Wow, that was detailed, thanks. I'll probably have to build myself a load soon enough as it seems that there's more to do. And I have another amp project on the way

Anyhow it was a bit early to pop the champagne for the amp. I had the chance to play with it a bit longer today and it seems that once it's warmed up (by playing, not by idling) it begins to rattle (scared the hell out of my 4-year old son) and distort stronger signals (low E strummed hard). After cooling it works again for a while.

Am I correct if I suspect that this comes from the new thermistor?

Is it accurate enough to measure the current by using a current clamp on the mains cable, or should I connect a multimeter in series somewhere within the the amp circuit? And what would be the easiest way to  generate a suitable dummy load? What comes to my mind first is pulling the speaker plug out for a while but I guess it's not what you had in mind

I finally got the parts, replaced the two electrolytes and thermistor,  and now I have a working amp!

http://aijaa.com/iLIxBb
http://aijaa.com/Qp3yud

If somebody needs a 100ohm NTC thermistor, you can get one here

https://www.elfa.se/elfa3~eu_en/elfa/init.do?item=60-305-89&toc=19509

I also bought replacement transistors (10 euro on Ebay) but obviously I didn't need them. Yet

Once more, thanks for the help Roly!

I finally managed to desolder the other leg of the intact thermistor and it gave a value of 100ohms. The first measurement gave 93ohms but it could have been that the thermistor was warmer than ambient after being in contact with heat?

So my question is that is this value (100ohm) in line with what it should be and would it be more likely to be a PTC than a NTC? Or is a total waste of time looking for a replacement, meaning I should go with the normal resistor at half the value?

You're probably right in that the transistors (mine are TIP33A and BD245A, original 2 x C840) and some other parts have been replaced, the board had some home-made soldering traces on the other side.

Thanks again for taking the time to reply..! I'll look into the business more next weekend and report back then

Thanks Roly!

While waiting for a reply I studied the amp in more detail, just in order to understand the circuit and to find out the differences compared to the schematic I have.

Here are some images

http://aijaa.com/ILCPZg
http://aijaa.com/P4mXXW
http://aijaa.com/cByEEq
http://aijaa.com/mcgdSU
http://aijaa.com/ZdjIDy

As you can see, the output stage's 25V/1000uF cap before the speaker is gone and so is the smaller ceramic capacitor (?) beside it. I had a hard time identifying this component on the schematic. All I know is that it's connected from the other side to the the broken 25/1000 which means that it's obviously the "1" or "25/11". However the sizing might have changed compared to the schematic. Luckily there's a similar one on the other side of the output transformer (also in the photos) that shows the color marking. Any ideas?

Hi all,

I have an old Japanese Companion A-200G 20W solid state amp (also sold under different brand names; Shin-Ei, Univox U-150R, Teisco) from the late 70s/early 80s that stopped working after my friend yelled a bit too hard through it with a normal voice microphone.

After switching it on it works (clean tone, full volume) for a few seconds and then fades down to a low output which is heavily distorted. Switching it off and back on does not help, ie. it is distorted from the second I turn it on. However, if I leave it unpowered for a longer time, it works for a few seconds after switching on (cold start).

I have some experience with electronics but I'd really appreciate if you could share some ideas where and how to start troubleshooting. Here's a schematic I found on the internet for a newer model with different power transistors and some other minor differences

http://www.aijaa.com/v.php?i=6727635.jpg
http://www.aijaa.com/v.php?i=6727633.jpg


The amp has a single clean channel meaning that this has nothing to do with channel switching problems.