Ampeg SVP Pro not switching on

[d3hne]

Hi guys.
I recently bought a pre owned SVP Pro preamp. While playing it at the owners place via headphone it sounded very nice. The outside also is in a nice condition.
Then after playing it at home connected to a preamp and a cabinet, it started to crackle really hard (there was no more tone, only the crackling) and the power LED flickered / went dark. This happens almost all the time. Sometimes it disappears after playing really quiet for 10 to 20 minutes.
I noticed that this crackling happened more often if there was sth lying on the preamp (bigmuff for example).
I thought it may be casued by vibration, so I started knocking softly onto the table the preamp was placed on.. and it also happened. Now it sometimes doesnt even switches on, sometimes soft knocking on the table helps. So I think its a loose connection or sth like that.
Maybe someone here had similiar experiences or you can help me to isolate the problem
Thanks!

[d3hne]

bump

[BogeyBass]

You going to have to do some sort of visual inspection of the inside.

Sounded like it could just be a bad output jack connection.

But if the Led is going out or the whole unit shuts down, then its most likely a loose or corroded connection some where around the main power supply.

look for the obvious loose/broken wires, or any power supply connection like a spade connector or a crimp joint.
or look for components where the solder connecting them to the board has broken loose or cracked around the component leads.

you can poke around with something non conductive when its powered up to find bad connections, like a chopstick or piece of long plastic or wood. if you find the bad connection power it down before doing repairs and give it about 20/30 mins to discharge the capacitors they can hold a charge even without power.

i dont think it would be a bad tube, but see if all the heaters glow at the same relative brightness. if one is obviously dimmer or just not working replace the tube. you can tap the tubes lightly with a pencil using the eraser end, and if the tapping is amplified by a large amount or makes a crackle then you got a bad tube that is microphonic.

most likely its a loose/cracked connection

[bassgod0dmw]

I'm not sure if this is the problem, but it happened to mine and I've heard of another so it's probably worth checking.

There is a fat ribbon cable connecting the front circuit board to the back circuit board. It connects to each board via a push in style connector mounted to each board. The cable just pushes into it. During shipping it has a tendency to come loose. Check to make sure it's seated properly.

It's really a poor design on Ampeg's part...

[d3hne]

hey guys. thanks for the tips!
i just disassembled the boards and had a look at the circuit and the soldering joints. they look like they should.
Only the transformers soldering joints werde a bit "dark", but i think that is flux since the pins are bigger than the others..
i also noticed that theres a "cracked" capacitor near the gain pot. i will replace that.
i will have a closer look later and tell you what else i can find.

[SGD Lutherie]

Originally Posted by d3hne

i also noticed that theres a "cracked" capacitor near the gain pot. i will replace that.
i will have a closer look later and tell you what else i can find.

Take a look for any bulging electrolytic caps. Some years back there was a problem with aluminum can electrolytic caps that caused them to swell and often leak their electrolyte.

This started back in 1999, and has still been reported up until current times, according to this Wikipedia article:

Capacitor plague - Wikipedia, the free encyclopedia

I had a fairly new DVD player that stopped powering on, and when I looked inside I found a swollen electrolytic cap in the power supply section. I replaced it and all was well.