Ampeg SVT 1000 low volume fault please help

[evil bunny]

 :trouble
I have an Ampeg SVT1000 head that needs repairing.
Suddenly the amp has almost inaudible output. 
Cab is fine have tried with another head. 
I am adept with a soldering iron and have no problem replacing which ever part may be at fault.
I have a good multimeter and voltmeter and am a qualified electrician.  Electronics is more of a hobby.     
have looked inside the amp and no apparent signs of overheated components or bad solder joints. 
Whats stumping me is that there is some volume when all pots on max.  about the same as if playing the bass unplugged. 
any suggestions on how to diagnose or if anyone has experienced this fault with any ampeg gear or anything solid state. 
Relatively sure I should be looking in the output stage.  any suggestions or experience muchly appreciated. 

[Enzo]

Couple places to check.  First, there is an FX loop with send and return jacks.  Poke a plug in and out of the return jack a few times.  Did that wake it up?   Plug a spare cord from FX sende to FX return, any help?   Also there are power amp in jacks as well as a couple selector switches.  Make sure the full range/biamp switch is set to full range.  Also make sure the sterteo/mono switch is at stereo.  In fact, flick each back and forth a couple times before setting.   And poke both power amp jacks to see if they wake it up.  You can plug the guitar right into the power amp IN jack and it should come out the speaker.

[evil bunny]

Ahhhaaaa!
Fantastic I'll give all of these a try.  Dont have the amp with me it is in the studio but will check all of these during the week. 
If this turns out to be the case is it a matter of some contact cleaner or swapping out the jacks then?
Thank you for your reply Ive been searching on line for days and not found a single helpful comment until now. 

[Roly]

If this turns out to be the case is it a matter of some contact cleaner or swapping out the jacks then?

Basically, yes.

[evil bunny]

Hey Enzo,
thanks for the insight, tried the fx loop thing no joy.  Plugged straight into the power amp, worked fine  so led me too the pre-amp part of the chain. 
Was wiggling each dial/knob/switch on the front and nudged the style select dial near the input jack and it sprung into life. 
turns out just some bad connections between the 5-way selector dial and the board.  So a bit of re-soldering and she should be good as new. 

Ever grateful

=:B

[Roly]

Excellent.   :dbtu:

Five will get you ten it's a fractured solder joint.  It may be a good idea once you are in there to touch up the others to front panel PCB mounted controls, and particularly all the PCB mounted sockets which are often a source of trouble.