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Serviceman's Log

There's a lot of high-end audio gear around from the late 80s and early 90s but try getting it repaired these days. It's no wonder many people are now electing to tackle repair jobs themselves rather than consign what was expensive gear to the scrap heap.

Items Covered This Month

• Technics stereo system

• Onkyo TX-DS474 A/V receiver

• Resurrecting a BWD 804
oscilloscope

• Faulty PC power supply

• Philips FW-380C mini hifi system

Although it may now look a little dated, hifi audio gear from the 1980s and 1990s was capable of truly excellent performance. But what do you do if something goes wrong? Ditch it and buy something else (with possibly inferior performance) or try to get it repaired?

Fortunately, there are some people who elect to do the latter, even if they have to do the job themselves.

My first story this month comes from M. B. of Parramatta, NSW and concerns his battle with a 20-year-old Technics stereo amplifier. Here’s how he tells it . . .

Technics stereo system

The cost of electronic equipment has dropped dramatically in recent years, making it less and less cost-effective to get gear repaired. If you have something that’s a few years old, you can probably get an exact replacement on eBay for a lot less than the repair price. What’s more, electronic repair shops are virtually extinct as I discovered when I tried to get a stereo amplifier repaired recently.

So why do we bother thinking about getting anything electronic repaired?

Click for larger image

I have a 20-year old Technics stereo system (made up of separate units) which I hadn’t used for quite a while. However, after a recent move, I decided to start using it again but as luck would have it, the main amplifier would only work for five minutes before it started to sound distorted and noisy.

I have worked on high-end medical electronics for some time now. This involves mostly sub-assembly replacement rather than component-level repair. Nevertheless, I thought it’d be interesting to have a crack at repairing the old amplifier.

The first thing to do was track down a circuit diagram. Trawling the internet turned out to be fruitless, so I had no option but to get it repaired. The next problem was to find an electronics repair business but that turned out to be a lot more difficult than I initially thought.

Eventually, I found a service centre and got the amplifier repaired. A number of electrolytic capacitors in the power supply had dried up and, on reflection, I could have just “shotgunned” it and replaced these myself, as they are always prime suspects. Anyway, I took it home where it worked well for a while.

Unfortunately, there was another problem lurking in the works because after a few months, the amplifier wouldn’t turn on. Unwilling to spend another $100 on an old system, I once again started trawling the internet for circuits. I didn’t really expect to find anything but decided that another 10 minutes of investigation wouldn’t hurt. Eventually, however, I came across a site which supplied me with the service manual I required for $20.

The block diagram in this manual indicated that there was a power-on signal which came from the tuner, ie, the system is turned on by a switch on the tuner. A quick check of the amplifier’s power supply indicated that it was OK but the enable/power-on signal to the control chip was missing.

Tracking down the power switching transistor soon revealed the cause of the problem. This was a type of transistor I’d not come across before and is called a “BRT” or “Bias Resistor Transistor”. These have bias resistors built into the transistor substrate. Apparently, the idea was to reduce the parts count on the PC boards.

Anyway, I noticed that the collector of this switching transistor was permanently low. By removing a few components, I eventually found that a daughter board, used for switching video signals, had a tiny piece of metal (source unknown) which was shorting between ground and the enable line to the power switching transistor. Removing this metal with a small-bladed screwdriver soon sorted that out.

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