larschr
04-13-2008, 01:57 PM
The story starts in May 2006. I went to the dump to get rid of some old computer stuff, and as I unloaded the car I spotted an amplifier with late 70s look. It looked like it had been thrown several meters into the trash container, and all the knobs were missing. But it looked like a high-class unit, so I grabbed it anyway. When I came home and removed the tape that held it together, I got a happy surprise: Inside it there was a plastic bag containing all the knobs, some screws and the cans from two blown capacitors. It was well equipped; Darlington push-pull output stage (STK-0059 ICs), midrange control, high- and low-filters, two tape connections with dubbing in both directions, 5pin DIN tape I/O on the front, two phono inputs and an output mode switch with mono L, mono R, mono L+R, Stereo and Stereo reverse-settings. Except from the blown caps it looked pretty good technically. The chassis had some big dents, but the main structure was intact. I replaced the caps, connected a set of speakers and a tape recorder. Turned it on, and I got decent sound in one channel, and very weak and heavy distorted sound in the other. After some testing with the oscilloscope I concluded that the signal was OK on the input pins of the power amp board, but not on the input pin of the power amp IC. If I applied a signal directly to the IC input pin I got an amplified, but distorted signal out. I gave the amp a good cleaning, and put it in storage in my mothers apartment and forgot it for a year.
I rediscovered it last Christmas, and decided it was time to get something done. So I took it out of storage and brought it to my home. After a close look to the power amp board I discovered a drop of solder shorting one of the capacitors to the chassis. I removed the short and powered it up again, and guess what! Both channels worked, but suddenly the sound became distorted and died out. And this is where “hell” begins. I immediately thought that one of the 30 years old capacitors probably had shorted out, and ordered some new. When I replaced them, I also resoldered a lot of other connections. I powered it back on, and the same thing happened. The signal was lost somewhere after the output pins on the amplifier ICs. If I turned up the volume with a load on the speaker terminals, it suddenly kicked in and almost blew the speakers, but I died out again a short while after I turned the volume down. I took it apart one more time and sprayed all the switches with contact spray for the dozenth time, and soldered in pieces of wire as a replacement for the printed circuits on all places with high loads. I also replaced two power resistors with higher wattage resistors and mounted them to the bottom plate – in the original place the heat made two of the capacitors so hot that you couldn´t even touch them. Time for a new test – it worked for a few minutes before it faded out again with lots of distortion, like when the batteries in a portable radio die. At that point I was just about ready to throw the amp on the railroad track and watch it getting crushed. Then something struck me. I took off the bottom plate and shorted the speaker output relay with some wire. I turned it on, hooked up the record player and put on a record – and the sound was amazing. I ran it for several hours, and it worked perfect. Now I have used it as the main amp in my system for almost 4 months, and I am very happy with it.
I rediscovered it last Christmas, and decided it was time to get something done. So I took it out of storage and brought it to my home. After a close look to the power amp board I discovered a drop of solder shorting one of the capacitors to the chassis. I removed the short and powered it up again, and guess what! Both channels worked, but suddenly the sound became distorted and died out. And this is where “hell” begins. I immediately thought that one of the 30 years old capacitors probably had shorted out, and ordered some new. When I replaced them, I also resoldered a lot of other connections. I powered it back on, and the same thing happened. The signal was lost somewhere after the output pins on the amplifier ICs. If I turned up the volume with a load on the speaker terminals, it suddenly kicked in and almost blew the speakers, but I died out again a short while after I turned the volume down. I took it apart one more time and sprayed all the switches with contact spray for the dozenth time, and soldered in pieces of wire as a replacement for the printed circuits on all places with high loads. I also replaced two power resistors with higher wattage resistors and mounted them to the bottom plate – in the original place the heat made two of the capacitors so hot that you couldn´t even touch them. Time for a new test – it worked for a few minutes before it faded out again with lots of distortion, like when the batteries in a portable radio die. At that point I was just about ready to throw the amp on the railroad track and watch it getting crushed. Then something struck me. I took off the bottom plate and shorted the speaker output relay with some wire. I turned it on, hooked up the record player and put on a record – and the sound was amazing. I ran it for several hours, and it worked perfect. Now I have used it as the main amp in my system for almost 4 months, and I am very happy with it.