Gibson GPA100 repair, where to start

[BigPolishJimmy]

Hi all,

I searched the forum for previous posts on this amp, but didn't find anything useful, so here goes.

I have a Gibson GPA 100 head circa 1971 that doesn't work.  I got it off of freecycle from a person who gave up trying to fix it.  I'm still pretty much a noob to electronics, but I've been fixing simple things like switches and input jacks for years.  I've owned a soldering iron for something like 25 years, you know electric guitar players, it's like learning how to restring a guitar.

Anyhow, back to the PA, ... I've been unsuccessful in locating a schematic for this, so I don't know the values of the various components unless they're printed on them. 

What are the most common problems that are likely to happen to a solid state amp from the 70's?   I'm assuming caps drying out is there.  What about transistors?

On inspection--after draining the caps(just in case)--I noticed a few things:

* One of the larger capacitors is somewhat loose on the board.  It doesn't look that old so I'm going to re-solder it to see if that helps.   

* It looks as if one set of the transistors have been changed out.  The old ones are hard to read, and there is a set of NEC181's on the other side. 

* It has a weird circuit breaker setup in it.  I'm used to seeing fuses, so I don't know if this is even good or not.  I suppose I need to lift part of it out of the circuit and test continuity on it to find out.

I'm going to try to find a schematic for another similar year Gibson SS amp to see how they did things in hopes of finding some common ground for how this amp is set up.  There's a lot to look at as the head has 8 inputs with adjustments for each.

The layout is not user-servicable friendly.  I've got it partially apart, but I'm going to have to free all the inputs and pots from the front face in order to get a look at the bottom side of the board, at least 24 nuts to unscrew plus the power lamp & switch.

I'll try to take some pics tonight and post them so you can see the layout. 

Any suggestions or guidance is greatly appreciated.

[J M Fahey]

The power amp section is probably the same as the one in
http://www.gibson.com/Files/schematics/G100A.pdf
Download and compare to yours.
Plus a couple pictures, of course.

[BigPolishJimmy]

Ok, lets see if I can get some photos to show up.





To the left you can see the thing that I assume is a circuit breaker.  The red dot is out and can be pushed, however it doesn't click in or out like a push-button switch.  I'm not familiar with this type and don't know if it's good or toast?  Do these blow and stick out like a turkey timer?  I'm used to fuses.





Here you can see the spring reverb and another portion of the board.  Question, what are the two large items mounted on the back? 



Here's the cap that's loose on the board.



Here's the NTE181



Not sure if you can see it, but the older transistors have a rounded top.  If it were a cap I'd expect it was swollen and faulty, however with a transistor, and the thickness of the metal, I'm thinking it was just built that way.

[guitarfreak666]

those 2 items are caps, if you touch them your probably death

[BigPolishJimmy]

Good thing I drained them with the other caps.  I was unsure because they have more than 2 leads coming from them.  Yep, I'm a noob, ...but a safe noob.

[guitarfreak666]

still i wouldnt touch to much caps without measuring them, you cant take too much risk

[J M Fahey]

Those silver cans are the filter caps.
Those NTE transistors were probably replacements in an earlier repair.
Shape is not important.
Compare the amp you have with the schematic I posted, it *should* be similar.

[tonyharker]

The red button just looks like a lamp with the fixing bezel broken off.  Thats why it can be pushed in?  Can you show the back of it in closeup?

[BigPolishJimmy]

Thanks Juan, I'll be using the schematic to compare things.    As I look at this a bit more, things begin to make sense.  Tracing the cord in the back, one side goes first to a connection at the breaker, then it goes to the rectifier diodes, next the filter caps, then on. 

The circuit should be fairly simple, but the board just looks overwhelming with 8-inputs and the associated adjustments related to them.  On the front half of the board you can see the pattern of 4 sections that are mirror images of each other.  While there are 8 inputs, they are delt with in pairs, thus 4 sections of volume, reverb, treble, and bass.  On the far side of the front of the board are a few caps and resistors that coorispond with either the master volume and/or reberb controls.  My focus therefore should begin with the back of the board, get the whole unit working and then sort any problems with the individual mix controls.

Tony, that really is a breaker or fuse of some type.  There's no lamp, the back is just a metal enclosure about 1/4 the size of an altoids box.  The power indicator lamp is on the front panel.  Still I'll try to get another pic of the back innards to show tonight.

[J M Fahey]

Yes, that´s a circuit breaker, a small version of what you have in your house.
The bad thing is that they were much slower than a regular fuse; because overcurrent must heat something and it takes a couple seconds; the good thing is that you can't put there the bad value fuse, one pulled from your car, some rolled up cigarette paper or as I learned yesterday, a bent bobby pin.
As of the amp, good that it makes sense.
Be certain that it does.
The 120 V line wires do not go straight to the board (it would kill you) but first to a transformer, which both isolates you from death, but also lowers voltage to around 60V, used by the amp.
Don't worry (yet) about the preamp , your big problem lies in the power amp.

[BigPolishJimmy]

yes, you are correct the line comes into the transformer first.  I misspoke there, talking from memory about a circuit I'm only beginning to understand.  I didn't have time to mess with it yesterday due to homework, but hopefully this weekend I'll get the chance to free up the board so I can secure that cap and then test the circuit breaker.  After those simple things, I'll have to see where the board takes me.

[J M Fahey]

Build a lamp bulb limiter (search here) and use it.
So if your amp works perfectly, fine, and if you missed some small obscure part, it won't burn again your expensive just replaced power transistors.

[joecool85]

Here is the Light Bulb Limiter thread.

For future reference, it is at the very top of the screen on the "Amplifier Discussion" board since it is set as "sticky."

[BigPolishJimmy]

Thanks!   I saw the lightbulb limiter thread but thought it was mainly for tube amps because they're older.  I've got all the parts laying around except for maybe the bulb, but I might have that too.  I'll build one before I get too far as it seems like a smart and easy thing to do.