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Fried Ampeg SVT-200T

Started by sewage666, July 03, 2012, 01:26:40 PM

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Try plugging the guitar into the *Effects Return* if the crackle stops and the signal sounds clear then the Gain pot is likely stuffed.
You will have a bit less volume but the rest of the front panel controls should all function normally.

A common issue is FX loop switches.
The switch is built into the socket and with older gear that contact corrodes and fails to pass signal.

The schematic that *g1* kindly posted shows 2 fail points, the FX loop and there is also the *Line in/Line out* which is the same setup.

If you suspect fx loop problems then use 2 short patch cables and bridge those 2 loops if the amp then works fine you likely have an easy fix.


And really, it would be so much better to start a new thread for your amp instead of adding on the end of a thread that has been dead for five years.


Wow! Thanks for the quick replies!

Phil, I'll try your suggestion when I get a chance in the next few days.

Enzo, you're right, a new thread would have been a better idea (given how old this one is). But I've found so little info on this amp, I was trying to write to people well versed in it and get their attention.

I'm any case, I'll post results once I get it tested and we'll go from there.



Well, plugging into the FX loop had the same effect: crackling and popping until it just stopped. Nothing from the speaker after that. Except for a quiet hum - but you've gotta put your ear about 2-3 inches from the grille.

I've got the case open, amp sitting on a plastic crate. A closer look showed that there are globs of solder at 3 different resistors and 2 pair of the resistors are in parallel ...pretty sure they aren't supposed to be that way because of the equivalent resistance and the solder is on the wrong side of the PCB [see pics, if they attach]. Also, are the transistors supposed to have screws through them? [Phillips heads, last pic.] I installed the one with the larger washer on it because it was either missing, or shouldn't be there.

If I understand correctly, the corrosion failure in the FX loop switches MAY be contributing to this issue. If so, cleaning the corrosion off should allow a signal to pass through them. Yes?

I'm assuming that they were right on the verge of not passing a signal and by inserting the cable, the contact area may now be fouled completely.

—> I'll get that board out and try to clean them as best I can. Granted I'm assuming a few things, and we all know what happens when assumptions are made, but it's one place to start.

More details as it evolves.



YES, If the contacts in those switching sockets are dirty it WILL cause crackling. 8|

You need to bridge *BOTH*,, cable from *FX loop out to FX loop in*,,,
as well as another cable from *Line out to line in*.
Both could be faulty.

The switch is on the *return* and *line in* sockets only so get some electrical cleaner spray and give them a good blast, Also you may want to slide the plug in and out a few times to get contact again. if it's crackling try gently wiggling and reinserting the plug,, see if you can get the signal to play clean.

If you have access to those sockets you can bridge the terminals right on the sockets.

Regards those extra resistors soldered on the top side;
Someone has modified the circuit it's often easier to add a new value to the top side,, saves pulling the whole amp apart you just add the mod on top,, no big deal and I doubt it would cause the problem you describe.
You need to establish that those 2 loop switches are working or not FIRST,,, then you move on to other things.
Those 2 TO220 transistors don't need the screws.


Hi again,
Had some free time this morning, so I got the PCB (with the pots and inputs on it) out of the case and ohmed out the 0dB, -15dB, Effects Send, and Return jacks ... and they were all high resistance due to corrosion on the contacts.
Got 'em cleaned up with a small file and contact cleaner, checked the tension on 'em with the patch cable (making sure they opened and closed - several times), and rechecked the resistance: 0.40 ohms on 'em all.
Put it all back together...

It works. Still a little crackle from the Squier due to patch cord contact at the output of that bass, but the Dean sound comes through nice and clear.

Guessing the years of use before I got my hands on it was the culprit.

Thank you guys so much for your help. I really didn't know where to start, but now she's up n running and I'm happy.



toast resistors, fried amps, fused relays......Why're geeks always speaking so FuNnY?
In my opinion the problem might be caused by neutral wire breakdown in the club , which could lead to a 380-volt high voltage on mains.
It's tragic but, even switching power supply modules can't handle such a CRAPPY voltage so always use a surge protecting extension cord