Ampeg SVT 200T Problem!

[boogi]

Hi, I am new to this forum and hoping some kind person out there can point me in the right direction for finding the fault on this amp? It has low/distorted output. Fault definitely on power amp PCB. ( Have verified with another working same model amp ). I found Relay T90 was fused short. I managed to release the contacts and the relay is now switching open and closed ok. But fault remains. So far all checks and test are ok? Please help?

[Jazz P Bass]

Ampeg SVT 200

Thr relay contacts may be damaged.

[Enzo]

Click the relay on, and while it stays on, measure resistance from input to output side of the contacts. Mechanical works is no guarantee of electrical works.

So plugging a signal into the power amp in jack makes the low distorted output? Apply a steady signal to the power amp in, and scope the result at the op amps out pins. What does it look like there?

[J M Fahey]

But fault remains.

Of course it does.
Unsticking those leaf switches, (really, you should replace the relay itself), will do nothing to repair what caused that fusing in the first place.
Probably the output stage burnt, passing DC to the speaker, the relay fused trying to interrupt that heavy DC current.
Might even guess the following scenario: output cable was shorted somehow, the previous owner turned the amp on and off a zillion times, first making some output transistor short, and going on until the overheated relay contacts fused together.
If somebody can post that power stage schematic, we will try to help you.

[boogi]

Thanks for the feedback guys. Much appreciated!

Can't believe i didn't check that Enzio! I was brain numb at that point!! But I will check as soon as I can. And yeah, Power Amp In is low/distorded output. I have narrowed the fail to the power amp pcb. First I used another ampeg and pre-amp out to power amp in and vicaversa. Then when I came across another SVT 200T I swopped over Power amp pcb's and all was ok. So fail is on Power amp pcb. So far comparisons between working SVT 200T and failed one havn't shown any descrepencies as yet.

Presuming continuaty is ok across closed relay? What would you suggest to test next? I am pretty sure that some other component fail caused the relay to short, but havn't worked on this type of amp before. Any more suggestions will be great thanks.

I have requested schematics from Ampeg. I will need board layout so hope they have one. This board dosn't have any component ref's on. It will be a swine to fault find just with the circuit diagram.

[Enzo]

My suggestion above - apply a signal and trace it through the amp. WHat does that signal look like coming out the op amps?

[boogi]

Cheers Enzo, will do soon as. Will post outcome. Thanks for now.

[wilsoncat]

Hello, new user here. I've got a SVT-350T (the combo version of the 200T head) and have a similar issue as this. After the amp is played at volume for 10-20mins the amp cuts off / distorts. I can't seem to find power amp section schematic anywhere on the internet but I am pretty sure that is where the problem lies. I'd hate to lose this amp as I've had it since the early 90s and it sounds great when it works. Any help would be appreciated.

[boogi]

Hi, that sounds like a dry joint to me. Most likely on the main PCB. If its the same layout as the SVT450H you may be lucky and find the dry joint or joints? without having to remove the PCB and turn over? Take your time, good light and magnifier if possible. Could be anywhere.
Good luck & cheers, Robert

[eze7782]

leaving the schemo for this great amp!

Fahey! groso!

saludos.

[wilsoncat]

leaving the schemo for this great amp!

Fahey! groso!

saludos.

Wow, thanks guys for the replies. I'll see what I can do.

[J M Fahey]

1) Gracias EZE, muy amable.
2) Hi wilsoncat.
You have a *great* amp, very well made, generously designed, in the great tradition of classic Ampegs.
They don't make them like these anymore. eze7782 was too kind posting not only the schematic but the layout too.
*Pamper that board*, whenever necessary desolder well, finish cleaning with solder wick, don't wiggle parts when pulling them to avoid pulling pads or tracks.
First do what Enzo suggested, send preamp signal to another amp , then send the other amp line signal into the Ampeg, to confirm where the problem lies.
3) Does it cut off as in no sound out (maybe a very distant "mosquito" comes through) or keeps sounding but weak and didtorted?
The first may indicates a failing relay or a cracked and failing connection; the second points to the circuit itself.
The best would be a scope, but if you don't have one, touch the heat sinks for abnormal temperature (if your hand can't stand the heat more than a few seconds or if it fizzes like a steak, it's *too* hot).
If you do not see a thermal problem, check the signal gets to C1 in the power amp.
Suspects: "from preamp" connector, "line in" jack contact and the jack itself (cracked solder connections).
Other suspect: maybe you have sound before the relay but not after, when losing sound bridge its contacts momentarily with a short piece of wire. If the speaker clicks or pops loudly and you hear strong hum through it, disconnect it at once.
Well, now you have a few things to keep you busy, good luck.

[wilsoncat]

Hi Fahey, I am experiencing the first scenario you described, it sounds fine then after playing about 10 mins the sound gets gets distant and distorted AND cuts out. Once it gets to this point you only hear sound intermittently and it's low and buzzing / crackling.

Unfortunately, I do not have a scope (I do not have much knowledge in that area anyway). The heatsinks are warm, not scolding hot so I do not believe that I have a thermal issue here. When I get home this evening I will try routing the signal out of the pre into my other amp and see what happens.

I want to also say that the information I've received on this forum has been outstanding. Thank you again.

This issue arose back in the early 90's and I never got around to actually messing with it. As you can see from the picture above, The "head" was transplanted from it's original combo (2X10 w/horn) into a new head case. I actually bought one of those knock off Behringer SVTs just to put this head in.

See pic:


I just barely fit (minor mod to case), now I need to find the Ampeg logo that I took off the cab so many years ago.

[J M Fahey]

OK, as a first test, play it until it cuts off for good, and without turning it off, plug a CD/MP3player or some other known working signal in the line in Jack.
*If* still weak/distorted you've narrowed it to the power amp itself or the output relay area; if now working, plug a known good cable from line out to line in and your bass up front. Check whether now you have sound or not.

[wilsoncat]

Well, I was playing the amp today to try to get it to cut off, but had no luck. I couldn't play very loud as my kids were sleeping so I guess I need to wait till a better time to push the amp. Thank you for the suggestions, as soon as this thing has the problem I will try.