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Old 6th July 2011, 12:33 AM   #11
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Quote:
Originally Posted by Mooly View Post
Just very quickly... and its not easy to read the circuit tbh... bit blurry

23 volts on Q705 collector ? Check the Base/Emitter volt drop across Q 708 (the driver) to the "top" output of the output pair. Is it open circuit ? Is the 5.6 ohm OK
Base/emitter going open circuit is a typical failure mode...

All the base emitter volt drops should not exceed around 0.8 volts max.
Getting 36.1v on Base of Q708. 24.7v on Emitter. 5.6 ohm reads 00.3 to 00.0v. These numbers are WAY out of line !
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Old 6th July 2011, 06:29 AM   #12
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Q708 is open circuit with 12 volts across its B-E junction.

Edit... maybe try MJE340 as replacement. I can't find any real data on the originals so check package and lead outs before fitting.

Also its good practice to power up initially using a bulb tester (100 watt bulb in series with mains) just in case there is more damage. It can save blown outputs etc.
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Last edited by Mooly; 6th July 2011 at 06:53 AM.
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Old 6th July 2011, 11:15 AM   #13
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Thanks for the help Mooly . I'll post my results once I find and install my replacement.
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Old 17th July 2011, 09:27 PM   #14
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Update:
Tried the MJE340, double checked the pinouts to match the service manual's and powered up. Still no left channel output . I'll take a closer look inside the beast again when I'm up for it.
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Old 18th July 2011, 06:13 AM   #15
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Carefully check the voltages on Q708 again. Waht are they now ?
There's absolutely no doubt over the original being faulty with those voltage measurements although thats not to say there aren't other problems either caused by or that have caused that failure. Any normal (which covers all in this amp) transistor with more than around 0.8 volts across its B-E when forward biased is faulty.

Was the original transistor the same physical shape as the MJE340 ?
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Old 18th July 2011, 11:29 AM   #16
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Morning Mooly,

The original transistor was quite different in design. It was a silver, coin shape device with "wings" if you will, for holding two screws. I used the schematic to position the new trannie so the BC and E were identically placed.

I plan on opening her up again today to get some readings.
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Last edited by Draal1000; 18th July 2011 at 11:50 AM. Reason: components for left channel (power amp)
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Old 18th July 2011, 11:37 AM   #17
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Afternoon...

That's an intriguing description for the transistor... I'll see what I can turn up. Is it on a small heatsink ?

Maybe a clear photo would help too showing it and the PCB etc.
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Old 18th July 2011, 11:45 AM   #18
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Hmmm... well I have found a picture of it and it shows as a T03 package. I'm surprised to see that for a driver transistor tbh. The MJE should certainly work though, if only to get prove and get the amp functional.

2sc895 search results

It would help to see how its mounted on the PCB. The case of the 2SC is the collector as is the metal tab of the MJE340.
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Old 18th July 2011, 11:52 AM   #19
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Just added a pic from the service manual 2 posts above . Hope that helps.
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Old 18th July 2011, 11:58 AM   #20
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Thanks... it's very strange though. Odd to see the drivers, an NPN/PNP pair as two different packages.

So we need voltage measurements again. See what is across the B-E junction now of the new MJE340.
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