Everything you are describing is pointing to some kind of oscillation.
A couple of things to look at but with a disclaimer... be careful, amplifiers that are oscillating can do so destructively. Perhaps use a bulb tester initially.
Look at C707 and C709.
1/ You could try increasing C709 (the 33pf) slightly to say 47pf or 56pf (and perhaps even a little higher)and observing the effect.
2/ If the above appears to do nothing then look at initially reducing C707 (22pf) to say 18 or 15pf. Any change ? If worse then increase to say 33pf and check result.
A couple of things to look at but with a disclaimer... be careful, amplifiers that are oscillating can do so destructively. Perhaps use a bulb tester initially.
Look at C707 and C709.
1/ You could try increasing C709 (the 33pf) slightly to say 47pf or 56pf (and perhaps even a little higher)and observing the effect.
2/ If the above appears to do nothing then look at initially reducing C707 (22pf) to say 18 or 15pf. Any change ? If worse then increase to say 33pf and check result.
Thank You so much for the help
So all i had on hand was 33pf
So i changed c707 from 22pf to 33pf, And the oscillation seems to be gone
And now the bias voltage at test points with load connected is going up to 8mv
But with no load i get the full 24mv for bias, and with the 8ohm load it goes down to 8 mv.
So its definatly getting better
Should I first try to change the c709 to 47pf - 56pf
or is it ok that i started with c707
So all i had on hand was 33pf
So i changed c707 from 22pf to 33pf, And the oscillation seems to be gone
And now the bias voltage at test points with load connected is going up to 8mv
But with no load i get the full 24mv for bias, and with the 8ohm load it goes down to 8 mv.
So its definatly getting better
Should I first try to change the c709 to 47pf - 56pf
or is it ok that i started with c707
Keep using the scope to confirm what is happening. If there is no oscillation and if the bias current changes (alters load vs no load) with a load attached then that points to a simple (probably normal) DC offset.
If the bias adjusts OK with no load (and no oscillation) and there is no oscillation when a load is attached then its probably fine.
Its impossible to give absolute advice on the caps without analysing the circuit for real and seeing how it behaves. If you go to large on the 22pf cap then a point will be reached where it actually causes instability. Increasing the 33pf cap (C709) will be less of a problem but may not "fix" the issue but rather just "fiddle it" so it seems OK.
What I would do given these unknowns is to set the amp up with a 10kHz squarewave and look at what is happening. That's a good guide as to whether you've overdone things with the cap sizes... or not gone far enough.
If the bias adjusts OK with no load (and no oscillation) and there is no oscillation when a load is attached then its probably fine.
Its impossible to give absolute advice on the caps without analysing the circuit for real and seeing how it behaves. If you go to large on the 22pf cap then a point will be reached where it actually causes instability. Increasing the 33pf cap (C709) will be less of a problem but may not "fix" the issue but rather just "fiddle it" so it seems OK.
What I would do given these unknowns is to set the amp up with a 10kHz squarewave and look at what is happening. That's a good guide as to whether you've overdone things with the cap sizes... or not gone far enough.
Thanks again for all your help - im learning so much invaluable information by repairing this amplifier
So I run a 10khz stabalized sine wave into the aux input. And my scope measures a 59hz sine wave coming out of the speaker terminals
At low volume the sine wave looks clean both sides.
When I turn volume up the Channel im working on gets fuzzy.
So im going to continue to try some different cap values till i find the best values that clean up the oscillation?
Only going to change c707 and c709 for now
I changed the c707 back to 22pf and changed the 33pf c709 for a 51pf mica film cap( thats all i had) and it helped at low volume but at higher volume still oscillates
It works better with the c707 at 33pf. So ill keep trying
How do i determine what frequency the oscillation is at
So I run a 10khz stabalized sine wave into the aux input. And my scope measures a 59hz sine wave coming out of the speaker terminals
At low volume the sine wave looks clean both sides.
When I turn volume up the Channel im working on gets fuzzy.
So im going to continue to try some different cap values till i find the best values that clean up the oscillation?
Only going to change c707 and c709 for now
I changed the c707 back to 22pf and changed the 33pf c709 for a 51pf mica film cap( thats all i had) and it helped at low volume but at higher volume still oscillates
It works better with the c707 at 33pf. So ill keep trying
How do i determine what frequency the oscillation is at
It sounds like you have problems with the replacement devices having different characteristics to the originals. Often device type isn't critical but on these older amplifier is seems it is. Its a problem I've encountered before on the forum with replacement devices.........
To determine the oscillation you need to expand the timebase on the scope so that you can make out the individual time period of the high frequency signal. The pictures here might help, 3rd one shows the trace expanded.
http://www.diyaudio.com/forums/anal...u-have-checked-see-its-stable-havent-you.html
To determine the oscillation you need to expand the timebase on the scope so that you can make out the individual time period of the high frequency signal. The pictures here might help, 3rd one shows the trace expanded.
http://www.diyaudio.com/forums/anal...u-have-checked-see-its-stable-havent-you.html
Eureka
This will be a happy new year indeed.
The amplifier is fixed . After your saying that my oscillation may be from the new different parts i installed, I checked the specs on all transistors
When I repaired this amp I had replaced the 2sd381(2) Q710 in the VA stage with a fairchild c2073. most of the specs were good except the fT the 2sd381 were 40 mhz.
The c2073 were 4mhz
And the Q714 and Q715 look like predrivers( I think) I replaced with same
Q714 2sd381 I replaced with a c2073 With a spec of 4mhz
Q715 2sb536 I replaced with A940 With a spec of about 4mhz
The originals were specked at about 40mhz -----------
So i replaced those with the original part numbers.
I also replace c707 with the 33pf cap. And now it seems too work great
So I guess the fT was causing some problems. Im sure there are other issues with the different parts i installed. But with thoes changes all is well
Thank You so much
My Nikko 1415 is back to life - I will post pics as soon as I put back together
This will be a happy new year indeed.
The amplifier is fixed . After your saying that my oscillation may be from the new different parts i installed, I checked the specs on all transistors
When I repaired this amp I had replaced the 2sd381(2) Q710 in the VA stage with a fairchild c2073. most of the specs were good except the fT the 2sd381 were 40 mhz.
The c2073 were 4mhz
And the Q714 and Q715 look like predrivers( I think) I replaced with same
Q714 2sd381 I replaced with a c2073 With a spec of 4mhz
Q715 2sb536 I replaced with A940 With a spec of about 4mhz
The originals were specked at about 40mhz -----------
So i replaced those with the original part numbers.
I also replace c707 with the 33pf cap. And now it seems too work great
So I guess the fT was causing some problems. Im sure there are other issues with the different parts i installed. But with thoes changes all is well
Thank You so much
My Nikko 1415 is back to life - I will post pics as soon as I put back together
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