dead ampeg, need help troubleshooting

[Joe N]

I originally repaired this amp because I thought it had blown it's output transformer. I replaced the OT, the output jacks, the fried screen resistors, and patched the carbonized traces. I got some garbled sound out of it for about a minute, and then nothing again, not even hum. nothing smells burnt inside, and there are no more carbonized traces. I hope the ot isn't fried again. all the b+ voltages with tubes loading it is above 400vdc, even on the preamp sockets which isn't correct. any ideas on what could be going on with it, or where to start troubleshooting?. It is an Ampeg VL501. I used the schemes found on the blueguitar FTP site to trace the circuit. (the schemes are for the vl1002, but the boards are nearly the same)

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[stokes]

"all the b+ voltages with tubes loading it is above 400vdc, even on the preamp sockets which isn't correct."If by this you mean all the B+caps have identical volts,then your tubes are not loading down the power supply for some reason.Could be open plate or cathode resistors.

[Joe N]

"all the b+ voltages with tubes loading it is above 400vdc, even on the preamp sockets which isn't correct."If by this you mean all the B+caps have identical volts,then your tubes are not loading down the power supply for some reason.Could be open plate or cathode resistors.

The power tubes have about 500+ on thier plates, and the preamp tubes have 400+. None of the RP's or RK's look bad, and they all measure fine. what worries me is that ther is no sound period! no hum, I hope the ot is fine

[J Martin]

Check your grounding. There may still be an open trace somewhere. It would be strange to blow ALL the cathode resistors.

[Mark Black]

Just a thought -

From my limited experience with the VL's - look for any QD wiring connectors which show signs of browning due to heat at the wire crimp. And for that matter tug on the wires to all of them & make sure the wire doesn't just pull right out of the crimped connection.

Also my "usual" solder caveats apply heavily to these (see "Marshall DSL" in earlier posts)...

[Enzo]

The first thing to mind if none of the tubes are conducting is not all the cathode resistors burning out at once, it would be a loss of heater current.

Are all the tubes lighting up?

If not, follow the wiries from the PT. They will have to meet the board somewhere, and quite possibly move from board to board via ribbon cable or other wiring and connectors. These high current connections like to burn out. I guess that was what Mark was talking about.

If there is a 400v B+ and it is still 400v at the plate of a preamp tube, check the cathode. If it is zero - and at that point I'd be surprised if not - then teh tube is not conducting. Very few reasons for that condition. And if it is more than one, then I can think of only two - heaters and ground return. I vote heaters.

[Joe N]

The first thing to mind if none of the tubes are conducting is not all the cathode resistors burning out at once, it would be a loss of heater current.

Are all the tubes lighting up?

If not, follow the wiries from the PT. They will have to meet the board somewhere, and quite possibly move from board to board via ribbon cable or other wiring and connectors. These high current connections like to burn out. I guess that was what Mark was talking about.

If there is a 400v B+ and it is still 400v at the plate of a preamp tube, check the cathode. If it is zero - and at that point I'd be surprised if not - then teh tube is not conducting. Very few reasons for that condition. And if it is more than one, then I can think of only two - heaters and ground return. I vote heaters.

Enzo,
al the tubes light, and I deffinatley have voltages at the cathodes. I also read about 430 vdc feeding the preamp tubes, and there is drop across those resistors at the tube plates, the voltages also do decrease slightly from PI to input tube. I also read about 7v for the heater voltages.

[Enzo]

OK then circuits are working. Start at the start now.

Speaker OK? Unplug the speaker from the amp and touch a 9v battery to the plug tip and sleeve momentarily. Spkr should go thump each time you touch. DOn't hold it there though. This tests spkr AND wire.

Connect a different speaker to the amp output, just to check.

You had 500v on the power tube plates, but how about the screens? Check every one. And verify the cathodes are connected to ground.

There is a switch marked power attenuator, is it OFF?

Verify the wiring from the OT to the speaker out jacks.

With the amp running, touch something metal to the power tube grids - a screwdriver or your meter probe. SHould make some sound if the power stage is running.

The PI tube is next to the power tubes. Touch pin 7 on it with something metal. SHould make more hum.

Work back through the amp.

Alternatively, apply steady signal to input, center controls, and trace signal through the stages.

[Joe N]

OK then circuits are working. Start at the start now.

Speaker OK? Unplug the speaker from the amp and touch a 9v battery to the plug tip and sleeve momentarily. Spkr should go thump each time you touch. DOn't hold it there though. This tests spkr AND wire.

Connect a different speaker to the amp output, just to check.

You had 500v on the power tube plates, but how about the screens? Check every one. And verify the cathodes are connected to ground.

There is a switch marked power attenuator, is it OFF?

Verify the wiring from the OT to the speaker out jacks.

With the amp running, touch something metal to the power tube grids - a screwdriver or your meter probe. SHould make some sound if the power stage is running.

The PI tube is next to the power tubes. Touch pin 7 on it with something metal. SHould make more hum.

Work back through the amp.

Alternatively, apply steady signal to input, center controls, and trace signal through the stages.

Enzo,

you have been a great help so far, and I really appreciate it. Everybody who has responded has been. What will work for putting a steady signal through? the only thing I can come up with is a Metronome set to an A-440 reference pitch. will this be strong enough to measure? if so, what Vac range should I be looking for?

thanks again

Joe.

[Amp Kat]

It wants abouit 100 to 150 mv RMS but anything lower or higher will work . You may also just have to crank it up or down some to compensate.

[Enzo]

That's fine. Even a steady hum works, like toughing the end of a guitar cord sound. When I want a go/no go test, I use my finger all the time. I touvh the input jack, or touch something with a screwdriver. It injects the hum noise. That's good enough to see if the thing functions or not. In the shop, I have a small stereo tunes to a music station. From a line out on the rear panel, I run a cable around to the bench. That radio music is my standard test signal working on amps. I even get stereo. Then when I am done, I play the guitar through it. Point is most anything works, especially if you just want to get sound or not. Like your case. Got a synth? patch to some organ setting or somethign and stick a matchbook between a couple keys to wedge one down. Ooutput from the synth is now a test tone. Just exactly like your metronome is.

I don't know what voltages to expect, but any reasonable input level should amplify to several volts after the first stage. Turn the test tone off and on and watch the voltage reading. If it goes up and down at the same time, you are reading signal.

But please do all the other things first, they tell you so much about what you are facing.

Believe me, if someone brings a silent amp to my shop, I am not going to get the sdcope on it and start tracing test signals. I am going to do all those little diagnostics I mentioned first. DOn't need a scope to tell me the speaker died.