Urgent help req !! - guitar amplifier channel switch fault.

Time to get out the DVM and start looking for s/c parts ... D2 would be right in the path...Start there ..

It will take an experienced tech to find the exact failure. Most likely one or a number of the transistors in the control area are probably damaged. This area would have to be properly serviced.

Look in to who their authorised service rep is for your area, or you may have to ship the unit out for proper service.

For add-ons to your equipment, it is best to buy comercialy made units that the manufacture approves, rather than building your own, unless you have the proper knowledge of what you are doing, and a solid background in electronics, and a detailed familiaraty with amplifier circuit design.

Jerry G. ======

Didn't see Kips response until now and I think he hit it right. That

3.9volt zener is a protect device and if it shorts, it grounds SW2 making it useless. May or may not be the fail or the only part bad. You're going to have to check the discrete parts on the switched input side of SW2 to be sure.

Well, D2 is toast. Shorted. I'm hoping that's all that's gone. The transistors close by (TS1 & TS2) are all surface mount, so I'm not planning on ripping them out just to test them. TS3 seems ok as LED1 still lights.

I'll get another zener diode in the morning, and keep my fingers crossed.

Just out of interest - is there a quick way I can test TS1 & 2 whilst still in-circuit ?

Cheers,

Kev.

R4, R5, R7 and R20 should have protected the four transistors. 10k should be enough to prevent enough base current flowing and causing damage with 9v applied.

I have to say though, it's a poor design that allows a fault condition to be produced, when a ( comparitively ) small voltage gets accidentally applied to an external user input. This sort of thing is bound to happen on a regular basis with this type of equipment.

A 1k resistor in series with the tip connection, would prevent the zener from failing, whilst still allowing it to clamp to 3.9v

Arfa

I've realised my error that caused it to blow... The switch box I built worked fine for weeks, but I needed to see visually which channel was on and the status of the reverb, as it's not always easy to tell when you have two saxophones blasting in your ears on stage ! Sooo... I added a couple of LEDs switched by the other poles of the DPDT footswitches and powered from the 9v supply I use for my effects pedals. I used a 2.1mm connector, metal body, mounted in the metal footswitch case. Effects pedals are almost always 9v DC, negative tip. The shield of the 1/4" socket on the footswitch (that connects to ground on the amp via the cable) was connected to the footswitch case too - via a metal 1/4" jack.

This meant that as the power is negative tip, the footswitch case was now 9V positive, and 9v went up the cable to the shield of the jack socket on the amp - then also to the ring and the tip as I pressed the footswitches. Without the guitar signal cable connected to the amp, it worked fine - BUT... when I plugged that in (from the effects pedals) - I had a direct short, as the shield of the guitar cable is connected to the negative pole of the 9V psu, but I was sending 9V positive to the ground of the channel switch jack !

...keeping my fingers crossed it's just the zener !

Thanks for everyone's input on this so far. It's much appreciated.

Cheers,

Kev.

Howdy Kev.......D2 didn't have any limiting resistor when the tip saw +9 vdc so probably blew. It doesn't protect as much as limit the "on" current of TS2 and TS4 (the affected stages). Putting +9 vdc to the ring shouldn't have hurt the non-inverting input of that op amp in A6 via the pot and putting +9 vdc to the sleeve was just a short of itself. With D2 blown, if open, TS2 and 4 will be turned on all the time and may be warm (depending on their specs). You should be able to reverse their switching (test them) by merely grounding the ring terminal of the remote jack OR by activating SW2.

What doesn't work as it stands now?

Gord

FIXED ! :-) It was only D2, the zener, that had gone. My amp's back to normal (just as well, as we have band practice in two hours !), and I'm grinning like a fool. :-)

Many thanks to everyone who helped here.

Cheers,

Kev.