Ampeg SVT4 Pro repair

[chymmylt]

I guys, I'm hoping you guys can point me in the right direction here. I have an Ampeg SVT4Pro bass head which has seen much better days. Unfortunately I had a house fire recently and lost nearly everything. Sad story aside, the amp itself was not IN the fire but did have a TON of water dump through it from the firefighters (I have NOTHING but respect for these heros!)

The amp was not plugged in at the time. I removed the lid (it's the rack mount version) to aid in it air drying and allowed it to dry completely and then some.

I cleaned it as best I could but when I plugged it in and turned it on - nothing. It didn't even blow a fuse. Just no signs of life

Everything on the inside looks all and all pretty good - no rust or contamination though clearly something is not good here.

I have an ok understanding of electrical repair and am comfortable with a soldering iron (i.e. I've replaced power connectors on a laptop motherboard, replaced cap's on a pinball machine, modded my xbox, rebuilt the electronics in a number of guitars etc). However, I have very little experience with amplifiers. I feel very comfortable replacing parts but don't know where to begin testing.

I know I should probably have it repaired by a professional but as I said, unfortunately the house burnt down - no insurance money and I have lots of things that need to be replaced before this and not the money to do it. The way I see it at the moment is the amp is blown and I will not have the money for a LONG time to have it professionally fixed. I feel comfortable enough in the skills that I would not make things any worse and really need a working amp.

Thank you so much for any help you can give!

Chy

[Enzo]

Sorry to hear your story. I live in a beat up 100 year old house in the country. I live in constant fear of fire. We don;t allow open flames in the house - candles or anything. SO my best wishes.

And welcome to the forum.

As a rule if no power is applied, water doesnl;t really damage electronics stuff. In fact, when they make circuit boards, they are often sent through, for all the world, a large dishwasher to clean the chemical residues off the board.

But your amp is dead, and it doesn;t matter why it got that way. Whatever is wrong is wrong.

Either power is getting into it and it doesn't work, or power is not getting into it.

With the unit plugged into the wall, plug something into the convenience outlet on the rear of it. That will tell you if AC is getting up the power cord and in the back door. If it is dead, check the solder under the MAINs cord connector. There is not many places for this problem to be.

Unplug the amp, turn the main power switch on, and hold the power plug in your hand. Set your meter to resistance and measure between the two main prongs of the plug. If all is well, you will be measuring the resistance of the power transformer primary. if it measures opn, there is a break.

Not much to it, there is a circuit breaker and a power switch in series with the transformer primary. One of those three or the connections between them is open. Hopefully not the transfomer.

If that is OK, then there is trouble on the secondary side.

NO signs of life points to the low voltage supplies. There are fuses inside this amp, not just outside. And you really need to pull them and check them with your ohm meter.

[chymmylt]

Thanks for your message! Boy can I relate on the 100 year old house factor - that's about how old the house I was living in. Half of the wiring in the house had been update but the other half of the house had old wiring that randomly without warning let. I'm just happy everybody got out safe - possesions (mostly) can be replaced (or hopefully repaired!)

I have to run a bunch of things this morning but when I get home this evening, I will test out your suggestions!

Thanks again, I really appreciate it - it's very nice/helpful to have a direction to start with!

Chy

[chymmylt]

Just did a quick check before leaving - power makes it to the convience outlet and testing the power cable comes up open. I haven't had a chance to open it yet - as I said I have to run but will open it up when I get home.

Thanks again for your help!

[chymmylt]

Ok - well I ended up having a few spare minutes before I had to leave - took off the top and tested the 3 fuses which were good but think I might have traced down at least the first problem - it seems that there's the power switch is bunk. I've removed it and tested it and throwing the switch doesn't seem to make any connection on the side that it's wired (looking from the back of the amp in towards the back of the switch - the wires are on the top and bottom on the left. There is nothing conencted to the right side of the switch and testing the right side SEEMS to work but I've not hooked up the amp wires to that side. Am I safe assuming that I can move the wires over to the right side or is there something about this switch I might not know?

Thanks!!!

Chy

[rf7]

AFAIK you can use either side of the power switch. I think you'll be OK. I see those go bad quite a bit actually.

[chymmylt]

Can I just say you all rock!!! I hooked it up to the other side of the switch and low and behold the amp is back to life!!! Then I turned it off to connect a speaker cab and it didn't work when I threw it the second time but after I threw it 2 or 3 more times it worked again and has been since. I'm thinking the other side of the switch is having intermittent issues. It all seems to be working though every single pot is dirty as heck (tons of static when turning them) - I'll try to clean them more but might have to replace them. Still - even if I have to replace the switch (it seems as that is very likely) and every pot in the amp (hopefully not) - I'm still in FAR better shape then I was this morning!

Thank you so much!

[Jazz P Bass]

pot cleaner

Try DeOxit in the pots. Radio Shack carries a small can.