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Old 6th February 2013, 09:05 AM   #11
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Hi.
My eyes are not t0o good today im having a little difficulty reading the schem.. from what you say i believe R31 acted as a fuse . If the output devices are ok i would first be looking for a faulty semi not too much of a pita as only a few to check. You say pin 14 has 30 or so volts . Just to check is the pin 14 you mention on the left of the pwr amp scem.. If so im sure it should be conected to ground ??

Last edited by madtecchy; 6th February 2013 at 09:10 AM.
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Old 6th February 2013, 09:08 AM   #12
mjona is offline mjona  New Zealand
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The protection circuit contains a single four pole relay which stops switch on and switch off noises getting through to both speakers.

It also prevents excessive d.c. getting to your speakers where the heat would pose a serious fire risk to the extend of losing your speakers and even having a house fire. A d.c. fault in either channel will prevent the relay from activating which is almost certainly the reason you are getting no sound on headphones.

As Mooly has pointed out, also alluded to by me, you need to quantify the d.c. voltage at the output of the faulty channel.

R31 has burnt out because too much current has been drawn through it and this raises the question of damage to transistor junctions through which this might have passed. There are two transistor whose collectors connect to R31 and these drive an NPN power device whose PNP counterpart was replaced at some time in history because of some fault and the NPN device may now be suspect.

You have replaced R31 but that has not cleared the fault so you need to know what level of current is passing through that now. Similarly you need to know what level of current is passing through the emitter resistors for the power devices.

You should be able to spot any problems by measuring the voltages across all these resistors, and comparing them with the circuit equivalents in the other channel.

If you would note these respective measurements and report your results it will help with further suggestions on solving your problem.
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Old 6th February 2013, 09:31 AM   #13
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IMHO i believe the main cause of this fault can be found with the power off. All them voltage measurments on both boards pose a big risk at this stage it would be easy to end up with two dead channels as for the other channel it is only assumed to be working so can not realy be trusted as a good ref point..
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Old 6th February 2013, 01:16 PM   #14
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Thank you all for your time and advice. Problem is I'm on and off on this job.
Seems I still have problems with my power amp section.

I've powered it through the series lamp to measure voltage at "pin3". It was -3.5V (quite off) while on "pin23" it was -0.12V. I also measured the voltage between the 2 complimentary pairs at "pin E" where it should be -38V. There the reading is 0V!! At the collectors (pins 36 & 27) the voltage is 37V which is very close.

You made me realise that pin 3 and 23 are the input to the protection circuit so it's beginning to make sense, at least logically if not technically. Now, how do I get pins 3 and 23 to 0V? VR3 and VR4 perhaps?

Thanks again for your help.
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Old 6th February 2013, 05:57 PM   #15
Mooly is offline Mooly  United Kingdom
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Your fault is almost certainly related to the driver transistors, possibly the outputs and related circuitry.

With the amp OFF you need to confirm that the following are OK.

D5 and D7 and the pot could be suspects as to the initial fault. You can apply a short (the blue line) using a piece of wire. The amp is safe to power up with this applied and will work correctly. This forces the output stage to zero bias current. The amp will still work and all the voltages can still be checked with it like this.
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Old 6th February 2013, 08:51 PM   #16
mjona is offline mjona  New Zealand
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Pin E is an earthing point and there should be no connection with the power rails. There are decoupling capacitors for the power transistors between collector and earth. These are there for stability reasons.

You appear to have measured the positive rail on one channel for voltage with the negative rail on the other. 37 volts is not necessarily out of spec as some variation in mains could account for that. All four devices should register that +/- values at that level.

You will not be able to eliminate the -3.5 volt d.c. by adjustments to a trim pot. You can do that when you have eliminated the fault. You can then do both channels together. Minus 0.12 volts d.c. is somewhat out of spec but it will not cause any harm to your speakers.

In my experience in repairing d.c. faults the most likely cause is a problem transistor.

In addition to measuring the resistors and diodes in circuit with the power off you can check the diode junctions of the three transistors mentioned by Mooly under the same conditions.

In each of these there should be a forward diode junction between collector and base and between base and emitter. The junctions should conduct only in the forward direction. You will be able to use the diode check function on your meter for that.

You have three transistors in series and any may affect another in circuit and you may get some strange results especially if there is a fault. However if the other channel is working correctly which I think is the case the equivalent readings will be valid and you can cross reference the results to account for any disparities you find.
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