about CRT's.QuoteYes - I've attached some. Some of them are showing same area. I don't have a macrolens.

author: cs.dk
author_rank: Unknown
timestamp: October 22, 2013, 04:38:23 pm
content: Quote from: sync on October 22, 2013, 03:53:48 pmGreatI'll try that. ThanksEDIT: The beam-find button, what does it excatly?

author: eurofox
author_rank: Unknown
timestamp: October 22, 2013, 04:44:54 pm
content: Quote from: cs.dk on October 22, 2013, 04:38:23 pmPut the "beam spot" visible on the display

author: cs.dk
author_rank: Unknown
timestamp: October 22, 2013, 04:49:22 pm
content: Quote from: eurofox on October 22, 2013, 04:44:54 pmI have figured that one out - But what about the measurements mentioned above? - Should the beam-find work under all fault-conditions? My question was more, what does the button "override".

author: sync
author_rank: Frequent Contributor
timestamp: October 22, 2013, 04:52:25 pm
content: LOL! I never saw so many bodges. There are many ICs with 1986 date code. The service manual says "first printing mar 1987". I think your scope is a very early model.

author: sync
author_rank: Frequent Contributor
timestamp: October 22, 2013, 05:06:14 pm
content: Quote from: cs.dk on October 22, 2013, 04:37:11 pmYes, there is some high voltage at switching on/off. But it's hard to say if it's right when the scope runs.

author: cs.dk
author_rank: Unknown
timestamp: October 22, 2013, 05:10:25 pm
content: Quote from: sync on October 22, 2013, 03:53:48 pmSpot on - 2 are glowing.. Picture is attached.I can't find the testpoints that you mentioned. The silkscreen is all unlabeled :/ I'll try to figure them out via the servicemanual.Thanks again

author: sync
author_rank: Frequent Contributor
timestamp: October 22, 2013, 05:30:51 pm
content: Ok, these neon bulbs are between the CRT cathode and G1. When they lit there is too much voltage. The CRT can't work then.Don't measure anything in this circuit while the scope is running. It's on -2.7kV potential!I will reply later.

author: eurofox
author_rank: Unknown
timestamp: October 22, 2013, 08:31:39 pm
content: Quote from: cs.dk on October 22, 2013, 04:49:22 pmOff topic but TOP from your country

author: sync
author_rank: Frequent Contributor
timestamp: October 22, 2013, 08:52:47 pm
content: There is too much negative voltage on the control grid. This simply cuts off the beam.The grid control voltage comes from the z-axis (intensity) amplifier (marked blue) and is then transposed to the -2.7kV cathode potential by a level shifting circuit (marked red). See attachment. The circuit on the right side of C2711 and C2712 is on -2.7kV. Be careful.I would first measure the output voltage of the z-axis amp. This is test point 7G. See fig. 9-5 of the service manual for locating it. The voltage should be between +14.7V and +39.5V. The A intensity knob should change the voltage.

author: Stonent
author_rank: Super Contributor
timestamp: October 23, 2013, 03:38:04 am
content: Replace the 8088-2 with an NEC V20. Man that takes me back, my first PC processor upgrade.PS - I'm not saying replace it for any reason to fix the problem, it's just an old upgrade trick from the 80s

author: cs.dk
author_rank: Unknown
timestamp: October 23, 2013, 03:51:47 am
content: Quote from: sync on October 22, 2013, 08:52:47 pmThis is quite difficult, when there is no silkscreen on the boardAs far as I can see, it must be one of the two, marked on the picture? Sorry for asking so much, but i really don't wanna probe -2,7kV

author: sync
author_rank: Frequent Contributor
timestamp: October 23, 2013, 02:45:41 pm
content: Don't worry about asking. It's the blue one. On the schematic the test point is at R2718. It has 75 ohm. The blue marked resistor is 75 ohm, the red marked is 10k.Be careful with the trimmer cap. It's the thing with the screw. Don't touch it, don't do any mechanical load on it, e.g. pulling/pushing it with a test lead. Or you can de-adjust it.All of the -2.7kV circuit should be under the plastic shield. Reassemble it.When I scared about high voltages then I do measurement this way:Disconnect the mains (pull the plug).Wait until the circuit is discharged or discharge it.Clamp the probes on the test points.Switch the device on (still disconnected from mains).Connect it to the mains and do the measurement WHILE keeping distance from the device and the measurement equipment.Disconnect it from mains and wait until it's discharged/ discharging it.

author: cs.dk
author_rank: Unknown
timestamp: October 23, 2013, 03:17:28 pm
content: Quote from: sync on October 22, 2013, 08:52:47 pmI've got 15.3V on the resistor, regardless what i do with the A and B intensity-pot.

author: cs.dk
author_rank: Unknown
timestamp: October 24, 2013, 03:44:00 pm
content: Any clues to come further, bringing this thing alive

author: sync
author_rank: Frequent Contributor
timestamp: October 24, 2013, 05:01:34 pm
content: I just finished some adjustments on my 2246A.I also measured the voltage on that test point 7G. It varies between 14V (low intensity) and ~60V (high intensity). I can change it with the A intensity knob. But when there is no trace (norm trigger without triggering) it stays at 14V. The neon bulbs didn't glow.Can you repeat the measurement and be sure that there should be a trace? Set the trigger to auto.But even if you get a non changing 15.3V again the neon bulbs should not glow. There is something wrong with the high voltage circuit or the CRT. The neon bulbs indicate that there is too much voltage between the CRT cathode and the control grid. It could have following causes.The z-axis level shifting is broken.The CRT has a short/ leakage from the control grid to something on lower potential.I would check the components of the level shifting circuit. Do you have experience checking components in-circuit?And remember this circuit is at -2.7kV. Only work on this while it's reliably discharged!

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: October 24, 2013, 05:24:21 pm
content: One of the DC blocking caps, C2711 or C2712, has probably died, taking other components along with it. They look iffy in one of the previous photos.

author: sync
author_rank: Frequent Contributor
timestamp: October 24, 2013, 06:28:27 pm
content: Possible. What is looking iffy to you?

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: October 24, 2013, 06:38:59 pm
content: Quote from: sync on October 24, 2013, 06:28:27 pmSeems the outer insulation is beginning to break down. Check the two areas in the background of this image, where they touch right at the edge.

author: sync
author_rank: Frequent Contributor
timestamp: October 24, 2013, 07:26:36 pm
content: I bit hard to see but you could be very right. On my 2246A the middle cap is mounted on the back side of the PCB. Maybe this cap was a common problem.cs.dk, please look at these caps for something unusual. Burn marks, strange patterns, etc. Disconnect the scope from mains. Wait 5min for discharging. Carefully separate the caps with a non-conductive stick. Inspect the area where they touched.

author: cs.dk
author_rank: Unknown
timestamp: October 25, 2013, 09:59:04 am
content: Quote from: sync on October 24, 2013, 05:01:34 pmWith the scope triggered i get ~15V to ~55V. I didn't know the scope should be triggered.Quote from: sync on October 24, 2013, 05:01:34 pmNot muchI'll look at the caps. Don't have an ESR-meter yet. Would be handy now?

author: sync
author_rank: Frequent Contributor
timestamp: October 25, 2013, 03:09:35 pm
content: Quote from: cs.dk on October 25, 2013, 09:59:04 amFine. The z-axis amp is working. So the problem is in the level shifter, the CRT or the high voltage supply.QuoteAn ESR meter doesn't help. These are 1-10nF caps. ESR meters are for bigger capacity electrolytics. If the caps are guilty then it's an isolation/ breakdown problem. Testing this require high voltages.First just make a visual inspection of the area were the caps are touching. Make photos of them.

author: cs.dk
author_rank: Unknown
timestamp: October 26, 2013, 08:43:30 am
content: Here are some photos of the caps.

author: sync
author_rank: Frequent Contributor
timestamp: October 26, 2013, 12:34:38 pm
content: What is this white stuff?Is that a crack in the diode?Only work in this area when it's discharged. Disconnect the mains. Wait 5min. There are resistors that would discharge the caps. BUT resistors can fail. To be sure discharge the cap with a screwdriver which is connected to the scope chassis. Look at this video from 4:00 to 7:00.You should not see sparks because it should be discharged already. If you see sparks please tell us.

author: cs.dk
author_rank: Unknown
timestamp: October 26, 2013, 11:45:45 pm
content: The white stuff between the neons may be some rebounce from the flash. The board look clean.I don't have a macro-lens, this is the best shot i can get of the "cracked" thingy. I need a magnifier, it is impossible to see with eyes, if the thing is cracked.Tried to discharge the HV caps, they don't give any sparks. Not even 2 sec. after power-off.

author: sync
author_rank: Frequent Contributor
timestamp: October 27, 2013, 05:52:44 pm
content: Quote from: cs.dk on October 26, 2013, 11:45:45 pmOk, there is no crack.QuoteThat means the resistors works or that there is no high voltage. Please don't rely on the resistors. Discharge the circuit with the screwdriver every time you work on it.For the next steps i would suggest separating the two touching caps (see attachment). Please gently bend these caps that there is about 2mm gap between them (where the arrow is). Then power up the scope. Are the neons still glow? If not, do you get a trace on the CRT?If the neons still glow then check every resistor and diode in the red marked circuit.Disconnect mains and discharge it. Measuring them in-circuit is a bit tricky. Sometimes you won't get the nominal values because the surrounding circuit affect the measurement. A resistor could measure a lower value because there is resistance in parallel with it and so on. The readings maybe take some time to stabilize. Test each resistor a second time with swapped testleads to reverse the polarity.A resistor with a too high measured value is likely broken. The diodes should test normally. About 0.7V forward biased and open reverse biased. Please report the measured values.

author: cs.dk
author_rank: Unknown
timestamp: November 02, 2013, 08:01:14 pm
content: Hi, sorry for the late reply, work takes all my sparetimeCan my board be an old revision? The schematic does not seem to fit exactly.First of all, i've bend the caps a little away from each other. No visual damage to them.I did find four diodes, that measured the same both ways. The diodes are, as far as i can see CR2713, CR2714, CR1003 and CR1004 (I marked them i the attached picture) - Can they have something to do with the problem? If yes, can i replace them with some 1N4007 i have in my junkbox?Diodes CR2701, CR2702, CR2716 and CR2717 is fine. ~0.685 Vf, nothing backwards. Other diodes just nearby i found, seems to be good.The two big 6,2M resistors (R2724 and R2751) measure ~6,6M.R2720 (20K) measured ~12,55K - I Can't find the diode next to it. It should as per the schematic be right next to one of the HV-caps. Is it possible R2719 is a trimpot? Can't find any other 20K in that area.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: November 03, 2013, 06:42:31 am
content: Quote from: cs.dk on November 02, 2013, 08:01:14 pmNope, not immediately part of the problem. CR2713, CR2714 and CR1004 *should* measure nearly the same both ways, like a slightly asymmetric anti-parallel pair of diodes. These 3 are protection diodes for the transistors near them, protection the basis/emitter diode from excessive reverse emitter/basis voltage. As long as they measure 0.6-0.7V in both directions, then they are fine.CR1003 is a bit different, but still not part of the core problem with the illuminated neon bulbs in the grid driver circuit. That diode is an over voltage protection diode for the Z-axis modulation input amplifier. If it is dead/shorted, then it will cause the Z-axis amp to permanently blank the beam traces, except for the on-screen readout (letters, numbers).So it cannot cause the neon bulbs to light, but it still may need replacement if it is dead. You cannot use an 1N4007 as a replacement, you need something like an 1N4148 or a similar very fast, low capacitance small signal diode. 1N914 etc.Edit: CR1003 is probably fine, unless you measure an open or a dead short. You are probably just reading CR1004 through the low resistance of the +7.5V power rail.Quote from: cs.dk on November 02, 2013, 08:01:14 pmR2719 is a trim pot, it has been moved out of the area, toward the rear of the board edge, for easy access and identification. It is one of the service adjustment points. It may be hiding below the thick, shielded cable running along the back edge of the board, toward the right in your photos. Careful with that cable, as it is the delay line, which is not easily replaced if damaged.R2720 probably measures low due to the DMM reading the parallel resistance of R2720 and R2719 (through the +130V supply rail to ground). The +130V rail has about 9K resistance to ground, so you should read about 20K || (20K+9K) ~ 11.8K. So that reading is probably as it should be.

author: oldway
author_rank: Super Contributor
timestamp: November 03, 2013, 06:49:49 am
content: Hello,As the two neon's bulbs are glowing, this means that voltage between cathode (pin2 of the CRT) and whenelt (pin 3 of the CRT) is greater dan - 100V.With so great polarisation, the CRT is fully blocked and there is no beam.Normal voltage should be between -15V (high beam intensity) and - 40V (no beam).NB: do'nt try to measure this voltages: DANGER: high tension of 2700V !)What's the fault then?Try first ajust R2719 (grid bias trimpot) and look if the neon's bulbs stop glowing and if you can see a trace on CRT.If not, there is a leakage on the whenelt side of the circuit.It can be:- leakage of C2711 - remove it and try to power the scope on. Be careful not to burn the CRT , switch it off imediately if you see the beam.- leakage of C2712 - remove it and try to power the scope on. Be careful not to burn the CRT , switch it off imediately if you see the beam.- leakage to ground of the wiring between the print and pin3 or in the socket of the CRT- leakage in the CRT between Whenelt and ground.

author: cs.dk
author_rank: Unknown
timestamp: November 03, 2013, 07:11:17 am
content: Quote from: ElectroIrradiator on November 03, 2013, 06:42:31 amThanks,R2719 should be the lower blue trimpot, right next to the HV-caps. Can i adjust it without somethings goes wrong, or do i need to measure a voltage at some point, when doing it?QuoteCan i just cut or desolder one leg to make it open circiut and power it on, or do i need something else?

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: November 03, 2013, 07:14:59 am
content: Quote from: oldway on November 03, 2013, 06:49:49 amAdjusting the grid bias pot will do nothing at this stage, except possibly ruin its calibration. It cannot explain the illuminated neon bulbs. The illumination requires either dead DC blocking caps, C2711 or C2712, or an internal short/leak within the CRT.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: November 03, 2013, 07:23:03 am
content: Quote from: cs.dk on November 03, 2013, 07:11:17 amI wouldn't touch it at this stage.PS: If you run the scope without the case for extended lengths of time, then I'd consider pointing a small, external cooling fan at the hybrid chip and the driver transistors in the vertical deflection amplifier. They won't receive any forced air cooling when the case is off, and these chips are notorious for dying from overheating.It is the large DIP chip with the metal top, plus the pair of TO-5 metal cased transistors with attached round heat sinks, just below/down and to the left of the circuit you are now working on. Near the left case edge when the scope is the right way up.

author: oldway
author_rank: Super Contributor
timestamp: November 03, 2013, 07:44:31 am
content: QuoteYes, of course, but do'nt cut it. Nothing else, only be careful, discharge de condensator with a 1k resistor before and be aware not to burn the CRT.QuoteNothing very important: calibration is only to ajust this trimpot for just visible trace with intensity at min.I agree that leakage is probably the issue but i tried to be shure there is not blanking overdrive.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: November 03, 2013, 08:05:37 am
content: Quote from: oldway on November 03, 2013, 07:44:31 amI do not disagree with your analysis. However, unless the scope has developed two independent problems, then chances are it is properly adjusted. So tweaking it now may just make it harder to spot if everything is sorted, once the leakage problem has been fixed.

author: cs.dk
author_rank: Unknown
timestamp: November 03, 2013, 08:13:16 am
content: I'll leave the trimpot for now.Next step must be the two caps. Can i lift both at the same time?

author: oldway
author_rank: Super Contributor
timestamp: November 03, 2013, 08:29:12 am
content: First measure de resistance between pin3 of the CRT and ground. (with scope swtched off...)You should find a high value, some Mohms.No, lift one at the time to know witch of them is leaking, if this is the issue.QuoteI allready repaired a lot of analog scopes of differents brands ( Philips/Fluke, Tektronix, HP, Gould, Hameg, ...) and that's what i would do first: try to ajust grid bias to see the trace, so i know if time base and vertical deflection are working or not.

author: cs.dk
author_rank: Unknown
timestamp: November 03, 2013, 08:53:59 am
content: The pins on the CRT isn't labeled eitherCan i measure from DS2702 neonbulb or another testpoint on the PCB to ground?EDIT: From one pin of DS2702 there is ~11,70M to ground, the other pin is open circuit.

author: mos6502
author_rank: Frequent Contributor
timestamp: November 03, 2013, 10:41:24 am
content: To be sure, measure from the anode of CR2701 ... anything that's directly connected to pin 3 of the CRT and that you can visually identify. If it's 11.7 MOhms, it's probably good.An LCR meter would be handy for checking those caps

author: cs.dk
author_rank: Unknown
timestamp: November 03, 2013, 11:03:36 am
content: From CR2701 i've got 11.45M to ground.I have a LCR-meter in order

author: oldway
author_rank: Super Contributor
timestamp: November 03, 2013, 12:01:49 pm
content: Ok, these value of resistance shows that there'is no short, but not that there is no leakage.LCR will not help to detect such leakage.Try now to lift one pin of each condensator.

author: sync
author_rank: Frequent Contributor
timestamp: November 03, 2013, 03:06:47 pm
content: Quote from: oldway on November 03, 2013, 06:49:49 amWhat I don't understand is how a leakage could produce to much negative voltage on the control grid. When the cathode is at -2.7kV then the control grid is the most negative part in the circuit. Every leakage should put the grid more positive, right?The neons indicate that the grid is more negative than the cathode. The grid side of the neons glow.

author: sync
author_rank: Frequent Contributor
timestamp: November 03, 2013, 03:20:48 pm
content: Quote from: cs.dk on November 02, 2013, 08:01:14 pmYes, your board is likely older than the service manual release.QuoteAs ElectroIrradiator already explained this is ok.Did you checked CR2703 and CR2704? See attachment, the ones marked with the red arrows.QuoteYes, it's a trimpot (GRID BIAS).

author: oldway
author_rank: Super Contributor
timestamp: November 03, 2013, 03:43:07 pm
content: Quote from: sync on November 03, 2013, 03:06:47 pmThat's right, i am wrong, leakage to ground let whenelt positive in reference with cathode...intensity would then be very hight.It would be interesting to measure de voltage between cathode and whenelt to know his value and polarity. (but this has to be done with caution)I have a Philips 30KV high voltage probe and always check those votages and also post acceleration 14Kv voltage.If whenelt is positive compare to cathode, the only possibility is that there is no post accelaration high voltage (defective HV multiplier)(NB: deflection vert and hor plate voltage must be more or less the same)If whenelt is negative compare to cathode, the tube is blocked bij too high blanking pulses.It would then be interesting to see if grid bias trimpot has some effect of not.

author: sync
author_rank: Frequent Contributor
timestamp: November 03, 2013, 04:07:58 pm
content: Quote from: oldway on November 03, 2013, 03:43:07 pmYes, measuring the high voltages would be good. But cs.dk has no high voltage probe and is scared of measuring these voltages.QuoteThe wehnelt is negative to the cathode. The neons are connected directly to them and indicate this. The z-axis amp output is already tested and looks ok. That would rule out the blanking. It could be a fault in the DC restorer. When the clamping does work then there could be to much voltage at the wehneltBut maybe there something wrong with the high voltage supply. But i have no idea how to test this without measuring the voltages.QuoteYes.

author: oldway
author_rank: Super Contributor
timestamp: November 03, 2013, 04:39:05 pm
content: Quoteneons work as well with both polarities....There is no diode in serie with them. How could they indicate what'is the polarity?QuoteI do not agree. If something go wrong with dc restorer, voltage can only be lower, not greater.Only a problem with dc bias could make this voltage greater.My experience show that defective HV multipliers is a commun problem.As "brute force" , he could shorten cathode and whenelt and power on o short time to see if there is high voltage and something visible on the screen.

author: mos6502
author_rank: Frequent Contributor
timestamp: November 03, 2013, 05:03:50 pm
content: Quote from: oldway on November 03, 2013, 04:39:05 pmWhen you apply DC to a neon lamp, only the negative cathode lights up ...I know little about analog scopes, so I might be off track here, but I'd check the HV caps with an ohmmeter just to make sure they haven't turned into resistors.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: November 03, 2013, 05:11:33 pm
content: Quote from: mos6502 on November 03, 2013, 05:03:50 pmcs.dk already did this by measuring the resistance from pin 3 of the CRT and ground, which was fine. Note that this is no guarantee against leaking high voltage caps. The only way to test their high voltage performance ... is with high voltage.An LCR meter will only be able to tell if the cap is a dead short, or has changed capacity. A cap, which tests good on a low voltage LCR meter, may still fail once HT is applied.

author: mos6502
author_rank: Frequent Contributor
timestamp: November 03, 2013, 06:02:10 pm
content: Quote from: ElectroIrradiator on November 03, 2013, 05:11:33 pmCorrect. To be absolutely sure, you'd have to apply the rated voltage to the cap, with a current meter in series.Quote from: ElectroIrradiator on November 03, 2013, 05:11:33 pmAn LCR meter will tell you a lot more, especially the dissipation factor (DF aka tan delta aka loss tangent)/ESR, which is a very good measure of capacitor health.The only thing an LCR meter can't detect is a leaky cap. For that you need to measure the DC current, as described above. Luckily, that's a rather rare failure mode, at least in low voltage caps. Maybe it's different with HV caps.

author: cs.dk
author_rank: Unknown
timestamp: November 03, 2013, 07:16:48 pm
content: Now the screen lits up - The display is flickering abit, but i turned it off, as i see the trace, for not damaging anything. The trace is _really_ bright.New question; Where do I get these caps?One is stated to be .0012 +/- 10% 4000V DC, the other .01 4000V DC - Both says 430P, and are labelled "Sprague"

author: sync
author_rank: Frequent Contributor
timestamp: November 03, 2013, 07:23:20 pm
content: Quote from: cs.dk on November 03, 2013, 07:16:48 pmNice!Which caps have you removed?And it's not clear that the caps are bad. See my next post. Have you checked CR2703 and CR2704?

author: sync
author_rank: Frequent Contributor
timestamp: November 03, 2013, 07:32:56 pm
content: Quote from: oldway on November 03, 2013, 04:39:05 pmLook at the attachments. If CR2704 (or R2745, R2719) is open then the voltage will be greater.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: November 03, 2013, 07:52:07 pm
content: Quote from: mos6502 on November 03, 2013, 06:02:10 pmYep. Measuring DF in a 1200pF high voltage film cap should be easy.sync,Could you try running your simulation with an internal resistance of 10K in the -2.7KV supply, and with 1.5nF for C2712, please? Looks like 1200pF is not an easy value to find.WIMA makes suitable low loss, high voltage film caps, but they are much too large...

author: mos6502
author_rank: Frequent Contributor
timestamp: November 03, 2013, 08:18:32 pm
content: Quote from: ElectroIrradiator on November 03, 2013, 07:52:07 pmNo need for the snarkasm. If you carefully reread my post, you'll notice I wrote "DF/ESR". For such a small capacitance, you'd obviously measure ESR.Anyway, 4kV caps are probably hard to find, so just use two 2.7n 2kV caps and two 22n 2kV caps in series, respectively.

author: oldway
author_rank: Super Contributor
timestamp: November 03, 2013, 08:21:13 pm
content: QuoteThose components belongs to the bias ajust circuit and not to the dc restorer.Remember:QuoteNB: with thoses condensators in place.

author: sync
author_rank: Frequent Contributor
timestamp: November 03, 2013, 09:26:04 pm
content: Quote from: ElectroIrradiator on November 03, 2013, 07:52:07 pmI added a 1nF run. (I used 0.99 and 1.6nF because the simulation stalled with 1 and 1.5nF.)

author: cs.dk
author_rank: Unknown
timestamp: November 04, 2013, 04:23:18 am
content: Quote from: sync on November 03, 2013, 07:23:20 pmThe removed caps are C2711 and C2712.CR2704 is fine. I can't find CR2703, thats a resistor hiding under C2712.This one must be C2711http://www.tedss.com/X675-.01-4000-10and C2712 herehttp://www.talonix.com/shop/item.aspx?itemid=11544#.Uncj2xCoTg0- Is it possible to get them from a european distributor?

author: cs.dk
author_rank: Unknown
timestamp: November 04, 2013, 04:41:37 am
content: Quote from: oldway on November 03, 2013, 08:21:13 pmI'll try to put the caps back in, and adjust the trimpot.

author: sync
author_rank: Frequent Contributor
timestamp: November 04, 2013, 02:13:02 pm
content: Quote from: cs.dk on November 04, 2013, 04:23:18 amThese caps isolate the control grid (wehnelt) high voltage. If they are leaky then the control grid would be more positive than the cathode (-2.7kV). But the control grid is too much negative. So it's unlikely that the caps are the fault.Please make photos of the area of the removed caps. So we can see differences between your unit and the service manual.Then put the caps back in. Try adjusting the GRID BIAS pot (R2719) like oldway suggested. If this doesn't help then lift one leg of R2722 (yellow arrow). This will disconnect the 130VAC to the DC restorer. Without this the DC restorer can't work. You should get a very, very bright trace. If you get this the caps are not the fault.

author: oldway
author_rank: Super Contributor
timestamp: November 04, 2013, 03:03:25 pm
content: It would be very important to know what is the signal at test point 7G (intensity modulation) on R2718 but this must be checked with another scope.Perhaps, DC restorer is allright, whe have a high negative whenelt voltage because we do not have any intensity information at test point 7G.

author: sync
author_rank: Frequent Contributor
timestamp: November 04, 2013, 04:00:28 pm
content: Already tested. There is 15-55V on 7G.https://www.eevblog.com/forum/testgear/tektronix-2246-no-beam!/msg316438/#msg316438

author: cs.dk
author_rank: Unknown
timestamp: November 04, 2013, 06:14:41 pm
content: Quote from: sync on November 04, 2013, 02:13:02 pmI put the caps back in, messing around with R2719, no luck. :/When lifting R2722 i've got a really bright trace, and the neons turned off.Side question, what is wehnelt? I'll google that. EDIT: Googled that phrase, and become wiser

author: sync
author_rank: Frequent Contributor
timestamp: November 04, 2013, 07:22:00 pm
content: Good. The caps are not the problem. Please check the missing CR2703. You don't have to find it. It's connected to C2711 and C2712. Make a diode test at the leads of the caps (yellow arrows).

author: cs.dk
author_rank: Unknown
timestamp: November 04, 2013, 08:15:16 pm
content: Quote from: sync on November 04, 2013, 07:22:00 pmThanks.It did measure "nothing" both ways, with the diode-test. Even at 20M-scale, the leads of the caps are open.Because of trouble with soldering these bastards in again, i lifted C2711, i thought it was a cold joint - Guess what, there is something hiding under there, more correct it is hiding under C2724 hidden by C2711.Could this one be the problem?Why are this thing so hard to solder? One end of both caps went fine, the other is just impossible. I did use flux, the solder just don't want, what i want it to. Need to clean up with some wick, and try again, when the cap need to be soldered in again.What diode do i need for replacement?

author: sync
author_rank: Frequent Contributor
timestamp: November 04, 2013, 09:39:29 pm
content: That's a hidden problem!And a bad design. The diode is likely broken due mechanical stress from the cap.Yes, it could be the only problem. I done a simulation with an open CR2703 and it shows to much negative voltage. The problem of your scope.Regarding the diode replacement type. According to the service manual it's a 225V 200mA signal diode. A BAV21 would be a close match. An UF4004 is also good. Any fast silicon diode with >=225V and >=200mA should work. A 1N400x is too slow.For replacement I would cut the rests of the broken diode from the board. Put back the caps and solder the new diode across the cap leads. Cathode to the smaller C2712 and anode to C2711. Just another bodgeNot worth the trouble to get the new diode between the caps. Fortunately this is the low voltage side of the caps.About the soldering. I don't know. Maybe the leads are oxidized?

author: cs.dk
author_rank: Unknown
timestamp: November 05, 2013, 04:16:39 am
content: I've ordered some UF4004, with some luck they are here thursday.

author: cs.dk
author_rank: Unknown
timestamp: November 05, 2013, 03:14:36 pm
content: It's alive!!!I did nip one of the ribboncables though, so i miss the main-menu.It's the one on the side, i didn't noticed it, so i cared about them in top-front. All measurement menus are there, so it must be fixable, maybe one pin is not correctly seated. I'll have a look at that.I did find an old Compaq hotswap server-PSU, an got some parts from it. I could not wait for the new diodes to arrive. I took a chance, and installed aEGP20D 200V 2A diode, it seems to work. Worst case the smoke would let out, and i'd have to wait for the new UF4004.The horizontal is a little of scale, the vertical seems to be pretty close.Thanks alot folks, without your help it was not being a scope again

author: BravoV
author_rank: Super Contributor
timestamp: November 05, 2013, 03:50:59 pm
content: I have been following this thread closely, its just I dont have anything to contribute, just glad to see a happy ending.Congratulations ! Great job and very well deserved !

author: sync
author_rank: Frequent Contributor
timestamp: November 05, 2013, 04:18:49 pm
content: Congratulations! Glad it's working again.About the horizontal. The calibration output is not very precise (1kHz +/- 25%). Do a self calibration and check it with a accurate source.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: November 05, 2013, 04:58:01 pm
content: Glad to be proven wrong about the pair of HT caps.Now get the iso out and clean her up a bit, she deserves it.Congratulations and enjoy.

author: cs.dk
author_rank: Unknown
timestamp: November 06, 2013, 04:42:19 am
content: I'll test with a more stable signalsource, to see if it needs adjustment.Yeah, some cleaning would not harm herNow i just need a new set of probes. Does all Tek probes feature the "auto 1x-10x" function?

author: Unknown
author_rank: Unknown
timestamp: November 06, 2013, 05:10:55 am
content: No, look for the 'readout' function in specs or datasheet. For example, inthis listit's indicated as "R.O.". Note that 1x probes will never have a readout pin since 1x is the default.

author: pgross
author_rank: Contributor
timestamp: November 06, 2013, 03:25:20 pm
content: Glad to see that you managed to get the scope fixed. Kudos to you and to this forumI 've recently restored my 2246 and now its good for another 25 years...My scope is completely recapped (mainboard and PSU) with some fresh electrolytics from Panasonic and Nichicon.The backup Lithium battery (mounted on the solderside of the readout PCB) is also replaced.Now your scope is alive, have you tried to run the self test from the setup menu?One thing I have noted is that the readout on my 2246 screen tends to flicker - more or less in accordance with the sweep time.I dont know if this is a normal behaviour - I have tried to adjust the readout jitter for optimum performance, but somehow the flickering can't be completely eliminated.Please feel free to send my a PM if you have any comments or questions, preferably in danish/Peter

author: cs.dk
author_rank: Unknown
timestamp: November 06, 2013, 06:06:32 pm
content: Quote from: alm on November 06, 2013, 05:10:55 amThanks. I'll have a look at Ebay for some probes.Quote from: pgross on November 06, 2013, 03:25:20 pmNice - With a recap, it will sure last a long time.If self-test is equal the Diagnostic, it says PassedMine is also flickering, not much though. I haven't noticed the sweeptime vs. flickering yet.

author: viperrepair
author_rank: Newbie
timestamp: November 06, 2014, 10:56:49 pm
content: HI just want make correct Sync's suggestion to solder the replacement diode's cathode to c2712 it is actually supposed to go to the resistor under c2712 not to c2712. almost made that mistake luckily I caught it.Regards.

author: viperrepair
author_rank: Newbie
timestamp: November 07, 2014, 11:05:56 am
content: Quote