marshall valvestate vs100 issues

[methodofcontrol]

a friend of mine found this amp out by the curb for the garbage man while walking his dog. 1x12 combo w/ footswitch and everything. pretty clean condition.
now the only problem is that it doesn't work properly.
so he gave it to me to check out. i got the schematic a few months back but didn't really get around to it until now.
the initial issue is that it makes sound but it's almost inaudible. w/ all volume controls on 10 you can still speak comfortably over the guitar sound coming through.
so, i tried injecting my guitar signal at R1 on the power amp board to determine if the problem is pre- or poweramp related. my signal came through about as stong as it does from the input jack. so i thought...it must be the poweramp!
then i start checking the other obvious things like that one 12ax7 in there. it doesn't even look like it's glowing, so i pull it and try it in a known good amp and find it works fine. i put it back in the vs100 and turn off all of the lights and turn the amp on....sure enough, there's not even a faint glow from the tube, so i start checking voltages....i'm supposed to see 450vdc as the main rail in the preamp, i checked it at various points and it's reading Mv all around.
i checked the 500vdc i'm supposed to see just off the rectifier on the poweramp board and i get 38vdc.
so...does this mean the tranny is shot?
what's my next step?

[Enzo]

Well, the heater of the tube runs on either 6v or 12v, not the 500v, so find out why the heaters are not on first. There is a fuse just for them. If the B+ is not operating, check the B+ fuse. Pull it out of the clip and measure it with an ohm meter, no eyeball work please for either fuse.

And since this does not appear to be a multiplier PS, what AC voltage is across the wires? Orange wires I think? B+ should be about 1.4 times the AC.

[methodofcontrol]

tube heaters run through F2 to the tranny (BU1, BN1). the fuse looks more like a capacitor soldered onto the board, but in any case i measured for continuity between BU1 and pins 4, 5, and 9 of the tube socket. fuse seems fine.
the mains fuse between the AC cord and power switch is also fine according to the meter.
also, i checked the other fuse that looks like a cap, (F1) i checked continuity between OR1 and the anode of D5 and i get a short, so that seems fine as well.

still on the preamp board: from OR1 to ground i get 123vdc. at OR2 i get 120vdc.
AC voltage across OR1 and 2 is 226vac.


not sure what i just did...but i looked at the tube and found that now it's glowing.and it has been for a bit as it's warm.i plugged the guitar in and the amp appears to be working fine now. voltages are still as i reported previously so far.

i will go play the amp for a bit and report back. please let me know if you have other suggestions. i definitely heard a pop when i was measuring voltages...i thought perhaps my probe wandered and touched something i wasn't aiming for. i'll check for bad solders again when i'm done playing if everything still works well.

[Enzo]

You restored a connection. SOmewhere in ther heater line. COUldn't tell you where. I would be flexing the board and trying to cause the failure again. Until you find the actual problem and repair it, you will always have an intermittent amp.


If the heaters are working now, then their fuse is OK, it would not have healed itself if it were blown before. However, when you measured it while still in the circuit, your meter was not just measuring IT< you also had the rest of the heater circuit in parallel. So even if you removed that fuse, your meter would still read continuity on the board. In other words, your meter would measure the path from one end of the fuse, back through the very low resistance of the winding, through the low resistance of hte tube heater and thus to the far end of the fuse. In heater circuits ESPECIALLY you need to pull the fuse to check it.

[black_labb]

the (-)38 volts you measured is probably the bias (you probably accidentally measured the bias as opposed to the high voltage.

as the others say, it seems to be an issue with the heater.

[Enzo]

BL, this is a "Valvestate" Marshall. One lonesome 12AX7 in a solid state amp.

[drewl]

There's nowhere near 500v in that amp...
anyway, clean all the pots and jacks, reflow critical pc board connections..

[Enzo]

More like 300v, but still plenty. The filters are 450v caps.

[methodofcontrol]

ok guys...so it looks like the specific problem was the brown wire coming off the tranny, (i forget if it was BN1 or BU1 on the schem).
once i flipped the pre amp board i saw bare wire and a thin shell of solder, i reheated and applied more solder the firmly connect it. while i was in there i figured i'd toughen up the amp some, so i resoldered every wiring harness pin attached to the board and every component that stuck out the face or rear of the amp, (jacks, pots, buttons, etc...).
since everything has been beefed up and inspected i haven't had any problems.
thanks for the help! now i have to get this god-awful thing out of my house before my tube amps beat it up.

[Enzo]

BN1 and BU1 to me mean Brown 1 and Blue 1, respectively.

[Amp Kat]

Take off that awful tube retainer as it digs into the 15 volt supply and shorts out the regulators. Everyone of these I've seen does that. Check your 15 volt regulators LM78 &7915 +/- 15 Volt regulators and make sure they are supplying the op-amps with those voltages and nothing being drug down.

[methodofcontrol]

thanks for the tip, i'll check those regulators. so far so good...no problems since resoldering.

[axeshredder42]

Do you have an electronic copy of the VS100 schematic you can send me? Having some issues with my amp.

Thanks

[methodofcontrol]

i do have a schematic i can email you. it's a PDF if i remember correctly. i just need an email address.

[Mike551]

Method...Could you email me a copy of that scematic as well? I have the same amp and its givin me some problems