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Old 27th August 2004, 06:36 PM   #11
anatech is offline anatech  Canada
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Hi jkramer,
The power transformer is likely okay except for the blown thermal fuse.
Both channels of the amplifier are blown. Fix them first. If you pull the outputs and power up you will take out many more parts. The drivers & bias transistors probably need replacing now, maybe more. Do not just power the unit up. A variac will be your best friend here.
If you do replace these parts and plan to just plug it in - throw the unit out now and save yourself the money on parts.
-Chris
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Old 28th August 2004, 02:26 AM   #12
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The fact that both sides of the output are toasted tends to indicate that something happened to the bias pot on the amp similar to the Sony that I repaired. I would strongly advise replacing the bias pots before powering on the unit again, otherwise you will most likely immediately blow any replacement drivers. Just cranking the bias pot around might restore the unit to operation, but I liken that to resetting the fuse on a time bomb. If you can get to a place where you can properly cut the +/- DC supplies to the amp, it would probably be a good idea to wire some automotive bulbs in series to soak up any shorts. This might save you some drivers.
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Old 28th August 2004, 02:36 AM   #13
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Default cold solder joints

Sony is famous for cold soder joints. I repaired a 555 due to cold solder joints. Variac is a must
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Old 28th August 2004, 03:36 PM   #14
anatech is offline anatech  Canada
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Hi wrenchone,
Both sides blown would normally indicate they were connected together, or to another amplifier. It is always good practice to check the bias controls as I always do. They may be damaged when the output stage fails, resulting in a burned carbon element. When they are intermittent, either the slip ring is oxidized, or the tension has failed on the wiper. The former case requires replacement.
Given the difficulty in getting exact replacement parts, I only replace them if I have good reason. There is no reason to use a conductive plastic control for this, the carbon types handle surges better.
-Chris
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Old 28th August 2004, 04:03 PM   #15
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Default popped outputs

Another scenario is that the outputs were shorted, a voltage line spike caused a voltage spike on the secondary of power transformer exceeding breakdown of one output, or any other way either output could have shorted. If the filter caps are large, and hold a lot of charge, then when one output fails, the feedback of the amp circuit will cause the other to saturate and the current surge through the unshorted output will be huge and will melt the unshorted transistor long before a fuse breaks! and then you have no good outputs.

Anyway you should take the amp with all of the outputs removed. Place a resistor in series with a diode, 10to 100Ohms or so, from were the base and emitter of each output connection is removed, and power it up with no load. This should somewhat represent the forward PN juction and re (resistance of base to emiter) of the output transistor, with re 100X what it would actually be to limit current in case the outputs are not the only failed componant. Measure the output voltage relative to ground, should be 0V, and then check the voltages between the bases of the outputs. If something still smokes, then the drivers are probably melted too. Look for this base to base voltage to be no more than 1.5V or 2V. Maybe slightly more if the outputs are darlingtons. You should be able to adjust this voltage with the bias pot.

Then again I may be totally wrong, I don't have a scematic for this amp.
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Old 28th August 2004, 04:17 PM   #16
anatech is offline anatech  Canada
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Hi Cunningham,
I've done that with STK driver IC's to check them before giving an estimate (two diodes + resistor). In this case, it is easy to check the drivers individually. I would recommend that route. Also, aways replace transistors one stage back from the last damaged part. This means, the drivers go for sure.
It's a good short cut for more experienced techs. At least outputs don't go poof!
-Chris
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