Marshall Short Part II (old subject new thread)

[Jazz P Bass]

Just a suggestion: reread what I posted & take a few carefull voltage measurements.

Consider first that the circuit is a Class AB type.
In pure Class B the output transistors are 'Off".
This can lead to crossover distortion, so the outputs have a small voltage on there bases to put them into Class AB, thereby eliminating the xover distortion.

To understand the circuit, you have to know how an NPN ans a PNP transistor work.(in this circuit)

Basically the collectors get tied to a high voltage.
The base of an NPN, when at a low voltage, is 'Off'.
The base on an NPN needs about .560 Vdc to 'turn on'.

A PNP base is normally a high voltage to be considered 'off'.
Lower the base by .560 Vdc & it starts to turn 'On'

TR5 & 6 are biased a tad different.

[sweatyk]

Well Enzo it looks like I will have to adhere to your advise on put it down for a while. My mom has went into the hospital so this might take a while. I will return sometime soon when things get back to semi-normal. Not giving up!

[g1]

Best wishes for your Mom.

[sweatyk]

Well mom’s out of the hospital but I still don’t have time yet to dig into this amp. However I have been pondering something and what Enzo said made my mind click. As with anything else to often people treat the symptom and don’t look for the cause. Keeping that in mind every time I have put this through the test without the limiter the fuse blows and TR8 & 10 short. But I’m testing the amp as a whole. Plugging in to the input running the signal thru the entire amp. Shouldn’t I isolate the preamp circuit from the power amp side first? So my thought is insert the signal after the preamp. If it blows I can at least eliminate the preamp as an issue correct? You think up all kinds of stuff sitting in a hospital all day. So once I get the rails correct and all is good on the limiter then plug into it after the preamp and no limiter and see what happens???.......Thoughts? Is my thinking all wrong?

[Jazz P Bass]

I don't know if the preamp itself can cause the fuse to blow.

[sweatyk]

Just an update.....I managed to peel back a little time to inspect the traces on the PCB probably for the 50th time.There were a couple of spots where the traces had lifted due to getting hot but was still intact. As a precaution I scraped back on some good trace and soldered in a clipped lead to stabilize it for positive continuity. The worst two were coming off of the Zeners. Might not have anything to do with whats going on but at least they wont be a suspect anymore.

[sweatyk]

I just wanted to conclude this thread. Unfortunately this will go unresolved. I have invested too much time and effort into trying to repair this amp. I was really hoping to do it seeing how the last couple of threads for the same issue went unresolved. I wanted to thank you all for your input, opinions, patience and suggestions. I wished that I was more educated in electronics to have done this. I’m going to reassemble the amp and place it on craigslist as a parts or repair item. At least I can get a couple of dollars back out of it. I almost was going to ask anyone here like Enzo if they would be willing to repair it for me but after thinking of all of the cost involved it just doesn’t make financial sense. Once again thank you all and I’m sure I will be back with a different amp and issue…..lol