Help! Problem With My '74 Concert Bass

[FranF]

My first post! Wish it was good news. I recently refurbished this amp to like-new, both sonically and visually. Worked great until I did something (apparently) dumb. I was A/B-ing a new bass through the Sunn and my B-15, when I accidently touched metal parts on both amps at once. Actually I was just turning off the Sunn, so I didn't know there was a problem until weeks later when I fired up the Sunn again. No damage to the B-15, which i kept using the night of the incident.
   Now, the problem. Obviously some voltage came into the Sunn's input stage from the Ampeg (which is two-prong ungrounded BTW). The Sunn now just makes a very faint distorted sound. All controls appear to work, and the amp section does "pop" on power-up, and hisses normally at idle, and it does increase as you turn it up. Nothing on the PCB looks burnt. All fuses appear intact. I suspect the voltage took out something very early in the circuit. Problem is, how do I find it? I understand that solid state devices are very hard to diagnose, especially in the circuit. Any advice on how I can get this amp running again? I really miss it!  I'd love to save some $ and do it myself, but will consider shipping it to someone who has good feedback from anyone here, as long as I can keep the repair as cheap as possible.  Thanks in advance for any help!!!!!     Fran

[Don T.]

Here is the back yard fixer method:
No guarrentees expressed or implied.
Take a set of 6oo ohm headphones and jumper a 10 mf NP capacitor to the tip of the plug on the headphone cord. Jumper the shank to chasis ground on the amp (it will have to be out of the cabinet so you have access to the guts). Put some signal into the input from a cd player. Now follow the signal through the circuit (starting at the input jack) by touching the unconnected capacitor lead to the components. DO NOT TOUCH MORE THAN ONE LEAD OF A COMPONENT AT A TIME. When you touch a power circuit the headphones will click. When you touch signal path you will hear the music. The loudness will go up and down depending on where in the circuit you are. Eventually you will come to a transistor (most likely the first gain stage FET) which will have signal coming to it but distorted signal (or none) coming out and that will be your problem. REMEMBER: there is lots of voltage around the power supply so be careful.
An amp tech will do the same process only using a scope instead of the headphones and of course he knows more of what he is looking at on the circuit board.
Have the schematic handy to help follow the signal. And have someone who knows about electronic stuff help if possible. I used this method to do repairs when on the road BUT it is possible to smoke the amp if you short traces and leads together. It takes a steady hand and an eye for detail.
Good Luck
Don T.

[FranF]

Don, thanks for the speedy reply! Sounds complicated, but I'm gonna try it. Any other opinions or tips? It may take several days, but I'll report back with either good news, or worse! Wish me luck!

[youDEVILyou]

wear a rain coat and some golashes.

maybe a tire around yoru waist

[skippy]

before you electrocute yourself, get a copy of the schematics (you can still order them from fender if you can't find them online).  my concert bass did the same thing, but for a different reason.  there is a noise capacitor, and all it does is reduce hiss.  30 cents and a soldiering iron, and you don't need health insurance!  quick fixes are like cheap deals, you pay shit, you get shit! get the schematic and do it right.

[FranF]

Skippy, thanks for the input. I do have the schematic. At least the one offered here. You think that one's good enough? I'm sure this will be a "work in progress" for a while. I hope people keep chiming in with little ideas here and there!!!

[FranF]

Anybody else have any opinions on my sick puppy? I'm getting ready to dive in!  Gotta get it working again!