Archyx Shapetail (archyx) wrote,
Archyx Shapetail
archyx

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Replacing a transistor

Some of you readers might already know that my Sansui 771 tuner-amp doesn't quite function the way it should. The amplifier side does work perfectly but the tuner's +12 volt supply doesn't always stick to its preset value, instead it drops down to +6 volts or less. I traced the problem to a (2S)D330 transistor in a TO-220 package on the PSU board.

Now, I'll disassemble the tuner-amp, take pictures on the way, and describe the procedure here.

The first thing to do here is to dig out the amp from the middle of the audio stack, the second is to get rid of the top cover which, by the way, is real wood.

Now, that we see inside the unit, the hunt for the flaky transistor starts! The "power board unit" must be attacked first because that board is what supplies the bad +12 V line. From there we can locate the flaky 2SD330E. I was going to fit the replacement part with a heatsink on it but at this point it seem to be rather impossible.

Desoldering of the flaky component requires the bottom cover to be removed. Since the transistor has been desoldered before for identifying it between BD330 and 2SD330 (both can have only "D330" printed on the face of the component), locating the correct soldering pads isn't that hard at all.

The new and the old component look very similar. When testing with a multimeter they both seem to be NPN transistors but the increased and rather random voltage drop measured between base and collector/emitter of the old transistor was clear evidence of internal breakdown. Little bending of the leads makes the installation of the replacement component easier.

I decided not to solder the new component as deep among the others as the old one was. This makes installing the heatsink possible. Initial testing without the heatsink proves that the regulator circuitry works with the new transistor. After installing the heatsink the output voltage became more stable and didn't fluctuate that much when warming up.

And it works...

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