Hi guys
I have this InterM MA-930 here. Schematics attached.
The original problem was one of the C3856 and one of the A1492 output transistors were short circuit
Having replaced them, I still had a fault where one bank of output transistors (the A1492 side) were being turned hard on at power up
I traced that to Q110 having been replaced by some other repairer with the wrong type device. It was an electrical equivalent but not a physical equivalent (source and drain reversed). I replaced that with the correct J78 and the amplifier then worked so I returned it to the owner
However, I am not able to test these amplifiers at high power at the workshop (until I get some large load resistors). It worked fine until the owner got the amp up to 0db - no clipping - when it suddenly blew the 10A main fuse
I got the amp back here with one A1492 short circuit collector emitter (not the one I changed previously) and one C3856 short circuit base collector (the one I had replaced previously)
At the moment I have the two faulty output devices removed but there is still a problem
On the good channel I read open circuit Collector Emitter on all output devices
On the bad channel I read open circuit collector Emitter on the A1492 bank, but I read about 1.5V-1.6V on my diode test range on collector emitter of the C3856 bank (+ve to collector)
I removed all the low value emitter resistors R140-R145 but it makes no difference.
I then removed all the remaining C3856 and they all test OK on my Atlas DCA55. Actually it says they are Germanium Power transistors for some reason but otherwise they are all fine.
I then resoldered all the C3856 and used one volt meter to measure B-E voltage on C3856 while I apply diode test on my other multimeter across C-E
I get about 0.5V forward bias B-E which explains why the output transistors appear to be 'leaky' Collector Emitter. They are actually being turned on somehow by my test meter on diode mode.
I then removed Q108 (C4370)and found that the 0.5V forward bias on the C3856 bank has gone away and they are no longer reading leaky C-E
I tested Q108 on the DCA55 - it is fine
I then removed Q109 (actually a K215) and tested that on the DCA. It's fine
I replaced Q108 and Q109. The forward bias comes back again.
I then removed Q106 (A949) and the forward bias goes away. Q106 test fine on the DCA.
I then tested all the 1N4148 diodes in this area on my DVM diode test mode, they read OK in circuit.
I've got as far as I can but I just can't see where this forward base bias is coming from when I test across the output bank C-E
I'm sure one of you clever guys can figure it out.
best regards
Richard
I have this InterM MA-930 here. Schematics attached.
The original problem was one of the C3856 and one of the A1492 output transistors were short circuit
Having replaced them, I still had a fault where one bank of output transistors (the A1492 side) were being turned hard on at power up
I traced that to Q110 having been replaced by some other repairer with the wrong type device. It was an electrical equivalent but not a physical equivalent (source and drain reversed). I replaced that with the correct J78 and the amplifier then worked so I returned it to the owner
However, I am not able to test these amplifiers at high power at the workshop (until I get some large load resistors). It worked fine until the owner got the amp up to 0db - no clipping - when it suddenly blew the 10A main fuse
I got the amp back here with one A1492 short circuit collector emitter (not the one I changed previously) and one C3856 short circuit base collector (the one I had replaced previously)
At the moment I have the two faulty output devices removed but there is still a problem
On the good channel I read open circuit Collector Emitter on all output devices
On the bad channel I read open circuit collector Emitter on the A1492 bank, but I read about 1.5V-1.6V on my diode test range on collector emitter of the C3856 bank (+ve to collector)
I removed all the low value emitter resistors R140-R145 but it makes no difference.
I then removed all the remaining C3856 and they all test OK on my Atlas DCA55. Actually it says they are Germanium Power transistors for some reason but otherwise they are all fine.
I then resoldered all the C3856 and used one volt meter to measure B-E voltage on C3856 while I apply diode test on my other multimeter across C-E
I get about 0.5V forward bias B-E which explains why the output transistors appear to be 'leaky' Collector Emitter. They are actually being turned on somehow by my test meter on diode mode.
I then removed Q108 (C4370)and found that the 0.5V forward bias on the C3856 bank has gone away and they are no longer reading leaky C-E
I tested Q108 on the DCA55 - it is fine
I then removed Q109 (actually a K215) and tested that on the DCA. It's fine
I replaced Q108 and Q109. The forward bias comes back again.
I then removed Q106 (A949) and the forward bias goes away. Q106 test fine on the DCA.
I then tested all the 1N4148 diodes in this area on my DVM diode test mode, they read OK in circuit.
I've got as far as I can but I just can't see where this forward base bias is coming from when I test across the output bank C-E
I'm sure one of you clever guys can figure it out.
best regards
Richard
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