freestompboxes.org

I thought I'd get my ass kicked repairing a little solid state amp which has very low volume. Good news is there were already a few people with the exact same problem:

https://forum.speakerplans.com/hh-elect ... tml#447660
https://www.ssguitar.com/index.php?topic=2595.0

But neither threads comes provides a solution. Symptoms are: big pops when plugging in and out, clearly not loud enough and a lot of background is. I know somebody has worked on the amp, because the legs of the first BF245C are sticking, not clipped off. Somebody has been messing with the input stage, the jacks. The inputs are wired in a way which is mildly confusing to me... but I know this is not some backward amp company, they supply the solid state power amps for the BBC
The high input has 3k4 resistance to ground, while the low input has 2M2? That work has been done also shows by the replaced capacitors...
The preamp stages seem to have BF245C transistors made in 1979..., or am I seeing things?
But the power amp in contained in this little brick:
Then I found out that this STK-080 module is mostly used in audio equipment, hifi gear. Then I felt a bit cheated by HH Electronics resume, but I guess you cannot live off of one good contract with the Beebs. The datasheet it's best run at 27V and has a maximum supply voltage of 39V. However, while measuring the voltage on the power amp, getting readings of 35V on one of the pins I shorted something out with the probes and killed the thing completely.

Luckily I was able to order a replacement quite cheap. Let's hope it's not a fake...
I guess my main question for now is: can you kill a power amp transistor by simply shorting it? Having spotted quite a lot of hifi repairs dealing with these modules and them being readily available still, make me put my money on that.

Oh yes, the cabinet is absolutely stunning and remarkably modern with the bare finger joints... it almost screams "rebuild me into a tube champ!"

Hi,
I was the guy who posted the original question that you linked to. Still haven't fixed the amp and I've had a few people send me pics of their amps for reference and they are all a bit different. Did you solve the problem?

cheers Steve

Yes, I replaced the first JFET BF245C -- check whether you have credible voltage on all transistors, i.e. 15-18VDC.

Thanks, I'll give it a try though I may have already messed it up hacking it about with replaced capacitors etc. Is the BF245C Transistor you mean the one marked Q1 on the board close to the input jacks?

cheers

steve

Hey Steve,

If the filter capacitors are bad, you are always going to hear it... unless something else is wrong. This amp also had the filter caps replaced, but also that first stage fet. It is located near the red and blue input signal wires. I have some video footage of this repair I really should find the time to edit...

Yes do post it. There are a few people who come across these nice little amps and just can't fix them. I bought this amp when I left art college in about 1980 and left my WEM clubman behind (silly move). It would be nice to get it going again though I'm a valve amp user these days. Interesting that it's really just a hi fi amp packaged up but I used to be able to get some nice gain when the master was down and the volume right up.

Here's it is, it's not much. I was trying to get rid of some of the amps wrecks I have been able to revive over the last few years. One guy who bought a bunch of my stuff brought this with him while picking up a tube amp. When it came in it has really low volume at max. Trying to measure voltages on the amp module, I shorted it and blew two fuses. I ordered a new amp module (thinking I damaged it) immediately before realizing the fuses had blown. After replacing the fuses however, the amp did not come back life, so I did replace the power module eventually. No change... no sound.

So I started measuring transistor voltages (which I should have done first) and found the first FET to be lacking the 18V supply voltage, so I replaced it. Amp came back to life. Tellingly this FET had already been replaced once before. I was suprised to find that the high input directly connects the guitar to the gate of the FET, no input resistor or capacitor.