Marshall Valvestate problems

[shaunp]

Hi all,

I've just resoldered a loose input jack on my Marshall VS100R and had to remove and reattach the PCBs to do it. A new problem popped up afterwards and I'm at a loss about what the cause is.

The problem is that while the amp sounds good with the reverb knobs turned down, but with them turned up I get a noisy, crackling sound, even with no instrument cable attached. Also, it smells like a component is overheating after ~15 seconds of power-on - regardless of the reverb knobs settings - so I don't want to plug it in for more than a few seconds to get voltage/current measurements for the repair work.

I'm really hoping this'll be a quick one for you amp gurus out there. Any ideas on what I can do or where to look (I've got a show next weekend and our tones are dialed in for this amp)? I found a link to the schematic if it'll help.

http://www.drtube.com/schematics/mar...v100-60-02.pdf

This may be a clue - I've noticed that the resistors R21 and R20 (180ohms @ 3W) in the voltage regulator circuit are dissipating over their wattage range (between 4-5W) since I measure 30V and -28V DC dropped over them, respectively. I'm thinking there's a problem with the caps in the regulator circuit keeping them from charging to 100% of the supply voltage. Possibly there's a problem with the regulators themselves, but I'm very rusty on my electronics know-how. I would love to get a second opinion before I pull out the PCBs again.

The schematic has R21 and R20 on the bottom right of page 2.
http://www.drtube.com/schematics/mar...v100-60-02.pdf (same PDF)

I've found that PSU+VE is +40V DC and PSU-VE is -40V DC so I'm really starting to think that the capacitors C9,C10,C11,C12 and possibly the voltage regulators REG1 and REG2 need replacement. Does this jive? See anything I'm missing?

Thanks very much in advance,
Shaun

[Enzo]

Yes, unplug the reverb pan and see if it stops. If you got the cables reversed, you will ground off the reverb drive and it won't be happy. The quick way to sort it out is to pull the cables from the reverb pan, turn the reverb up part way and touch the tip of each cord. The one that hums when you touch it goes into the OUTPUT jack of the reverb pan.

But I fear the damage is already done. Just make sure to figure out which is which before connecting the pan once this is fixed.

ANd you may have got it right and the reverb chip just decided to fail now on its own.

This amp isn't old enough for ALL those caps to fail. Those two resistors get hot because too much current is flowing.

IC1 the reverb chip probably died, so that is the noise you hear when you turn up the reverb. The IC is powered up whether the reverb control is advanced or not, so if it is cooked, it is dragging the supplies down - whuch heats those resistors.

If the power rails are 40 volts, and 30 volts are dropped across those resistors, the 12 volt regulators can't possibly put out their rated voltage.

The trick? After the amp is on a moment, touch each IC on the board - start with the reverb chip just to humor me. The one that burns your finger is bad. Visually inspect the chips. ANy IC with a crack in it is bad. Your power supply voltages will restore themselves once the errant load of a bad chip is removed from them.

[wolf1419]

VS65R Preamp o.k. - no output to phones or spkr

Sorry, I need a new post on VS65R

[wolf1419]

Amp treble control not functioning....HELP!!!!

See Treble control dead!!!

[fbodysilly]

Hi all,

I've just resoldered a loose input jack on my Marshall VS100R and had to remove and reattach the PCBs to do it. A new problem popped up afterwards and I'm at a loss about what the cause is.

The problem is that while the amp sounds good with the reverb knobs turned down, but with them turned up I get a noisy, crackling sound, even with no instrument cable attached. Also, it smells like a component is overheating after ~15 seconds of power-on - regardless of the reverb knobs settings - so I don't want to plug it in for more than a few seconds to get voltage/current measurements for the repair work.

I'm really hoping this'll be a quick one for you amp gurus out there. Any ideas on what I can do or where to look (I've got a show next weekend and our tones are dialed in for this amp)? I found a link to the schematic if it'll help.

http://www.drtube.com/schematics/mar...v100-60-02.pdf

This may be a clue - I've noticed that the resistors R21 and R20 (180ohms @ 3W) in the voltage regulator circuit are dissipating over their wattage range (between 4-5W) since I measure 30V and -28V DC dropped over them, respectively. I'm thinking there's a problem with the caps in the regulator circuit keeping them from charging to 100% of the supply voltage. Possibly there's a problem with the regulators themselves, but I'm very rusty on my electronics know-how. I would love to get a second opinion before I pull out the PCBs again.

The schematic has R21 and R20 on the bottom right of page 2.
http://www.drtube.com/schematics/mar...v100-60-02.pdf (same PDF)

I've found that PSU+VE is +40V DC and PSU-VE is -40V DC so I'm really starting to think that the capacitors C9,C10,C11,C12 and possibly the voltage regulators REG1 and REG2 need replacement. Does this jive? See anything I'm missing?

Thanks very much in advance,
Shaun

I too just resolidered an input jack and have discovered problems.
I just picked up this v100 combo amp and knew about the input jack going in, (it had screeching feedback noise which stopped when you grabbed the guitar plug).
I also knew note the reverb tank was bad (both in and out coils open) I ordered one and am waiting for it.
So...
I was happy when I found the bad joint, and upon completing the correction tested the head outside the cabinet.
It worked great!
Then I screwed it into the cabinet. The power light came on but the channel led's did not and no sound. My happiness faded.
I pulled the head and plugged it in again. nothing. I tapped on the Power amp board lightly and it came on. so I decided to pull the board an look for bad joints again.
Found the 30v AC connector a little loose (didn't think it could cause a problem) and one transistor (T11 or T12) looking suspicious.
I touched them up and put it back together. Still the same-power light no channel selector and no sound.
Transformer outputs seem fine. I checked the channel selector switch... it's ok.
I had a extra valve and tried that... Still the same. Tested the glass, F1 and F2... all ok.
I tested the 3 ribbon connectors... all seem fine.
I pulled the reverb PCB and noticed the IC has already been replaced once.
The amp was working without the tank. doesn't seem the reverb pcb could take out the preamp.
Any Ideas?
Thanks in advance.
Tony