ok. as pic above : left: nut - 40.3 right : nut - -00.1
E - -00.1 E- -40.3
B- -00.1 B- -40.3

Rabbitbreth didn't show the + sign, but I suspect that left: Nut is +40.3 v.

 

I think the 2M2906 (metal case) driver transistor is actually a 2N3906 (as shown) so it matches the 2N3904 driver transistor.
Then both little transistors are overloaded and produce distortion and smoke at high output levels and if the 2N3055 output transistors have normal but low gain.

Did the 2N3055 transistors come with transistor mounting hardware?
Did you use the insulating washers to mount the transistors to the heatsink?
Remove any insulating washers. The case of the transistors must be electrically bonded to the mounting nuts and bolts, i.e. the metal heatsinks are live, one at +40V and the other at speaker output voltage.

no mounting hardware . no insulating washers.


if I touch the heatsink with my probe I get a signal

 

I see something black under that screw head.

 

I see something black under that screw head.

View attachment 307293

that's a black metal spring washer that's on all four screws. They were with the old transistors , I put it back the way I found it

 

With the amp power cable unplugged, use the DMM on ohmmeter range and measure the resistance between the right most transistor case and the +40V rail at the bulk capacitor. You ought to be reading close to zero ohms.

 

With the amp power cable unplugged, use the DMM on ohmmeter range and measure the resistance between the right most transistor case and the +40V rail at the bulk capacitor. You ought to be reading close to zero ohms.

View attachment 307303

yep O.O1 ohms

 

ok. as pic above :
left: nut - 40.3 right : nut - -00.1
E - -00.1 E- -40.3
B- -00.1 B- -40.3

We misinterpreted the sign on your readings.
To avoid confusion, don't use -ve sign for anything else except to mean negative.
Don't leave out the + sign when the reading is positive. Sign is absolutely important.

BTW, 0.00V reading is not nothing. When the meter shows 0.00, say so. Do not say nothing.

Now we have to go back and analyse what all this means.

 

We misinterpreted the sign on your readings.
To avoid confusion, don't use -ve sign for anything else except to mean negative.
Don't leave out the + sign when the reading is positive. Sign is absolutely important.

BTW, 0.00V reading is not nothing. When the meter shows 0.00, say so. Do not say nothing.

Now we have to go back and analyse what all this means.

ok . got it . sorry about the dashes.

 

so: left nut + 40.3 right nut : - 0.1

left E : -0.1 B : -0.1

right E: -40.3 B : - 40.3

 

We want to check the bias voltages on the base of Q4 and Q5.




We are expecting to see the base of the upper transistor Q4 to be around +0.8V while the base of Q5 to be around -0.8V.

 

We want to check the bias voltages on the base of Q4 and Q5.

View attachment 307328
View attachment 307329

We are expecting to see the base of the upper transistor Q4 to be around +0.8V while the base of Q5 to be around -0.8V.

the upper transistor on the pic above , is +0.86, and the lower is -0.48

 

Two base-emitters in series make 1.2V to 1.4V.
There are a total of four base-emitters in series then four diodes are needed to avoid crossover distortion.

 

Two base-emitters in series make 1.2V to 1.4V.
There are a total of four base-emitters in series then four diodes are needed to avoid crossover distortion.

3 actually, but there are only two on the board, and by the voltage measurements they seem to be plain old single junction diodes. Remember this is a 'guitar' amp - low distortion may not be a priority.

 

Ylli, thanks for correcting me about the Sziklai pair instead of the PNP darlington that I wrongly was thinking about.

Distortion? Guitar speakers do not produce the high frequencies of crossover distortion and guitar people probably are deaf to high audio frequencies.

 

Is it possible that you installed the 2N3906 incorrectly?
The original was BFR79.



This is the pinout for 2N3906.

 

Is it possible that you installed the 2N3906 incorrectly?
The original was BFR79.

View attachment 307372

This is the pinout for 2N3906.

View attachment 307368

well you did warn me Mr Chips so I took care to put it back the way it came out . I will take a pic if I can so you can see

 

so , there's the old one placed to the right . middle pin goes back ...hard to see .

 

Ok. So you did replace it with BFR79. It looks like it is installed correctly.

 

Ok. So you did replace it with BFR79. It looks like it is installed correctly.

yep . ok. phew .

 

Can you do signal trace check at these two test points?