Marshall Valvestste VS100H amp

[Enzo]

2SA872 is a 90v 50ma signal xstr, PNP of course.

MCM sells them for 26 cents each.

They are a differential pair, so if you sub one, sub them both.

I don't have a direct sub for it offhand, though I am sure there are plenty of 100v low current asian TO92s out there.

If I had to do it fast, I would likely grab an MPSA92 or 93 and use it there. I'd have to turn it sideways for the leg arrangement to work. Otherwise I'd just order the right part from MCM or someone - NOT from Marshall.

[e_c_mccollum]

Are those little yellow oval capacitors (100n) found in this amp the same as disc ceramic capacitors? I believe they are, but the size difference puzzles me.

I found these capacitors on the mouser site, but they are more expensive.

[e_c_mccollum]

Never mind, found my anwser.

[fleshtwister]

Can anyone help me? I have a Marshall VS100 combo. I found this thread and it seems people here might have an idea of what's wrong.

I replaced all of the transistors that are along the heat-sinks with the ones mentioned earlier but it did not change the problem.

The problem is, when I use the amp (once it warms up) it has a lot of crackle in it and the volume drops up and down but never as loud as it should be. If I pull up on the PCB near the transformer (see attached photo) the sound gets very loud like it should be. As soon as I let it come back down it goes back down to nothing almost. After a few minutes, the sound cuts out all together and nothing will bring it back except turning it off and back on. When I do turn it off I get a VERY loud pop through the speaker.

I have replaced the tube and that had no effect at all.


Thanks in advance for any help or suggestions.
BTW, I do have the schematics but I'm not very verse on all the electronic terms so you may have to dumb it down a little.

[Enzo]

HI flesh, welcome to the forum. You would do yourself a favor by starting a new thread on your amp - more people will see it that way.

My first guess is the headphones jack, it has a cutout contact for the main speakers. With the amp running and sounding low and crappy, look down on the phones jack in there. Press down on the contact nearest the rear panel - I just use the point of my meter probe for this. Does that bring back the sound? If so, that contact either needs to be cleaned and maybe reformed (classy word for bent) or replace the jack.

Most likely otherwise is one of the jacks on the rear panel, recheck the solder under each one of them. And see those white cement power resistors? resolder them too. Especially check R29, 0.33 ohm 7 watt.

[fleshtwister]

Thanks Enzo. I thought it might be the headphone switch from reading other posts so I tried it with the headphones plugged in and it still cut out after a few minutes. I could only get the sound back by turning the amp off and back on.

I guess I'll start a new thread. Thanks for the suggestion.

[fleshtwister]

Hey Enzo,

I did a little messing with the output/input jacks in the back. They didn't make a difference when the speaker was plugged in but I plugged my headphones in and I was able to play for at least 30 minutes. I stopped there. In the past, I had it cut right out with the headphones so I think you're probably right.

I ordered new jacks since that seems like one of those could be the culprit. They're cheap enough to just go a head and replace all of them. Also, I remember the guy I bought this from saying "the footswitch can be a little tricky at times and you might need to fiddle with the jack when you first plug it in order to get it to bite".

So, I'll let you know in a few days when the parts arrive. I'll also double check R29 when I'm there soldering.

Thanks again!