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DC Offset with Cap Coupled Amp
DC Offset with Cap Coupled Amp
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Old 23rd September 2021, 03:51 AM   #11
dbxdx5 is offline dbxdx5
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Truly appreciate the suggestions and intel indianajo. In addition to what Osvaldo suggested, this gives me some useful direction for areas to check.
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Old 23rd September 2021, 03:52 AM   #12
rayma is offline rayma  United States
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Try some freeze spray, on one part at a time. The input stage is more likely to have the problem.
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Old 23rd September 2021, 04:09 AM   #13
JMFahey is offline JMFahey  Argentina
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Quote:
The peaks that I've measured occur so quickly that I can't snap a photo of my old analog scope screen fast enough to capture them.
Ok, I had understood that they happened continuously, like tick - tick - tick ..... , every few seconds .... "motorboating".

You may try this:
* first look at screen to check you can see it.
* put camera pointing at screen,
* turn room lights off, so practically only light source is screen trace
* no flash of course
that will make all cameras, some phones might do the same, choose a "long" exposure, say a couple seconds, instead of , say, 1/30th of a second.
It may catch the peak.
I also was a Photographer long time ago and s similar trick was used to catch lightning and fireworks, go figure.

If not, but you can see it, draw it on a piece of paper.

* scope supply rail, at or near Q110 collector, is it there?

* what if you short R13 (3k3) with, say, a 10uF to 100uF cap? Does it disappear? (on screen or audibly)

* what if you short R4? (same thing).

we are trying to find where does it get into te amp.

PS: fwiw I also have an old analog scope
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Old 23rd September 2021, 06:31 PM   #14
dbxdx5 is offline dbxdx5
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Thanks Fahey.
I haven't tried everything that's been suggested yet, but here's what else I've found since my last post.
  • Video of what my scope is showing at the speaker output of one channel. This is with an 8 ohm dummy load. 0.1V/div 20m/div. Scott Amp - YouTube
  • All of the power supply voltages measure within 1V DC of what's in the schematic, with the except of the B3+ rail. This is 75V vs. 70V. All look stable and free from any oddness on my scope.
  • I disconnected the lead going to C1 on the amp board in order to separate the preamp board. (Someone check me on this being the correct place, but I believe it is, since the preamp seems to go out to the volume/loudness and then to C1 negative side.) No noticeable change to the waveform with the lead disconnected.
  • I tried shorting R13 and R4 with a 100uF cap. This would be jumper from non-ground side of each resistor to positive side of cap and negative side of cap to chassis ground, correct? If so, the waveform became significantly worse. If this isn't right, let me know how I should be shorting them.
  • The voltages at the collectors of Q110 and its counterpart Q10 are ~75V. This sometimes moves up slightly, on the order of .5V, I assume due to fluctuations in the line voltage.
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Old 23rd September 2021, 08:46 PM   #15
indianajo is offline indianajo  United States
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There are 4 inputs from the Riaa & tone control sections to Q1 & Q2. Bass treble, loudness, and balance. Any of those could be source of the jumping. I'd take a scope and look at each, for amplitude of any jumping. Don't know what you are doing with capacitor to R13 & R4. R13 establishes operating point of Q3, I wouldn't monkey with it. Except to push on connections with a stick and see if big popping occurs. A trash speaker on output will alert you to what is going on, if your eyes blink or the pop happens between scope sweeps.
Slow variation of +70 rail at collector of Q110 shouldn't cause popping. It should sag in voltage as the amp puts out more wattage, that is normal. But such sags are below 10 hz usually and inaudible.
Again, locating first stage where the voltage changes suddenly with no input is kind of where you are, since your power supply is okay. Shorting the input is a problem on this amp since it has 4 of them.
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Old 23rd September 2021, 09:42 PM   #16
dbxdx5 is offline dbxdx5
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Ok, I checked the four inputs. Those at C3 and R15 are both showing fluctuating millivolts on my scope and meter. I then checked around the caps for the bass pot and my meter displayed the same thing: voltages spiking to over 200mV.

Probably not coincidentally this is the same problem area that led me to that reversed C3 cap that was causing the bias in one channel to be all over the place. I had checked the voltages at the bass pot then, and found a few volts in spots (see attachment). When I replaced that mis-installed C3 cap with a new one, the bias calmed down and those grossly high voltages were gone, however, I didn't pay much attention beyond that. Something else I had noticed was that pressing on the knob of the pot had initially caused those voltages to spike. I retightened the panel nut for the pot and that seemed to help. At this point I'm thinking either one or more of these .068 caps is bad or there's a mechanical issue with the pot itself.
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File Type: jpg Bass pot voltages.jpg (111.6 KB, 92 views)
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Old 23rd September 2021, 10:11 PM   #17
indianajo is offline indianajo  United States
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Film caps are usually pretty reliable, but maybe not in 1963-68. I've had good luck with Mallory and Goodall film caps in hammond 1964-68 organs, and sprague in 1966 wurlitzer organs.
Pots, especially those on the front panel, are high wear items. Hard to buy these days though, clarostat & CTS and all mil-spec 1/4" shaft pots, the factories have been closed & demolished.
Then again, don't forget to push connections in that area with a stick while listening. These were hand soldered.
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Old 23rd September 2021, 10:34 PM   #18
JMFahey is offline JMFahey  Argentina
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Good.
I trust capacitors, specially film types, but pots and any mechanical connections, not that much.

Indianajo: I suggested shorting (AC grounding) those resistors not as a "solution" or "messing" but to check whether they killed the mystery spikes.
Itīs a "diagnostic" action

That problem worsened makes me strongly suspect grounding (actually loss/worsening of it) or loss of supply decoupling.

Old high ESR rail decoupling caps come to mind.
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Old 23rd September 2021, 10:52 PM   #19
dbxdx5 is offline dbxdx5
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Yep, old film caps. No markings as far as brand. It would be nice if one or more were the problem, because as noted finding a replacement pot is going to be tough. But I'm not optimistic it's the caps.
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File Type: jpg Bass pot caps.jpg (388.6 KB, 83 views)
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Old 23rd September 2021, 11:13 PM   #20
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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I continue suspicing of motorboating because of a bad cap in the supply or decouplig filter.
Film caps work preferably with higher frequencies.
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