Marshall VS100

[Valvehead]

vs100 head = no audio , just loud low hum . I smelled something getting hot , found that R21 is getting super hot , but still reads 180 ohms. I tried swapping out the 2 regulators Reg1 / Reg2 with new ones , no change. R21 is not going to ground
On the schematic where it says +LT i have +40vdc
If i unplug CON3 the resistor doesnt get hot . CON3 goes to the output pcb CON14

ill attach a pic of the reg readings

what to check next ?

side note= Reg 1 is a 7812CT ,, all i had was a L7812...is that ok?

https://www.thetubestore.com/lib/the...-Schematic.pdf

[TomCarlos]

For reference -

Reg 1 is a 7812. Here is the datasheet.

Reg 2 is a 7912. Here is the datasheet.

Just want to make sure you used a negative regulator for Reg 2.

Looks like you do not have the correct input voltages to make these work.

[g1]

R20 should be getting hot too. Is it open? Or are you missing the -40V rail?

[Valvehead]

Correction--both resistors are getting hot..in about 15 seconds they are cooking at around 200 degrees . both are 184 ohms

i do have +40v / -40v at them . What voltages should i see at reg1/reg2 ? +15 / -15 ?

[g1]

Correction--both resistors are getting hot..in about 15 seconds they are cooking at around 200 degrees . both are 184 ohms

i do have +40v / -40v at them

Can you recheck voltages at both regulators? If 7912 has -3V at input, I'd expect R20 to be quite a bit hotter than R21.
In any case, if you are using IR thermometer, also check for any hot IC's.

[TomCarlos]

Hmmm... here is something interesting. The 7812 and 7912 are 12 volt pos and neg regulators. But the schematic shows plus and minus 15 volts. Can you look at the original part and tell us what it shows on the marking?

[g1]

The schematic has an error(s). +/- 12V is correct for the low voltage regs. and supplies.
The error often comes up in the VS100 threads.

[Valvehead]

ok..i am a complete idiot..........i had Reg 2 installed backwards....



here is what im getting at the regs after fixing that - could C9/10/11/12 be causing mayhem?

[Enzo]

R20/21, they serve to drop voltage entering the regulators. Measure voltage at both ends of each. Measure RIGHT AT THE PART, not elsewhere. Your negative Vreg is not getting any voltage. Well really neither is now. 7v is not enough. The Vreg needs a good 3v or more to work. I mean 3v above the rated volts. so a 12v Vreg wants a good 15-18 volts or so going in. If you only feed it 7v it cannot possibly put out a smooth 12v, eh?

You should have 40v on one end and something in the 15-20v range on the other. Now if we have plenty of voltage on both ends, then we look to see why it never reaches the regulators. If one end is 40v and the other only 7, then we can assume it is being loaded down.

Your 12v rails power the ICs. All over the schematic where it says 15v, it is really 12v. But if BOTH supplies are loaded down to 7v, then the most likely thing is an IC. SO after it has been on a brief warm up, touch each IC and see if it is really hot. ICs should not get hot. If you do find a hot one, remove it, now the 12v return? Or not?

[Valvehead]

r20 -7.8v / - 40v
r21 +7.3v / +40v

i went over the entire amp with the laser , nothing else is getting hot . Everything was 55 to 65 degrees . After 30 seconds or so you can smell those resistors burning and actually saw smoke out of 1 of them today ...they are over 200 degrees and i saw 275 at one point

[g1]

If the IC has a dead short it will not get hot. You will probably have to desolder the supply pins of all the IC's and see if the regulators then go to proper voltages.

[g1]

Before you desolder any IC pins on the preamp board, disconnect the daughter board that only has 3 IC's on it, see if that makes a difference at the regulators.

[Valvehead]

Unplugged that , now i have +7.6v / -8.3v

[The Dude]

I agree with others. Sounds like a shorted op amp since both supplies are effected. If you have a good desoldering tool, it's not too difficult to unsolder pins 4 & 8 of op amps until the voltages come up. If not, it can be more difficult. Most commonly, the first op amp from the input is at fault from someone plugging in a signal that's far too hot, so I'd start by unsoldering pins 4 & 8 of IC4, or remove it completely and recheck supply voltages. Shorted rails are a PITA because it can be anything attached to the rails.

[Valvehead]

ive got a solder sucker.......i guess ill try it