Peavey VTM 120-- please help

[jarvis1227]

I have a peavey VTM 120 from the 80's. The power and standby lights come on but i have no sound. the preamp tubes are lit but the power tubes are not!! all fuses check out ok. the speaker cable and cabinet is good, the guitar and cable are good. i swapped all tubes with replacements that i have to no avail. i have had this problem before but i don't feel like spending almost 200 and waiting 2 weeks for repair.

here's what happened. the past few times i've really pushed the amp with some overdrive and distortion my volume would all of a sudden go way down. i could shut the amp off for a few minutes and i'd be back in business. sometimes it would happen only after 5 minutes of playing other times after an hour. now today i can't get any sound out of the amp at all.

please help.

thanks!!

[Enzo]

It has nothing to do with how you played through it. The heater voltage is missing from the power tubes, that's all. In my shop this would probably cost you about $60. I can't imagine how someone could get the repair bill up to $200.

There is an 8-pin ribbon cable from the preamp tube board to the power amp tube board. The two pins on ine end are the heater connections sending the heater supply from the preamp board to the power tube board. I'd bet my lunch money that on one end or the other of that ribbon cable there is a burnt pin or pins. SImply repairing that connector or just soldering wires in its place should solve the problem.

[jarvis1227]

Enzo,

you were right, of course..... where do i send the $60.00? j/k. but seriously the places that repair amps around here are really expensive - over $100 just to put it on their bench.

thanks a ton

[Gtr_tech]

Yes its the heater ribbon. Request that it gets hardwired and you won't see that problem again. PV's screw up by using that in the first place for high-ish continuous current.

[WholeToneMusic]

Spandex, mullets and molex

Back in the 80's some companies decided to use Molex connectors on the heaters to only have them melt. If I see one that comes my way I just hard wire it.

[rw2k73]

I apologize for resurrecting an old thread, but I have the exact same symptom with my VTM 120 that I recently acquired.

I played through it for a while and shut it down. After an hour break, I came back and turned it on. After letting it warm up, I turned the standby off and there was no sound, no hiss, nothing. There was also no glow from the power tubes. When I turned it back on standby, the green status light wouldn't go out either.

My question is, if I notice that it is in fact the ribbon cable, what type and gauge of wire would you use to hard wire it?

[Phostenix]

Check out this series of videos from Lee Jackson on youtube on how to service Ampeg amps that had the same type of heater wire issue.

The first one is here:
[ame="http://www.youtube.com/watch?v=Pg2IrwZ6b4U"]YouTube - Ampeg VL-1002 Service and Bias by Lee Jackson Part 1[/ame]

There's also a series of videos on his XLS-1000 amps that I think have the same issue (it's been a while since I watched them).

[ame="http://www.youtube.com/watch?v=yUY8NQ36PCk"]YouTube - Free XLS-1000 Service and Tube Heater Mod by Lee Jackson PT1[/ame]

[Gtr_tech]

Hah....his bench looks like mine does....piles of parts I'm too lazy to clean up

I don't recommend soldering to the header pins like he says can be done. Sure its possible, but normally the reason those connectors fail (aside from heat) is cracked solder (which can create more heat). So kids.....don't solder to those pins. Pull them out (you can heat the pin from the solder side of the board and pull out individual pins from the plastic keeper) and solder some 18awg wire from each end of the connector if there's 2 ends.

[rw2k73]

Thanks. This helps a lot.

[rw2k73]

Okay, upon visual inspection, the MOLEX connectors and wires look good; the fuses all look good; and the filter caps look good; however, I believe the output transformer is bad. You can see in the pic that someone welded some ears onto a replacement at some point. You can also see several blisters and hot spots on the tops.

Is this a pretty straight forward fix? I'm prepared to do it. I'm just not sure if there are obstacles that I should be aware of.

I'm going to call Peavey for info. on the part.

[Enzo]

Well, let us stop and think for a moment. You have tube heaters that don;t light up, right? How can that be the fault of the output transformer?

Transformer in the photo looks pretty stock to me. Peavey welded mounting plates on a lot of transformers. And the drippy bubbly stuff just looks like sealer to me.

[rw2k73]

Well, let us stop and think for a moment. You have tube heaters that don;t light up, right? How can that be the fault of the output transformer?

Transformer in the photo looks pretty stock to me. Peavey welded mounting plates on a lot of transformers. And the drippy bubbly stuff just looks like sealer to me.

What are you thinking?

[Enzo]

I am thinking I am getting older by the day, and my joints all hurt way too much.


Or did you mean something else?

If your heaters are not glowing, then nothing else is going to happen. Kinda like if the battery in your car is dead, nothing else is going to work.



Please verify, are you reporting that the orange glowing heaters in the center of each tube are dark... not glowing?

If so, look at the power supply board inside the amp.

PLEASE BE CAREFUL, the voltages inside the amp can KILL YOU.

The power supply is the board with the several fuses on it. Look on the schematic for the board layout. Two heavey wires from the power transformer will come to connector J7. I believe they are yellow wires. Set your meter to AC volts and probe down into Molex J7. Got 6VAC?

Next to that connector is the 8 amp fuse F4. It is probably a ceramic fuse, so you can;t see into it. PULL that fuse from its holder (with power off), and measure its resistance. If it checks open, the fuse is blown, if there is continuity through the fuse, it is OK, and reinstall it to its clip. You cannopt successfully measure the fuse still in its clip.

Immediately next to that fuse is four pin Molex J4. Follow the traces on the drawing from J7 and from F4 and you see they gop to the two pins on J4 furthest from J7. That sends the 6VAC through the little gray ribbon to the end of the main preamp tube board nearby. Got 6VAC on the two end pins at both ends of that short ribbon?

On the preamp tube board, that four pin Molex was on the end, and right around the corner from it is an 8-pin Molex with a ribbon to the power tube board. Pins 7 and 8 of that ribbon carry the heater voltage from the preamp tube board over to the power tube board. Again we want to determine if there is 6VAC at pins 7 and 8 of each end of that ribbon.

Now here are some clues. Look closely, if the power tubes are dark, but the preamp tubes actually have two little glow spots in each, then that means the 6V is getting through the four pin cable to the preamp tubes, but not making it through the 8-pin one.

Now both these ribbon cables have a Molex at each end, and that heater voltage pin on either end of either one could be bad. Usually the white plastic Molex housing is discolored around the pin. But the pins in the Molex housings are all female pins, but don;t forget there is a male for each, and that means you will have to raise tjhe boards as needed to expose the underside to inspect the solder to those male pins.

[Enzo]

Oh, and that 6VAC? I am measuring that from pin to pin, NOT TO GROUND. So there are two pins on J7, so 6VAC from pin to pin. And so on.

[Gtr_tech]

Okay, upon visual inspection, the MOLEX connectors and wires look good; the fuses all look good; and the filter caps look good; however, I believe the output transformer is bad. You can see in the pic that someone welded some ears onto a replacement at some point. You can also see several blisters and hot spots on the tops.

Is this a pretty straight forward fix? I'm prepared to do it. I'm just not sure if there are obstacles that I should be aware of.

I'm going to call Peavey for info. on the part.

That xfmr's just fine. PV liked to weld mtng brackets on xfmrs for some reason. Look at the pwr xfmr....welded mounts. The bubbly stuff is just varnish....every transformer has it. It will appear bubbly if its really slathered on.

All you need to do is get the 6.3vac to the pwr tube heaters and you'll be back in business. Heater pins on a 6L6 are 2 and 7. Use yer meter set to either ohms or continuity. Pull *all* tubes from thier sockets. Remove the 2 wire connector J7 (it has 2 yellow wires on it). Verify that pin 7 on any pwr tube socket reads close to "0" ohms or gives a continuity beep at one pin of J7. Repeat from pin 2. Verify with your meter that fuse F4 is good.

This is a common problem woth PV tube stuff because they insist on using those Molex header connectors for ease of construction.