Gallien Kruger MB150E-III Bias issue

[Arbutt]

Hello all
I have MB150 on the bench which Blew Q20-Q19 Replaced them and Tested all working ok, But seems to be getting hot
Checked the bias and the Bias details on the schematics
according to the schematic i should be reading 10MV across J3.
Turned R35 down anti clockwise and im getting 390-410MV see picture
Both new transistors were replaced like for like and come from the same reliable source
Checked all 0.22R all ok
Turning R35 its increasing the bias voltage across J35
across R43 and R47 = 121mv
across R41 and R39 = 128mv
across R38 and R40 = 80mv
across R44 and R48 = 71mv

Im i getting worried over nothing or what
Many thanks for your help in advance gallien-krueger_mb150-iii_pwramp_sch.pdf

[Jon Snell]

I expect the replacement transistors have a different Vbe reading.
Check R38 is not damaged!
If it was replace it and measure between J3-1 and J3-2.
Replace R36 with a 680 Ohm 5% 1/2W MO resistor if still too high..

You will not be able to accurately measure the resistance of the 0R22 resistors. It suffices to say that each emiter has an 0R22 resistance to the output rail making approximately 20mA bias rating if 10mv is accross them.
Always disconnect your speaker load and have no signal.

[g1]

Turning R35 its increasing the bias voltage across J35

What does this mean, where is J35?
Which page of schematics is the correct revision for your unit?
I am very surprised if Q19 and Q20 blew but there were no other parts damaged.

[Arbutt]

J35 is the bias test points . Marked on the schematic. I will look into what version it is .
the 2 transistors that went short were changed like for like and came from a uk supplier i have used for years .
after I changed them the amp is working OK. Just running hot .
I did check everything else in the circuit , everything tested OK.
Will spend a bit more time on Monday
Thanks again

[Arbutt]

I expect the replacement transistors have a different Vbe reading.
Check R38 is not damaged!
If it was replace it and measure between J3-1 and J3-2.
Replace R36 with a 680 Ohm 5% 1/2W MO resistor if still too high..

You will not be able to accurately measure the resistance of the 0R22 resistors. It suffices to say that each emiter has an 0R22 resistance to the output rail making approximately 20mA bias rating if 10mv is accross them.
Always disconnect your speaker load and have no signal.

Thanks jon I will take a look at what you said on Monday

[g1]

J35 is the bias test points . Marked on the schematic.

Sorry but I can't find it? In the bias procedure notes they refer to J3. At the test points they show J3-1 and J3-2. Nowhere do I see J35.
That's partly why I asked which page schematic you are using.

[Arbutt]

Sorry but I can't find it? In the bias procedure notes they refer to J3. At the test points they show J3-1 and J3-2. Nowhere do I see J35.
That's partly why I asked which page schematic you are using.

Sorry man that was my mistake. Just got into the workshop after the weekend off. Its J3 Not J35
I have found the varient 206-0211-A3 page 15

[Arbutt]

I expect the replacement transistors have a different Vbe reading.
Check R38 is not damaged!
If it was replace it and measure between J3-1 and J3-2.
Replace R36 with a 680 Ohm 5% 1/2W MO resistor if still too high..

You will not be able to accurately measure the resistance of the 0R22 resistors. It suffices to say that each emiter has an 0R22 resistance to the output rail making approximately 20mA bias rating if 10mv is accross them.
Always disconnect your speaker load and have no signal.

Morning jon
I have tried what you said about , R36 was ok so i removed it and went to 680R. The bias got hotter. So i fitted a 1.2k and slowly increased til i found a 2.2k allowed me to 8-9mv at the lowest setting ( Fully anti clockwire)
This is with no speaker or signal
whats your thoughts on this

[Jon Snell]

8 - 9mv is a reasonable value.

Now you know how to fine adjust the pre set bias control.
Electronics is fun and not complicated at all.
I suggested 680R as a pointer for you ... and now it is a simple case of deduction, which I knew you could do. I hope that helped you on your way to becoming an electronics expert. Practise is the best way to learn.