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Bad output device?
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Young Croc
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Points: 1174 		Post Options Post Options   Thanks (0) Thanks(0)   Quote APW Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2018 at 8:42pm
Unless TS4 conducts you will always have the same voltage on all 3 pins of TS3 as no current will flow through R25, R26 and P1 so the potential difference across them will be zero.

Re-check TS4 if Ok Put it back in the circuit and see what the base/emitter voltage is.
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Young Croc
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Points: 1174 		Post Options Post Options   Thanks (0) Thanks(0)   Quote APW Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2018 at 8:47pm
One other thing to check, TS6 is the current limiter transistor for the negative half cycle of the output stage; if this is short it will stop TS8 and TS10 conducting and will allow the output to swing positive.

Edited by APW - 18 April 2018 at 8:48pm
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Young Croc
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OK, thanks for explaining that! I will check all that tomorrow and report back.
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Young Croc
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No problem, let me know how you get on.
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Originally posted by simonp1100 simonp1100 wrote:

Originally posted by AJordan AJordan wrote:

Originally posted by Muckerbarnes1 Muckerbarnes1 wrote:

It's 2x NPN bipolar as in the OP. No FETs. 

The fwd test on the BR of 0.7 and nowt in reverse;
Was this the same on all 4 diodes in the BR? 


Yep, all four.

As I said the problem is not the rectifier.
Never..
Billy Dawg.
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Young Croc
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Points: 1013 		Post Options Post Options   Thanks (0) Thanks(0)   Quote AJordan Quote  Post ReplyReply Direct Link To This Post Posted: 19 April 2018 at 5:21pm
OK, R19 checks ok, I'm not sure about D12 as I don't really know what a zener should read. I'm getting 0.7 one way and 1.68 the other.

I thought I was on to something, TS6 tested bad in the board but when I took it out it tested OK.

Originally posted by APW APW wrote:

Unless TS4 conducts you will always have the same voltage on all 3 pins of TS3 as no current will flow through R25, R26 and P1 so the potential difference across them will be zero.

Re-check TS4 if Ok Put it back in the circuit and see what the base/emitter voltage is.


Does the above still apply if TS3 is disconnected as well? It is btw.

I have rechecked TS4 and it still tests OK. I haven't put it back in yet as I remembered that when they were in the board all three legs of TS3&4 were around the +/- 43V level.
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Young Croc
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Points: 1174 		Post Options Post Options   Thanks (0) Thanks(0)   Quote APW Quote  Post ReplyReply Direct Link To This Post Posted: 19 April 2018 at 5:41pm
do you have about 15volts across d12 when the power is applied, if yes it’s OK, you won't be able to test properly it without applying power, you can only really test them for short circuit with a meter..

TS3 controls the bias current through the output stage if you power the amp without TS3 fitted you run the risk of destroying the output transistors.

Edited by APW - 19 April 2018 at 5:43pm
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Young Croc
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Will check D12 tomorrow, gotta go out tonight. Output trans are not in the board either.
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Young Croc
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Only just had time to look at this again. D12 checks fine. Decided to check the other diodes again and don't know how I missed it last time but D13 seems to be a short. Could this be the problem?
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Young Croc
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yes. Tongue
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Young Croc
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Cool, cheers! I'll fit a new one and report on the outcome. What would be the effect of a short on D13?


Edited by AJordan - 25 April 2018 at 7:32pm
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Young Croc
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Big smile and it worked!! Massive thanks to everyone who gave advice!! I would have been totally lost without it! Been an enjoyable learning experience though. First lesson learnt - when you think you've checked all the components, have a good look around to make sure you haven't missed any then check em again. Second lesson learnt - I still have a hell of a lot to learn!

Are bias trim pots normally so sensitive? I thought I would check it before I finished and it was at 6mV. Schematic says 10-20mV. The slightest touch of the pot would send it from 0 to 300mV. After half an hour of trying to get it right I left it at 9mV(the closest I could get) will that be OK?
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