Ampeg VT120 help pleae

[phat_vinny]

Hi guys. I am new to the forum but not too new to tube amps. I have an Ampeg VT120 that I like the sound on for about 2 minutes. After that it goes to channel A by itself and then pukes out for sound. If I leave it on and play for a while the preamp goes back to the channel it shoud be on. I know the problem is in the preamp as I can plug a pedal into the line in and drive the power amp nuts.


It seems to be some where in the switching. I am thinking electrolytic filter cap for the "F" power supply may be the problem. I'd like to have other thoughts about this if I may before I open it up as it is a chore to lift, open and work on.

Thanks in advance for any assistance.

Vince

[Enzo]

Why do you think it is the F filter cap?

if you think that, then measure the voltage across it. is it more or less 10VDC? Set your meter to AC volts and remeasure. Is it about zero volts AC, as it should be? or scope it - is it free of ripple?

If the 10v rail is OK then, move on. Does it actually go to channel A, switching control functions and all, or does it just turn on Ch A light? When it goes to A by itself, does the Pull A switch have any effect? DO ALL the channel A controls now have effect, or only some? DO ALL the channel B controls stop functioning or do some remain live?

Look at the switching schematic. Note the footswitch jack - FS from now on - has cutout contacts for the tip and ring. The cutouts are grounded through the pull switches. With those switches closed, measure resistance to ground from either cutout contact. SHould be about zero ohms, right? Is it? We are testing the switches and the circuit board pathways. Then measure to ground from the tip and ring contacts themselves on that FS jack. Do not apply downward pressure that might close loose contacts. Measure from the base of the legs. Still zero ohms? In fact I prefer measuring from the solder side. I prefer that because I need to check the solder ther anyway, and I am assured my probe pressure will not affect the closing of the FS contacts.

ANything more than half an ohm across the FS cutout contacts is not good. They then must be cleaned or the jack replaced.

Note the drawing shows 0v or 3v at each contact. Does activiating those switches or using the FS make those voltage changes?

All the switching is done with LDRs and JFETs. Any one of these could be bad. But they also need to be controlled. SO look at all those bipolar xstrs Q15-20. Their bases are controlled by the 0-3v signals. Check at the collector of each. The collector circuits all feed from the +10v. SO when each xstr is OFF, then that collector should rise to about +10v. Q15,16 excepted. When each xstr is ON, then the collector should drop to a low voltage. Q15,17,18 to about zero, the others a little above due to the diodes in their emitter leads. Note those collectors that control LDRs by "OC" numbers. If the collector circuits don;t drive the LDRs they can't work right. Of course the LED side of an LDR can work without the photocell side working, but that is the next step. Note also the collectors of some xstrs also control the gates of JFETs with those numbered refernces 1,2,3,4.

When it sounds crappy, shake the reverb to make the springs crash. Does that sound loud, strong, and clear like normal, or is it crappy too? probably OK, but it does check one stage further back than the FX jacks.

JFETs are normally ON and have a low resistance source to drain. I don't know, maybe 100-200 ohms. When you apply a voltage to their gate they turn OFF and go to a very high resistance state. SO by clicking the pull switches or stomping the FS, you can turn them opff and on, hopefully, and just measure resistance across each as you do. Yes, measure resistacne while the amp is running. Just don;t measure "resistance" anyplace else while it is running. One side of each is grounded already. The "hot" side of all of them should be free of DC. If you find any DC there. you probably have a leaky JFET - the gate voltage leaking through onto the signal path. And that could explpain your crappy sound. There are only 5 of them outside of the reverb, check them.

Then the LDRs the resistor part goes to a relatively low resistance when the LED inside is lit. I doubt they are bad, but you can check the same way your JFET was checked.

And all that switching stuff aside, you also have DC voltages to check at each triode.

[phat_vinny]

Enzo You are so right

You are so right. I just jumped to a conclusion. Why did I suspect that filter cap? I guess it was because I see so many electrolytic caps fail. I did replace it and now it doesn't jump to channel A but it does have other issues. Lucky guess. I should have followed proper troubleshooting processes. I will for the rest of the issues.

Thanks for the insight on the surrounding components. I will check them before I put that beast back together. Do you know of a simple and safe way to bypass the opto-isolators temporarily? Can I just jumper the LDR side with a small resistor for troubleshooting?

I guess sitting around for 4 years in an Arizona garage will do take its toll on equipment.

Now that I have the switching working it's time to find out where the crackle and noise is coming from...

I will post the findings and Thank You again.

[luthierwnc]

I had one of those I took in trade for a while about 11 years ago. That amp, and the VT60, both had famously bad traces around the filters and bias circuit. I found out the hard way. It was serious enough that there was an official fix-it notice out on how to repair them -- kinda like a recall notice on a car. It required a little scraping and some jumper wires. When it blew it arced a nice black spot between the traces. I'd highly recommend you do some research and see if that repair sheet is still available.

Good luck, Skip

[phat_vinny]

Thanks Skip.

I emailed SLM per your advice. I will post their reply.

I am not to keen on jumpering but if it needs to be done, it needs to be done.

I may just point to point the power amp and use the board for the switching circuitry. I know that my old beater Fenders have stood up well to road use, flights and being knocked off stages now and again.

I will learn how to fix these things again. I am sorry I sold my scope, variac, load box and signal generator. It is tough using a DMM and battery powered amp with a scope probe attached to it to fix stuff.

Thanks again,

Vince

[Enzo]

That simple test equipment will fix 95% of what we work on.

The resistor part of the LDR is just that, a resistor. It is an element in the signal path. I can;t imagine how parallel resistance would harm anything. COnsider if two of them turned on at the same time insrtead of just one. What would happen? Well, you;d have sound from two preamp channels instead of just one. Go for it.

[Roy Blankenship]

We had a dealer in Columbus, OH, who sold Crate/Ampeg stuff like crazy. I have fixed many of those, they earned the title, "Most Repaired Amp in the History Of Music". The channel switching thing is often cracked solder joints, which any Crate/Ampeg (or any amp made with a single sided circuit board) will suffer from at some point. Anywhere there is a sizable connection, the solder will need reflowed, sockets, potentiometers, jacks, the blade standoffs, and molexes in particular. Good luck!

[phat_vinny]

Thanks Roy.

Thanks Roy,

I have not touched it in a couple of weeks. I did clean up some of the solder joints and the power amp now works reliably and I have to get to the pre-amp. Since I work on it in the garage and I live in AZ, it has been too hot to work on but it is cooling off. I will look at the rest of the tube sockets and the other larger solder joints.

Thank you again for the reply.

Vince