Ampeg SVT Classic goes into standby on its own

[PRNDL]

I have an Ampeg SVT Classic Bass amp that goes into standby in the middle of playing. The standby light goes on, and you can get it out of standby by flipping the switch twice, or pounding on the amp a few times.

Another tech reflowed the solder joints on the board, cleaned and tightened the power tube sockets, which helped for a while, but the problem is back.

I notice the standby switch is different than other vintage amps.
It is between the AC and a time delay relay to the power transformer.
There is a separate heater transformer connected directly to the AC.

Also,the LED is complicated.

STANDBY/POWER/FAULT INDICATOR LED: This is a multifunction LED. In Standby Mode, it glows red. In the On mode (when the high voltage comes on) it glows green. If it does not turn green in the On mode, there is no high voltage present and the unit needs servicing. If the amp detects a fault in the power tube circuit, the high voltage is turned off and the LED flashes between red and green.

Does anyone know if these switches or the relay tend to fail with age and have problems like this?

Is there a common issue to look for?

Does it have an over-current protection / shut-off?
If so, it might have some bad power electrolytic caps that are shorting and tripping the switch.


Thanks!!

[Enzo]

If you suspect the switch and relay, test them. Even if they "tend to fail," that doesn;t tell us whether yours did.

[jhoueayn]

Could you please help me on this. I bought my Ampeg SVT Classic amp through my american friend in States and shipped to me here in the Philippines. I have a problem with my amp now. We dont have a service center here in the Philippines. my amp "STANDBY/POWER/FAULT INDICATOR LED: it does not turn green in the On mode, the unit needs servicing. Is there anybody would like to share and help me on how could i fixed my 2 year old amp? With kind regards to all here in music electronics

[Prairie Dawg]

@PRNDL, on which board did your service person reflow the solder joints? there are three boards I think-main, power tube, and power input. Reflowing the solder joints on the power input board did the job for one i worked on. Of course you've got to have a known good set of tubes before you do anything in that line.

A matched set of known good tubes is a necessity when troubleshooting Vietnamese made Ampegs, and even then, it may or may not fix the problem that you have. It's the requisite starting place though, which is why I don't work on them any more-I hate telling people they've got to drop $150 just to start the troubleshooting process.

[Jazz P Bass]

Ampeg SCT CL Fault Mode

The amplifier may be In Fault Mode or the circuits asigned to it are not functioning.
I have attached a cropped version of the Preamp Fault circuit & the Power Fault Circuit.
Check the voltages as indicated on the schematics. to verify that the circuits are functioning.
If they are, than the amp is in Fault mode.
Let's see what is what.

[Gregg]

These SVTs will also turn off if tubes (ore only one of them) get too mismatched.

[jhoueayn]

Hey Jazz,

I cant open the file you sent. There is an internal error to it. Can you please resend the file? With thanks.

[Steve Conner]

Yes, both PDF files open as blank squares on my machine (acrobat 9.4)

[Jazz P Bass]

Try this past post.
I don't know what happened.
Guess I should preview my posts
Past Post:http://music-electronics-forum.com/t16921/

[jhoueayn]

@Gregg::So how I could fix it if in case with instances.

[Gregg]

If you have mismatched tubes or some of them is dead you'll need to replace it/them. As far as I know if tubes get mismatched by more than 7-8mA these amps will automatically shut down. The other thing you can do is regroup the tubes in a way to avoid such differences if possible.

[jhoueayn]

@Greg: Thanks for the response. What i'm experiencing is once i plug the amp into an AC source the "STANDBY/POWER/FAULT INDICATOR LED: it does not turn green in the On mode and stays in RED. So. i can't proceed if processing with above you mentioned. Appreciate your help.

[Gregg]

The other reason for the amp staying in standby mode could be you're not getting high voltage on preamp tubes and the voltage divider feeding the opamp circuit doesn't feed any voltage to it. I would check the whole automatic shutdown circuit to see if the voltages from the schematic are present and/or for bad opamp/s.

[jhoueayn]

Thanks Gregg>> With your help, hope i can get fix this pricy amp.

[jhoueayn]

Hi Gregg> Did you got a chace to take a look on the schecmatic> Do you have schematic diagram for this SVT CL? Maybe i could check too? thanks.