Gallien Krueger MB200 Problem

[kynsi]

Hi all,

I have a Gallien Krueger MB200 in for repair and it's driving me crazy. The customer complaint was: "it got very warm and smelled funny".
I Switched it on and it seemed to work ok. Just to be sure I decided to measure the output transistors and I coudn't measure any bias for one of the TIP33C output transistors. So I replaced both TIP33C's (Q10, and Q11) and this is when the trouble began.
It's a rev E model and whenever you desolder a component the solder-traces come loose and or break. So, it turned out I had to do a lot of repairing traces before the new transistors were connected to the right parts again. Still no luck. After blowing some fuses, Q9 (TIP30C) turned out to be lose and broken so I replaced it.
Still no luck. After turning it on again one of the other output transistor had blown (Q21, TIP34C). I just replaced it, checked the connections, switched on the amp, and the fuse popped. Turns out the transistor I just replaced has blown! It is parallel with Q20 and that one is still ok.
Any ideas how to proceed? Although I've used the lightbulb current-limiting technique it keeps blowing fuses and components. What should I do? Where to start??

I already managed to check the preamp and that is fine.
When swithing on the amp there was no input and no load attached.

Thanks!

[kynsi]

Small update: just decided to check Q10 and Q11 and Q10 is also broken, again..

[52 Bill]

These amps usually are not that hard to figure out. And I use a light bulb limiter with them all of the time with no problems.

I don't like mixing new and old outputs in any amp. It sounds like you have been just replacing the dead ones? At least replace each type in pairs.

If it was working when it came in, then something happened after you did work on it. Double check all of your work, make sure there are no solder bridges, that the repaired traces are connected to the right spots, all of the replaced transistors have been installed correctly, etc.

Be sure that the bias circuit around Q12 and Q13 is all working and correct. Did you test all of the drivers?

[kynsi]

No, I replaced the output transistors in pairs. The one that has blown was a new one.

The main problem is that the output transistors keep blowing. So before I can measure voltages the amp is dead. All drivers measure fine in circuit, but as I explained, removing them might damage the board.

[J M Fahey]

The lamp bulb limiter prevents chain burning outputs, so I guess you are doing something wrong.
What power and type bulb are you using?
Does it shine?
As of lifting pads and tracks, it can always happen of course, but GK boards are well made so it shouldn't happen easily.
Wiggling a part leg while trying to pull it is a definite no-no.
Use a good solder pump to clean it well, if necessary finish with solder wick.
Transistors should be easy to lift without undue force.

[kynsi]

Thanks for your tips.

The lifting of pads is actually a known issue with these amps (the 200MB by the way, not mb200, which is a modern one), it's even in the servicemanual. Later revisions (G upward) had much better boards.

I'm using a 100W lightbulb, but honesty presses me to confess I didn't use it all of the time As I found a loose connection I figured that would have been the problem, but after restoring all the broken lines something is obviously still wrong.
I am getting a slightly strange reading around Q13, so will try to remove it with extreme caution and measure it out of circuit.

[kynsi]

Replaced Q12 (was marked Q13 on the bottom, but actually Q12 was blown). Replaced all output transistors, turned it on with lightbulb and no problems. Left the amp on for a while, checked bias and everything was stable.
Turned it on directly connected to mains and everything still ok! Rechecked bias and amp is still stable. Now I have to test with a load.

[kynsi]

Tested it with speaker, and is ok! Fixed!
Thanks J M Fahey!

[Mario nicolas]

Buenas tardes amigos, estoy necesitando ayuda con un cabezal gk mb200, lo que hace es lo siguiente..
despues de un tiempo de estar encendido y con carga se va a falla y corta la salida de rf y la verdad es que no tengo idea a que se debe, ¿alguien me podria orientar? Ya solo soy un simple aficionado de la electrónica y mi perfil esta orientado a lo industrial
Desde ya les agradezco!

[Enzo]

Here, this may help...

Good afternoon friends, I am needing help with a gk mb200 header, what it does is the following... after a while of being on and with load it goes to failure and cuts off the rf output and the truth is that I have no idea what it is due to, could someone guide me? Now I'm just a simple fan of electronics and my profile is oriented to the industrial I thank you in advance!

[Mario nicolas]

Querido amigo enzo, soy electricista industrial de profesión, no tecnico electronico por eso mi pedido de ayuda.