Laney VC15 channel switch problem...??

[acorkos]

both the panel channel switch and the footswitch are not working correctly. LED comes on when drive channel is active. LED is supposed to go off when clean channel active...but the LED stays on, and when the clean channel is selected, there is no sound. any ideas on what to check?

[Jazz P Bass]

It appears that the switching circuit is not switching.
Q1 (J117 fet) over at V1B is normally on/ 300 ohms to ground.
That will disable the clean channel.
Permanently if Q1 does not get a gate signal.
And that comes from TS4, transistor.
Either the front panel or the foot switch should pull TS4's base low, which will drive Q1 gate low.
But the LED function comes off of TS2 & TS3!
So there are two issues involved.
I would start looking at base voltages & see where it is failing.
You may find blown traces or components, seeing that it is a footswitch input.
(you plugged what, where!)
Or maybe failed solder joints.

[acorkos]

thanks

i pulled up the pcb...many of the switching components (all except relays) are surface mount on the underside of the pcb. any trick to working with these things? i'm a little nervous about working on a live amp with the pcb dangling in the air

[Jazz P Bass]

The first thing you could look for is transistors that are blatantly shorted.
Power off.
Then with power on, I would suggest that you measure the value of each base voltage of the transistors for the switch circuit.
Here is a transistor pinout sheet (not to be taken as 'bible', but the surface mount transistor is pretty standard)

[acorkos]

The first thing you could look for is transistors that are blatantly shorted.
Power off. Then with power on, I would suggest that you measure the value of each base voltage of the transistors for the switch circuit.

transistors Q1, TS1, TS2, TS3, TS4 not shorted w/power off

TS1 and TS3 showing only trace base voltage w/channel switch in either position

[Jazz P Bass]

O/k.
Here is the switch logic.
TS2 is what 'controls' TS1 & TS3.
In it's normal unmolested state, TS2 base is held high through R2. (transistor On)
That high on the base translates to a low on the base of T1 & T3. (transistor Off)
Closing the footswitch or the front panel switch should pull T2 base low.
Which will let T1 & T3 base go high. (relays On/ Led's On)
The logic for Ch 1:
TS4 base is normally high (Transistor On)
Q1 Jfet is normally closed, Ch 1 is muted.
With a switch closure TS4 base will go low (Transistor Off).
That will let Q1 Jfet gate go high, turning it off. (Ch 1 unmuted)
So get out the DVM & see what is not true.

[acorkos]

both relays aren't switching correctly when drive switch is engaged

base voltages:
ts2 clean/2.7v, drive/8.3v
ts1 clean/.3v, drive/.1v
ts3 clean/.1v, drive/0v

ts4 clean/0v, drive 15.7v

q1 @r67 clean/0v, drive/15.7v

[Jazz P Bass]

It appears that I had the logic states backwards.
Q1 JFet mutes the dirty channel.
So Q1 is getting it's signal.
That leaves T2, 3 & 4.
I do not see how the Led's are even lighting with Drive/ 0 volts on the base of T3.
At any rate start looking at resistor values.
R3 may be open or the connection path is.
That is were the base voltage comes from for T2 & 3.

[Enzo]

TS2 could be shorted, if the bases of the other TSs are stuck low, then TS2 collector must be, its base moves but not its collector. That should turn off the LED and the relays. If the base has insufficient drive but the load remains energized I worry that TS3 might be shorted. And your open R3 would certainly do it as well.

The mute works off the switch through TS4, which is totally separate from the TS2/3/1 guys. That explains why the dirt goes off with the switch even if the TS2 things don;t happen.

How's this sound. Switch mutes the dirt like it should, but TS2 is unable to pull in the relays, so the relay contacts never transfer over to the clean signal path. So no sound.

[acorkos]

...start looking at resistor values. R3 may be open or the connection path is.
That is were the base voltage comes from for T2 & 3.

r3 meters 4.7k

TS2 could be shorted, if the bases of the other TSs are stuck low, then TS2 collector must be, its base moves but not its collector. That should turn off the LED and the relays. If the base has insufficient drive but the load remains energized I worry that TS3 might be shorted. And your open R3 would certainly do it as well.

The mute works off the switch through TS4, which is totally separate from the TS2/3/1 guys. That explains why the dirt goes off with the switch even if the TS2 things don;t happen.

How's this sound. Switch mutes the dirt like it should, but TS2 is unable to pull in the relays, so the relay contacts never transfer over to the clean signal path. So no sound.

that sounds good, but ts2 has similar meter readings to ts3 and ts4...i don't think it is shorted

[acorkos]

i resoldered all the SM joints in the switching circuit....hurray! it's working now.
thanks for the help!