Marshall Short Part II (old subject new thread)

[sweatyk]

Did a little checking just now. Pulled both TR8 & TR10 out......both shorted across C and E as was in the circuit. Also read at the solder connections on the PCB for both.....no shorts. Both R118 and R96 read .6 ohm out of circuit. Checked the rest of the transistors and diodes in circuit and no shorts found there. That's all I had time for tonight unless there are some suggestions?

[Jazz P Bass]

Where did you purchase the TIP transistors?

They may have been fake.

[sweatyk]

Mouser

[sweatyk]

Should I go ahead and install the new outputs or are there some things that should be checked prior to doing so? Obviously everything we have done up to this point still has not identified what is causing the outputs to blow.

[DrGonz78]

Personally I would put the transistors on there and start measuring every voltage test point across the output circuit. Of course with the limiter on and with/without a load to see if there are differences. I would be testing the amp with signal using the limter in place to make sure it won't blow the amp out. Then again the limiter might not protect the output transistors from shorting out, but it is still safer than running it without one. I would be monitoring that 15v supply to make sure it is stable over long periods of time. Glad you are not giving up yet!! There are many here in this community that would like to see you fix this amp and find out what was the problem.

Perhaps other members here will have different advice but that is where I would start. It seems we did not check voltages on all the transistors in the power amp section.

[sweatyk]

Replaced TR8 & TR10. Hooked thru the current limiter with 75 watt bulb. No speaker load. All controls turned to zero. Here are what I read at idle on all transistors with respect to ground/earth:
TR1 E= 2.5mv.....B= 1.37v......C= .62v
TR2 C= 15.5v.....B= .62v........E= .42v
TR3 E= 0v.........B= 1.3v........C= .6v
TR4 E= -43.5v...B= -43v........C= -1.1v
TR5 E= -.7v.......B= -1.3v.......C= -43v
TR6 E= 1.2v….B=1.75v…….C= 43v
TR7 E= 43.6v….B= 43v…….C= 1.06v
TR8 B= 1.06v….C= 45v…….E= 23.3mv
TR9 B= -.47v…..C= 1.06v…..E= -1.1v
TR10 B= -1.1v….C= -45v…..E= 19mv
TR11 C= -.95v…..B= 20mv….E= 21mv
TR12 C= .90v…..B= 22.5mv…E= 21.7mv

The following was also measured with respect to ground/earth:
R83= 15v
R78= -15v
R64= -15v
R65= 15v
ZD1= -15v
ZD2= 15v

The following were measure across the part:
R118= 2.2mv
R96= 2.2mv
R73= .63v
R74= 1.5v

TR8 & TR10 measured emitter to emitter= -4.7mv
TR8 & TR10 measured base to base= -2.1v

[DrGonz78]

My feeling looking at the voltages is that TR6 base voltage seem a tad bit higher and that is something to consider. Not much else to nitpick in the voltage table. A while back I was looking at one of your pictures that you uploaded and looked very carefully at TR6. Now I know you can't really look at parts to see if they are bad or not... However, perhaps it was stressed on the many failures occuring and looking at the pic of it made me think it might have been stressed. Check the voltage drop over R83 compared to R78 too see if the resistors are not working the same. I would expect to see 1.3v on each of the bases as far as I know. The voltages will go up without the limiter in place. I am not sure completely but I would expect that the voltages on TR5 and TR6 should be matching on the bases.

[sweatyk]

Well as far as TR6 is concerned there are no physical signs of stress. It must be the angle of the picture. Also would like to note that I have replaced it at least twice in the earlier change outs. I know that doesn’t mean that it is fine right now but I wanted to point that out. On the base voltage of both TR5 & TR6 you said they should be the same. My readings were TR5 -1.3v and on TR6 1.75v. Now are both supposed to be positive or negative volts? Or the opposite in polarity but reading identical numbers? Also R78 & R83 with the polarity difference read the same 15v. I wanted to also note that these voltage readings were taken cold. Shall I repeat the process after a 5 to 10 minute warmup?

Throughout this process even before I came to this forum I had shotgunned out numerous transistors, resistors, and IC’s in an effort to carpet bomb the problem. Not the best way to defined what is bad but in the beginning it was about repair and give it back to the owner. I’ve had it for a year now still trying. I had a conversation with a local tech and it makes sense at least to me that there seems to be a resistor that is jumping out of tolerance somewhere as the signal pushes harder thru the circuit at higher volumes. I certainly could be wrong about that because after all you guys have WAY more experience than I do or I wouldn’t be here. An amp identical to this one recently came available on CL and I bought it and gave it to the customer so I now own this one. I have invested more than it is worth in time and dollars but now it’s about principle. Having seen several related threads unsolved I want to see this thru no matter what for the good of the organization! So I am so ready to accept guidance and hope that we can get to the bottom of this.

[Enzo]

My working assumption is that we are looking at the symptom and not the problem. This thing is burning up outputs. What causes a transistor to blow? Excess current/heat. The transistors should handle peaks to its load, which leaves me looking for the bias circuit going open. The outputs are wired to the power rails. Their bases are pulled towards those rails which would turn them on. Both sides turn on, we now have a output transistor sandwich heating up between the rails. The bias circuit grabs the opposing output bases, and pulls them together to pinch off current through the outputs. When the bias fails, both sides are free to turn on hard, shorting across the power rails and destroying the outputs. TR9 is the bias transistor. My suspects are intermittent opens, not shorted transistors. The amp plays until something triggers its self destruct.


You need to limit the time you let yourself spin your wheels. When you are going over the same stuff over and over, you lose focus. Best to set a time limit, then put it aside for a week.

[sweatyk]

Not fully understanding circuits is the bias section consisting of R73, R74, and TR9? Is there more than that? And are we really to be looking upstream in this circuit for an issue? Enzo thanks for chiming in. My local techie friend (goldtop5) has mentioned you from time to time. As for you closing advise that's all I can do. My life plate stays so full that it is all that I can do to carve out a couple of hours a week to work on this relying heavily on you guys for guidance and suggestions.

[Enzo]

Those three parts are the basic bias circuit, but don't forget all the connections between them and the rest of the world. D1,D2 are also bias parts. Really suspicion is on everything between D1 and TR9

[sweatyk]

Well interpreting schematics is one of my weak spots for understanding them completely. So let me know if I have this incorrect. The components that make up the bias circuit starting at D1 thru TR9 are as flows:
D1
D2
TR4
TR5
TR6
TR7
TR9
R80
C47
C49

Please let me know if this is correct.

[sweatyk]

Well interpreting schematics is one of my weak spots for understanding them completely. So let me know if I have this incorrect. The components that make up the bias circuit starting at D1 thru TR9 are as flows:
D1
D2
TR4
TR5
TR6
TR7
TR9
R80
C47
C49

Please let me know if this is correct.

Hello?....enzo?....anyone? Any comment on my last post?

[Jazz P Bass]

Here is my take on the circuit.

IC 8 output is a sine wave which drives TR 6 & TR5.
They in turn will cause TR7 & TR4 to conduct according to the polarity of the sine wave.

TR7 & TR4 are the driver transistors for the outputs.
They are biased barely on by the base resistors to the power rails.
This 'barely on' condition is what sets up the output transistors (TR8 & TR10) to also be barely on. (Class AB)

The collectors of TR4 & TR7 are connected by the bias circuit, through TR9 and it's associated resistors.

TR11 & TR12 are the 'protection citrcuit'
If the output swings too high, they will 'steal' base current from the output transistor bases.

I hope this helps some.
Your mileage may vary.

[sweatyk]

Thanks Jazz for that explanation. I got some mileage out of that. However I still feel a little lost on what to look for now. Just looking for some direction.