GK 400 rb

[Jazz P Bass]

For sure something is amiss on the negative rail side of the output section.

I have a schematic labeled 'Very Early'. attached.

[catstrat]

Thanks. I'm pretty sure i have the right schematic. It's the board layout i'm unsure of. A board layout makes things a lot easier. I should start looking at the negative rail and everything powered by that rail. Right ? From what i hear i should have less than 100 mv dc. at output. Let me go over the schematic again.

[catstrat]

400 rb

I'm locked into about the 14th battle with this thing. Schematic shows neg 7 volts at pin 6 of U1. I have neg 4.5 v. I also have neg 14 volts at pins 3 and 5. That would be a shorted op amp. You're getting voltage where voltage shouldn't be. I removed the op amp and soldered in a socket. Before i installed the op amp back in i checked the voltage at the O P and had negative 48 volts. I re-installed the opamp and now i have neg 8.5 volts at o/p and no voltage at pin 6, but i still have voltage at pins 3 and 5. Could the op amp be shorted? I don't stock those so what are the suitable replacements. Pin 6 connects to the base of Q1. I tested Q1 both in and out of the circuit last night and two of the legs tested as open. I replaced it cause i have tons in stock. I just don't know how many problems i created before my limiter came along. I have too much time in this thing to give up now. Thanks.

[52 Bill]

Pin 6 connects to the base of Q1. I tested Q1 both in and out of the circuit last night and two of the legs tested as open. I replaced it cause i have tons in stock. I just don't know how many problems i created before my limiter came along. I have too much time in this thing to give up now. Thanks.

The output of the opamp connects to the emitter of Q1. not the base. Be certain that you have installed it correctly.

The LF351 is a jfet input single opamp. It is obsolete now, but can still be found or substituted for with another one. Maybe a TL071 would work there.

You still have a problem in the power amp section. It doesn't matter if it was there originally or was caused by powering it up without the limiter. You just have to check things until you find all of the problems.

[The Dude]

If you suspect two of the op amp pins shorted, why not take it out and see (simple continuity test)? You've already installed a socket, so it wouldn't be that difficult.

[catstrat]

400 rb

I did some research last night and found the main reasons for the extreme dc offset. It can be either a leaking /bad driver or predriver or O/P transistor.Something out of wack in the bias circuit. Or a resistor in the circuit that has overheated to the point of causing it to go out of tolerance, usually up in value. I'll know what to check for next time. I checked Q 10, 15031 in circuit this morning and got an open between c and e. I pulled the transistor and still got an open. I compared it with a new 15031 and it tested the way it's supposed to. I'll replace that transistor. I also have some toasty resistors but they are still within tolerance. I'll probably replace those for piece of mind. I'll try those things and get back. I'll say one thing, this has been a learning experiance for me. I hope the troubleshooting skills i'm learning on this amp can be applied to all transistor amps. I'm also learning the do's and the don'ts of working on solid state stuff. Thanks again folks.

[catstrat]

I installed a socket and subbed an 071 and it didn't change things. I just replaced q10 and again no change. I'll keep looking. Out of curiosity, what should the going rate for solid state repairs be. I don't have a clue. Some guys charge more than for tube amp repair. I know What to charge for tube amps but not transistor.

[Enzo]

Same as I charge for anything else - time and materials. Soldering is soldering, parts cost whatever they cost, troubleshooting is troubleshooting.

[catstrat]

400 rb

I just confirmed the board layout i have is completely different from the schematic. The schematic shows a Q 2 but the layout has no Q2. So here we go. This is one of the reasons i have such a hard time with solid state amps. 40 per cent of the time on solid state stuff the layout doesn't even match the schamatic. I know tube amps well enough to diagnose with out even a schematic but transistors,not so. Unless the component number is silkscreened on the board, i have a hard time. This is one of the reason's to, i end up replacing random parts. Without a proper layout matching the schematic, it get's tuff for me. So, does anyone have the proper layout. The board is numbered 60041A and on the back a sticker says 3-85. That sounds like a build date. I'll contact GK and see if they'll help.

[catstrat]

gk 400rb

Thanks Enzo. I always wondered about that. Dis regard my earlier post. Apparently the board layout is, at least, close to what i have. I saw you Enzo on another forum and you were kind enough to list the transistors. Let me get back at it.

[Enzo]

My experience is contrasting to yours. I find solid state circuits to be pretty consistent. The tube stuff is what changes all the time. I have one model Gibson tube amp in my files with THREE different factory published schematics, PLUS three more that I hand drew from circuits. That is a total of at least SIX different schematics for the same model.

When I do not have a schematic, the first line of attack to me is contact the manufacturer.

My old GK files skip over 60041, the archive goes from 60040 to 60043. You say your schematic is right, but not the layout? Does the schematic say 60041 on it?


I never concern myself with dates, but if you must, the controls usually have date codes on them, as well as many of the larger components.

Here, I found yours in the 400RB file. It is too large to post here. But if I have it, so does GK. I see a row of TO220 transistors, Q2 is the one on the left end.

[catstrat]

400 rb

Alright, i'm done. I've had it wasting time on this when there is stuff sitting there i know i can fix.. So this one is going back to the customer un fixed. He can send it somewhere else for all i care. Problem is, through my ignorance of transistor circuits i probably caused additional problems. It's time for me to bail on this one. Thanks for all the support. You all have been helpful.