Red Knob Twin hum until V9 removed

[52 Bill]

Bill, The voltage at 1&2 is 1.9vdc with channel 1 switched on and down to .01 when channel 2 is switched. Yes, the channel lights switch correctly. Also pins 13&14 are at 1.9 but I don't have a foot-switch to check "on" voltage there.

Are these reading with LDR1 in circuit?

LDR1 is turned off by floating the led at a "high" voltage, probably about 3 vdc. If the control voltage at pins 1&2 doesn't get high enough the led will turn on enough to lower the resistance of the cells and lower the volume of the channel. Can you read the voltage across the led of LDR1? When channel one is selected, there should be little or no voltage across it. When Channel two is selected there should be a "high" voltage when the led is turned on.

The voltage at pins 13&14 only turns on and off the reverb. The voltage there should normally be low, but it should rise when there is nothing plugged into the inputs or when the footswitch is used.

[01redcobra]

Are these reading with LDR1 in circuit?

LDR1 is turned off by floating the led at a "high" voltage, probably about 3 vdc. If the control voltage at pins 1&2 doesn't get high enough the led will turn on enough to lower the resistance of the cells and lower the volume of the channel. Can you read the voltage across the led of LDR1? When channel one is selected, there should be little or no voltage across it. When Channel two is selected there should be a "high" voltage when the led is turned on.

The voltage at pins 13&14 only turns on and off the reverb. The voltage there should normally be low, but it should rise when there is nothing plugged into the inputs or when the footswitch is used.

The led portion of the LDR is still in circuit. The other 3 leads lifted. Reading across the led, I am seeing 1.5vdc with channel 1 engaged and 1.65vdc with 2 engaged. Not so good eh?

[52 Bill]

The led portion of the LDR is still in circuit. The other 3 leads lifted. Reading across the led, I am seeing 1.5vdc with channel 1 engaged and 1.65vdc with 2 engaged. Not so good eh?

Out of curiosity, did you check to see what the voltages at pins 13 & 14 are with and without a plug inserted into the input jacks? What is IC101, LM339? Do you have one to swap in?

[01redcobra]

I checked the voltage at 13 & 14 and it's 1.94vdc without and 1.87 with a plug in the input. As far as the LM339, I don't have one here, but I should be able to pick one up local tomorrow if need be.

[52 Bill]

I checked the voltage at 13 & 14 and it's 1.94vdc without and 1.87 with a plug in the input. As far as the LM339, I don't have one here, but I should be able to pick one up local tomorrow if need be.

It seems that none of the voltages are correct on the outputs. Check all of the diodes and other parts connected to IC101 (if you haven't already done it) and if there is nothing there that is wrong, then change the chip and see if it acts normally.

[01redcobra]

It seems that none of the voltages are correct on the outputs. Check all of the diodes and other parts connected to IC101 (if you haven't already done it) and if there is nothing there that is wrong, then change the chip and see if it acts normally.

Thanks so much Bill! I checked all the resistors and diodes like you said. Everything looked good, so I replaced the IC and we are good to go. No more hum and channel switching and reverb are very happy. So am I. And thanks again to Jazz and G-One for your help too.

[52 Bill]

Glad to hear you got it sorted out.

[Labjr]

Hi. I'm new here.

I have a red knob twin with the exact same problem. It had no reverb. I clipped three leads from LDR101 and the reverb came back but now has a loud hum unless I pull V9. I gather you changed the LDR and the LM339 IC? The LDR is now hard to find. I wonder if the original LDR is till good but just the IC is bad?

I know wires will start breaking when I pull that board so I hope I only have to do it once. This amp is a real PITA. Thanks