Champ 12 Switching voltage woes

[Mr_bibbles]

Been slamming my head against a desk for a couple days now. Had a Champ 12 come in that has the clean channel always on with volume up to 10 no matter where the controls are.

http://www.thevintagesound.com/ffg/s...p_12_schem.gif

Replaced the Overdrive Indicator LED, the two LDR's, a defective foot switch jack, I can now switch channels and control the volume with the potentiometer...but I am still having issues... the voltage rail for the switching is too high!

Should be
CLEAN: 1.68
DIRTY: 3.28

IS: (on limiter)
Clean : 12-14VDC
Dirty: 4VDC

While the dirty channel runs, I have about 4v on the control rail while on a limiter, but it jumps up to a whopping 12-14V on the clean channel. I don't want to leave it on to get an exact voltage rating as I'm afraid of frying the last LDR I have on hand after toasting a fresh replacement.

I have the proper 27V at the Grid, where they tap off the control voltages. R31, R32, and R33 all measure within spec.

I am stumped... what could be driving the voltage up that high?

[The Dude]

When in clean mode, the cathode of the LED in LDR1 is grounded providing a current path. So, whatever voltage is supplied should drop to whatever the voltage drop across that LED is supposed to be (in this case, about 1.68V). There aren't many options for the cause of the fault. Some are:

1) The LDR is not getting grounded via the switch. Check the switched ground side of the LDR to make sure it is being grounded by the switch.
2) The LED inside the LDR is bad. Check the diode pins of the LDR with diode check on your meter.
3) The LDR is installed incorrectly. Check orientation.
4) Connection/Solder/Trace issues. (For instance, maybe the supply is not making it to the anode side of the LED in the LDR).
5) Things I haven't thought of.

[Mr_bibbles]

When in clean mode, the cathode of the LED in LDR1 is grounded providing a current path. So, whatever voltage is supplied should drop to whatever the voltage drop across that LED is supposed to be (in this case, about 1.68V). There aren't many options for the cause of the fault. Some are:

1) The LDR is not getting grounded via the switch. Check the switched ground side of the LDR to make sure it is being grounded by the switch.
2) The LED inside the LDR is bad. Check the diode pins of the LDR with diode check on your meter.
3) The LDR is installed incorrectly. Check orientation.
4) Connection/Solder/Trace issues. (For instance, maybe the supply is not making it to the anode side of the LED in the LDR).
5) Things I haven't thought of.

Thank you Dude; I have done all of these steps but this was an incredibly helpful way to make me retrace my steps. It was 2


2. This LDR is indeed shorted... this is the second one I had installed, so I had just assumed it was working correctly. It may not have been defective though, I'm going to solder in a regular LED to the top board and see if it lasts 10 minutes to ensure the new one won't get fried.

Thank you so much!

Question... if the Overdrive LED indicator light was not installed, leaving no connection to ground for LDR #2 (overdrive connection), would that raise the voltage enough to damage LDR #1 (clean) , since it wouldn't be dropping voltage through the two overdrive switching LED's? I installed the LDR's without first replacing the LED indicator diode...,

[The Dude]

If the LDR was shorted, the supply voltage in clean mode wouldn't be 12V, it would 0V because the supply would be grounded. Did you mean open?

[Helmholtz]

The voltage across C17 should be around 13VDC.
R33 is the current limiting resistor for the LEDs.
One LED drops around 1.7V and two in series around 3.4V. So these are the voltages you should see after R33 if everything is fine.
If you see 13V in clean mode, either the LED inside of "LDR1" is open (or reverse polarity) or the footswitch doesn't close/connect to ground.

[Mr_bibbles]

If the LDR was shorted, the supply voltage in clean mode wouldn't be 12V, it would 0V because the supply would be grounded. Did you mean open?

Ahh yes I did indeed mean open. Thanks guys!

It's worth noting an amature has "modded" the amplifier, including the power supply so I thought perhaps something was dreadfully wrong. I believe I'll be able to fix it from here, just simple open LED in the LDR.

Thanks!!!