SVT 4 Pro issues

musicman724 · #1 05-30-2011, 03:18 PM

I have an svt 4 pro and only the B side is working properly. When in bridge mode or using the A side it produces a distorted sound. I have changed the preamp tubes, plugged the bass straight into the amp A input and I took the amp apart and reflowed the solder joints on the MOSFETs, tube sockets and most of the the other joints on the boards. After putting it back together it is still doing the same thing. Can anyone help?

Ncognito · #2 05-30-2011, 06:25 PM

I had a bad solder in the effects loop and it did the same thing. Check those solders too!

musicman724 · #3 05-30-2011, 06:30 PM

Were you using the effects loop? I will check em.

Ncognito · #4 05-30-2011, 06:37 PM

I never use the effects loop.

musicman724 · #5 05-30-2011, 06:55 PM

Yeah I just tried it and still has the same noise. I hope someone has the answer for this. I don't wanna have to put it in the shop. I'm technically inclined but I do not know all the ins and outs of working on amps.

musicman724 · #6 05-31-2011, 02:36 PM

charlie monroe · #7 05-31-2011, 05:18 PM

Quote:

Originally Posted by Ncognito

I had a bad solder in the effects loop and it did the same thing. Check those solders too!

Hi Musicman, it sounded like you just bypassed the EQ and went and plugged into the effects return, and the distortion was still present.

I think that Ncognito is saying that even though he did not use the effects loop, a poor solder joint there affected the signal anyway.

I apologize if I misunderstood. If I got it right, you should check the connections on your effects loop.

I have a 4PRO from the nineties, and thankfully no prob so far. Just out of curiosity, what year is yours? You too please Ncognito.

musicman724 · #8 05-31-2011, 05:47 PM

I did check all the effects loop solder and connections. Mine is a 2005 I believe. That is if I understand the serial number code.

Ncognito · #9 05-31-2011, 06:28 PM

My Ampeg SVT-4 Pro has about 2000 shows on it. I've used them since 1997. Probably been in the shop for repairs about 10 times and it's always been for solder repairs. If the amp takes a hard hit, a solder somewhere seems to be the culprit. I've never had a Mosfet go bad.

musicman724 · #10 05-31-2011, 06:56 PM

Maybe I will have to get a heat gun and do a real reflow. I just used my soldering iron and reheated the connections and added solder where needed.

musicman724 · #11 06-01-2011, 09:17 AM

Jerrold Tiers · #12 06-01-2011, 11:24 PM

If the A side is a problem and the B side is OK, then the preamp is not the issue, nor is anything else that is not a part of the "A" amp side.

Bridge mode is distorted because the "A" amp is distorting, and bridge uses both.

There are a large number of possible reasons, too many to list.

If the amp is basically working but distorted, and it were on a bench for me to fix, I'd look at the output waveform on a 'scope and try to get a handle on the type of distortion, which will lead to the likely cause.

If you are not familiar with amp repair, it's better not to get into it, you may mess up more than is now bad. Sorry if that sounds like an insult, it isn't intended. I am pretty mechanically and electrically savvy, but I still take the car to a mechanic. There's nothing like experience to make the job easier.

BTW, I don't particularly recommend wholesale reflowing with a heat gun..... I know people do it, but it can lead to MORE bad joints in many cases..... it's nothing like a wave solder machine, no flux, and no extra solder available.

musicman724 · #13 06-02-2011, 09:14 AM

Thanks!