Oscilloscope B&K Precision 1472C Intensity problem ! - Page 1
Forum section: electronics / repair

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 03, 2024, 01:44:26 am
content: I have an old B&K Precision 1472C that is working except when I turn the intensity knob nothing happened, the line is always at full brightness. The service manual says the line should disappears when the knob is at the 9 o'clock position but it does not !!This is what I have done so far:1: Replaced all the electrolytic capacitors.2: Replaced all the infamous 2SC458C transistors with 2SC536NF.3: Replaced some other transistors.4: Replaced IC201, 202, 203 and 204.5: Replaced diodes D311, D312 and D313 V06E with UF4005.Check every resistors, diodes on the power supply board. I even check with my multimeter on the solder side, EVERY traces to make sure none were broken and on the component side EVERY connections between them to be certain there was not a cold solder joint somewhere. Very time-consuming !!!!Voltages measurement:Connector P3031: +86V2: +83V3: NC4: NC5: -1900V6: -1895V7: -1895V8: -1895V9: -1560VCRT socket (removed or not from the CRT, I get the same readings)1: -1895V Heater2: +86V Astig3: -1900V Grid4: +102V Horizontal plate5: NC6: +116V Horizontal plate7: -1895V Cathode8: -1560V Focus9: NC10: +92V Vertical plate11: +83V Astig12: +78V Vertical plate13: NC14: -1895V HeaterI know it's a lot of numbers but I would really appreciate if someone could have a look at the schematic and try to tell me what maybe the problem because I'm running out of solutions !!!

author: bdunham7
author_rank: Super Contributor
timestamp: January 03, 2024, 02:01:42 am
content: Are you adjusting VR307? If so, and you can't get other than full brightness, then the next thing to do would be to check the voltages at VR2 and VR307, seeing what they are and if they change at all when you adjust the control or trimmer. Which version do you have?

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 03, 2024, 03:16:39 am
content: Thank you very much for your help bdunham7 !!!No matter how I turn VR307, all I get is full brightness.I check VR2 (500 Ohms) itself, not for voltage and it is ok, goes from 2 Ohms to 500 Ohms.Connector P3091: +5.05V2: +5.06V (no matter how I turn the knob)3: groundDirectly at the VR2 potLeg 1: +5.03VMiddle leg: +5.04V (no matter how I turn the knob)Leg 3: fully CW +5.05V, fully CCW +4.96V (From pin 2 of connector P202, I think it's the blinking pulse ??)VR307Fully CCW +146V, fully CW +85VI have the later version with mini board X77, I replaced almost every components on that little board !!!

author: bdunham7
author_rank: Super Contributor
timestamp: January 03, 2024, 04:20:48 am
content: That schematic makes my eyes water. I haven't figured out what everything does yet, but I would disconnect the side of VR2 that goes to J202 pin 2 and tie it to ground through a 50R or so resistor, then try it to see if the intensity control responds and is adjustable. Once you know that, you know which direction to concentrate your efforts.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 03, 2024, 07:13:35 pm
content: I disconnected pin2 from P202 and tied it to ground through a 50 Ohms resistor and now pin 2 of P309 goes from +0.4V to +5.04V, but no matter how I turn the knob, line is always at full brightnessThe service manual shows what voltage should be present on each CRT pins. It says pin 3 and 7 should be at -1400V but they are at -1900V and -1895V respectively !!I really hope that together we are going to find the problem, because all those hours troubleshooting it, all the money for the parts, I could have bought a better working oscilloscope. Well mine is working but not the intensity LOLPlease don't let me down ......

author: bdunham7
author_rank: Super Contributor
timestamp: January 03, 2024, 07:52:26 pm
content: Quote from: guizmo1967 on January 03, 2024, 07:13:35 pmSure you can buy a scope, but where's the fun in that?I merged the 3-piece schematic in case anyone else wants to jump in. Can you tell me whether your unit actually has a Z-axis input on the back?

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 03, 2024, 08:18:31 pm
content: Yes it has a Z-axis input on the back and I did try something with that.I used four 12 volts battery to make a -24Vdc to +24Vdc power supply, connected that to a 100KOhms pot and the middle leg to the Z-axis input and still nothing happened !!I told you, I spent many hours on that scope .....

author: bdunham7
author_rank: Super Contributor
timestamp: January 03, 2024, 09:30:24 pm
content: Do you know where the Z-axis input is on the schematic? I don't see it.Can you verify whether P302 has a shorting plug installed? And if removing it changes anything?Try this experiment: Set the scope up so that it is triggering and showing a trace, then adjust the horizontal position so that the trace is starting on the left side but well onto the screen. Then remove the signal or stop it from triggering and sweeping--is there a bright dot remaining on the screen?I haven't really figured out exactly how the high voltage section works on this. Do you have another scope for testing?

author: Zenith
author_rank: Frequent Contributor
timestamp: January 03, 2024, 09:33:45 pm
content: I don't find the circuit diagram easy to follow. I think pin 3 of the CRT (pin 5 on P303) is the first grid of the CRT and should be several volts negative to the cathode for normal operation, just like with a vacuum tube. Possibly at -10V it totally cuts off the beam and at -5 it's as bright as it should be. The brightness control and flyback blanking affect the voltage on that grid.As a hunch I'd change R346, a 22M resistor, for a suitable high voltage replacement.If your luck's out, you have an internal disconnection between pin 3 on the CRT and the grid. You should still see the voltage on pin 3 change, with respect to the cathode, when the brightness is adjusted.You could measure the voltages, and with a codge force pin 3 to be say, 18V negative to the cathode. That should at least dim the display.I'm sure I don't need to tell you that oscilloscopes can bite, so be extra careful.Quote from: guizmo1967 on January 03, 2024, 08:18:31 pmConsider it an investment of time rather than an expenditure, and all part of the joy of learning.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 04, 2024, 12:46:08 am
content: Quote from: bdunham7 on January 03, 2024, 09:30:24 pmJust look to the left of the CRTQuoteYes it has shorting plug but ounce removed, still no change !QuoteIf I turn the SWEEP TIME / CM knob to CHB, I get a really bright (over-bright) spot in the center of the screen.QuoteNo I don't !

author: bdunham7
author_rank: Super Contributor
timestamp: January 04, 2024, 01:05:43 am
content: I can't see the Z-input anywhere. Is it on the schematic that I attached? What is the closest component to it?Let's try the sweep thing another way. Set the scope up for NORMAL and LINE trigger, only Channel A selected and 1µs/div sweep rate (this should dim the trace a bit, but not for helpful reasons), then move the trace to the right so that it starts on the screen. Is there a bright dot at the point where the trace starts?Not having another scope often makes these things tough, but this just might be solveable.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 04, 2024, 01:07:51 am
content: Quote from: Zenith on January 03, 2024, 09:33:45 pmPin 3 of CRT (pin 5 of P303) is gridPin 7 of CRT (pin 8 of P303) is cathodePin 8 of CRT (pin 9 of P303) is focusPin 1 of CRT (pin 6 of P303) is heaterPin 14 of CRT (pin 7 of P303) is heaterIf you look at my voltage measurements in my first post, CRT pin 3 is at -1900V and CRT pin 7 is at -1895V. That's the reason for the full brightness, but why !!!QuoteI already changed that resistor !QuoteI don't think the CRT itself is the problem because if I removed the adapter from the CRT and measure each pin of that adapter, I get the same voltage readings as in my first post.QuoteHow do I do thatI'm sure I don't need to tell you that oscilloscopes can bite, so be extra careful.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 04, 2024, 01:27:47 am
content: Quote from: bdunham7 on January 04, 2024, 01:05:43 amIt's connector P302, R363QuoteOK I turned SWEEP TIME knob to 1us/div, TRIGGERING LEVEL knob to normal, not AUTO and if I turn the HORIZONTAL (POSITION) knob I can see a really bright dot moving across the screen.

author: bdunham7
author_rank: Super Contributor
timestamp: January 04, 2024, 01:40:28 am
content: Quote from: guizmo1967 on January 04, 2024, 01:27:47 amDo you have a different schematic? I still don't see it....OK, you don't have LINE trigger, but what you did confirms that you have no blanking as well as no intensity control. I'll think about what to do next if you don't have a scope. If you can, check the voltages at the emitter and collector of Q321, both with the intensity turned up and turned down. Leave it set up with the wire at VR2 grounded through the resistor for this so you have a varying voltage at the center of VR2.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: January 04, 2024, 02:04:29 am
content: Check the voltage of the anode of D312.There will be a little bit of a high voltage there so be aware of that so you don't blow up your meter. At least 500V, likely higher. Look at the schematic for more details.When you adjust the intensity control, there should be a voltage change.If there is none, check the cathode of D313. The voltage there is low so adjust your voltmeter range accordingly. The voltage there should change when the intensity control is adjusted.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 04, 2024, 02:23:59 am
content: Quote from: bdunham7 on January 04, 2024, 01:40:28 amI have the same schematic as you. The image you posted, it's right there INT MOD, shorting plug !!!!Voltage at Q321 when I turn VR2:Emitter: from +40.5V to +86.5VCollector: from +123.5V to +128.5V

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 04, 2024, 02:43:45 am
content: Quote from: BlownUpCapacitor on January 04, 2024, 02:04:29 amVoltage at anode D312 is -1908V, and no change when I adjust VR2Voltage at cathode D313 goes from +84.6V to +91.2V when I adjust VR2

author: bdunham7
author_rank: Super Contributor
timestamp: January 04, 2024, 03:34:02 am
content: Quote from: guizmo1967 on January 04, 2024, 01:27:47 amOh! I was expecting a BNC type connector and slightly different wiring. I thought that was an internal connector! So the Z-axis input is not DC coupled, which means using batteries won't have any effect. Is there any way you can come up with an AC voltage of about 10VRMS? If so, set it up so that the signal is displayed on CH A with as much amplitude as possible without clipping, then simultaneously feed the same signal to the Z-axis input. When you do, you should see the tops of the waveform dim and the bottoms brighten (or vice versa).The voltage change at the emitter of Q321 shows that the circuit is working (somewhat, anyway) that far up. Your problem is likely in the area of Q318, but I'm not sure how to test that without a scope.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 04, 2024, 04:12:49 am
content: QuoteWell if you are talking about a function generator, I don't have one yet !Just to let you know, Q318, Q319, Q320 and Q321 were replaced.Will get back tomorrow, here it is 11:00pm, time to go to bed .... I really hope that we will find the problem.

author: bdunham7
author_rank: Super Contributor
timestamp: January 04, 2024, 04:28:48 am
content: Quote from: guizmo1967 on January 04, 2024, 04:12:49 amWere all those parts replaced trying to fix this specific issue?The 10VAC can come from a transformer, 60Hz should be fine. Maybe a 9VAC wall wart?

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 04, 2024, 05:28:00 pm
content: Yes I replaced all those parts trying to fix this issue, like I said I ran out of solutions ...I have a transformer laying around, it's a 27Vac center tap. Black probe on center tap does not give -13.5Vac and +13.5Vac, they are both at +13.5Vac or +27Vac when I only measure both ends.Service manual says:INTENSITY MODULATIONVoltage: 20 p-p minimumInput resistance: 470KOhms

author: bdunham7
author_rank: Super Contributor
timestamp: January 04, 2024, 07:13:43 pm
content: Quote from: guizmo1967 on January 04, 2024, 05:28:00 pmThere is no + or - with AC. Go ahead and connect the center tap and one winding to Channel A, get a signal on the screen and then connect it to the Z input as well. Make sure you connect the same wire (the center tap would be best) to the grounds. You can alternate which non-center-tap wire goes to the Z-axis and the dimmed part should appear on the top or bottom depending on which one you select.

author: carlitos
author_rank: Contributor
timestamp: January 04, 2024, 07:45:19 pm
content: One of the Neon, N301 and N302, may be leaking.Capacitor C343 or C333 open, without capacitance.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 04, 2024, 07:58:20 pm
content: Ok this is what I did:ground clip to center tap, probe to one winding connected to CHA, sine wave on the screen.Z-axis BNC ground to center tap, Z-axis input to same winding as my oscilloscope probe.Result = sine wave at full brightness !!!

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 04, 2024, 08:04:28 pm
content: Quote from: carlitos on January 04, 2024, 07:45:19 pmThose two caps has been replaced as well !Since I never had this oscilloscope before, first time I turned it on without the casing, I noticed that the two Neon were just slightly glowing at the tip and I ask myself should they be completely offDo you think that since the beginning, the problem was just the NeonWell the problem is not solved yet, but just asking ....

author: p.larner
author_rank: Frequent Contributor
timestamp: January 04, 2024, 08:09:50 pm
content: I had an old tek 453 given to me years ago,it had the same issue,it was a transistor o/c in the Z amp.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 04, 2024, 08:32:50 pm
content: Have a look !

author: bdunham7
author_rank: Super Contributor
timestamp: January 04, 2024, 08:48:12 pm
content: Quote from: guizmo1967 on January 04, 2024, 07:58:20 pmOK, now try the same thing but moving the ground from the center tap to the other winding so you're giving it 27VAC, adjusting the scope to keep the signal on the screen. If that doesn't do anything, turn the scope off and let it cool a bit, then take the connector off of the CRT and measure the resistance from pin 3 to pin 7 on the CRT itself.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 04, 2024, 09:01:47 pm
content: Moved the ground clip and Z-axis ground to other winding and still at full brightness.Resistance from CRT pin 3 (Control grid) and 7 (Cathode) = OL on my multimeter !I have a question ? When I measure resistance of R352, I get 0 Ohm ! Is that because there is an inductor parallel with it ??

author: bdunham7
author_rank: Super Contributor
timestamp: January 04, 2024, 09:29:43 pm
content: Quote from: guizmo1967 on January 04, 2024, 09:01:47 pmYes, the inductor will have a very low DC resistance. Can you also measure between pin 3 and pin 1, then pin 7 and pin 1? Also measure R347 if you haven't already?In addition the neons glowing are an issue. When you reassemble it enough to test it, can you observe if the neons change at all if you vary VR2?

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 04, 2024, 09:38:25 pm
content: Pin 3 (Grid) to pin 1 (heater) = OLPin 7 (Cathode) to pin 1 (heater) = OLR346 replaced and R347 = 46.7KOhmsPin 1 to Pin 14 (heater) = 3 OhmsFrom the beginning I ask myself, are those neon bulbs supposed to glow ?? They glow even if the connector to the CRT is removed !No, the neons do not change when I vary VR2.

author: carlitos
author_rank: Contributor
timestamp: January 04, 2024, 09:45:46 pm
content: I do not know if the neon lamp have to glow, that circuit two neon lamp in series with a resistor in parallel i think it acts like a voltage limiter.Is it possible to adjust the -1900V with VR306?

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 04, 2024, 09:49:41 pm
content: Quote from: carlitos on January 04, 2024, 09:45:46 pmYes I have before.In the service manual, pin 3 and 7 should be at -1400V.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 04, 2024, 10:04:50 pm
content: This is from the internet:"Many many years ago when I was in the Navy, the ship I was on had a radar repeater that used a NE-2 neon bulb as a voltage regulator. The neon bulb failed fairly often, maybe about every three months. Often enough that I could remember that it had failed before. So I bought a NE-2 from Radio Shack and used it instead of one from the Navy supply system. And it did not fail again while I was on that ship. Just pointing out that old stock may not be reliable.I would try to come up with a circuit that used one neon bulb across part of a voltage divider to do the regulating. I think trying to use 5 or 6 in series would lead to very erratic behaviour."I'm beginning to think that those NE-2 neon bulbs are the problem because I replaced almost every components around them !!!

author: carlitos
author_rank: Contributor
timestamp: January 04, 2024, 11:16:09 pm
content: Between pin 3 and 7 you have almost 0V because neon is conducting, to start conducting that voltage have to rise above 160V and this is a way to not damage CRT.Q318 was changed and D316? R349? R358?Verify connection between transformer L301 and base of Q318.Minimal leakage on this circuit could lead to neon lamp glow.

author: bdunham7
author_rank: Super Contributor
timestamp: January 04, 2024, 11:58:38 pm
content: Quote from: guizmo1967 on January 04, 2024, 10:04:50 pmThe neon bulbs on circuits like these (which are common on CRT oscilloscopes) are there to limit voltages across those diodes and you typically only see them flash when you are turning the scope on and off or something is wrong. If a neon bulb does fail, it typically doesn't 'strike' and the voltage across it will be too high, not too low. However, you measured the voltages at those points and there was only a few volts of difference, right? Not 80-100 volts or more?This is where I'm not sure how to proceed without a second scope. I'm guessing there is a lot of AC voltage across the bulbs, probably at the frequency of the HV oscillator. Have you looked at C329 and C330? Do you have a way of testing them? (or perhaps replacements at hand?) With the DC voltages you measured, the Z-axis input really should have made a difference.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 05, 2024, 12:02:37 am
content: Quote from: carlitos on January 04, 2024, 11:16:09 pmReplaced D316 with 1N4148R349 and R358 were good.There is good connection between L301 and base of Q318 (base voltage = 0V)

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 05, 2024, 12:25:54 am
content: Quote from: bdunham7 on January 04, 2024, 11:58:38 pmI did not measure the voltage across both neon bulbs !I have replaced all the 2KV ceramic capacitors C329, C330, C333, C343, C332 and C335.Ok, can someone explain me how to do this:I did not know nothing about CRT before but because of this problem, I read a lot!! As I understand the cathode needs to be at certain negative voltage. For the electrons to pass through, the grid as be MORE negative that the cathode about 5V to 35V (5V means a lot of electrons and 35V almost none). The focus as to be LESS negative than the cathode. In my case this is the voltage I get:Pin 3 (Grid) = -1902VPin 7 (Cathode) = -1895VPin 8 (Focus) = -1560V and this one is working perfectly because VR3 act as a voltage divider and goes directly to CRT pin 8 !Only 7V difference between grid and cathode, that's why full brightness.Is there a way to make the cathode more positive for testing purpose, let's say about -1875V to have a difference of 27V to see if the line disappears?

author: bdunham7
author_rank: Super Contributor
timestamp: January 05, 2024, 12:46:12 am
content: Quote from: guizmo1967 on January 05, 2024, 12:25:54 amYes, that is what I was trying to achieve by having you supply the 27VAC to the Z-axis input.At this point I'd recommend temporarily removing C333 and C343 and observing the results and redoing the Z-axis input experiment.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 05, 2024, 12:52:52 am
content: Ok, will try that tomorrow, enough testing for me today, my head is about to blow off !

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: January 05, 2024, 02:32:17 am
content: Check D312

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 05, 2024, 09:19:35 pm
content: Come on guys, we can repair this scope !!I know a lot of you don't read all the posts, this is for you:D311, D312, D313 (V06E) replaced with UF4005D315 (WZ-050) replaced with NZX5V1BD316 (1S1555) replaced with 1N4148D314 (1S1705), the only diode I did not replaced yet.Today I removed C333, C343 then turned on the scope and again a line at full brightness !Connector on the CRT:Pin 1, 14 = -1900VPin 3 = -1906VPin 7 = -1892VPin 8 = -1563VConnector removed from the CRT:Pin 1, 14 = -1894VPin 3 = -1899VPin 7 = -1893VPin 8 = -1563VTo me, there is nothing wrong with the CRT itself, nothing is shorted !!!Tonight, I will redo the Z-axis experiment with my 27Vac transfo !

author: bdunham7
author_rank: Super Contributor
timestamp: January 05, 2024, 09:26:50 pm
content: Quote from: guizmo1967 on January 05, 2024, 09:19:35 pmAre the neons still glowing with the capacitors removed?QuoteJust curious--why not?

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 05, 2024, 09:33:14 pm
content: Yes !

author: bdunham7
author_rank: Super Contributor
timestamp: January 05, 2024, 09:43:36 pm
content: So there appears to be only 6 volts difference between pin 5 and pins 6 or 7 of P303, those go to opposite sides of the neon bulbs and yet they glow. Something is wrong with that picture.Can you tell us exactly how you are measuring that voltage--including the make/model of the meter and the HV probe I assume you're using?Can you safely measure the voltage at the junction of D311 and D312, then both sides of R346?

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 05, 2024, 09:46:37 pm
content: QuoteI have to say someone did try to repair this scope before. The D314, is supposed to be a 1S1705 and I could not find online the datasheet that's why I did not replaced it. But the guy before me replaced it with some kind of glass diode that looks like a point contact germanium diode with color band on it like a resistor. The color are Violet, Black and Green meaning 70E. I have no idea if it's the equivalent replacement for the 1S1705

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 05, 2024, 09:54:17 pm
content: I have an auto-ranging digital multimeter, unfortunately not a Fluke with a HVP-40 probe.I don't think it's the problem

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 05, 2024, 10:12:38 pm
content: QuoteJunction of D311 and D312 = -1931VR346 side of N301 = -1902VR346 side of N302 = -1910V

author: bdunham7
author_rank: Super Contributor
timestamp: January 05, 2024, 10:12:46 pm
content: Quote from: guizmo1967 on January 05, 2024, 09:54:17 pmI'm not saying that it is defective or "the problem", I'm trying to think if it might be having an effect on the circuit and I'd like to know the model so that I know what features it has in case there might be some other measurement that could be made. The HVP-40 shouldn't load the circuits signficantly, even the oscillator part so you're good there. However, it is only good for DC and maybe 60Hz measurements.

author: bdunham7
author_rank: Super Contributor
timestamp: January 05, 2024, 10:17:49 pm
content: Quote from: guizmo1967 on January 05, 2024, 10:12:38 pmDoes your meter have a diode check function? I'm assuming you have not reinstalled those capacitors?

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 05, 2024, 10:18:47 pm
content: of course ! No, the capacitors have not been reinstalled !

author: carlitos
author_rank: Contributor
timestamp: January 05, 2024, 11:40:36 pm
content: In my opinion those capacitores have to be in place because the blanking pulse do not work and could give identical symptoms.Diode 1S1705 can be replaced with BAV21 following information on this page:https://www.web-bcs.com/diode/dc/1s/1S1705.php?With VR2 in mid position what are voltages on transistor Q318?

author: bdunham7
author_rank: Super Contributor
timestamp: January 06, 2024, 12:38:32 am
content: Quote from: guizmo1967 on January 05, 2024, 10:18:47 pmOK. The try this:Step 1: With the scope off and cooled, using the diode check feature put the negative lead at the junction of NE2 and D312 and the positive lead at the junction of D312 and D311. If you get a forward voltage indication of 0.7V or so, then move the positive lead to the other side of D311 and see if you get 1.4V or so (+/- 0.2V depending on the exact meter parameters).Step 2: If you got an OL or OPEN reading in the first part of step 1, then do the same thing with the leads reversed. Report the values.Step 3: Put the negative lead at the junction of D311 and D312 and the positive lead on the other side of each of the diodes. Reverse the leads and do the same thing again.This is turning into an interesting but frustrating problem. I hope I'm not overlooking anything simple, but I'm speculating that D312 may be installed backwards or some damage has occurred. Is is possible to post a closeup photo of the diodes or the whole HV area around them?

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 06, 2024, 12:41:32 am
content: Quote from: carlitos on January 05, 2024, 11:40:36 pmThank you for the information.Q318Emitter = +0.076VBase = 0VCollector = +68V

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 06, 2024, 01:46:10 am
content: Quote from: bdunham7 on January 06, 2024, 12:38:32 amStep 1: OLStep 2: 0.472V, the other side of D311, 0.914VStep 3: Negative at junction of D311 and D312, positive other side of D312 = 0.472V positive other side of D311 = OLPositive at junction of D311 and D312, negative other side of D312 = OL negative other side of D311 = 0.472V

author: bdunham7
author_rank: Super Contributor
timestamp: January 06, 2024, 02:13:17 am
content: OK, I think that proves that the diodes are installed correctly and not damaged. I'm stumped for now--perhaps someone else has an idea? I may be missing something simple, but I can't understand the junction of D311 and D312 being 21V more negative than the other side of D312 as the difference should be limited by the diode. Also, the neon bulbs glowing when there only appears to be a few volts across them seems to indicate there's some issue with measuring these voltages or there is some ripple present. Circuit loading could be a possible source of measurement error, although 2KV and 1G-ohm gives you only 2µa, right? In any case, is there any change in the glowing of the neons or the CRT display when you actually take those measurements?Anyway, don't give up yet. And I'm assuming the 27VAC Z-axis experiment yielded no result?

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 06, 2024, 02:34:53 am
content: Quote from: guizmo1967 on January 05, 2024, 10:12:38 pmQuoteWhen you ask me to measure those voltages, since D312 and R346 are very close to N301, I could see N301 dim to almost not glowing and the screen seems to flicker as I was not in front of it !!!

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 06, 2024, 02:41:51 am
content: Do you want me to still do Z-axis experiment with capacitor C333 and C343 removed

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 06, 2024, 03:41:07 am
content: ****** UPDATE ******Ok, when I probe the cathode of D312 and touch N301 with a little bit of wiggling (I don't know if it's the right word, English is not my first language) I can make N301 turn completely off and then the line dims by half while N302 still glows a little bit like before !!! Are the two neons bulbs leaking ? Are we finally going somewhere

author: bdunham7
author_rank: Super Contributor
timestamp: January 06, 2024, 03:41:44 am
content: Quote from: guizmo1967 on January 06, 2024, 02:41:51 amGo ahead and try it. Also, when the neons are glowing, can you see if both electrodes are glowing or just one (on each bulb)?

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 06, 2024, 03:49:35 am
content: Quote from: bdunham7 on January 06, 2024, 03:41:44 amLook at my last post !

author: bdunham7
author_rank: Super Contributor
timestamp: January 06, 2024, 03:52:10 am
content: Quote from: guizmo1967 on January 06, 2024, 03:41:07 amI've been wondering if the neon bulbs might be a red herring (an obvious but irrelevant clue) but I'd have a real close look at all the connections, traces and solder joints in that area.

author: bdunham7
author_rank: Super Contributor
timestamp: January 06, 2024, 03:54:05 am
content: Quote from: guizmo1967 on January 06, 2024, 03:49:35 amThe posts crossed, but your's didn't fully answer my question. I'm talking about looking closely at the two individual electrodes within each bulb (if you can see them) and whether the orange discharge is around just one electrode or around both.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 06, 2024, 03:56:05 am
content: One thing is for sure now, those neon bulbs must be completely off ! Tomorrow I will reinstalled capacitor C333 and C343 and check the connections around that area, but it's getting late !Problem not solved yet, but we are getting there

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 06, 2024, 04:04:32 am
content: Quote from: bdunham7 on January 06, 2024, 03:54:05 amIt is only one electrode in each bulb that glows (only the tip), the ones that are connected to the diodes, not the two electrodes in series.

author: bdunham7
author_rank: Super Contributor
timestamp: January 06, 2024, 04:16:38 am
content: So if you do the Z-axis test, here's what you're looking for. This is from a different scope that I was demonstrating, the Z-axis is modulated by a square wave here to demonstrate speed, but if yours works it will look similar but without the sharp transitions.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 06, 2024, 10:36:55 pm
content: Well, my hopes were high yesterday, but today it's gone .....Today I did the Z-axis test with C333 and C343 removed, no changes on the line !Reinstalled C333 and C343, check every connections around those bulbs for continuity, found nothing wrong. Removed the jumper that was holding down the neon bulbs flat on the PCB and bend the bulbs straight up. Turn power knob and to my surprise, the neon bulbs were completely off, don't know why ??The line now seems a little less bright, I check voltage at pin 3 (grid) and pin 7 (cathode) and now the difference is 16V instead of the 8V earlier, less electrons !If I turn VR2 CCW to CW fast, I can see a very slight dimming of the line !!I'm relying on you guys, because I don't know what to do next

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 06, 2024, 11:08:30 pm
content: I don't know if this could help !I connected CHB to my audio amplifier speaker terminals, selected MODE CHA and CHB and took two pictures, one with intensity knob (VR2) to min and another one with VR2 to max. You can see on the picture the line of CHA (nothing connected to it) and the sound waves of CHB. There is something weird, hope you can see the difference.

author: carlitos
author_rank: Contributor
timestamp: January 06, 2024, 11:16:46 pm
content: I can't see any clue on pictures, but i think blanking pulse is not working.Resistor R359=47k?Can you measure voltages on Q314 and Q313 with VR2 in mid position?

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 07, 2024, 12:21:22 am
content: Q313Emitter: +14.56VCollector: +19.36VBase: +15.23VQ314Emitter: +10.25VCollector: +15.23VBase: +10.84VR359 = 46.6KOhms

author: carlitos
author_rank: Contributor
timestamp: January 07, 2024, 01:16:42 am
content: That reading does not differs too much from schematic.On oscilloscope pull Trigger Level knob to select Auto. With multimeter selected to AC Volt measurement, measure voltage on Pin2 of connector P202, then measure voltage on emitter of Q321 and with VR2 check minimum and the maximum value.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 07, 2024, 01:44:12 am
content: Pin2 connector P202 = +5.04VQ321 VR2 minEmitter: +40.4VCollector: +128.3VBase: +40.8VQ321 VR2 maxEmitter: +40.2VCollector: +128.3VBase: +40.6V

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 07, 2024, 03:17:31 am
content: If someone could explain this to me, I already mentioned it, but no one answered !If you look at page 30 in the service manual where you can read all the voltages, CRT pin 1 and 14 (heater) should be at -1900V, pin 3 (grid) and 7 (cathode) at -1400V, pin 8 (focus) at -1500V.The -1900V comes from L301 through D310 and R343. Can someone explain me where or what parts on that schematic makes the voltage go from -1900V to -1400VI don't think B&K Precision made a mistake in their service manual !!!Please, it's getting really frustrating

author: bdunham7
author_rank: Super Contributor
timestamp: January 07, 2024, 04:08:50 am
content: Quote from: guizmo1967 on January 07, 2024, 03:17:31 amIf I get a chance I'll try and skim the manual in the next few days to see where the discrepancy is. However, I think the voltages should be about -1.9kV and you're right, there's nothing there that would drop them and plenty to prevent the filament and grid/cathode voltages from being different. And after reading what you did, I'd guess the neon bulbs glowing was indeed a fake-out. You might try cleaning that area of the board with alcohol just to make sure there isn't any residue on any kind on it.As for your frustration, I've fixed a fair amount of stuff--including oscilloscopes--and sometimes you just have to set them aside and think for awhile. Maybe in a few days when I'm not busy I'll write out an explanation of how I think each part of the circuitry works and if I'm wrong about something--which would impede troubleshooting--someone can point it out. The manual is pretty sparse in that regard, the schematic is pretty much your only map.Do you know what the AC bandwidth of your DMM is?

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 07, 2024, 04:28:14 am
content: No, there is no mention of AC bandwidth in the user manual.If you have the time to skim the service manual, I would really appreciate itBecause I'm out of solution and the -1400V, I don't understand

author: bdunham7
author_rank: Super Contributor
timestamp: January 07, 2024, 04:37:57 am
content: Quote from: guizmo1967 on January 07, 2024, 04:28:14 amWhich meter is it? Is it TRMS?I'll let you know.

author: JeremyC
author_rank: Regular Contributor
timestamp: January 07, 2024, 05:15:40 am
content: Check the C337 (1uF/250V) capacitor. btw what are AC and DC voltages on this cap?What is B-E voltage on the Q318? Check both, DC and AC.Check the R358 resistor and -10V.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: January 07, 2024, 09:23:23 am
content: HiI have tried to follow this posting but have got lost in it,Back to fundamentals.To produce spot cut off for the CRT used the CRT Grid must be -25V to -66V more negative than the Cathode according to the CRT manufacturer , your original figures show it to be only -5V more negative, hence full brilliance.Most later oscilloscopes use the Z modulation/trace blanking circuitry along with some DC restoration to control the CRT brilliance, this is probably where your problem lies.G Edmonds

author: carlitos
author_rank: Contributor
timestamp: January 07, 2024, 03:01:00 pm
content: Quote from: guizmo1967 on January 07, 2024, 01:44:12 amStrange values, with these voltages there is no blanking pulse.To confirm, make sure there is no signal in BNC input, better move switch to GND and with Trigger Level knob, switch between Auto and Normal, trace disappears? If not disappers confirms no blanking pulse.Quote from: guizmo1967 on January 07, 2024, 03:17:31 amIn simple words, to obtain -1500V at pin 8 (focus) it is used a simple resistive DC voltage divider, R344, R345 and VR3.At pin 3 and pin 7 -1400V is obtained with a resistive/capacitive AC voltage divider, R346, C333, C343, L307 in parallel with R352, controlled by a square wave on Q321 emitter.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: January 07, 2024, 04:38:53 pm
content: HiIs this the early version or the late version? See manual.G Edmonds

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 07, 2024, 05:20:48 pm
content: Quote from: bdunham7 on January 07, 2024, 04:37:57 amIt's a Mastercraft (Canada) 52-0052-2 and not TRMS !All the other post before this one, give me some times to test your suggestions

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 07, 2024, 06:47:35 pm
content: Quote from: JeremyC on January 07, 2024, 05:15:40 amC337 has been replaced. DC = +116V AC = reading all over the place, not steady !!Q318 B-E voltage = not accessibleR358 good and -10.26V

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 07, 2024, 06:55:00 pm
content: Quote from: carlitos on January 07, 2024, 03:01:00 pmShorting plug on Z-axis BNC input, switch to GND. When triggering knob to AUTO line across the screen, NORMAL line disappears and a really bright dot appears on the far right of the screen.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 07, 2024, 06:55:37 pm
content: Quote from: George Edmonds on January 07, 2024, 04:38:53 pmLate version with the mini board X77

author: carlitos
author_rank: Contributor
timestamp: January 07, 2024, 09:01:11 pm
content: You do not have another oscilloscope and i do not know the behavior of your DMM, but lets try this.Put Trigger Level in AUTO and you see trace, then rotate Sweep Time to 0,5s/cm and you see a dot moving left to right. If happens as expected you will see also a very quick movement of dot from right to left. Now, measure DC Volt on pin2 of connector P202 when dot moves left to right and from right to left.I'm expecting that moving dot left to right be 0V, and 5V from right to left if this variation is slow enough to DMM.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 07, 2024, 09:18:41 pm
content: Quote from: carlitos on January 07, 2024, 09:01:11 pmSWEEP TIME/CM at .5s (even slower if I rotate CAL on SWEEP TIME), the dot only moves right to left with a quick movement left to right, almost a line. DC volt at pin2 of P202 while doing that = always at +5V (VR2 middle), no fluctuations (pulse). With VR2 at min DC volt = +4.96V and VR2 at max DC volt = +5.04V

author: carlitos
author_rank: Contributor
timestamp: January 07, 2024, 09:45:11 pm
content: With same position of controls measure if exist fluctuations on pin1 and pin2 of IC204, pin6 of IC202.

author: bdunham7
author_rank: Super Contributor
timestamp: January 07, 2024, 09:52:19 pm
content: I had a chance to look at the manual and I think the -1400V thing can be ignored for now. There's a another section of the manual that contradicts this and says you can adjust the -1.9kV supply with a probe at pin 1, 7 or 14 of the CRT, implying that a cathode voltage of -1.9kV is correct. Also, there's no way there would be 500V difference between the filament and the cathode.I was going to write more later, but for now can you check the the voltage on both sides of R349? And can you measure that using the DC, AC and frequency modes of your DMM?Also, I think I've misunderstood part of the circuit because I missed that D315 is a zener (of unspecified voltage). Could you measure the voltages at both sides of D315? And then the voltage directly across it?

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 07, 2024, 10:15:31 pm
content: Quote from: carlitos on January 07, 2024, 09:45:11 pmIC204Pin 1 +2.331V right to left and with fluctuations when quick movement from left to rightPin 2 +0.854V right to left and with fluctuations when quick movement from left to rightIC202Pin 6 +3.803V right to left and with fluctuations when quick movement from left to right

author: carlitos
author_rank: Contributor
timestamp: January 07, 2024, 10:52:10 pm
content: Now we have something. Is it possible to see value or direction of fluctuation?IC 204 was changed, original or equivalent?Now we have to find why there are no fluctuations on pin 3 of IC204.Voltage of pin 14 of IC204.Resistance of VR2 because i don't remember you have done, and R355, R356, R357, C339 and TC302.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 07, 2024, 10:54:21 pm
content: Quote from: bdunham7 on January 07, 2024, 09:52:19 pmD315 was WZ-050 (5.0V) replaced with NZX5V1B. Cathode side +41V, anode side +36.2V and across +4.72VR349 DC voltage:+195V on one side and the other +42.8VR349 AC voltage:Reading all over the place, I don't have a TRMS !R349 Frequency:+195V side = 0 Hz and the other side 25.78KHz

author: bdunham7
author_rank: Super Contributor
timestamp: January 07, 2024, 11:10:47 pm
content: Quote from: guizmo1967 on January 07, 2024, 10:54:21 pmTRMS isn't important here, but bandwidth is. Can you set your DMM to the 200V range and try again to measure AC voltage at the lower side of R349? It doesn't have to be a good or stable reading, even knowing whether it is 20-ish or 80-is would help. This may not work, but it is worth a shot.Also, earlier I had you disconnect the end of VR2 that goes back to IC204 and ground it so that you'd get a good variable voltage. Is it still like that or did you put it back?

author: George Edmonds
author_rank: Frequent Contributor
timestamp: January 07, 2024, 11:16:27 pm
content: HiI have now had time to paste the three A4 partial scans of the schematic into its original A3 size.A previous post stated that the voltages around Q321 were incorrect. This is a critical clue, the CRT spot intensity is controlled by the ratio of the +5V supply against a pulsing 5V blanking signal coming from Pin 3 of IC202 on the time base board. The ratio being controlled by VR2. the intensity control. The resulting voltage is then amplified by Q320, Q319 and Q312, the resulting pulses are then further conditioned and DC restored to provide the negative grid voltage for the CRT intensity control. The signal that should be present on the Emitter of Q321 is given in Fig 5 on page 12 of the manual.I suspect that there are no blanking pulses being generated by IC202 on pin 3. This gives you two problems, one just what is a TD-3472AP (IC202) as I can find no data on it. The second one being without a working Scope it will be almost impossible to fault find yours.I cannot stress how important the correct Fig 5 waveform/signal is present on the Emitter of Q321 before you go any further. Given the amount of previous work done around the CRT supplies there could well be additional faults now present.G Edmonds

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 07, 2024, 11:21:28 pm
content: Quote from: carlitos on January 07, 2024, 10:52:10 pmLower when quick movement from left to right.QuoteIC201, IC203 and IC204 replaced with SN74LS00N and IC202 with SN7472NQuote+5.02VQuoteR355 = 40.5K OhmsR356 = 3.28K OhmsR357 = 3.64K OhmsI pretty sure those resistors once removed from the board will be right on !C339 and TC302 no idea ??

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 07, 2024, 11:27:44 pm
content: I cannot tell you guys how much I appreciate your help, thank you very much and let's find this problem !But I have to say my head is about to blow off with all the tests you made me do ....

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 07, 2024, 11:40:21 pm
content: Quote from: George Edmonds on January 07, 2024, 11:16:27 pmI replaced TD3472AP with SN7472That's how I see it too, you adjust VR2 to your liking and the blanking pulse do the rest !

author: bdunham7
author_rank: Super Contributor
timestamp: January 07, 2024, 11:42:43 pm
content: Quote from: George Edmonds on January 07, 2024, 11:16:27 pmMy theory or assumption is or was that the blanking and intensity controls (eventually) resulted in a DC bias being applied to D314 and related components, a higher voltage for blanking the beam and a lower one for a bright beam. I also was thinking that the blanking control worked by raising the low side of VR2 and thus bypassing it as a control. Q318, switched by the 1T signal from the transformer, would chop the DC bias supplied through D314 and the resulting AC component would be passed up, clamped and DC restored by C333, D311 and D312. C343 would serve mostly as the filter for the half-wave DC coming through D312. If my version is incorrect, then my attempts at diagnosis haven't been so helpful!I agree that this is very tough with limited instruments! Regardless of how the scope works, I'd really like to see what is going on at the collector of Q318. We've observed that there is no blanking (refer to some earlier posts where I had the OP move the trace start onto the screen and there was a large, bright dot) but every thing I've suggested to try and alter the beam brightness has failed, including the Z-axis input which should be independent of all the other systems. 27Vrmsis 38 volts peak, that should have been enough to cause at least some observable change in beam brightness.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 07, 2024, 11:46:12 pm
content: Quote from: bdunham7 on January 07, 2024, 11:10:47 pmThe AC voltage measuring in this problem gets me confused because to me the voltage coming out of L301 is rectified so it is DC. When I measure DC, black probe on ground and red probe on the parts I want to measure. Where do you want me to put the probe for R349 ??The end of VR2, I put it back.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: January 07, 2024, 11:51:55 pm
content: HiSorry I have been calling it IC202 it should be IC204.You are basically correct in how this scope intensity control actually works, It is a very common way of controling CRT intensity as it enables the intensity control to be at low voltage. There is a good explanation on the Tek Wiki site when used in 2000 series scopesG Edmonds

author: George Edmonds
author_rank: Frequent Contributor
timestamp: January 07, 2024, 11:57:59 pm
content: HiThis is a dynamic system, DC measurements are by and large invalid, again I say the waveform at the emitter of Q321 is the keyG Edmonds

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2024, 12:12:11 am
content: Quote from: guizmo1967 on January 07, 2024, 11:40:21 pmOn scopes I'm more used to (Tek) the blanking, intensity and Z-axis input are all combined into a Z-drive circuit. Here they're more separate functions.The problem with thinking that the blanking pulses are controlling intensity is that the scope has an X-Y mode (turn the timebase selector all the way clockwise) where there shouldn't be any blanking pulses because the beam is on continuously. Now that I think of it, perhaps put the scope in that mode and see what it does. You can put some input on both channels and you should get something on the screen.

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2024, 12:20:25 am
content: Quote from: guizmo1967 on January 07, 2024, 11:46:12 pmPut your black to ground and the red to the lower side of R349. The actual signal will be a mixture of AC and DC, but your meter should register just the AC part--which is also what will be passed by C333.

author: carlitos
author_rank: Contributor
timestamp: January 08, 2024, 12:24:14 am
content: The service manual have a section of troubleshooting and in fig.5 has a waveform of Q321 emitter. Lowest voltage 40V, goes up to 90V, after a small time goes down to 40V. Pin 3 of IC204 commands that rising to 90V dropping voltage to 0V.I'm thinking this not happen.

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2024, 01:26:39 am
content: Quote from: carlitos on January 08, 2024, 12:24:14 amYes, it goes to 90V during sweep (bright time) and then to 40V (blanked). But earlier in this thread I had the OP disconnect VR2 from IC204 and ground that wire, then check the voltages at the emitter of Q321 as he adjusted VR2. He got approximately the required 40 to 90 volts, yet the intensity didn't change. And then the fun started....https://www.eevblog.com/forum/repair/oscilloscope-bk-precision-1472c-intensity-problem-!/msg5258538/#msg5258538He has since reconnected VR2 to IC204. The scope should be blanked if it is just sitting there with no trigger and not in X-Y mode, so he can check this again with everything connected and see if he still gets reasonably close to 40-90V on the emitter of Q321.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 08, 2024, 01:34:59 am
content: QuoteFor sure I saw that troubleshooting chart, but unfortunately I don't have another scope !QuoteDMM on AC, reading fluctuates a lot but the higher I saw was +27.5Vac

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 08, 2024, 01:41:57 am
content: This is important I think !Mode CHA only, line on the screen, voltage at junction of R349 and D314 = +42.4V no matter how I adjust VR2 !Mode CHB only, line on the screen, voltage at junction of R349 and D314 = +42.4V no matter how I adjust VR2 !Mode CHA & CHB, two lines on the screen, voltage at junction of R349 and D314 = from +42.4 to +66.2V when I adjust VR2

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2024, 01:47:58 am
content: OK, try it in XY mode--rotate the timebase all the way clockwise to the CHB position.And measure on AC as well, see if you can spot any variance.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 08, 2024, 01:49:09 am
content: QuoteEmitter of Q321 only varies when MODE is in CHA & CHB, else it is stuck at +42V

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 08, 2024, 01:58:47 am
content: Quote from: bdunham7 on January 08, 2024, 01:47:58 amSWEEP TIME at CHB (x-y), dot on the screen, junction of R349/D314 and emitter of Q318 stuck at +40V or so, no matter how I adjust VR2.

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2024, 04:45:51 am
content: Quote from: guizmo1967 on January 08, 2024, 01:58:47 amSo 40V or so would represent the CRT being blanked if the service manual is correct, but I'm beginning to have doubts about that. Or perhaps there's a flaw in my reasoning, but here it is anyway.The way the circuit around Q318 appears to work is that the signal from the 1T winding of the HV transformer turns Q318 on and off at about 25kHz. If no current flowed through D313 or D314, then the current through R349 would be around 2mA when on and none when off, the voltage at the collector of Q318 would vary from a few, perhaps tens of volts when on and about 195V when off, yielding an AC signal of perhaps 150-180Vp-p--if there were no load losses--and an average of 75-90V. That signal goes through C333 where the positive (above average) swings go through D311 and are absorbed by the lower impedance -1.9kV power supply and the negative swings go through D312, charge up C343 and cause the grid circuit to go more negative.The circuits involving D313 and D314 limit these voltage excursions further, reducing the amount of voltage differential between grid and cathode. VR307 sets a maximum, since anytime the voltage goes over what VR307 is set at current will flow out through D313. The whole Q321/D314 circuit sets a lower limit, since anytime Q318 manages to pull the voltage down below Q321's emitter, current will flow through D314. For some reason I'm not sure of, this latter thing appears to be happening since you have the same voltage at both points. However, as mentioned earlier by others, DC readings are only going to go so far here.I'm really not sure how to proceed at this point. I'd really like to see what some of these points look like on a scope. You may have more than one issue now as well, since the voltage at Q321 doesn't really respond appropriately to VR2. When it did respond properly earlier when I had you ground one side of VR2, the intensity still did not respond.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: January 08, 2024, 07:45:07 am
content: HiOne thing is being forgotten, this is a capacitor coupled pulse circuit. With one side of VR2 grounded where do the AC/Pulses come from?G Edmonds

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 08, 2024, 04:41:45 pm
content: You, George and bdunham7 have more knowledges that me, but I can follow your thinking on how it is supposed to work ! To me, it's all a big voltage divider made of VR3 on one side and the other side with R349, emitter of Q321 and collector of Q318.I think you are right on one thing, as long as I don't have another scope to verify all the waveforms on page 12, I will never find the problem. For that I also need a function generatorNow with everything we tested together, do you think the problem is still with the power supply board (X68) or the time base board (X74) ??

author: George Edmonds
author_rank: Frequent Contributor
timestamp: January 08, 2024, 05:07:50 pm
content: HiWhen fault finding if all possible cut the problem in two, the logical place to do this is the Emitter of Q321 if for no other reason than the information given in the manual. With one side of VR2 grounded the DC behaviour of Q321 appears to be reasonable so you must suspect the signals coming from IC204 as a first step, this is not to say that there are not problems post Q321.In the next hours I will try to analyze just what happens around IC204 where I believe the basic problem is, but without another scope you are probably up a creek without a paddle. It is a pity that it is the CRT blanking and intensity circuit that is the problem as the vast majority of scope problems can be simply fixed with just a DMM.Please take heart you are not the only one that has found the Intensity, blanking and Z modulation circuitry of a scope very difficult to understand, the main problem being using low voltage circuits to control the exact voltage on The CRT grid when it is at some +2KV above ground.All the best for nowG Edmonds

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2024, 05:42:11 pm
content: Quote from: George Edmonds on January 08, 2024, 07:45:07 amIn my version, the AC that is coupled through C333 comes from Q318 being switched on and off by the 1T winding of the HV transformer at 25kHz or so.

author: carlitos
author_rank: Contributor
timestamp: January 08, 2024, 06:05:09 pm
content: I have come with an idea to try to simulate square pulses that are missing.Put VR2 at mid position, rotate Sweep Time to see almost a line but you can see the dot moving, solder a wire on pin 2 of connector P202 and other side to touch GND or solder a pressure switch between pin2 and ground.Then with oscilloscope turned on you will touch GND with loose wire 1 time per second and increase the touch to see if trace dimes. If you can you could try adjust VR2 at the same time.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 08, 2024, 06:47:23 pm
content: Quote from: carlitos on January 08, 2024, 06:05:09 pmIt's funny you mention that, because I thought of that sometimes ago !!Since I don't have the test equipment to verify the pulse coming out of pin3 of IC204, I thought I can make my own by simply programming a microcontroller, all on a breadboard and feed that pulse directly to VR2. Is this achievable ??What would be the frequency (High and Low times) of that pulse ? The amplitude would be +5V, since Vcc of the microcontoller is +5V !

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2024, 06:58:16 pm
content: Quote from: guizmo1967 on January 08, 2024, 06:47:23 pmThe frequency of the blanking pulse would be related to the sweep speed but the duration is probably fairly constant. The manual is showing an example with a timebase setting of 0.1ms/div, or 1ms sweep time and about 0.4ms of blank time. So your test circuit would pull the +5V low for 400µs every 1.4ms to replicate that. However, if you were using a 10ms/div sweep rate, as you might for looking at a 60Hz signal, then the pulse would be more like low for 400µs every 100.4ms.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 08, 2024, 07:10:38 pm
content: So you want this:1.4ms = 0.0014s = 714.28 HzHigh = 1000usLow = 400usIs 714.28 Hz enough ??

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2024, 07:19:50 pm
content: Quote from: guizmo1967 on January 08, 2024, 07:10:38 pmEnough for what? This is what it would have to operate the blanking for the return and retrigger period, so you're just replicating the conditions in the manual. I don't think it has anything to do with the AC coupled intensity control, but since we've been unsuccessful so far and I have no idea where the issue is I don't see the harm in trying.

author: carlitos
author_rank: Contributor
timestamp: January 08, 2024, 07:46:44 pm
content: Quote from: guizmo1967 on January 08, 2024, 06:47:23 pmMy idea is to quick and simple verify if error comes from power supply board or time base board.Then, confirmed that there is no pulse from IC204, yes, you can program a microcontroler to investigate where stop that pulse.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: January 09, 2024, 12:15:16 am
content: Hi allAt this point I feel that it will be helpful to clarify some points.First on the matter of blanking pulses, ignoring XY for which no blanking pulses are required, the scope has two dual trace modes. The first one being Alternative traces, in this mode the beam must be blanked during trace fly back and when moving between traces. The second mode being Chopped where the beam draws a small portion of the first trace and then moves and draws a small portion of the second trace and so on, blanking must be in place for fly back and the movement between traces. This movement between traces occurs at a rate of 200kHz, see block diagram on page 6 of manual.The more I look at the schematic the more I am convinced that the intensity control is implemented used the pulsed output of IC204 on pin 3, one further clue is the frequency compensation trimmer TC302 across R356. In my considerable experience the small winding on L301 feeding the base of Q318 is use to provide a form of synchronised DC restoration only not the rectification of the blanking/intensity pulses. This rectification is done by D311 and D312.I am at a considerable disadvantage because the parts list in the manual is wholly inadequate (read total Crap), is C333 a 4700pF capacitor a high working voltage one?Although about as clear as mud from the manual the Mini Circuit Board or its wiring could also be the problem, it contains a pulse shaping or limiting circuit between the output of IC202 on pin 6 and the input to a gate circuit in IC204, pin 1, the one that outputs on pin 3, that drives the intensity control.It would be much simpler with a second scope.All the bestG Edmonds

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 09, 2024, 03:17:22 am
content: Quote from: George Edmonds on January 09, 2024, 12:15:16 amMy DMM has a FREQUENCY function for up to 400KHz. On pin3 of IC204 and pin12 of IC202, there is no pulse, my DMM shows 0 Hz. IC202 has been replaced with SN7472 and IC201, IC203 and IC204 with SN74LS00N.For your information, I replaced all 2KV ceramic capacitors including C333. I also check all resistors on mini board X77, check Ground and +5V and while at it, I replaced all diode with 1N4148.I know a second scope would help a lot, but I don't have one.So making a pulse with a microcontroller, a good idea or not according to you ??

author: George Edmonds
author_rank: Frequent Contributor
timestamp: January 09, 2024, 09:28:27 am
content: HiNo it will probably add to the confusion, a simple logic probe would be of more, but limited, helpG Edmonds

author: bdunham7
author_rank: Super Contributor
timestamp: January 09, 2024, 07:38:06 pm
content: Quote from: George Edmonds on January 09, 2024, 12:15:16 amThat's a good point about the CHOP function, and a similar issue arises at the higher sweep/refresh rates where the ~25kHz frequency of the whole Q318 system would not be fast enough to function effectively. I think that L307, C343 and R352 form a compensation system whereby the higher frequency components (not rectified) of the blanking signal are transmitted directly up to the grid and the lower frequency components and DC are transmitted up and rectified via the Q318-C333-D311/D312 circuit. The trimmer capacitor would be there to adjust the transient response to help the two systems blend together well enough to make an acceptable display.

author: p.larner
author_rank: Frequent Contributor
timestamp: January 12, 2024, 06:19:36 am
content: any headway on this yet?.

author: guizmo1967
author_rank: Regular Contributor
timestamp: January 16, 2024, 09:13:37 pm
content: Well, maybe !!Since I don't have a second scope, I have to take the long road ....I removed the sweep time and the vertical amp board once again to test every resistors and diodes with my DMM, something I did not do before. No open resistor found, maybe some's were out of spec but once removed they were good. No bad diode either.I decide to look at the production changes for the 1472C and to my surprise not all the modifications has been doneMod 1: was already doneMod 2a: was already doneMod 2b: was already doneMod 2c: C139, C140, R222 and R223 by following the trace, not even there !!Mod 3a: was already doneMod 3b: was already doneMod 3c: was already doneMod 3d: It was indeed a 6.8K with no C248 ?? Replaced with a 4.7K and added C248Mod 3e: C249 and L209 not even there !! I have to buy those parts.Mod 3f: was already doneMod 3g: D213 was missing ?? I added a 1N4148 from IC204 pin4 to pin12.I put everything back together even if mod 2c and 3e are not done yet. Turned on the scope and now I have a pulse (max I saw was 318KHz) at P202 pin2 that varies when I rotate SWEEP TIME/CM, that's a small progress !!! When I adjust VR2, the voltage at P309 pin2 goes form +2.7V to +5.04V.When I play with MODE (CHA, CHB, A&B, A+B, A-B) and VR2, I can see the intensity changes depending where MODE is.I still don't have a pulse at pin 12 of IC202That's all I can say for now