Marshall Vasvestate VS100R Head

[JohnnyVieira]

Greetings (is it polite/standard to start a new thread with this word? 1st question: 2nd grade english!)
Well, hello,
Introducing myself: a greenhorn when it comes to electronics. I can kinda read a schematic and kinda know what is the purpose of each component, so, I'm sorry in advance.
Yesterday I bought a VS100R Head and the correspondent speaker, the price was incredible not to buy. The seller told me the head had something wrong with it.
I dismounted it, cleaned it, and saw nothing. It was working: i could turn it on, and play guitar, but with a really low volume (emphasis on "really"), even with the volume knob at maximum. Oh, and in every channel (clean, distortion 1 and 2) it distorded.
I went searching for a solution online and found that usually, for this kind of problem, it is cold connections (bad soldering). So, I looked again, and in the power amp circuit it had some transistors lacking solder. I reinforced it, and now i have no guitar sound, just a hum that do not change if i change the knobs positions.
I opened it again and saw something that was there before i resoldered (i took pictures before and it was there already): a burded resistor, and perhaps, a dead transistor. The resistor was clearly burned, but the transistor just a "little", maybe because they are right next to each other, the burning sines on the transistor are just burning "facts" of the resistor. I changed the resistor for an equal valued one, and turned the amplifier on. It burned really quick (the new resistor). There is a shortcircuit somewhere. Viewing the schematics, could it be that the transistor is really dead and causing the resistor to burn? Everything else is looking as it was new. Transistor=T8, Resistor=R20

[52 Bill]

There is a shortcircuit somewhere. Viewing the schematics, could it be that the transistor is really dead and causing the resistor to burn? Everything else is looking as it was new. Transistor=T8, Resistor=R20

Welcome to the place. T8 may be damaged, but it is more likely that T9 or T10 is shorted and is causing R20 to burn. You need to test all of the transistors in the output section. Do you have an ohm meter or multi meter with a transistor test function?

You probably should build yourself a light bulb limiter as well.

[DrGonz78]

Yeah I had that exact amp for repair one time and never had anything like that happen. It was more the effects loop or some other jack that was bad and needed cleaning. In this case do just as Bill has said... Light bulb limiter first! Did any fuses pop? Could be something bad on the regulators too. Still test those main output transistors to see what we have here... Those things are tougher to get at on certain models and yes it is wise to take the output transistors out to test them out of circuit.

[Jazz P Bass]

And please do not connect a speaker at this time.
Wait until the output section is stable.

[JohnnyVieira]

Ok, I figured out what was wrong: one of the output transistors was shorted. Still, I changed every output transistor, replacing the BDV65 for TIP142 and the BDV64 for TIP147. The R20 isn't burning anymore. I changed the tube too.
The problem is still there: really low volume and distortion in every channel. What the hell could it be?

[g1]

Bad speaker or speaker cable? If this is the case you may end up blowing the power amp again, so be careful.

[JohnnyVieira]

No, I've already tested it with another one. The problem is the same if I plug the headphones, so, shoudln't it be in the pre amp section?

[Enzo]

Burnt looking parts are usually bad, but MOST bad parts look the same as good parts.

No. It might be, but might not be too. The headphones come from the same power amp that drives the speakers. If that power amp is bad, then both speaker and headphones will sound bad.

Isolate the problem. Turn the amp on, turn the reverb up half way. Now rock the amp to crash the reverb springs. Is the reverb spring noise also weak and distorted, or is it loud and clear?

Plug a guitar into the normal input jack. Plug another cord into the FX send and run it to some other amp. How does that FX send sound in the other amp, loud and strong, or weak and distorted?

Inside, check all power supplies. The power amp uses V+ and V- which are probably 35v or so, are both clean and up to voltage? The ICs run on +15 and -15, are those up to voltage and clean?

The tube has its own high voltage supply, so is the tube heater glowing? Is there high voltage on the tube plates? Did you try a different tube?

And finally, there is a J111 JFET at the input to the power amp, T16. It could be defective or stuck on. It is a mute. There must be voltage on its gate to turn it OFF. Is there? If not, that is a problem. If there is gate voltage, it can still be defective, so with the amp running measure RESISTANCE across it. Looking at the drawing a convenient place to do that would be from the right end of R1 to ground. If there is a low resistance, the JFET is stuck or shorted.

[JohnnyVieira]

Turn the amp on, turn the reverb up half way. Now rock the amp to crash the reverb springs. Is the reverb spring noise also weak and distorted, or is it loud and clear?

Yes, the problem still occurs.

Plug a guitar into the normal input jack. Plug another cord into the FX send and run it to some other amp. How does that FX send sound in the other amp, loud and strong, or weak and distorted?

I will check this.

The tube has its own high voltage supply, so is the tube heater glowing? Is there high voltage on the tube plates? Did you try a different tube?

Yes, I did. i bought a new one. It glows, but sounds the same.

And finally, there is a J111 JFET at the input to the power amp, T16. It could be defective or stuck on. It is a mute. There must be voltage on its gate to turn it OFF. Is there?

I will check this too.

Thank you very much Enzo!

By the way, in one hour I'll be with a friend with the same amp, but the combo version.
Is it ok if I connect his pre amp module to my power amp module and vice versa? This way I could figure out where is the problem: power or pre module.
The question is: is it safe for his amp?

[Enzo]

Another amp is another amp, same model or not. SO you can connect your FX send to his FX return, certainly.

[JohnnyVieira]

SO you can connect your FX send to his FX return, certainly

It sounded good, loud and strong!
This means that the problem is for sure in the power module, right?

[JohnnyVieira]

Ok, major discover: I measured the big resistors (the ones with a white thing above it, that I suppose is a "passive cooler") and got the following results:
R29 (0R33, 7W), measured 1 ohm with the amplifier off;
R24 (0R33, 5W), measured 1 ohm with the amplifier off;
R25 (0R33, 5W), measured 240 ohm with the amplifier off;
R19 (0R33, 5W), measured 170 ohm with the amplifier off;
R21 (0R33, 5W), measured 250 ohm with the amplifier off, and 310 ohm with the amplifier on.

T10 was dead (I already replaced it), and when I bought the amplifier it had TIP142 (equivalent to BDV65) instead of TIP147 (equivalent to BDV64) in T12 and T11, maybe it killed those resistors?

[52 Bill]

R21 (0R33, 5W), measured 250 ohm with the amplifier off, and 310 ohm with the amplifier on.

Don't try and read resistances with the amp turned on. You can do damage to the meter and the amp if you don't know what you are doing.

Test the output transistors for short circuits and replace the bad 5 watt 0R33 ballast resistors.