Ampeg SVT IV Pro - fan Q and schematic request

[gcaveney]

Hi all,

1. Am I right in thinking my fan should run constantly from power-up and not be heat or input/output sensitive?

2. Just had output transformers replaced, and the amp is running fine, except the fan, which is intermittent. Is this because the output problem is now sorted, or because it's on the wonk?

3. If, as I suspect, it should run continuously - any ideas why it isn't? And does anyone have a schematic so the poor sod who's dealing with it can get to the bottom of the prob? Can post serial no if this helps.

Many thanks.

George.

[Jazz P Bass]

Ampeg SVT 4 Pro

I do not think the SVT 4 Pro has output transformers.
(3) 12AX7 tubes in the preamp.
MosFet transistor output.
The fan circuit reacts to temperature change.
Here is a schematic. Test Point 13, 16 & 17 should be checked.
IC2 (LM35DZ) is the temperature change output IC.

[gcaveney]

Much appreciated. I'll print this off and pass it to the dude who understands it.

[alexwfm]

svt 4 problem!

Hello there,
I'm repairing a svt 4 pro.

The main problem is the A poweramp's relay that stays always in protection mode.

There is also a strange thing: the fan don't spin otherwise i touch or tick the boardside near the temperature sensor IC1. It seems to be a bad contact or something like that.

I'm asking.. there is something i can check before i strip down it all??
Thank you in advance!
Alex

[Enzo]

You won't see anything on the output side of the relay, but check on the amp side of the relay. Is there DC on the output bus?


So resolder the temp sensor. Also gently tug on its legs to see if either is broken off.