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Ampeg SVT-3 Pro, advice wanted

K

kaygee

Jan 21, 2008
11
1
UK
#1
I have had a SVT-3 for 2 years and am very happy with the sound and it is easily loud enough for me.

The problem is the graphic eq sometimes switches on and off repeatedly on it's own, causing the sound to go louder and quieter, and the eq light flashes on and off. I have the eq level at maximum and the eq on all the time. This occurs when a footswich is plugged in and also when it is not plugged in.

I bought the amp second hand so it is no longer under guarantee, any suggestions of causes or possible solutions would be gratefully received.
 
#2
Sounds like it might be an intermittent contact on your EQ switch in the amp. Try messing with it and see if it helps. (Try wedging it in with a toothpick and see if it helps. If it does, theres your problem) Perhaps the insulation on one of the wires has worn through and is making contact with the board and bypassing the switch.

Does it do it when you are playing it or will it do it with the unit sitting idle?
 
#3
Thanks for the suggestions. The problem occurs when the amp is idle and also when I am playing.
I opened up the amp tried gently moving the body of the EQ switch to see if it had a bad solder joint (it is PCB mounted) but it is fine, moving the switch button side to side also did nothing. I also tried wedging the switch in place with a tooth pick but that had no effect.
One other thing that happens when the EQ light goes on and off is a relay sounds like it is opening and closing, could this be the protection relay that delays the power amp output a few seconds when you turn the amp on?
 
#4
That unit uses FET switching to cut in and out the EQ.

Even if the switch is fine, a loose connection elsewhere can cause the switching. Because you say it is a clean switch on/off, I am inclined to think it is in the 'drive" to the switch circuit.

Could also be the footswitch jack. The EQ switch is routed through the footswitch jack on its way to the switching circuit, because you can footswitch the EQ.

try plugging in/out a few times on the footswitch jack.
 
#5
Thanks Jerrold, I tried plugging in and out the footswitch jack but that did not cause any problems.
How can I check the drive to the switch circuit, it would make sense if this switched on and off repeatedly when the problem occured. Do I need to remove the PCBs from the case and inspect them for dry/broken joints or is there anything else I can do?
 

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