Gallien Krueger 250ML (Unit 2)

[sweatyk]

First off this is not the same amp that I posted about earlier this year. I have 2 of them. I will refer to this one as “unit 2” as to not confuse with the previous one I’m still working on. I bought this amp 15 years ago for the speakers from a repair shop in Orlando Fl. selling as unrepairable for parts only. Decided to dig it out of the closet and give it a go. And as I have said before I’m a hobbyist/novice and not a well vetted tech as most of you are.

These are the items that I found initially at fault. Either shorted or burnt open and physically cracked.

Q13 & 17 shorted

R98, 112, 131, 145 – All 100 ohms burnt

R123 emitter resistor had been replaced with two 2 watt .47 ohms in parallel, one side loosely wrapped together not making good contact.

Both volume pots not original, loose and had the traces lifted and broken around them plus one had a broken shaft.

Found a solder bridge on one of the circuits and corrected it.

I source replacement pots and have made solid connections where the traces were broken….Thumbs up

I replaced Q13 & 17

Originals = TIP33C & TIP34C with TIP35C & TIP36C

After reading serval comments it was apparent that the driver transistors should be replaced also.

So Q12 & 16 were replaced with the original type TIP29C & TIP30C

All of the 100 ohm resistors were replaced

R123 emitter resistor was replaced with the proper .33 ohm 5 watt resistor.

I lifted several of the diodes out of circuit to check for shorts and all were reading as they should.

I hooked it up through my current limiter and powered on. No shorts per the bulb. Plugged in my guitar and it played but something wasn’t quite right. With the chorus and echo on it wasn’t as rich in sound as it should be. Upon inspection I noticed that I had overlooked a switching transistor literally cracked open. It was Q25, I had a replacement for it. Once replaced I powered it up without the limiter and got immediate smoke from R128 & 132. After replacing them I decided to sweep over all of the transistors again to look for shorts. This time I found Q31 shorted and R157 open. So, I repeated the process that I did with the other side of the amp. Both outputs and drivers were replaced along with R157. Also, since I replaced Q25 I went ahead and replaced Q24.

Powered up with the limiter and no shorts. Played the guitar through it while on the limiter and it sounded like it’s supposed to. The sound was very rich.

So after that I decided to check the bias of each amp side. I used dual meters and hook to both emitter resistors for the +/- of that side of the amp. Each was slightly off but adjusted to 5 mv. I let it idle each time for over 10 minutes for each amp side (with no speaker load). 2 things that I noticed and don’t know if that is normal. One amp side kept drifting slowly down approx.. 1 mv after the ten minutes. The other drifted but not as much. Seemed more stable.

The other thing I was monitoring was the temperature of the output transistors and the emitter resistors. The negative side of each amp was hotter than the positive sides. After 10 minutes R123 & 157 were reading 103 to 109 degrees F while R106 & 139 were reading 84 degrees. Being the novice that I am can you guys point me in the right direction of where to start looking to see why the temp is so different between the + and - sides?

[Jon Snell]

According to the schematics, and Kerchoffs Law, it makes no sense. The current feeding the components in each tail must be equal between the +ve and -ve supplies assuming there is no output loading, they must be equal and complementary.
Check the voltages across the emitter resistors, (2.5mV per resistor). They should read the same. If they do, all I can say is thermal dissipation is not linear avross the pcboard. Maybe.
The junction of R106 7 R123 must be 0volts NOT 6.5volts as described in the service manual!

[Helmholtz]

According to the schematics, and Kerchoffs Law, it makes no sense

As always, your technical contributions are excellent.
But the name of the German physicist is Kirchhoff.

[Jon Snell]

As always, your technical contributions are excellent.
But the name of the German physicist is Kirchhoff.

The joy of me being dyslexic.
I knew what I thought was correct.
Thank you for the correct spelling.

[nickb]

According to the schematics, and Kerchoffs Law, it makes no sense. The current feeding the components in each tail must be equal between the +ve and -ve supplies assuming there is no output loading, they must be equal and complementary.
Check the voltages across the emitter resistors, (2.5mV per resistor). They should read the same. If they do, all I can say is thermal dissipation is not linear avross the pcboard. Maybe.
The junction of R106 7 R123 must be 0volts NOT 6.5volts as described in the service manual!

It seems to me that the numbers in square boxes are AC readings. Without boxes it's DC. The gain of the power amp is about 40 so an input of 0.16V gives and output of 6.5V

[g1]

It seems to me that the numbers in square boxes are AC readings. Without boxes it's DC.

I think so too for this example. But they don't seem to follow any standard through the schematic. There are AC voltages in circles; DC voltages with or without boxes, some with polarity given, some not; and these that seem to be AC volts in boxes.

[nickb]

I guess I should have added...."except where they aren't" Gosh.... What a dog's breakfast.

Anyway the point is that the 6.5V in question is 6.5V AC.

[Jon Snell]

I guess I should have added...."except where they aren't" Gosh.... What a dog's breakfast.

Anyway the point is that the 6.5V in question is 6.5V AC.

It seems to follow like English language; I before E except after C sometimes.
I wonder if the emitter resistors had infact got the same voltage drop or not.

[Mick Bailey]

Do the supply rail voltages hold up OK? Also, is there any DC offset on the speaker output?

[sweatyk]

Do the supply rail voltages hold up OK? Also, is there any DC offset on the speaker output?

I measured at both the speaker out plug and on both sides it started at 4.5VDC when I first powered it on but rapidly started clocking down to zero.

On the 45VDC +/- I wanted to be sure I was checking from the correct place for each. I read from the negative side of C52 and the positive side of C51. What I read was +/- 4.4 VDC at each spot. That can't be correct. Am I at the wrong spot to check that?

[The Dude]

You should (says Mr. Obvious) be reading rail voltages on the filter caps- +&-45V. It's basically the same points as reading directly off of the rectifier. What are you using for a ground reference? Meter battery is good? Is there maybe a connector from transformer to board that is making a poor connection?

[sweatyk]

You should (says Mr. Obvious) be reading rail voltages on the filter caps- +&-45V. It's basically the same points as reading directly off of the rectifier. What are you using for a ground reference? Meter battery is good? Is there maybe a connector from transformer to board that is making a poor connection?

I also checked the voltages directly off of the BR and got the same reading. I lifted the BR up off the pcb and left the - leg out of circuit to check it with no load and got the same results. I am using the back plate the output transistors are mounted to for ground. Can I assume that is sufficient? Also the batteries in my meter are good.

[g1]

Measure right across each cap, not to ground. See if that makes a difference.
Are you on the bulb limiter? If so, is it lighting up?

[sweatyk]

Measure right across each cap, not to ground. See if that makes a difference.
Are you on the bulb limiter? If so, is it lighting up?

Well I measured directly off of the BR and no I didn't have the bulb limiter on. I used it for the initial firing up and there were no issues. I borrowed a functioning amp from a friend and his reads 43VDC +/- on both sides of the BR where my two are reading 4.5. The incoming AC voltage is the same on all of them. I think it might be safe to say the BR's are the issue on both of these. Looks like I'm placing an order to grab a few.

If the rail voltages are very low like this and the amp is being used because it will play, could that have been the contributing factor in the multiple parts failures that shorted the outputs and fried a few 100 ohm resistors in the process? Like causing them to over heat?

[g1]

I don't think it's possible the amp was making sound with voltages like that. I suspect the heatsink is not grounded, which you also wondered about. That's why I suggested measuring right across the caps, use one of the grounded leads for your meter ground.