Hi,

I've done a stupid thing with my new guitar amp, and sent 9v DC where
it shouldn't go, and now it won't change channels.

Please can someone help ? I've uploaded a schematic (866kb) he
http://homepage.ntlworld.com/anengineer/vc15.pdf

Scroll to the bottom page (page 3) (for some reason they included an
earlier, incomplete revision on page 2 !), and the relevant section is
grid ref A3 to A6.

I connected a badly wired external switch box to the 'Remote' jack in
grid A3, and sent 9v DC up all three connectors - sleeve, ring & tip.

The normal operation of the amp is that it has a 'clean' channel and a
'distortion' channel, switched by either the latching push-switch on
the amp ('SW2A' - grid A4), or by a remote switch via the jack in grid
A3. Either switch takes the line to ground to activate the relay (grid
A5/6), which switches the channel, and illuminates LED1 (grid A6).

Currently, LED1 is permanently ON and the amp is stuck on the
distortion channel. Neither SW2A nor an external switch has any effect
now, and there is no 'click' from the relay.

The 'Remote' jack also allows switching on/off of the built-in reverb
(short ring to sleeve [ground] for OFF) - but thankfully this seems to
still be working ok. - However, shorting tip and ring on the 'Remote'
jack also switches the reverb off - suggesting that the 'tip' line is
permanently shorted to ground.

Looking at the schematic, my guess is that D2 has gone short-circuit -
but then, that is an amateur's guess !

Please could someone suggest what I should test / replace ?

Many, many thanks to anyone who can advise.

Cheers,

Kev.

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

kip
wrote in message
ps.com...
Hi,

I've done a stupid thing with my new guitar amp, and sent 9v DC where
it shouldn't go, and now it won't change channels.

Please can someone help ? I've uploaded a schematic (866kb) he
http://homepage.ntlworld.com/anengineer/vc15.pdf

Scroll to the bottom page (page 3) (for some reason they included an
earlier, incomplete revision on page 2 !), and the relevant section is
grid ref A3 to A6.

I connected a badly wired external switch box to the 'Remote' jack in
grid A3, and sent 9v DC up all three connectors - sleeve, ring & tip.

The normal operation of the amp is that it has a 'clean' channel and a
'distortion' channel, switched by either the latching push-switch on
the amp ('SW2A' - grid A4), or by a remote switch via the jack in grid
A3. Either switch takes the line to ground to activate the relay (grid
A5/6), which switches the channel, and illuminates LED1 (grid A6).

Currently, LED1 is permanently ON and the amp is stuck on the
distortion channel. Neither SW2A nor an external switch has any effect
now, and there is no 'click' from the relay.

The 'Remote' jack also allows switching on/off of the built-in reverb
(short ring to sleeve [ground] for OFF) - but thankfully this seems to
still be working ok. - However, shorting tip and ring on the 'Remote'
jack also switches the reverb off - suggesting that the 'tip' line is
permanently shorted to ground.

Looking at the schematic, my guess is that D2 has gone short-circuit -
but then, that is an amateur's guess !

Please could someone suggest what I should test / replace ?

Many, many thanks to anyone who can advise.

Cheers,

Kev.

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.

www.techdata-kicksass.net

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.

wrote in message
ups.com...
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.