Marshall VS100H - Reached the end of my troubleshooting knowledge...

[AndyDufresne]

Hi, first post for me. I'm limited in troubleshooting, but I know how to read a schematic, how to solder, how to use a multimeter and how to work around electronics safely.

Here's the story. I bought a VS100H for twenty bucks. When I got it, it was humming bad, but it still had limited output. I figured twenty bucks - I've got nothing to lose. The guy who had it before me said that, "it had gotten wet" and that he had turned it on and played it for a few hours then it started humming. That's really all I know of the history.

I opened it up and nothing looked really bad except what I think are voltage regulators (correct me if I'm wrong) REG1 and REG2 looked like they've seen a lot of heat. I also noticed that R20 and R21 get hot to the touch.

So, I go over the boards checking for joints that look weird. A couple seem funny so I reflow them and I also reflow the regulators and the resistors. Also I've read that hum can come from the filter caps, so I reflowed them too. Nothing more than that.

Then I plug it back in and, while everything lights up, there is zero output, nothing from either speaker jack or the headphone jack.

Any advice on what I should check next? Thanks in advance.

[Enzo]

OK, loud hum is usually one of two things. DC on the speaker - a "blown output" - or a loss of filter cap on a power supply. COUld be a bad cap, but more often just solder to it.

Until you know the amp is NOT puting out DC< do not connect a speaker. DC will burn out speakers.

So. First thing, test the speaker. Pull the wires off it, or if this is the head version, unplug the speaker. Touch a 9 volt battery to the spepaker terminals. A working speaker will pop or thump with each touch. Don;t leave the battery touching, it will drain the battery and won;t help the speaker any.

Now fire up the amp. Is there DC voltage on the output? I'm not concerned over a few millivolts.

Now check power supplies. It is normal for Vregs and R20/21 to get warm or hot. The schematic says the Vregs are 12v, but they could be 15v ones in your amp. Are both of them putting out their voltage? Look at the various 8-leg ICs. As far as I recall, all of those will have power on pins 4 and 8. SO check a few. Is power distributed throuh the board to them? Those are the low voltage supplies. There are also +/- supplies for the power amp. I don;t know what they are, I'd guess 35-40v or so. Are both present and similar voltage to each other?

There is a tube in this thing. Is the heater inside the tube lighting up? There is a heater fuse if not. And also check pins 1 and 6, is B+ voltage present? I forget what it is, 250-350VDC I'd expect, just don;t want to see zero there. There is a fuse for that high voltage supply as well.


Look at the power amp page. By the input see R1 meets C2, with T16 to ground between them? T16 is a mute, it has to have a voltager on its gate to turn off the mute. Is ther voltage on T16 gate leg? And with the amp powered, measure resistance across that JFET. From the junction of E1 and C2 to ground. Is ther at least a few thousand ohms? Or is it something like 12 ohms, shorting out your signal?

If that is OK, plug a signal into the thing, and connect the FX send to some other amp. SOund come out over there?

[soundguruman]

Blown output, you got it.

[AndyDufresne]

Thanks for the help. I went through most of the tests and nothing seemed off. So I started poking around checking to see if there was voltage and resistance where they were supposed to be.

What's this? No voltage here? Well hmmmm...I wonder if it's because I didn't fully seat this ribbon cable into its socket! Feeling stupid, but relived. So it looks like the filter cap reflow fixed the hum and then actually putting the thing back together correctly this time got it working again.

I spent a lot of time trying to find out how to do the tests that you recommended. I wasn't sure how to test the voltage at the tube pins. Do you take out the tube and just put a probe in the socket? I had to do some research on JFETs to find out how to test them. Is there a rookie tutorial on how to safely test some of the more common electrical components?

Actually sounds better than I thought it would. Can you guys recommend a replacement tube? I like a tight, punchy, compressed sound. I usually turn the mids all the way up. Also can I just plug a replacement tube in or will I need to bias it too? Thanks.

[soundguruman]

Thanks for the help. I went through most of the tests and nothing seemed off. So I started poking around checking to see if there was voltage and resistance where they were supposed to be.

What's this? No voltage here? Well hmmmm...I wonder if it's because I didn't fully seat this ribbon cable into its socket! Feeling stupid, but relived. So it looks like the filter cap reflow fixed the hum and then actually putting the thing back together correctly this time got it working again.

I spent a lot of time trying to find out how to do the tests that you recommended. I wasn't sure how to test the voltage at the tube pins. Do you take out the tube and just put a probe in the socket? I had to do some research on JFETs to find out how to test them. Is there a rookie tutorial on how to safely test some of the more common electrical components?

Actually sounds better than I thought it would. Can you guys recommend a replacement tube? I like a tight, punchy, compressed sound. I usually turn the mids all the way up. Also can I just plug a replacement tube in or will I need to bias it too? Thanks.

You can put in a JJ for about $10
or a Groove tube for about $23

[AndyDufresne]

Do I just order it and plug it right in? No biasing?

[Enzo]

There is no bias adjustment for the preamp tube. Plug one in and see if you like it.

[soundguruman]

There's a huge variation in preamp tubes.
Some have much more gain, and some much less.
It's almost better to get several and try them all.
It's like 31 flavors of ice cream.

[MikeH]

For the record you don't have to use a 12ax7 either (Actually I don't know what tube is in there, I'm ASSUMING it's a 12ax7). You can use an 12a..7 tube around, like a 12ay7, 12at7, etc. They all have different flavors of tone. Although, honestly you will probably prefer the 12ax7.

[soundguruman]

I would use ECC83.

[Enzo]

ECC83 is simply the European system type number for the 12AX7.