Hi Al,
here the results:
BAD CHANNEL WORKING CHANNEL
Q23 (PNP) Q23 (PNP)
B (-) - E (+) 76 mV B (-) - E (+) 623 mV
Q25 (PNP) Q25 (PNP)
same as above 53 mV same as above 623 mV
Q22 (NPN) Q22 (NPN)
B (+ ) - E (-) 243 mV B (+) - E (-) 622 mV
Q24 (NPN) Q24 (NPN)
same as above 243 mV same as above 622 mV
Hope this helps.
Cheers,
Max
PS: I was ment to be a chart... but it seems I can't use the tabulation in here.. hope it's readable
here the results:
BAD CHANNEL WORKING CHANNEL
Q23 (PNP) Q23 (PNP)
B (-) - E (+) 76 mV B (-) - E (+) 623 mV
Q25 (PNP) Q25 (PNP)
same as above 53 mV same as above 623 mV
Q22 (NPN) Q22 (NPN)
B (+ ) - E (-) 243 mV B (+) - E (-) 622 mV
Q24 (NPN) Q24 (NPN)
same as above 243 mV same as above 622 mV
Hope this helps.
Cheers,
Max
PS: I was ment to be a chart... but it seems I can't use the tabulation in here.. hope it's readable
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Hi Max,
Do the same test with driver transistors Q20&Q21. If the bad amp, and working amp have similar results than most likely the OT is where the problem is.
If you're interested I have replacement parts for $10.00 plus the cost of shipping. That's four transistors matched and ready go.
Al
Do the same test with driver transistors Q20&Q21. If the bad amp, and working amp have similar results than most likely the OT is where the problem is.
If you're interested I have replacement parts for $10.00 plus the cost of shipping. That's four transistors matched and ready go.
Al
Hi Al,
I am underway until to tomorrow but took the 2 amp board and the DMM with me and will be able to check tomorrow evening (GMT).
I will measure both and will let you know.
Thanks for the offer on the OT, I didn't feel like buying a batch and matching them.
Cheers,
Max
PS.: Thinking about it I think that the leads of the power resistor under the board were a little to long and might have shortenend on the board... will see what the DMM says... leads have been shortened since.
I am underway until to tomorrow but took the 2 amp board and the DMM with me and will be able to check tomorrow evening (GMT).
I will measure both and will let you know.
Thanks for the offer on the OT, I didn't feel like buying a batch and matching them.
Cheers,
Max
PS.: Thinking about it I think that the leads of the power resistor under the board were a little to long and might have shortenend on the board... will see what the DMM says... leads have been shortened since.
Hi Al,
kindly find hereunder the measurements from Q20 and Q21 of the non working board.
The values are identical to the ones from the working board.
Strange thing is inverting the probes gives the same results...again on both the working and the non working board. So I guess it's ok, although I don't understand it.
Q20
B(-) - E(+) 248mV
Q21
B(+) - E(-) 248mV
Hope this helps,
Max
kindly find hereunder the measurements from Q20 and Q21 of the non working board.
The values are identical to the ones from the working board.
Strange thing is inverting the probes gives the same results...again on both the working and the non working board. So I guess it's ok, although I don't understand it.
Q20
B(-) - E(+) 248mV
Q21
B(+) - E(-) 248mV
Hope this helps,
Max
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Sorry Al,
I just re-read your post... yes I have shortened the leads and powered it up again.
But the fuses still blow, alternatively on both rails... I will swap the OT and let you know.
And as I will have to take the presumed bad OT out anyway I will check them again once out of the board.
Regards,
Max
PS: You have mail concerning a set of matched OT.
I just re-read your post... yes I have shortened the leads and powered it up again.
But the fuses still blow, alternatively on both rails... I will swap the OT and let you know.
And as I will have to take the presumed bad OT out anyway I will check them again once out of the board.
Regards,
Max
PS: You have mail concerning a set of matched OT.
Hi!
Have you assembled this amp? How does it sound...I'm doing a revision of the PCB which I download from one of these SymAsym thread, can't recall though which one
I have a question from the designer,
I have found PCB_Rev1.3 which uses a couple of power output and
PCB_Rev1.4 which uses 4 outputs but both variant uses 50vdc rail, what's the difference
....I meant in the specs
Short update on the broken channel.
After Al kindly shipped a set of matched OT I unsoldered the (presumably) fried output transistors.
Measuring them outside the circuit shows that Q23 was shorted.
The 3 remaining transistors measure OK but as I clipped the leads to unlsoder them without damaging the board they will get tossed.
Waiting for the postman now...
After Al kindly shipped a set of matched OT I unsoldered the (presumably) fried output transistors.
Measuring them outside the circuit shows that Q23 was shorted.
The 3 remaining transistors measure OK but as I clipped the leads to unlsoder them without damaging the board they will get tossed.
Waiting for the postman now...
Up again... for something like 2 minutes...
Hi,
I replaced the OT, maed sure that they were ok measuring them and comparing with a working amp board.
Then I started biasing the board up to something liek 40mV between emitters of Q22 - Q23.
While I was at it I re-measured the other amp board as I didn't remember my final bias.
Doing this smoke came from the just repaired amp board, I immediately cut the power, OT seems to still to be OK as per my measurements.
Unmounting the board and checking for visible burn marks showed nothing.
How should I proceed ?
Cheers,
Max
Max
Hi,
I replaced the OT, maed sure that they were ok measuring them and comparing with a working amp board.
Then I started biasing the board up to something liek 40mV between emitters of Q22 - Q23.
While I was at it I re-measured the other amp board as I didn't remember my final bias.
Doing this smoke came from the just repaired amp board, I immediately cut the power, OT seems to still to be OK as per my measurements.
Unmounting the board and checking for visible burn marks showed nothing.
How should I proceed ?
Cheers,
Max
Max
Think I found it...
Hi,
I think that my ground connector from the PS-board shorted the leads of the 10 Ohm power resistor. The resistor is mounted under the board, the leads sticking out on the top side...
I'll cut the leads, put a drop of hot glue on all of them , change the connector and give it a try again.
More to come...
Max
Hi,
I think that my ground connector from the PS-board shorted the leads of the 10 Ohm power resistor. The resistor is mounted under the board, the leads sticking out on the top side...
I'll cut the leads, put a drop of hot glue on all of them , change the connector and give it a try again.
More to come...
Max
Have you fitted a close rated mains fuse?
Have you fitted supply rail fuses after the main smoothing bank.
If the problem is oscillation when you bias up then these fuses might just save the output stage.
I think you need to watch (oscilloscope) the output signal as you bias up.
Is the input shorted.
Is the output unloaded?
edit:
hot melt glue will melt and release whenever the amplifier heats/overheats.
Have you fitted supply rail fuses after the main smoothing bank.
If the problem is oscillation when you bias up then these fuses might just save the output stage.
I think you need to watch (oscilloscope) the output signal as you bias up.
Is the input shorted.
Is the output unloaded?
edit:
hot melt glue will melt and release whenever the amplifier heats/overheats.
Andrew,
Actually the ground connector shorted the power resistor, I had not trouble biasing the amp after changing the connector.
I have indeed a drop of hot glue on the pins from all the resistors, but all connectors have been changed, no risk of shorting anymore.
I don't expect any further trouble, but will check for oscillation. I have a O-scope and a signal generator and should find the procedure here on the forum.
FIY, inputs were not shorted as Al indicated it wouldn't be necessary.
Edit: Amp up and running, Bias setted @ 40mV , "worst" offset per channel 7mV
Max
PS: your help is always appreciated.
Actually the ground connector shorted the power resistor, I had not trouble biasing the amp after changing the connector.
I have indeed a drop of hot glue on the pins from all the resistors, but all connectors have been changed, no risk of shorting anymore.
I don't expect any further trouble, but will check for oscillation. I have a O-scope and a signal generator and should find the procedure here on the forum.
FIY, inputs were not shorted as Al indicated it wouldn't be necessary.
Edit: Amp up and running, Bias setted @ 40mV , "worst" offset per channel 7mV
Max
PS: your help is always appreciated.
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