Don't use speakers. Work with no load initially. 3 to 5 V at output is way out of whack--- that's 150mV to 250mV, referred to inputs. There's something still defective.
Short the added cap so it's not under suspicion and get the amp back into the state you described in post #12, if possible. Is bulb dim at that point? If so, trim input and output offset, and please report output voltage, supply rail voltages, and voltages at bases of Q101a and Q101b. Next step will be to add load resistance gradually until misbehavior presents itself.
I've got to turn in for tonight but will check in the morning. Hang in there. 😉
Short the added cap so it's not under suspicion and get the amp back into the state you described in post #12, if possible. Is bulb dim at that point? If so, trim input and output offset, and please report output voltage, supply rail voltages, and voltages at bases of Q101a and Q101b. Next step will be to add load resistance gradually until misbehavior presents itself.
I've got to turn in for tonight but will check in the morning. Hang in there. 😉
(just on 'ac coupling' / DC blocking input)
From memory, the DC300 has an un-balanced input.
I would put the C/R coupling right at the input connector > something like .22uF & 47k ohm to ground.
At least that part would be done and finished.
I wish I could help you more, and will follow posts with interest 🙂
PS.
For power amps without any DC output protection, and maybe prone to fail >
it doesn't actually cost that much to capacitively couple the output using a 'bank' of capacitors
comprising a combination of 'high value electrolytics back to back' + some bipolars + some film type.
From memory, the DC300 has an un-balanced input.
I would put the C/R coupling right at the input connector > something like .22uF & 47k ohm to ground.
At least that part would be done and finished.
I wish I could help you more, and will follow posts with interest 🙂
PS.
For power amps without any DC output protection, and maybe prone to fail >
it doesn't actually cost that much to capacitively couple the output using a 'bank' of capacitors
comprising a combination of 'high value electrolytics back to back' + some bipolars + some film type.
With the new input cap shorted, everything works.
about 40mV of DC offset. 60V rail voltages. 0-30mV on the bases of Q101a/b. Not sure which is which at the moment.
How would I add load resistance gradually? I just have speakers and 8ohm dummy loads.
I don't think anything has failed, it just doesn't like my DC blocker. I'm assuming that if I did the same thing to the second channel, it would cause the issue too.
I should mention I also replaced all the small black transistors in the process of troubleshooting. Used KSC1845/KSA992.
Maybe I try it the way Mister Audio suggests. 0.22uF and 47k gives a pretty high cutoff though (15Hz), going by what this filter normally is.
If it is on the input side, I could let the gain pot be the R and just use a 1uF cap in series with the input?
If it is on the input side, I could let the gain pot be the R and just use a 1uF cap in series with the input?
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IMHO, the cap can go in either place, wherever it's most convenient.
But the 1uF cap is already a bit too small. The amp's input impedance is R101 + R103's wiper resistance to ground, but assume it's 10k for convenience. So the high pass time constant is 1uF * 10k = 10ms or about 16Hz. Better value would be 4.7uF or 10uF, non-polar. (The 150k resistor is almost irrelevant, as the driving impedance is set by the input gain pot and the impedance of the signal source; it can vary from very low to about 25k depending upon wiper position.)
The more vexing issue is behavior suggested by the Dim Bulb Tester. I can't explain why presence/absence of the blocking cap would make any difference. Maybe I lack imagination, but it makes me suspicious of an undiscovered issue in the amp triggered by a transient associated with the added blocking cap.
I'm very sympathetic with the desire to keep the DBT in place until all mysteries are resolved. So I suggest keeping the load disconnected and the blocking cap shorted until things are sorted out. I would make use of the amp's DC coupling and vary the amp output through 0V to +/- 30VDC using DC applied at the input. (Perhaps use a transistor battery and the amp's gain control to vary output voltage if you don't have anything more convenient.) The idea is to exercise the amp's output while looking for misbehavior or a glowing bulb. Compare with same experiment on the working channel for reference.
If nothing dramatic occurs, apply 1k load and repeat the experiment, then again with 100 Ohms, etc. The objective is to uncover a problem or gain confidence that the DBT can be eliminated. Use your instincts to add experiments with the blocking cap. Maybe try the cap on the working channel for comparison. There's no reason for misbehavior in a working amp, IMHO.
Keep us posted. Good luck!
But the 1uF cap is already a bit too small. The amp's input impedance is R101 + R103's wiper resistance to ground, but assume it's 10k for convenience. So the high pass time constant is 1uF * 10k = 10ms or about 16Hz. Better value would be 4.7uF or 10uF, non-polar. (The 150k resistor is almost irrelevant, as the driving impedance is set by the input gain pot and the impedance of the signal source; it can vary from very low to about 25k depending upon wiper position.)
The more vexing issue is behavior suggested by the Dim Bulb Tester. I can't explain why presence/absence of the blocking cap would make any difference. Maybe I lack imagination, but it makes me suspicious of an undiscovered issue in the amp triggered by a transient associated with the added blocking cap.
I'm very sympathetic with the desire to keep the DBT in place until all mysteries are resolved. So I suggest keeping the load disconnected and the blocking cap shorted until things are sorted out. I would make use of the amp's DC coupling and vary the amp output through 0V to +/- 30VDC using DC applied at the input. (Perhaps use a transistor battery and the amp's gain control to vary output voltage if you don't have anything more convenient.) The idea is to exercise the amp's output while looking for misbehavior or a glowing bulb. Compare with same experiment on the working channel for reference.
If nothing dramatic occurs, apply 1k load and repeat the experiment, then again with 100 Ohms, etc. The objective is to uncover a problem or gain confidence that the DBT can be eliminated. Use your instincts to add experiments with the blocking cap. Maybe try the cap on the working channel for comparison. There's no reason for misbehavior in a working amp, IMHO.
Keep us posted. Good luck!
I've put the cap between the input and the pot. This seems to work fine. I tried a 3.3uF cap, then 10uF, and these seemed to not block any DC.
Testing with my signal generator which has a DC offset control.
Tried 0.22uF and this might be about right? If I take the value of the pot as R, this gives a cutoff of around 7Hz. (most amps still have a much lower cutoff, around 1-2Hz.
It definitely seems this design didn't like having the cap after the gain pot. It wants to see the gain pot to get the impendance it is expecting. In every other amp I've worked on, the cap is after the gain control. But this isn't any other amp, it is an old archaic, weird design.
Testing with my signal generator which has a DC offset control.
Tried 0.22uF and this might be about right? If I take the value of the pot as R, this gives a cutoff of around 7Hz. (most amps still have a much lower cutoff, around 1-2Hz.
It definitely seems this design didn't like having the cap after the gain pot. It wants to see the gain pot to get the impendance it is expecting. In every other amp I've worked on, the cap is after the gain control. But this isn't any other amp, it is an old archaic, weird design.
Use your signal generator to measure low frequency -3dB point. I predict 0.22uF will be much higher than 20Hz.
And measure response with gain control at max. You're going to find that low frequency corner will be a function of gain setting.
And measure response with gain control at max. You're going to find that low frequency corner will be a function of gain setting.
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I've settled on 1uF. I wasn't as scientific as I could have been. But ran some signals through while watching the woofers, and the AC voltage. I can turn down the frequency and the response stays pretty flat until almost the bottom. I'm ready to call it good...
I put it through a bench test to full power again too.
I put it through a bench test to full power again too.
Sounds like the thing has an input bias current issue, possibly inherent to the design. 150 mV at the input with a 150K resistor smells like 1 microamp, which wouldn't surprise me from a design of that vintage. A look at the schematic shows the input opamp is a uA739, and indeed its datasheet shows typical input bias is 0.3 uA, 1.5 uA max. If this theory is correct, turning the input gain down then up would reduce then increase parasitic DC offset on the output.
The input has a couple of offset controls, which seem like an excellent way to smoke woofers if they get wrung out.
tl;dr: okay design for 1968, but the state of everything, from transistors through opamps to even resistors and capacitors, has vastly improved since then. I know a guy who had a few forty years ago and he thought there were two kinds of DC300s: ones that had taken out speakers, and ones preparing to.
The input has a couple of offset controls, which seem like an excellent way to smoke woofers if they get wrung out.
tl;dr: okay design for 1968, but the state of everything, from transistors through opamps to even resistors and capacitors, has vastly improved since then. I know a guy who had a few forty years ago and he thought there were two kinds of DC300s: ones that had taken out speakers, and ones preparing to.
Good work and value choice 🙂
Reducing/blocking infrasonic frequencies can be very good for very many ported speakers >
stopping them from 'flapping about' at frequencies they weren't designed to reproduce.
This can result in both reduced low frequency distortion and 'effective' power handling.