Marshall Short Part II (old subject new thread)

[sweatyk]

JM is what you're saying that we didn't isolate the part causing the problem but we found a way to change it's effect by adding the resistor on R74? Which helped stabilize it? And I also have one other issue with the amp once it's up and running.........the reverb doesn't work. The tank is good so it has to be in the circuit. But that's another thread after this thing is smoothed out. Tomorrow I plan to play it on the overdrive channel and see what happens.

[sweatyk]

Well it looks like I'm back to square one! It played a long time at lower volumes hooked to my 8 inch bench speaker without a problem. I hooked it to the 12 inch speaker with the channel volume at 5 and the master volume at 3 and got almost 5 minutes then it popped the fuse.........uggggghhhhhhhh. Gentlemen please give me direction.

[g1]

In post #54 you got 10 minutes out of it, no problem. Was that with the 8 inch ? Any chance there is a problem with the combo's speaker, cable, or connector?
Check if the blown fuse is caused by a short in the new output transistors.

[sweatyk]

I firmly believe that the speaker is not an issue here. I've had this hooked up to 3 different speakers during this troubleshooting. On a quick check both TR8 and TR10 are shorted across the collector and emitter. TR9 did not read a short. When I hooked up the big speaker today it was so that I could push the amp a little harder to see what it would do. I had my meter measuring the voltage across R118 as I was playing. At idle it was around 2mv but with the volume up pretty loud I watched it jump at times up to 180mv the harder I played. I had the volume up about half way. A little loud for this small room. That's all I'm going to do tonight. I will look into it further when I have a chance. Any suggestions are always appreciated.

[DrGonz78]

Hey Sweatyk sorry to hear the amp is not stable again . Anyway a thought just occurred about where are you sourcing your TIP142/147 replacements? In other words where do you buy them? Just gotta ask...

Did you replace the fuse and do more testing? Probably not since you are reporting the TR8 & TR10 to be shorted collector to emitter. I was just a little confused sorry. So lets see what your part source is too. Best regards.

[sweatyk]

The first three sets of TIP's came off of eBay from Hong Kong. The last two sets were from mouser. I still have one set left from mouser. And no I haven't had time to dig into it yet but I plan on it here in the next few days. A friend suggested that it could be a resistor that is going out of tolerance as it heats up. At this point I'm at a loss.

[DrGonz78]

One more question regarding the speaker... What is the impedance of the speaker?

[sweatyk]

8 ohm

[Jazz P Bass]

Did you replace the mica insulators under each output transistor or reuse the old ones?

[sweatyk]

Replaced and with fresh thermal compound.

[g1]

To be even more specific, the thermal compound is supposed to be a very thin layer, basically to fill any tiny pits in the metal. Excess thermal goop can be as bad as none. And TR9 must also have a good thermal contact with the heatsink as it tracks and compensates the bias according to heatsink temp.
As far as the speaker, I meant everything outside of the test setup, the cable and connector plug can also cause problems. Or were you using the combo's spkr. cable for your test speaker?

[sweatyk]

Let me start at the beginning to give the big picture. I’ll try not to make this too long and drawn out. Last year I helped a friend find this on craigslist. Everything worked fine but the reverb did not work. He kept it for a week and played it before going on vacation. He left it with me so that I could clean a lube all of the pots and jacks plus see if I could fix the reverb. I pulled the chassis out of the combo and set it up on my bench. It was late at night and probably shouldn’t have done this while being this tired but I began to look at the schematic and troubleshoot the reverb. I decided to see if there was any voltage on pin 7 of IC10. Don’t ask me why I started there. Just seemed logical from a greenhorns point of view. This is where the problems began. I didn’t take the time to place my meter where it was easy to view. When the probe touched the pin I had to look to the left at the meter and my hand drifted and something got touched causing a nice bright flashing light known as a fuse. This is how the outputs got blown and I’ve been chasing it for a year now.

[sweatyk]

Come on guys don't leave me hangin now........lol

[g1]

Actually, I think we may be all waiting for more data on what you found to be wrong.
Have you removed TR8 & 10? Are they both blown? How about their emitter resistors?
Also, what kind of wiring is used from chassis to speaker? Is there a jack and plug or is it hardwired?

[sweatyk]

As in post # 64 on a quick check TR8 and TR10 are short across C and E. TR9 is not reading any shorts. The insulators are new and I did not over do it on the thermal compound. The way in which I connected the speaker from the combo was using alligator clip leads between the speaker and the speaker connectors on the PCB. Never had a problem doing that in the past. I used the same method connecting it to my bench speaker for testing. This weekend has been kinda busy so I haven't had time to dig into it deeper. This has been a real brain fryer to me but I just can't give up. The other threads dealing with this kind of amp never got resolved and I want to conquer that! I'm not sure if it would serve any purpose but I have my son in laws VS100 head here for storage and it works perfectly. It's a little late in the evening here to start researching the schematic to see if there are any similarities that could be helpful. What do you guys think?