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Old 1st June 2009, 12:22 PM   #11
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Quote:
Originally posted by EchoWars
Talking about C05? It has been replaced. So has C03.

Ceramics are pretty reliable, and since both channels are oscillating (when each board is installed by itself, that channel oscillates) I keep looking back at the power supply, but I've never seen this kind of thing...

It is possible for bad supply caps to cause oscillation. You could try, as a test, to solder in fresh capacitors in parallell.
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Old 1st June 2009, 01:43 PM   #12
Bigun is offline Bigun  Canada
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don't count me as vet'...

Is it possible there is some interaction with the load (assuming you have one) ?

Can you isolate pieces, one at a time by taking out their supply etc a few resistors (e.g. R13) ?
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Old 1st June 2009, 09:10 PM   #13
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Default Re: more ideas...

Quote:
Originally posted by djoffe
have you separated the output board from the input preamp? What if the input preamp (F-2520) were oscillating, or passing through oscillations from the LV power supply (F-2521)?

If you can separate the two and the output oscillation goes away...you've narrowed your search considerably...
Yes, there's a connector that runs from the input board to the driver board. I can break this connection and it makes no change in the oscillation.

Quote:
Originally posted by megajocke



It is possible for bad supply caps to cause oscillation. You could try, as a test, to solder in fresh capacitors in parallell.
I thought about that, but there isn't a lot of room. I do have some 35,000f 80V caps here somewhere. I might install them and see what happens.

Quote:
Originally posted by Bigun
don't count me as vet'...

Is it possible there is some interaction with the load (assuming you have one) ?
No load right now.
Quote:
Can you isolate pieces, one at a time by taking out their supply etc a few resistors (e.g. R13) ?
If I take out R13, the oscillation will stop because I essentially disable the whole driver board. I'm not sure what that will prove...
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Old 1st June 2009, 10:09 PM   #14
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no load at all ? just the probe tip, is that what you are saying ?
if so, I'd suggest to connect a dummy load, 8 or 16 ohm resistor (several watts, no input signal of course)
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Old 2nd June 2009, 12:53 AM   #15
Bigun is offline Bigun  Canada
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I may have misread the schematic, but it looked like R13 just provided power to the LTP, removing that leaves the output stages powered up. What I'm really suggesting is that with the oscillations everywhere you can't see the wood for the trees. If you can disable pieces, systematically, you might narrow down key pieces that are essential to sustain the oscillations and get closer to finding the source.
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Old 2nd June 2009, 04:16 AM   #16
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Quote:
Originally posted by payloadde
no load at all ? just the probe tip, is that what you are saying ?
if so, I'd suggest to connect a dummy load, 8 or 16 ohm resistor (several watts, no input signal of course)
Well, it's never supposed to oscillate, load or no load. I do see what you're saying, but I don't think the lack of a load matters one way or the other. No oscillation, ever.

Quote:
Originally posted by Bigun
I may have misread the schematic, but it looked like R13 just provided power to the LTP, removing that leaves the output stages powered up. What I'm really suggesting is that with the oscillations everywhere you can't see the wood for the trees. If you can disable pieces, systematically, you might narrow down key pieces that are essential to sustain the oscillations and get closer to finding the source.
I can remove R13, if you think it will prove something, but I'd say that oscillation will quit because the feedback for the amp is now dead. I'll try tomorrow and see what happens.
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Old 2nd June 2009, 06:36 AM   #17
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Seeing as you did quote a fair amount of changes to the amp, and seeing all the caps installed into the circuit to stabilise it, I'd hazard a guess that there's some serious changes made to the open loop bandwidth/gain of the amp.

Just as a test, install a 150pF cap across the VAS transistor. That would be C5 across TR3 if I'm reading it right. This would be a pretty severy measure, but would eliminate so many variables. If the amp does staibilise, we can maybe try tweak it from there. If that doesn't help then there's bigger issues at work here.
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Old 2nd June 2009, 01:26 PM   #18
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Quote:
Originally posted by gbyleveldt
Seeing as you did quote a fair amount of changes to the amp, and seeing all the caps installed into the circuit to stabilise it, I'd hazard a guess that there's some serious changes made to the open loop bandwidth/gain of the amp.
Well, that's the way things started out...lots of changes. But after identifying the oscillation, I have put one of the driver boards (I'm troubleshooting with only one of the driver boards right now) back to totally stock configuration...only the output transistors are changed.
Quote:
Just as a test, install a 150pF cap across the VAS transistor. That would be C5 across TR3 if I'm reading it right. This would be a pretty severe measure, but would eliminate so many variables. If the amp does stabilize, we can maybe try tweak it from there. If that doesn't help then there's bigger issues at work here.
Makes sense. I'll try this first.

But, assuming that lots of compensation at the VAS stage stablizes the amp, the question is: what made it unstable when I've worked on so many of these amps using the same parts and had no problems at all?

Well, let's see what happens first.
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Old 2nd June 2009, 01:58 PM   #19
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It's hard to say at this point, but looking at the schematics they tried some fun things there to keep the amp stable without limiting the closed loop HF. I also think (just my opinion) that back in those days the transistors were a lot slower than the new fancy stuff we have now and that may wreak havoc on a design that was borderline to begin with. Again, just my opinion, but let's try and tame it first before jumping to conclusions
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Old 2nd June 2009, 03:36 PM   #20
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Well shoot...I thought I could use the 35,000f 80V caps I had here to swap out the four large power supply caps. That won't work, as the originals have an oddball mounting method for the terminals that requires the solder lug terminals. Michael Percy has the perfect replacement, but the cost to roll the dice on this fix is $200 ($50 for each cap). If I knew that four new power supply caps would fix the problem, I'd buy them in a heartbeat. But I'm not big on spending $200 on a longshot.

I'll try swapping out C5.
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