Amp troubleshooting question

Alright I have a Peavey Heritage vtx212 that I got for cheap and have been practicing through for the last few months. I was playing through it last sunday and stopped putting out any sound at all. The power led was still on, but status was off. I pulled it apart and discovered that my F1 internal fuse was blown. (http://www.schematicheaven.com/newamps/peavey_heritage_vtx.pdf)

Fast foward, I replaced the fuse, and ended up with a very over powering hum, which led me to believe my filter caps were shot. I pulled two of them and one tested good, the other was kinda sketchy, so I replaced all four. The hum remains, and is unchanged. Any help on where to start looking would be great. Thanks!

Btw, the hum is unchanged by any of the control knobs, or tube swapping. The tone of it does change slightly if I switch from low to high power. Thanks again, any direction as to where to start testing would be wonderful...

Ground fault? Could this be why the fuse went south in the first place? I'm a tron noob, but the auto & airplane mech in me makes me look for ground problems before I do anything else.

I'm way rusty on troubleshooting, but from what you're describing I wonder if you've got AC bleeding into the output (power tube) section. I suppose it could be in the power supply, but I'd look at blocking caps on the power tubes. I think I'd also be very careful about turning on until you get this diagnosed.

I bypassed the ground selection switch and had no success. The outlet I am using I know is grounded so I am pretty sure it is not a ground issue.

What caps are you referring to as blocking caps? C74 and C75? or C67? Forgive my ignorance, I have built few pedals and even make my own circuit boards but this is my first time to really get into an amp....

Okay, might have been overkill but I removed and tested every single diode and capacitor and resistor on the power board and everything tested good. (well I didn't test the transistor) All the switches (ground included) are working.

Now for the new stuff, I tested the power going to the tube power board and it is infact getting ac power through to there 47vac on one bank and 16vac on the other, I am worried it may be a transformer issue. Tomorrow I will try pulling some of the caps in the driver circuit and testing them. Any suggestions?

Loud hum is almost always power supply..somewhere there

is diode/cap/resistor out of spec..the problem with this darn

Peavey stuff (yeah I've got 'em we've all got 'em) is that

they mix transistors in with the tubes, which means you've

got double the power supply problem: power to the tubes

or transistors all require the diode/cap/resistor circuitry

and from lookin at that schemo, if yer not up to volt tracing

w/scope and multimeter, the task is too tricky..transistors

do crazy things in Peavey circuits..pony up the bucks for

a service repair, $65 for the hour and $15 parts. Now if

you like a 2x12 combo, save up for a vintage Fender and

you will be able to fix it all yourself in time, cause the're

just simple beautiful point-to-point layouts, all the mods

are available, parts are everywhere..leave the transistors

for the stompboxes and the consoles

If your 5 amp AC mains fuse is not open, then the power transformer has a good chance of being ok.

You mention the F1 internal fuse ( 1 amp ) being open.

Yes, the status LED will not illuminate in that case.

This is the fuse for the high voltage supply as shown on the schematic.

You mention testing components on the ''power supply'' board and nothing remarkable found.

The transistor ( Q1 ) that feeds to the status LED would not cause a open internal F1.

It has 440 K ohm in series with the base and is essentially just a switch to power the status LED.

Moving into the output valve section, several things.........

Remove the output valves and check from the valve plate pins to chassis common.

If a very low ohms is read on your meter - the primary suspect would be one or both of the damper diodes are shorted.

It is possible the output transformer could have a short to chassis on the B+ plate supply but I feel that is unlikely.

If no low ohms to chassis is read with the valves out, then I have seen numerous amps like this one have a shorted output valve. It could have been an intermittant arc within one or more of the valves.

What frequently happens is the cathode drive transistors will get blown when the tube shorts.

The outputs have a cathode drive instead of control grid drive.

The control grids have a fixed voltage feed and are bypassed to chassis common via C67 electrolytic cap.

Back to the cathode drive - if you have a crapped cathode transistor(s) - it can remove the voltage required on the cathode that is essential for keeping the control grid negative with respect to the cathode.

This is an unusual circuit but is how they drive the outputs, directly, with transistors.

Other peripheral components need checked in that cathode drive circuit if you get to that point of where the problem is located.

I have an amp here very similar, that I will eventually rework and create a more conventional drive with a 12AX7.

That rework is beyond the scope of a brief description.

Hope some of this may help.........

Good luck

Thanks djcgtr, I acctually understand how the damn thing works now... well sorta ...

On pins 1-8 of V1 and V2, I read aprox. 4k Ohm resistance, on the same pins for V3 and V4 I get infinity. This would mean I have a problem with the Q7 6465, and probably other components in that circuit as well, correct?

All of the other pins read really high, except for 3 which read 0 ohms.

Thanks djcgtr, I acctually understand how the damn thing works now... well sorta ... On pins 1-8 of V1 and V2, I read aprox. 4k Ohm resistance, on the same pins for V3 and V4 I get infinity. This would mean I have a problem with the Q7 6465, and probably other components in that circuit as well, correct? All of the other pins read really high, except for 3 which read 0 ohms.

V1 V2 Pins 1 & 8 with anything but infinity to chassis would point to a blown Q7.

Yes - I would say you are correct.

Since Q 7 has no power to be conducting - then you should not read anything collector to emitter..........

Good chance Q9 is maybe ok.......if you change one - change both Q's 7 and 9.

And yes check the peripheral circuit in the drive as you said.

If pin 3 shows zero ohms.........

That is the same as a clip lead from the plates B+ connecting to chassis common.

That really shorts the high voltage supply out. (will blow F1)

Looks like unhook CR 1 and CR 2 in the plate circuit and check them. One or both are possibly shorted - change both of them regardless, if one is bad.

Unsure if Peavey has the replacements still around. You would have to call them. Use the numbers like #2873 diode(s) when inquiring.

Then - I would suggest a total valve change once getting the drive and short problems out of the way.

Hate to see a new drive get crapped with a bad valve.

Sometimes a quick flash in a bad valve might crap the drive before the B+ fuse opens.......

This drive is unique, and it does work.

I have repaired many of these over the years, I don't like it but that is the design.

One suggestion, from me personally, is don't figure on going for ''power valve distortion'', by massive cranking volume and/or a power brake implementation.

The output section is a ultra linear design output ( xfmr with screen grid taps ).

It was made to be fairly clean in the output stage.

Let preamp gain or external pedal etc do your overdrive sound.....

You won't get the same results like you would in a Marshall or etc etc amp, by ''clipping'' or saturating the output valves........