CRT scope trace fades unevenly - Page 1
Forum section: electronics / repair

author: Mike99
author_rank: Frequent Contributor
timestamp: June 26, 2025, 01:14:19 pm
content: I'm restoring a 40 year old radio test set which includes an analogue scope with a 10MHz bandwidth.If I apply an input of around 100kHz or less the lower half of the trace fades before the upper half when reducing the intensity. If the input is at a higher frequency the trace fades uniformly.This happens whatever the vertical position so it's not a problem with the phosphor.I have replaced all the HV capacitors and all the electrolytics in the power supply.Could it be a mechanical issue? The only coil adjustment is for trace tilt to compensate for the earth's magnetic field.Mike

author: TERRA Operative
author_rank: Super Contributor
timestamp: June 26, 2025, 01:18:04 pm
content: Do you have schematics? It looks like an imbalance in the y-axis circuit, maybe one side of a push-pull arrangement is not the same as the other side?

author: Mike99
author_rank: Frequent Contributor
timestamp: June 26, 2025, 02:23:41 pm
content: Thanks for your reply. Schematic and block diagram attached.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 26, 2025, 04:08:10 pm
content: Mike, scope the grids and see if you see any waveform present that can modulate one of the grids and therefore, vary the intensity. It sound like some of the amplified input waveform somehow modulates (on the lower peaks) one of the grids and vary the intensity. At higher frequencies, it may not be so visible due to the persistence of the phosphor. You should check with other wave shapes as well.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 26, 2025, 04:22:39 pm
content: I checked it with a square wave and ramp, same result. I'm a bit wary of poking around at the back of a CRT. How many volts can I expect on the grids?Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 26, 2025, 06:59:03 pm
content: Since we are only interested in any AC component riding on there, you can put a suitable blocking capacitor (~ 1uF/600 V) in series with your scope probe to be safe.

author: m k
author_rank: Super Contributor
timestamp: June 26, 2025, 07:30:35 pm
content: Cathode and G1 capacitors are rated 2kV.

author: tggzzz
author_rank: Super Contributor
timestamp: June 26, 2025, 07:42:19 pm
content: What is the frequency of the signal shown on the CRT? If 50Hz, that's a big clue.What does a simple voltmeter indicate is the DC voltage and AC voltage (ripple) on- Q127 collector and emitter- Q128 collector and emitter- C141Possible values for CRT voltages (but do RTFM!): cathode voltage could be -2kV, and the grid-cathode voltage ~100V. The grid-cathode voltage determines the beam intensity.Does the fading pattern change when the input frequency changes, or is it synchronous with the input frequency? If the pattern is synchronous with the input frequency, then try to work out a mechanism by which the vertical deflection voltages are being coupled to the grid-cathode voltage.

author: D2236
author_rank: Regular Contributor
timestamp: June 26, 2025, 07:57:52 pm
content: Quote from: m k on June 26, 2025, 07:30:35 pmThe DC voltage after the doubler is about -1.4 kV, so just use one of the 100 nF HV caps you used for the replacements. Best option is to use a x 100 HV scope probe.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 26, 2025, 08:48:51 pm
content: Quote from: tggzzz on June 26, 2025, 07:42:19 pmThe input signal was 100kHz. Above that there is no asymmetric fading. I'll have a look at those voltages tomorrow.EDIT: Apologies, the input signal was 1kHzMike

author: Mike99
author_rank: Frequent Contributor
timestamp: June 26, 2025, 08:50:37 pm
content: Quote from: D2236 on June 26, 2025, 07:57:52 pmI kept the old 0.1uF 1kV and 2kV caps so I could use one of those but I only have a bog standard X10 probe.Mike

author: tggzzz
author_rank: Super Contributor
timestamp: June 26, 2025, 09:11:23 pm
content: Quote from: Mike99 on June 26, 2025, 08:50:37 pmBe very aware of your probe's voltage-vs-frequency de-rating curve.

author: D2236
author_rank: Regular Contributor
timestamp: June 26, 2025, 11:43:23 pm
content: Your standard probe used with the blocking cap will be good. Can you highlight all the HV caps you replaced on the diagram?

author: ondraN
author_rank: Regular Contributor
timestamp: June 27, 2025, 07:33:32 am
content: It can also be caused by the penetration of the measured signal into the Z axis. Try disconnecting one pin of C135. Then the backward path of the beam will also be visible, but if the eclipse of the lower part of the waveform disappears, it will be clear where to look for the error.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 27, 2025, 07:53:10 am
content: Quote from: D2236 on June 26, 2025, 11:43:23 pmC135, C137, C138, C139, C140. They were oil/paper and I replaced them with metallised polyester.Mike

author: m k
author_rank: Super Contributor
timestamp: June 27, 2025, 08:33:16 am
content: Is it the same despite signal's vertical height?Underside missing even if whole signal is on upper side of the screen.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 27, 2025, 08:53:21 am
content: Quote from: m k on June 27, 2025, 08:33:16 amIf you mean vertical position then yes, it's the same wherever the trace is on the screen.Mike

author: tggzzz
author_rank: Super Contributor
timestamp: June 27, 2025, 08:56:22 am
content: Quote from: Mike99 on June 27, 2025, 08:53:21 amIs the problem the same with a 10Hz, 100Hz and 1kHz signal; is it timebase dependent?

author: m k
author_rank: Super Contributor
timestamp: June 27, 2025, 08:59:01 am
content: Quote from: Mike99 on June 27, 2025, 08:53:21 amEven if amplitude is low?

author: Mike99
author_rank: Frequent Contributor
timestamp: June 27, 2025, 09:14:54 am
content: Quote from: ondraN on June 27, 2025, 07:33:32 amOK I disconnected C135 and with no blanking pulse the waveform fades uniformly at all frequencies. Does that offer any clues?Mike

author: m k
author_rank: Super Contributor
timestamp: June 27, 2025, 09:41:57 am
content: You have two possibilities.One is that intensity is deflection based.Other is that it is deflecting signal based.Make pictured amplitude low, maybe 1/4 of screen.When you move the signal vertically, is intensity locked to the signal or the screen?

author: Mike99
author_rank: Frequent Contributor
timestamp: June 27, 2025, 10:45:21 am
content: Quote from: m k on June 27, 2025, 09:41:57 amFor frequencies below about 100kHz with an amplitude of 1/4 of the screen the lower part of the waveform fades first whatever the vertical position on the screen.Mike

author: ondraN
author_rank: Regular Contributor
timestamp: June 27, 2025, 12:10:15 pm
content: Quote from: m k on June 27, 2025, 09:41:57 amOK. We can rule out the Z axis. In that case, the signal from the Y channel will probably modulate the voltage on the screen grids somehow. Most often, it is a conductive path on the socket at adjacent pins, on a connector or switch. I would use the oscilloscope measurement method through a HV capacitor, as someone already mentioned.

author: D2236
author_rank: Regular Contributor
timestamp: June 27, 2025, 01:08:31 pm
content: Since the blanking circuit operates of the rectified 150 V supply, and that isfiltered by C141/2, which also feeds one of the grids through R182, R173/4 combination,any ripple over R174 may modulate the grid. By lifting C135 you may reduce the load on the 150 V supply and have less ripple going to the grids.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 27, 2025, 01:43:00 pm
content: Quote from: ondraN link=topic=464697.msg5960409#msg5960409[/quoteTha thanks I'll have a look over the weekend.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: June 27, 2025, 01:43:42 pm
content: Quote from: D2236 on June 27, 2025, 01:08:31 pmOK will do, thanks.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 27, 2025, 01:57:17 pm
content: Probing over R174 will be safe for your x 10 probe since it will be less than 200 V. You may want to look at the blanking pulse amplitude over a longer sweep time, and see if the pulse amplitude varies according to shape of the input signal. The one side of the vertical output amp (C148) connects to the rectified 150 V.You don't want to see any of the effect as shown below on Q121.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 27, 2025, 03:15:35 pm
content: Good to know thanks. BTW I have already replaced C141/142.Mile

author: Mike99
author_rank: Frequent Contributor
timestamp: June 27, 2025, 07:40:37 pm
content: Sadly I have to abandon this quest for now as the blanking pulse generator has diedThanks for all your suggestions and sorry I wasn't able to try them all out.I'll come back when/if I get it fixed.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 27, 2025, 08:04:50 pm
content: That sucks! Hope you can sort it out quickly.

author: m k
author_rank: Super Contributor
timestamp: June 27, 2025, 08:15:58 pm
content: Is tone out ok?

author: Mike99
author_rank: Frequent Contributor
timestamp: June 28, 2025, 01:55:10 pm
content: Quote from: m k on June 27, 2025, 08:15:58 pmIf you mean the two-tone generator, then yes.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: June 28, 2025, 02:00:23 pm
content: Quote from: D2236 on June 27, 2025, 08:04:50 pmThanks, and I did, rather sooner than I expected. Both transistors in the blanking pulse generator were dead (Q120/Q121).I had another look at the blanking pulse with different input signal frequencies and noted that the baseline ripple varied from about 1V p-p at 1MHz to 6V p-p at 10kHz. Given that the uneven fading is frequency dependant, and disappeared when the blanking pulse was removed, could this be a factor?Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 28, 2025, 02:13:47 pm
content: Yes, that could certainty be causing your issue. I suspect the loading by the vertical output amp somehow causes that baseline to appear. Did you look at the voltage over R174?

author: Mike99
author_rank: Frequent Contributor
timestamp: June 28, 2025, 04:28:15 pm
content: I put the scope on R174 (grids 7 & 11) with input signals of 10kHz and 1MHz at the same amplitude.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 28, 2025, 04:36:36 pm
content: Can you scope the other grids, with your blocking cap? How does the ripple look over C141/2?

author: Mike99
author_rank: Frequent Contributor
timestamp: June 28, 2025, 05:17:14 pm
content: OK here you are.C141 has 350mV p-p ripple at 100Hz.Grid 2 has 250mV p-p ripple at 50HzGrid 4 screenshots attached for no signal and 10kHz.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 28, 2025, 07:15:57 pm
content: I looked at what effect ripple on the +12 V supply rail will have on the baseline of the blanking pulse. In my simulation, I applied a 1 Vpp ripple on this rail, and the effect is very visible. On the schematic the + 12V also feeds the vertical output amp with a connection near Q113_b. If the regulated supply is not stiff enough; the amp will modulate it. So, check the regulated +12 V and -12 V rails for signal ripple. Good idea to check C124, C143, C145, C146 in the +/- 12 V regulators if you haven't done it yet.There is also C109 (10 uF/25 V) for + 12 V filtering near Q102/4 in the vertical amp. Then the +/- 12 V regulator design uses + 12 V out as reference to set the -12 V. Any variation on the +12 V line will also show on the -12 V line.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 29, 2025, 03:20:30 pm
content: I changed C124, C143, C145, C146 a while ago. There is no discernable ripple on the +/- 12V rails nor on C109. The only discrepancy I found is that the +12V rail measures +11.1V while the -12V rail is -12.5V. However the band scope supplies are labelled as +11V and -12V so maybe that is correct.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 29, 2025, 03:59:27 pm
content: The 12 V rail is correct at about 11 V because the +12 regulator uses a 12 V zener, you lose about 0.7 V over the b_e junction. So something else is causing the baseline ripple that we have not identified yet. We are missing a clue somewhere that it is clean with blanking cap lifted. The fact that it only happens with lower frequencies seems to point to some filter time-constant.Do you see the fading with all signal amplitude levels, or over a certain threshold?

author: Mike99
author_rank: Frequent Contributor
timestamp: June 29, 2025, 04:03:04 pm
content: The fading occurs at all signal amplitude levels.Mike

author: m k
author_rank: Super Contributor
timestamp: June 29, 2025, 04:06:12 pm
content: Is there any differences of fading speed between amplitudes?

author: Mike99
author_rank: Frequent Contributor
timestamp: June 29, 2025, 04:17:19 pm
content: No, the fading is independent of signal amplitude.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 29, 2025, 04:18:58 pm
content: If you go back to the original blanking (with the retrace visible), is this problem still present?

author: Mike99
author_rank: Frequent Contributor
timestamp: June 29, 2025, 04:30:01 pm
content: If I disconnect the blanking pulse by lifting C135 the problem goes away.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 29, 2025, 04:32:32 pm
content: Sorry, I meant with the partially working blanking before we did the modifications.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 29, 2025, 04:35:03 pm
content: Ah, OK, stand by ...

author: Mike99
author_rank: Frequent Contributor
timestamp: June 29, 2025, 04:43:24 pm
content: I left Q121 as an MPSA44 and replaced Q120 with the original 2SA1015Y.The flyback trace has returned but now the trace fades evenly at all input frequencies ...

author: D2236
author_rank: Regular Contributor
timestamp: June 29, 2025, 05:38:07 pm
content: Very interesting... It appears, we were misled by the ripple shape, thinking it had to do with the vert-amp. Can you check what happens if you apply the same base resistor for this transistor? Have you replaced C117? I could not pick it up in the old posts.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 29, 2025, 05:41:44 pm
content: Yes I replaced C117. Do you mean base resistor for the current Q120?

author: D2236
author_rank: Regular Contributor
timestamp: June 29, 2025, 05:43:23 pm
content: Yes, so we can see if it behaves the same with the resistor.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 29, 2025, 05:59:11 pm
content: OK it will be a couple of hours.Thanks for sticking with this!Mike

author: m k
author_rank: Super Contributor
timestamp: June 29, 2025, 06:04:33 pm
content: Can you input a square wave?

author: Mike99
author_rank: Frequent Contributor
timestamp: June 29, 2025, 06:51:56 pm
content: Quote from: m k on June 29, 2025, 06:04:33 pmSine wave, square wave, triangular wave, I've tried them all.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 29, 2025, 06:54:29 pm
content: Quote from: D2236 on June 29, 2025, 05:43:23 pmThe resistor is in. After so many component changes I'm amazed the board hadn't disintegrated. Anyway, the waveform still fades uniformly but I can no longer get it to stabilise with the sweep control, it just free runs.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: June 29, 2025, 07:05:06 pm
content: I don't think I can justify spending much more time on this. I might go for the least worst option and put it back how it was originally. After all it's 40 years old so it can be forgiven for not working perfectly.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 29, 2025, 07:08:28 pm
content: One more thing. With the 2N3906 and base resistor, can you see what happens when you put a capacitor (100 nF to 470 nF) in parallel with the resistor?

author: D2236
author_rank: Regular Contributor
timestamp: June 29, 2025, 07:12:28 pm
content: Quote from: Mike99 on June 29, 2025, 07:05:06 pmThat is true, but it takes time to learn and understand why circuits behave a certain way.

author: m k
author_rank: Super Contributor
timestamp: June 29, 2025, 07:21:17 pm
content: Quote from: Mike99 on June 29, 2025, 06:51:56 pmAre you tuning sweep time according to the input frequency?

author: Mike99
author_rank: Frequent Contributor
timestamp: June 29, 2025, 07:29:34 pm
content: Quote from: m kYes.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 29, 2025, 07:48:19 pm
content: Quote from: D2236 on June 29, 2025, 07:12:28 pmIndeed it does, and I am most grateful for the time you have spent analysing the problem and advising me, but I have other projects stacking up behind this one and must call a halt to it soon.Any idea why speeding up the blanking pulse would affect the intensity in this way?Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 29, 2025, 08:35:20 pm
content: Here is my guess: I think it has something to do with not having the transistor hard saturated by inserting the base resistor. This helps with the blanking pulse with fast time-base sweeps, but with slower sweeps the base-line offsets in an oscillatory fashion causing the softer blanking on the peaks and hence the fading. With faster sweeps, the time is too short for it to come out of saturation before the next pulse, but on slow sweeps it moves upward. That is why I want to see how it performs when we bypass the resistor with a cap value (100 n - 470 n) large enough to keep it a bit more saturated with longer sweep times.It may be necessary to decrease the base resistor value to where you get acceptable blanking with minimal fading. Maybe the PFET option will work better after all.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 29, 2025, 08:52:16 pm
content: OK thanks. I'll try that over the next couple of days.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: June 30, 2025, 07:21:32 pm
content: OK it's confession time. When I made the last modification I unplugged a PCB connector for better access and forgot to put it back. That's why the trace wouldn't lock.So here are the latest results for different configurations of Q120.2SA1015Y with 27k in base: flyback trace visible and uneven fading.2N3906 with 27k||245nF in base: almost no flyback trace and uneven fading.BS250 with gate to Q118 base and extra diode: almost no flyback trace and uneven fading.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 30, 2025, 08:46:33 pm
content: So whenever we try to clean up the blanking, we introduce the uneven fading problem in all cases. This is very strange. We know the base resistor have some effect, since removing it gives flyback traces and even fading. What is unexpected is that we get the same with the PFET!? All theories about the cause are in the trash can for now...Can you find a happy median with reducing the base resistor, while obtaining acceptable flyback and fading?

author: Mike99
author_rank: Frequent Contributor
timestamp: July 01, 2025, 11:43:57 am
content: Here are some results with a 2N3906 plus base resistor for Q120 and MPSA44 for Q121.The minimum base resistance for no visible flyback trace at 1MHz is 18k. The maximum resistance for no uneven fading at 1kHz is 10k. Attached photos show flyback trace for zero ohms and 10k. The latter is probably my best option.Mike

author: m k
author_rank: Super Contributor
timestamp: July 01, 2025, 12:28:28 pm
content: Quite a bugger, there are not very many components left.Maybe function switch is not switching as it should, but would it affect clamping as is happening.I guess C111, C115 and C116 are also new.Polarity of C111 indicates that Band Scope -12V should be positive, tone circuit verifies that.Negative of C117 is also odd.So Cathode or G1 has an attenuation effect, but there is nothing that knows how input signal goes and attenuation goes with signal level, not with deflection level.What are the possibilities.That 500V drops because there is too much load, overall or doubler only.Higher input frequency should be consuming more.External sync can disconnect sweep from vertical side.X-Y mode can bypass sweep timing, but then artificial blanking is needed for elimination purposes.Maybe keeping fast sweep but inputting something slow can tell something.

author: D2236
author_rank: Regular Contributor
timestamp: July 01, 2025, 04:09:12 pm
content: We are dealing with a balance between two options. We need to get Q120 to turn off quickly. The base resistor helps keeping it out of hard saturation and speedup the turn off. The problem with this is that on the longer sweeps, we don't have a stiff base line, and it has time to move up causing the soft blanking symptom. It doesn't fully explain the same experience with the BS250, but I suspect that the threshold level for the gate is still marginal. Maybe another 1N4148 in series will push it over to turn on more fully and get lower RDS-on. We need to keep the base line of the blanking pulse closer to GND (under 5 V) to prevent the soft blanking to show up.I think it is fixable,BUTit may require modification using an optocoupler together with Q120. We will eliminate the influence of changing current through Q121 affecting the base current of Q120 and the turn-off time. If we get the led current dialed-in, we should be able to keep the base line of the blanking really low. Using something like the common 4N25 may be fast enough for this purpose.It would have been very useful to compare the blanking and other measurements with a similar working SM-220.

author: Mike99
author_rank: Frequent Contributor
timestamp: July 01, 2025, 04:15:40 pm
content: Quote from: m k on July 01, 2025, 12:28:28 pmYes it is a bugger! I have replaced C115 and C116 but not C111. The DC balance adjustment works OK so I didn't think it necessary.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: July 01, 2025, 04:20:23 pm
content: Quote from: Mike99 on July 01, 2025, 04:15:40 pmMuch as I would like to fix this I have to keep the modifications to a minimum as it's a collectable item and I'm planning to sell it. (However it's not worth the $1,000 that someone is asking for on that famous auction site!!). So I think that rules out an optocoupler.I have asked repeatedly on two forums for owners of this box to let me know how theirs behave but have had no replies.Mike

author: D2236
author_rank: Regular Contributor
timestamp: July 01, 2025, 04:35:23 pm
content: Yes, that is what I thought. Changing the internal image of the unit is not acceptable.Quote

author: Mike99
author_rank: Frequent Contributor
timestamp: July 05, 2025, 11:16:24 am
content: Quote from: m k on July 01, 2025, 12:28:28 pmThe service manual is littered with errors. C117 polarity is correct in the schematic but C111 is reversed.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: July 05, 2025, 12:01:58 pm
content: I really would like to understand what's going on with this scope, even if it can't be fixed.I have now replaced all the electrolytic capacitors.Q120 is a 2N3906 with a 100k pot in the base lead. Q121 is an MPSA44.With the pot set to zero ohms there is no baseline ripple on the blanking pulse and no uneven fading of the waveform. When the pot is increased to the point where uneven fading begins the baseline ripple appears very suddenly.So is the ripple on the blanking pulse causing the fading? If so, how can it be prevented?Mike

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 05, 2025, 01:27:09 pm
content: The pics you posted show that the sine wave is apparently increasing in freq. The distance on the screen horizontally from peak to peak is getting smaller from L to R. There is a problem in the horizontal amplifier that is making it look like this. Maybe this observation will help?

author: Mike99
author_rank: Frequent Contributor
timestamp: July 05, 2025, 01:52:56 pm
content: Quote from: Wallace Gasiewicz on July 05, 2025, 01:27:09 pmThank you for spotting that very subtle artefact, I hadn't noticed.That trace was a 1MHz signal. I've just observed a 10kHz signal and there is no increase in frequency.I calibrated the horizontal gain in accordance with the service manual so I suspect it may be a limitation of the circuit design.Mike

author: m k
author_rank: Super Contributor
timestamp: July 06, 2025, 02:58:10 pm
content: I assume the pot situation is with 1MHz input signal.Put sync to external and measure few voltages without any input signal.Bases of Q118-Q121 to GND and over C116, C117 and R144.How I understand it.First D108 is on through R138, Q118 is off, Q119 is on and C116 charged through D108.Q120 and Q121 are also off, Q120 base is pulled up through R140 and D107 keeps Q121 base low enough.When C115 is pulled down, D108 goes off and Q118 goes on when C116 is low enough.After that Q120 and Q121 go on and their bases are stable.Finally C115 is charged, D108 goes back on and Q118 goes off when C116 is high enough.After that Q120 and Q121 go also off and their bases are stable again.So R138 + D108 + (R141 || (C116 + R143)) can be one route of the problem, R140 + C150 the other one.R135 + D107 can't do it alone.You can also check how single sweep goes.Maybe it's a heat problem.

author: Mike99
author_rank: Frequent Contributor
timestamp: July 06, 2025, 07:40:56 pm
content: Quote from: m k on July 06, 2025, 02:58:10 pmYes the flyback trace is most noticeable at 1MHz. I don't think it's a heat problem as it occurs immediately after switch on.Thank you for the detailed analysis. I have a busy week ahead so it may be a while before I can investigate and report back.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: July 08, 2025, 02:24:53 pm
content: To all the wonderful people on this forum who have provided me with advice and suggestions on how to fix this problem, I have reluctantly decided to stop working on it as there are too many other things competing for my time. I never like to give up but don't have much choice now.After following your advice and making many measurements I was unable to identify a specific fault and now believe that it is a limitation of the circuit design, which is pretty basic. It is better than it was with a couple of simple component substitutions, and could probably be improved further with more extensive modifications, but as a piece of vintage equipment that I'm planning to sell I want to keep it as original as possible.It wasn't time wasted as I have learnt a lot from you guys. Thank you for spending the time to analyse the circuit for me and make your suggestions, it is much appreciated.Mike