I know we usually do pedals here, but I need some advice on fixing this amp

[ghostsauce]

I dug up a Gibson G80 amp, it's solid state but 40 years old and it has a really nice tone to it.. I suspect the transformer is bad, anybody here know how to figure out what model/brand the PT is? I'd just order a new one with the same specs.

Here's the schematic - http://ghostsauce.net/random/g80%20schem.jpg
and on the transformer it says PT 789 45

[R.G.]

A couple of questions first:
- what are the symptoms?
- what things make you think the transformer is bad?

These are because while the power transformer is the single most expensive part in the amp, it's also one of the most reliable. They rarely fail on their own. Generally something else failing overstressed the power transformer. It's no fun killing the replacement too.

There are ways to test the transformer to see if it's really bad. In general, with your meter set to measure ohms, you measure for continuity where the windings are supposed to be continuous, for isolation where they're not supposed to be connected, and test for internal shorts with an inductive kickback test. I have a description of these tests on my web page, geofex.com.

To test the transformer and power supply in the amp, you will need either a variable transformer ('variac') or a light bulb limiter to make turning it on non-destructive.

However, if you're listening to it at all (i.e. 'has a really nice tone to it'), chances are it's not the PT. That's why I asked the symptoms.

[digi2t]

Just a word of experience here....

My first amp was a Gibson G212. I bought it when I was 15, and I'm 47 today, so do the math. It's an excellent early 70's, 100W, SS combo amp, it's been my workhorse all these years, but....

the soldering in these units was absolutely abysmal. I got the amp cheap back then (100$), but before long, every time I would play clean through it, it would crackle like a lightening storm.

Luckily, I figured out which of the soldering iron to hang onto back then, and going around touching, and beefing up all the solder points cured it for good. There were several points where the solder was weak, and had cracked. You could see it plain as day with the naked eye. It's been going non-stop for the last 30 years. As a matter of fact, ALL my equipment has changed over the years, EXCEPT that amp. It's super clean sound allows you to pump any pedal through it, without coloring the tone of that pedal. I cherish it, like my first born.

So, I'll echo R.G.'s comment about "the great tone", and suggest that you carefully go over the obvious, before digging any deeper and assuming the worse. Especially with these early 70's SS Gibson amps. They were under CTS control at the time, and while the amps are quite good, quality control was obviously given it's walking papers.

Cheers,
Dino

[R.G.]

To reinforce that, once the immediate crisis is over, you'll want to make the amp reliable. There is a set of things to do that will do that, which include:
- replacing all the polarized/electrolytic caps for new ones
- remelting all solder joints with a bit of fresh 60/40 flux cored solder
- cleaning all contacts; this includes switches, pot wipers, knob/pot/jack/switch bushing mounting nuts onto the chassis

But that needs to wait til it works right and just needs polished.

[Gurner]

So first question. .. *is* there +35V, -35V, +14V & -14V present on the output of the power supply?

[R.G.]

So first question. .. *is* there +35V, -35V, +14V & -14V present on the output of the power supply?

I was hoping that he'd answer that with the "Und vat mate you think that ziss vas de proplem?".

[ghostsauce]

Sweet, I didn't want to bug you guys about it cause everybody here's been so helpful to me before. Ok, so when I first got it, it put out sound but it was barely audible. So I figured.. yeah, needs a cap job. So I pulled the board and fixed a crapload of joints (abysmal was a good description, lol. There was a solder ball as big as my pinky on one of the rectifiers) and replaced an electro cap that was obviously bad. It was supposed to be a 50uF but I only had a 47uF on hand so I put that in just to see if it'd work better, not intending to keep it... put it all back together and was sad to find out that now it passes zero signal, and hums very loudly.

The hum is not affected by the volume pots, so it's after that stage.. but given the schematic it seems the volume is fairly early on. So I've gone over it a million times and cannot find the problem, so I'm left to assume that it needs the 50uF and that is the only problem. Regarding the transformer though, it looks quite bad. It was taped on using electrical tape, and although I'm not familiar with trannies it appears that the windings are exposed and rusted-looking on one side. The windings also seem to be bent and touching other windings, etc.. I didn't test it because I don't really know how, but just looking at it I'd be surprised if it works.

If you guys don't mind explaining to me how to test it though I'd love to keep the original and attach it properly, lol.  I got the amp for free, somebody was just throwing it away. Cosmetically it's mint but the guts really need some work, it seems.

*Just re-read RG's first post, I'll check the tranny tomorrow then and see

[jimbanzini]

check for DC voltage at the speaker output.  sounds like you've got some fried components in the power amp that are letting voltage from the power supply pass through.

[R.G.]

Regarding the transformer though, it looks quite bad. It was taped on using electrical tape, and although I'm not familiar with trannies it appears that the windings are exposed and rusted-looking on one side. The windings also seem to be bent and touching other windings, etc.. I didn't test it because I don't really know how, but just looking at it I'd be surprised if it works.

That's a good reason to suspect it, and it changes my advice.

If it's obviously been abused, and you're willing to swap it out, my advice is to swap it out first. Exposed windings, bends, other mechanical abuse says that it's safety properties may be compromised. That being the case, you're up for some work, but it will make it vastly more reliable.

1. Replace the transformer. You want 48Vct to 50Vct secondary, about 150-200VA. 48Vct will do fine, and is more common. Because it works at a single frequency, its power handling are intimately tied to its physical size, so if you'll measure it, and get one about the same dimensions.
2. If the transformer was abused, it is also possible that the rectifier diodes are hosed, and I would replace those.
3. Dead rectifiers may have hosed the first filter caps. Be prepared to replace those, but wait and see how the transformer replacement goes.
4. I notice it's a two-wire primary. Might as well make that into a safer and quieter three wire AC cord as you go.

All of this presumes that you already know how to do AC power wiring safely. Don't get into that if you don't already know how.

[PRR]

> it has a really nice tone to it..
> it put out sound but it was barely audible

I'm confused. Was it "nice" or "barely audible"?

Iron-rust on a PT is NOT a problem.

If you really have exposed fine-copper, it's pretty sick.

The PT is 24-0-24VAC (48VCT) at 4Amps (maybe 3A). Like R.G. said (duh). On reflection, 4A is "too good", the original was probably a saggy 3A. Not an unusual part.

Now that it is really sick, you go through it step by step.

Like: car won't start. Is this new or has it been sick a long time? Is it going WURRRR or wrr or tic? If there's juice, is it getting to the spark coil? Is the coil kicking-up to make sparks? Do sparks get to plugs? Is there gas in the tank? Is the gas getting to the engine? If you put your thumb on the spark-hole and crank, is there compression?

In this case: what Gurner said. You should have + and - 35V at the main caps. (Ignore the 14V for now.) Since you have hum, I bet the PT is somehow working. If you have only one side of the two 35V rails, that's rectifier or cap. (Amp of this vintage and quality, I'd replace the FWB and the main caps on general mis-trust; also to verify that all solder is good (because it's mine, not some over-rushed low-pay worker).

Before that: put 100 ohms 10 watts in series with the speaker. Most likely faults will put DC on the speaker, possibly 35V of DC which will burn the speaker coil. With 100r in series, faults can only put a few volts on the speaker, and you can still hear something.

In the power amplifier stage: output should be under 1V DC. At the input Q3 Q4 (OMG, TIS98!), both Bases (connect to 47K) should be about -1V. Output transistors should be cool/warm... if hot, short the trim-pot/R23 for now. If still hot, we need to hack to find the fault. (The "tone" is probably not the power amp, and it is over-extended... if it isn't easy, I'd be tempted to smack a LM3875 in there.)

[ghostsauce]

The tone was barely audible, but sounded really good. Had the same quality to it as all the YT demos of the G-series amps, so I had something to compare it to.

I appreciate your help guys but I got a nice offer for it and I'm going to let it be somebody else's project. I'm gonna take the money and do something nice for my wife, as she has been really nice about me acquiring gear as of late. I've already got a JCM900 for big stuff, and a Class5 for home use and small church gigs so I don't really need anything else, especially not one that to me has to narrow a use. Thanks again for the info!

[Gordo]

Rats, this was just getting exciting.  Kudos for doing something nice for the wife though...