GK MBE150III Going into protection when pushed

[Arbutt]

Hello all
Ive posted a few things about this pain in the arse , So i thought i would start a new post
Brief History
It came in dead.
I found Q19 and Q20 burnt up
Replaced them and tested the amp. Amp working but running hot and couldn't bias down. Cutting a long story short
Found o/c R38 and a short between B-E on Q17. Removed Q17 and Q19. Transistors were ok. Cleaned the board/Area and short went away, Replaced with 2 new transistors Short gone
Must of been a solder splash ?? Biased the amp as the Schematic states.10MV ( J3)

Anyway tested the amp sounds good no issues apart from when you turn master and gain up, 12 oclock Ish the amp goes into protection and LED goes red, Wait 1-2 seconds and its back working again
seems to be worse if you use the drive function

I have monitored the Voltage rails no issues, when goes into protection , They remain stable
all voltage rails are correct as the schematic states
schematic below Page 15 - 206-211-A3
I have checked just about everything,
Hoping one of you more experienced techs can help and shed some light on this for me
Many thanks in advance
gallien-krueger_mb150-iii_pwramp_sch.pdf

[x-pro]

If you overload the input of an amplifier with more signal than it needs to produce its rated output power, any amplifier will go into limitation.
For accurate diagnosis, you need objective data.

[The Dude]

Did you bias the amp after repairs? If the amp is overbiased, excessive current may cause problems.

[Arbutt]

Did you bias the amp after repairs? If the amp is overbiased, excessive current may cause problems.

Yes the bias is set as per the manual 10Mv across J3
The amp is not running Hot at all,

[Arbutt]

If you overload the input of an amplifier with more signal than it needs to produce its rated output power, any amplifier will go into limitation.
For accurate diagnosis, you need objective data.

Thanks i don't believe that the input is being overload at all. At practice level is ok, But as soon as you have let say the master and Vol set to 1/2 way and play a few notes on the bass together it cuts out
I have done a few of these amps and i know they shouldnt do that

[mikeydee77]

I think is a familar story with these amps. I have attempted to repair around five of these over the years. Each one worked for a while but when taking off the current limiter or pushed then it failed. My theory is that it is do with output device matchng which I have not attemped.

[Arbutt]

I think is a familar story with these amps. I have attempted to repair around five of these over the years. Each one worked for a while but when taking off the current limiter or pushed then it failed. My theory is that it is do with output device matchng which I have not attemped.

Interesting . Only 2 of the outputs were changed both Toshiba and came from cricklewood electronics. Have you ever contacted GK and got any assistance?

The damn thing works well apart from this issue

[mikeydee77]

Nope I have not taken it further to be honest. I have wasted a lot of time on these amps trying to fix power amps hence why your thread caught my eye. It is probably worth contacting GK to see what the deal is. I think others on this forum have had success with them so hopefully they will chime in at some point.

[g1]

Possible some other component (likely a transistor) was stressed by the original fault and is only breaking down under higher drive levels. Maybe freeze spray would help.

[x-pro]

C19, D15 is good? Try putting 47uF instead of 33uF.
It is necessary to feed the signal from the generator and on the required load (equivalent) to see the signal distortion with an oscilloscope. And then think about it.

[Arbutt]

I have contacted GK, there is a email address on their site for tech problems

[Arbutt]

Hello People Sorry i'm back
I contacted GK support Not very helpful ,
Anyway i decided to continue with it., I have taken a few voltage measurement and I think i maybe on to something
powered on and no signal Bias is at 10MV across J3
I tested while on with a signal gen and dummy load with the levels up just under the amp switching in to protection
Measuring across
R40 = 18MV
R42 = 18MV
R44 = 22MV
R48 = 22MV
R39 = 88MV
R41 = 88MV
R43 = 44MV
R47 = 44MV

I did do a list of the transistor B-C-E voltages last week and now can't find now so i need to do it again
All voltages are correct 28V +/- and 14V +/- Tested all transistors in circuit, Nothing i can find, Also tested resistors and diodes in circuit, all ok

Mikeydee 77 stated that he has had this fault many times before and has never been able to cure it. His thoughts were on Matched sets of transistors
I look forward to your comments
Have a good day
Arbutt