Hmmm... firstly something in the left channel stands out as 'wrong' tbh.
Node 7 at +4v and node 8 at zero volts. That on the face of it is wrong because it implies 4 volts across the base/emitter junction of the lower 2N3055 and that isn't possible on a good transistor. The base/emitter voltage will always be in the 0.6 volt region (give or take a little either way but never much higher than that) as it is essentially behaving like a forward biased diode. So something is very amiss there.
No bias current on this amp would give around 0 volts across the base/emitter junction.
Just to be sure we're all looking at the same thing:
Are you sure you haven't mixed the channels 🙂
What does show for both channels is that the midpoint is correct (the 33v)
Can you measure that? Maybe measure from the resistors if that is easier but the reading must be across the junction for accuracy. Put your red lead on R1178 for example and black lead on R120 on the ends that go to the transistor.
For curiosity you can compare to the other channel.
Node 7 at +4v and node 8 at zero volts. That on the face of it is wrong because it implies 4 volts across the base/emitter junction of the lower 2N3055 and that isn't possible on a good transistor. The base/emitter voltage will always be in the 0.6 volt region (give or take a little either way but never much higher than that) as it is essentially behaving like a forward biased diode. So something is very amiss there.
No bias current on this amp would give around 0 volts across the base/emitter junction.
Just to be sure we're all looking at the same thing:
Are you sure you haven't mixed the channels 🙂
What does show for both channels is that the midpoint is correct (the 33v)
This transistor (or surrounding area) would give what you describe above if faulty. If it is operating correctly there would be approx this magic 0.6 volts across the base/emitter junction.
Can you measure that? Maybe measure from the resistors if that is easier but the reading must be across the junction for accuracy. Put your red lead on R1178 for example and black lead on R120 on the ends that go to the transistor.
For curiosity you can compare to the other channel.
Just a thought... was your node 7 really 4v or was it 0.4v which would be OK given the really low bias current these amps run at. The 2N3055 is barely conducting at all 🙂
Tr2 is open circuit. Disconnect the base wire and then do the diode test b/c and b/e. At idle it should be about 0.2V.
The centre voltage is OK but without signal that can be asserted by the driver or even the ore driver.
The centre voltage is OK but without signal that can be asserted by the driver or even the ore driver.
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Thank you both for your reply and advise!
I have to admit, I goofed, after re-measuring pin 7 twice, the correct value is 0.5V on the left channel. Don't know how I came up with 4V...???
But on the right channel no value at all at pin 7...
So I am going to do what you suggest ejp and I will report back...to be continued...
I have to admit, I goofed, after re-measuring pin 7 twice, the correct value is 0.5V on the left channel. Don't know how I came up with 4V...???
But on the right channel no value at all at pin 7...
So I am going to do what you suggest ejp and I will report back...to be continued...
Please also check TR103 that I circled above. If open circuit you would get:
1/ Centre midpoint voltage correct.
2/ Bias current at zero.
3/ B/E voltage on outputs would be zero.
4/ Positive output on a sine would be missing.
1/ Centre midpoint voltage correct.
2/ Bias current at zero.
3/ B/E voltage on outputs would be zero.
4/ Positive output on a sine would be missing.
I would like to reply to ejp, yes I meant zero.
I have disconnected the base wire on TR2R without load, no input signal.
This is what I measured: bc voltage 32V, be voltage 0V. ( I noticed that at first the be voltage is 0.5V, but after a while goes to 0V .)
Should I check also TR103? Or is it clear that TR2R is causing the problem?
I have disconnected the base wire on TR2R without load, no input signal.
This is what I measured: bc voltage 32V, be voltage 0V. ( I noticed that at first the be voltage is 0.5V, but after a while goes to 0V .)
Should I check also TR103? Or is it clear that TR2R is causing the problem?
If the b/e voltage is zero it means it isn't being driven, so the problem is upstream. Investigate Tr104 and Tr106. Measure c,b,e of Tr106. You can do that at the inboard ends of the resistors at the wire-loom end of the board.
Hello ejp, I have measured TR106. Here is what I've found:
e 0V
b 0.46V
c 33.36V
Isn't the base of TR106, also the collector of TR104, a bit to low?
e 0V
b 0.46V
c 33.36V
Isn't the base of TR106, also the collector of TR104, a bit to low?
Maybe, but the quiescent current being so low means that the drivers are only just on themselves. If there was a problem it would be worse than just 'a bit low'. Are you now able to set the quiescent?
I like to respond to ejp. I am not able to set the quiescent, remains zero.
And I should have mentioned it earlier, I do have the version 10 board.
Okay, I'll try to replace TR103.
And furthermore, thank you both, ejp and Mooly, for helping me so far!!!
And I should have mentioned it earlier, I do have the version 10 board.
Okay, I'll try to replace TR103.
And furthermore, thank you both, ejp and Mooly, for helping me so far!!!
I have replaced TR103 and TR107, but with no luck...
Actually, I noticed that the quiescent current of the "good board" isn't that stable.
I ran out of patience, so I decided to order new boards on ebay. And I hope to receive them in a short while.
Hopefully the output transistors are all right and I can finally enjoy the sound of this amp...
I like to thank you both for your effort and time!
Actually, I noticed that the quiescent current of the "good board" isn't that stable.
I ran out of patience, so I decided to order new boards on ebay. And I hope to receive them in a short while.
Hopefully the output transistors are all right and I can finally enjoy the sound of this amp...
I like to thank you both for your effort and time!
