Gallien Kreuger 800rb troubles...

[SCStowaway]

Was given this amp being told it had a bad power supply board and would not fire up. Got it home, plugged it in and gave it a shot. It fired up, but immediately made smoke and sizzling noises. Turned it off. Saw it had fried 4 or 5 resistors on the power supply board. Found a salvaged board online for 60 bucks. put it in, fired it up, hum for 2 seconds then fuse blows, amp off. all resistors seem to be intact. All the reading I've been doing leads me to think I should start by replacing output transistors, would you agree? Im not 100% sure which are the output transistors though. Do the prefixes (mj, tip, mje...) give you a clue as to their role or stage placement?? Any info would be much appreciated. Hopefully ill have better luck here than the other forum. I am fully aware that there are professionals I can take it to. I enjoy this stuff and I am trying to learn more about my amps. I was going to attach schematics, but don't have permission. Thanks in advance!

[nosaj]

i would first check the power supply first disconnected from the preamp and power amp.
Here is the service manual https://medias.audiofanzine.com/file...ual-472053.pdf
Page 14 has the power supply voltages, if they are not right here nothing else will be either. Forget about the rest of the amp till you get through the power supply.

An welcome aboard, this is a board where techs hang out and help everyone.

nosaj

[The Dude]

I'll add. Don't just start replacing parts. Troubleshoot the amp. Figure out what parts need to be replaced. Start by building a light bulb limiter so you don't keep blowing fuses and burning up parts. And, don't attach a speaker to the amp until you verify that it is working and there is no DC on the output(s).

[SCStowaway]

Ha ha! Ive been holding off on getting a cab till I have a working head, so no problem there. And thanks for the tip Nosaj, Ill give that a shot. Just so I'm clear and on the same page, Im gonna put the black lead of my meter to the amps grounding post then test for voltages on the points labeled A-G?

[The Dude]

Yep. That's a good start.

[SCStowaway]

Ok, so I disconnected the power amp and preamp, built my light bulb limiter and got to work. short lived. The second I turned it on there was a pop and a blue spark that literally looked like it came from nowhere. it was between the two boards and in front of the transformer. Fuse still intact. With meter set at 200, I would get readings that would start at .5 or so then drop to .1 then fluctuate everywhere in-between, on all the test points(being the solder points from where the wires would go to the rest of the components. I even tried probing right at the transformer, same result. Does this make sense?Are these symptoms of a bad trans? Thanks!

[nosaj]

Ok, so I disconnected the power amp and preamp, built my light bulb limiter and got to work. short lived. The second I turned it on there was a pop and a blue spark that literally looked like it came from nowhere. it was between the two boards and in front of the transformer. Fuse still intact. With meter set at 200, I would get readings that would start at .5 or so then drop to .1 then fluctuate everywhere in-between, on all the test points(being the solder points from where the wires would go to the rest of the components. I even tried probing right at the transformer, same result. Does this make sense?Are these symptoms of a bad trans? Thanks!

Well I can say the light buld limiter is not working correctly. If it was the light bulb would have lit up bright.https://antiqueradio.org/dimbulb.htm
nosaj

[SCStowaway]

Would it have stayed bright though? It may have lit up for an instant, it happened so fast. Im positive I wired the dim bulb tester correctly. frustrating

[nosaj]

Would it have stayed bright though? It may have lit up for an instant, it happened so fast. Im positive I wired the dim bulb tester correctly. frustrating

yes unless the bulb blew, it the bulb works in a regular lamp, your limiter is wired wrong.

See you can take a light bulb limiter and plug just a cord into it with the white and black wire twisted together and the limiter will glow bright because there is a short.
In a normally functioning amp plugged into the limiter. the bulb will light full power for a sec or 2 then go dim once the caps have charged up.

nosaj

[SCStowaway]

Ive plugged other items into the plugged in limiter and they get power. I did get a 200w bulb, i think cause the first video i watched on how to make one, that's what the guy recommended. Too high? Also, I can hear the transformer when plugged in and powered on, a very faint hum.

[g1]

Yes it's too high. You want to be using a 50 or 60 watt bulb.

[nosaj]

Ive plugged other items into the plugged in limiter and they get power. I did get a 200w bulb, i think cause the first video i watched on how to make one, that's what the guy recommended. Too high? Also, I can hear the transformer when plugged in and powered on, a very faint hum.

i typically use only a 40 watt bulb as a go no go test . Once it 's a go I take it off limiter, when I wire in something new I use bulb to test.
nosaj

[SCStowaway]

Awesome, Ill go get me a new bulb then. Thanks guys

[SCStowaway]

Alright, I got me a 40 watt bulb, and a new multimeter and I got some actual results. The light started somewhat bright on start up, but quickly dimmed to just barely on, where it would stay for the remainder of the test. And for the first time, there were no sparks when I flipped the power on. All of the measurements I got were pretty close to what the schematics said, all were a tad under. where it should have been +-15, it was +-14, for +-65, it was about +-60, and for +-85, it was about +-78. Is that within tolerance? Whats next?!? Thanks!

[The Dude]

Those voltages look good. They will be slightly low on LBL. For clarification sake: With the power supply disconnected from the rest of the amp, you blew a fuse without the light bulb limiter? That seems a bit odd since the voltages look ok on the limiter.