Small Repair Job: Old Amplifier. - Page 1
Forum section: electronics / repair

author: Mint.
author_rank: Frequent Contributor
timestamp: February 11, 2012, 09:08:02 am
content: So today I had a garage sale near my house, so I thought, why not go and maybe pick up some cheap electronics that I can salvage parts from. When I arrived there, I spotted a small amplifier. Curiosity got the better of me. I bought it for 2 bucks and happily took it home. Upon the first glance the condition of the amplifier is good. The 240VAC is removed/cutoff, and at the back there seems to be only room for two speakers and one which doesn't work according to the little tag. However the condition of the front panel was excellent, all it needed was some alcohol to clean it off and looked good as new! I continued to take it apart. On inspection I noticed that there are two exactly the same looking boards, which are the amplifier circuits for the left and right speakers. I continued to inspect the circuitry. The hot wire was connected to a large switch which is switched on at the front and then the wire from the switch went to the transformer. The transformer itself has two 115V input parts, the were connected in series so it can work with 240VAC from the wall outlet. The transformer had two outputs, one was 4V 0.2A for the on indicator lamp and the other one was 135V 0.5A for the rest of the circuitry. I assume that since it was made in Japan they had 115V there, hence the 115V transformer. The neutral wire went to a fast blow fuse which appears to be 0.5A and then connected later on to the transformer. The ground was connected to the chassis and to the surrounding metal bit of the capacitor. I continued to test whenever the fuse works. It does, just needed a little bit of a clean from oxidation. Then I tested the lamp, I noticed that it was powered using 4V from the transformer, but it was rated for 8V, I guess there wasn't any need for a brighter lamp. All of the boards were raised above the metal case to prevent short circuit. Everything was bolted on using a philips screws (+) and sometimes use of various bolts.

author: Mint.
author_rank: Frequent Contributor
timestamp: February 11, 2012, 09:10:50 am
content: PART 2The next part was to troubleshoot the circuit. Firstly I installed the mains wiring so I could properly check it, I bought a cheap extension cable for another 2 bucks, cut the end of and then installed it. Then I tested for continuity and then I had a look if there were any leaked, blown or damaged components on board. Everything was perfectly functional. After a few small solder repairs and change of various wires in some places, I started to clean the board, firstly I blew air on all the parts so they would be a bit cleaner. Then I cleaned some solder joints with alcohol and I also did the metal cover just to make it look good.

author: Mint.
author_rank: Frequent Contributor
timestamp: February 11, 2012, 09:14:07 am
content: PART 3After I finished the cleaning and soldering. I hooked up all the proper wires for input and output (speakers). Then I decided to give everything a little test and... it works! I tested the outputs on the scope and they both were functioning properly. So now what I have is a perfectly functional amplifier all for under 5 dollars.

author: Rerouter
author_rank: Super Contributor
timestamp: February 11, 2012, 10:18:47 am
content: so pretty much it would seem it was a cut cord and a dodgy few solder joints that were the problem, still for the apparent age, i'm impressed all the caps are healthy.

author: Mint.
author_rank: Frequent Contributor
timestamp: February 11, 2012, 10:34:40 am
content: Yep thats right. There was a decent amount of oxidation on some of the components. Bad soldering job in other places. Unfortunately I do not have an ESR meter to measure the health of the caps, but so far it works well

author: Bored@Work
author_rank: Super Contributor
timestamp: February 11, 2012, 10:48:45 am
content: It is not unusual to cut the cord to make it clear that something is faulty and not to be used.When we were still allowed to throw electronics in the normal trash people were asked to do that to make sure that dumpster divers won't happen to mistake a faulty, maybe dangerous piece of equipment, with a still working one. Well ...But I have also seen this in professional environments with proper waste disposal. E.g. when some equipment failed some isolation test. Cheap equipment and that beyond repair was taken out, the cord cut and the equipment placed into storage until it went to waste disposal. With the proliferation of C13/C14 mains connections this became rare.

author: ciccio
author_rank: Frequent Contributor
timestamp: February 11, 2012, 10:51:00 am
content: Nice: it must be more than 45 years old: look at the transistors, with their long, flexible, color coded leads, and at the driver transformers on the PC boards. Maybe they used no more than 4 transistors per channel: one for the input stage, one for driving the transformer and two for the output stage.Output stage bias adjustment was critical with those circuitries: it is better to keep it a little under-biased, and accept some crossover distortion.Now it would be labeled as "esoteric".Nice heatsink, too.No more than 5 W per channel, I think.Best regards

author: FenderBender
author_rank: Super Contributor
timestamp: February 11, 2012, 01:50:26 pm
content: Quote from: Rerouter on February 11, 2012, 10:18:47 amWell they are Rubycons!