Gallien Krueger MB150E-III on going bias issues

[Arbutt]

Hello
This is a continuation of a problem with this amp. A few weeks ago i had this amp in for dead fault. 2 transistors had gone Q19 and Q20. Replaced Toshiba branded and from a good UK source.
Amp was working ok But noticed it running hot
Checked the bias and at the lowest point it was 400mv .
After a chat on here i was advised to change R36's 1k value .. ended up with the bias @ 10mv across bias test points with a 2.2k in place of 1k ( R36)

original post
https://music-electronics-forum.com/...iii-bias-issue

Tested the amp all good.
Customer took it home and used it at home 20 mins, Took it to a gig that night, after about 30 mins or so it cut in and out.
Swapped over to his spare amp did the whole gig without issues
gallien-krueger_mb150-iii_pwramp_sch.pdf
I have tested the amp and all working ok, Using the dummy load and playing a bass Red LED comes on and cuts out

So im not happy with the resistor change mod. It clearly has a problem.
I have done all the checks i can . Looking for help froom you lot on this one
Board version 206-0211- A3

many thanks in advance

[Jon Snell]

Have you monitored the HV lines to see if the power transistors are switching as they should.
The red LED is the pre amp limiter, not power amp limiter.
Monitor the rails and check they both switch to high at the same time.

Did you replace the components that did the damage to Q19 & 20?

[Arbutt]

Have you monitored the HV lines to see if the power transistors are switching as they should.
The red LED is the pre amp limiter, not power amp limiter.
Monitor the rails and check they both switch to high at the same time.

Did you replace the components that did the damage to Q19 & 20?

Hi Jon
I know its been a while But i decided to get on too it and start of the reason i have to mod the bias
I Found 68R resistor ( r38 ) open Replaced that and found that there was a short between Base-Emitter on Q17
Removed Q17 and Q24 to find these were ok But a possible solder splash etc
Cleaned the area up and refiited the transistors
No Short between Base-Emitter
Replaced the 2.2k ( Modded ) R36 to the original 1k
Reassembled and Tested powers up and bias is now back to as it should be being able to turn down fully anti clockwise and turn up to the 10MV

But i still have the issue when using at gig levels it cuts out. LED goes red the comes back on again
I have checked all voltage rails , All ok
I have gone over the board looking for any other blown/short components Can't find anything
Monitored the HV rails both ok at 57+/- not moving even when switching to protection
Signal gen and dummy load, Can see the output signal is correct

running out of ideas

[Jon Snell]

Sounds like over temperature or over current to me.
Does it run hot ... there is a thermocouple rated for 85° that may be detecting over temperature.
Next time you use it, run a fan onto the heat sink and see how it goes, at least that may point you in the right direction.

[Arbutt]

Thanks jon . It's running cool. It will cut out even when it's been on 30 secs etc .
doing my little head in