Gallien Krueger 700rb

[Mars Amp Repair]

I wonder if anyone has seen this, or if there is a service bulletin for it. Unfortunately, GK is already covered here in the Denver area & needed no addl servicers.
When cold, this one puts out a negative going spike oscillation to the tune of about 750hz. The amp still reproduces sound & power.
If I cool Q20 in the power stage, the oscillation will slowly disappear...but from past experience, I doubt the xistor is the issue. The issue is not volume control dependent & is being produced after the pre-amp.
I have no lit for this model...all my GK lit is fairly old. Best I can tell from the pcb traces (which in a GK is very difficult at best), this transistor is somehow cascaded with 2 others of the same variety. 2sc2378 I think. I unfortunately don't have this xistor or the suggested subs...I also hesitate to replace this one with anything other than the same variety of those in the ckt, besides I'm thinking I'm only changing its gain by cooling it.
Has anyone had any experience with this issue? I know these types of problems are pretty impossible to figure over the web, unless someone has seen this...thanx, glen

[Enzo]

They may not need additional warranty repair shops, but did you ask them for a copy of the schematics? AMps last a lot longer than warranties, and they have sent me what I asked for in the past.

That transistor is a 70v 100ma NPN signal type, and most any other will work in its place. SOmething rated higher voltage might be easier to find, nothing wrong with a 120v, 100ma part. 70v being an unusual voltage rating for a signal xstr. I more expect to see 60, 80, 100v parts. But that's just me.

Your amp goes WOOO when cold? does it stop after warm up? I am confused when you say it does it when cold, yet cooling parts makes it diminish? If it stops after warm up, THEN is the time to spray cooler on there to see what makes it start again. Or did you mean it does it constantly without even having to warm up?

SOunds like the amp is unstable. I might be looking for a smallish electrlytic that is blown open, missing, loose from the board. I'd be looking for a drawing first of course.

[Mars Amp Repair]

Yeah,
I have japanese transistor spec & sub books that I got from MCM. Since most consumer stuff used japanese semi's years ago, I'm fairly familiar with their configs. What I'd really like to get my hands on is some spec books for the Motorola xisitors. Does anyone know where one can find that..or a site with that info?
Yeah Enz, I should clarify...it made the oscillation when the AMP was cold (first turned on) & then slightly freezing the transistor made the frequency increase & it's amplitude diminish & then finally disappear. When I heat the xistor, the oscillation quits (or if you let the amp get hot itself).
It really looked like a negative digital signal leaking into the audio, but I don't believe there is any digital stuff going on in this amp as well as I wouldn't expect it to be affected by heating a drive transistor in the power stage.

I agree on aquiring the serv lit, esp with GK & amps built similarly. I am always surprised how open & willing some of these companies are to supply lit. Alesis will e-mail me anything I need. I'm glad many of these companies must realize the easier they make it for musician's to get their stuff fixed means the better rep they get & more they sell. Seems like a 'no duh', but for years many companies treated their lit like it was 'top secret'.
I left a message w/GK today to see if they'll e-mail me a copy.

What happened to the days you could just shine a light under the pcb & follow the traces on the other side.

I'll let you know what it is when I find it as well as some of these others I seem to be getting plagued with these days...I think Murphy has parked humself on my bench for a while ...thanx, g

[Enzo]

You should be able to find old Moto data books, but if nothing else, that division is now called ON Semiconductor, and from their web sit you should be able to get selector guides. I love my old Moto selector guides. CHart form by package type, listed usually in ascending voltage and ascending current for power stuff. Signal stuff mught bve vy noise figure or whateve.

[Mars Amp Repair]

thanx Enzo,
Any idea where a guy could get ahold of an old guide like that? Sounds like just what I need. I'll have to check e-bay & the like..g

[Enzo]

I would start by asking everyone I knew who was involved in electronics. Ask for any extra copies of semi parts data publications they might part with. Moto publishes books - much like the tube manuals from RCA - on power semis, small signal semis, logic semis, etc. But what I find really good is the selector guide, which puts the parts in chart form for a quick scan.

[Mars Amp Repair]

Cool on selector guide. We have a part supplier here in Denver that has been here since the 50's. They have been buying out old shops since the 80's. I'l have to see if they have anything they'd be willing to part with.
I called GK today...was very surprised that the let me talk to a tech..many mfgs won't unless you're authorized. He hadn't heard of that symptom...I was hoping for 'oh yeah' we have a svc bulletin for that one...did get the schemo e-mailed to me. g