Ampeg SVT-VR turning on and off

[pontiacpete]

Has anyone every come across a problem like this, where the amp comes on momentarily and goes off in about 3 seconds and keeps repeating this every 10 seconds or so.


OK, something is weird here. I have 2 of these SVT-VR s and both are doing the same thing. So I must be hooking something up wrong. Using the 1/4" speaker jack not the speak-on, tubes are all lighting up but the voltages come up on the pre's and then drop off. mmm

[pontiacpete]

Found that the bias pots and balance pot get it come up.

[jmaf]

Speaker cable? Bad contact in speaker cabinet? Cabinet jack? It's unlikely both are doing the same thing. But then the bias pot and balance thing wouldn't make sense.

[pontiacpete]

Doesn't make sense to me either, but when I turn the bias 1 and 2 as instructed until LEDs come on then it works. What is making me wonder now is why if they had been in use, ( I think somewhat recently the owner just wanted me check them for a tour) do they have to be turned in the first place or does this procedure need to follow each time it is powered up? that can't be.

[jmaf]

does this procedure need to follow each time it is powered up? that can't be.

Of course that's wrong. There is probably something wrong with that tube protection circuitry. Have you tried removing some output tubes and see if it starts ok?

[pontiacpete]

I removed 2 output tubes and it starts ok. Could it be that these amps have been in storage?

[jmaf]

I removed 2 output tubes and it starts ok. Could it be that these amps have been in storage?

Hmm. Did you try to remove other pairs and see if runs ok as well? Perhaps you've found the offending pair at first try.....put it back in and remove another please, see if it works.

[pontiacpete]

Both of these are more or less working , it turns out that the bias was set so low it was switching on. Once biased they work. I'm just surprised they were so under bias when they arrived here.
Now I'm finding some other issues. One amp has a fan that's shot. Can't get that for monday. The other one has a working fan but the volume on the first channel will not go all the way down and there's a nasty hum but otherwise plays. I don't have a pot to put in that will look good , i.e. fit the knob and pcb. I'd like to have the proper replacement if that in fact is the cause of the hum, not sure yet. On the meter in circuit it goes from .2 ohms to 890 something so appears to be good.
I have to test some more.