Gibson GA-8T tremolo fault

[JonSnell Electronic]

DHi Ron, don't get confused!
You have proved the effect of lifting the resistor and that stops the oscillation.
Next step is to put it back on pin 8 and check the other end goes to the foot switch. You will then prove the switch operation, the other contact from the foot switch going to ground. Don't forget the switch is in series with the resistor and its ground!.

 

[JonSnell Electronic]

Pin 9 is the heater centre tap. I would expect to see at least 6v A/c and by the look of your drawing floating at a Dc voltage somewhere near your description.
On the capacitors I would expect to see quite a good amount of A?c due to the oscillator.

 

[JonSnell Electronic]

The DC voltage on the heaters probably means there is no centre tap to ground on the winding. Use a pair of 100R 1/2W connected in series across the 6.3V heater voltage (not the rectifier!) and connect the centre of the resistors to ground. This will balance the heater voltage and reduce any induced hum.

 

[osci]

Hi Jon

I've connected one end of the footswitch to the resistor while the end of switch to ground powered up and no oscillation.

I then connected all wires back to original, powered up and learn I've lost the tremolo except from about position 9 -10 on the speed pot.

when I switch the footswitch off the fast oscillation returns

 

[JonSnell Electronic]

"I've connected one end of the footswitch to the resistor while the end of switch to ground powered up and no oscillation." You have proved the circuit works. Don't put anything back just yet,
At that point, switch the switch and did it work?

 

[JonSnell Electronic]

"when I switch the footswitch off the fast oscillation returns"
The switch needs the wiring changed as on is off etc.

 

[osci]

Hi Jon,

Good news,I fixed the buggar....WOOHOO.......now its hard to describe what the actual was, however,

I had the footswitch wired correctly.....it was more the way that I had it "wired/connected" to the speed pot, I think It was actually bridging pins 1 & 2 of the tremolo speed pot??

what I did is put the footswitch in series with pin 1 of the speed pot, does that make sense?

So now I have virtually full range of tremolo from 1 thru to 10 and it sounds quite velvity.

thanks heaps for all the help & advice you & Enzo gave without it I would still be here next weekend trying to fix it

cheers Ron

ps: I just hope that I can repay this by giving advice to someone else in the boat

 

[Nigel Goodwin]

Well done

Sometimes you can spend ages, and then it ends up something simple and silly you've done yourself - it's like trying to proof read something you've written, when you check it your brain reads what it was supposed to say, and not what it actually does.

 

[osci]

Well done

Sometimes you can spend ages, and then it ends up something simple and silly you've done yourself - it's like trying to proof read something you've written, when you check it your brain reads what it was supposed to say, and not what it actually does.

Thats exactly how it was Nigel, you've summed it up to a T , felt like such a dill after I'd fixed it and stood there for a while pondering why I didn't get it sooner

 

[Nigel Goodwin]

Thats exactly how it was Nigel, you've summed it up to a T , felt like such a dill after I'd fixed it and stood there for a while pondering why I didn't get it sooner

I've been a professional service engineer for over 40 years, and at times you've spent hours on a repair, and on the next bench someone else has spent as long on their repair. So you swap over, and both get completed in a few minutes - sometimes you just can't see what's in front of you

 

[Enzo]

A miswire right in front of you can "look OK" and you pass it by, but in trying to fix something remotely, as we have been doing here, it can be particularly perilous, because we tend to assume the unit is the same as the schematic specifies. I have been doing this all my life, and I have been training technicians for 40 years, and yet, it happens to me too.

I am glad you found it.

And this is the perfect example to show novice techs that it isn't always a good idea to jump in and start replacing parts. We could replace every part in the amp, and if we never noticed the wire on the wrong end, it STILL would be doing this.