Red Knob Twin hum until V9 removed

[01redcobra]

I decided to do some checking before pulling the preamp/control board where LDR1 is located. Checked the voltage to the LM339N. As per the schematic, I should be finding negative voltage on pins 4,6,8, and 10. Is that correct? If it is then something is wrong, because I'm finding like 1.26+ volts on those pins. 5,7,9, and 11 show 1.9+vdc. Diode problem at D107 and D104?

[g1]

Don't see any voltages for the IC inputs on the schematic I have. But the only one's that should matter here are pins 1&2, 2.8VDC when Ch.1 selected, .59VDC for Ch.2. If those are correct, then try what JazzP said, disconnect LDR1.

I'm not understanding why the new vactrol for the reverb caused the diminished volume on channel 1.

Unable to rule out as coincidence yet.

[01redcobra]

Don't see any voltages for the IC inputs on the schematic I have. But the only one's that should matter here are pins 1&2, 2.8VDC when Ch.1 selected, .59VDC for Ch.2. If those are correct, then try what JazzP said, disconnect LDR1.

Unable to rule out as coincidence yet.

They are off. pins 1&2 are showing 1.9vdc when ch.1 is selected and .012vdc with ch. 2 is selected. But I do hear channel 2 come on when selection is made.

[g1]

Are you using a footswitch? If so, is it the right one for this amp?
The Twin Red Knob footswitch.pdf
If not, the footswitch jack's switch contacts must close with nothing plugged into it.
Also, check the "D" voltage readings shown under power transformer on amp schematic.

Edit: A failing opto (diode side) may mess up some of the readings anyway. So you may end up having to disconnect LDR1 regardless.

[01redcobra]

No footswitch with the amp. I do have 29vdc on pin 3 of the IC101. What should I be seeing at R132? Should that be 30vdc also? I am measuring only about 13.8vdc there. The same with the jack for the footswitch. And did you mean to pull the LDR1 altogether?

[52 Bill]

No footswitch with the amp. I do have 29vdc on pin 3 of the IC101. What should I be seeing at R132? Should that be 30vdc also? I am measuring only about 13.8vdc there. The same with the jack for the footswitch. And did you mean to pull the LDR1 altogether?

One side of R132 should have 30 volts AC and I would've guessed a pulsed 15 vdc on the other side, so the 14 volts that you get is about right.

Changing the reverb ldr should not have caused this, but it did happen after it was done. Recheck all of your work, etc.

If channel one is as loud as normal when you are plugged into the parallel inputs, then the optos are working fine the switching circuit isn't. Check all of those switching diodes, like D2, D3, D5, D6, etc.

[01redcobra]

I've rechecked my work and found nothing wrong. Checked the diodes with my Fluke, D2-3-5-6. All are ok. I am tempted to reinstall the original LDR for the reverb and see if it changes anything. With the input in the switched inputs I have a good signal in and out of V7B, but it's really weak going into V7A even with the volume up. Just leads me back to LDR1.

[Jazz P Bass]

Have you tried removing LDR 1?
(did I say that already?)

[52 Bill]

Scoped at V7 and I have a good signal into and out of V7b, but almost no signal into V7a pin 2. This is with the input in the channel switching inputs. If I send it in thru the paralleled inputs, it gives me a nice strong signal and it's volume seems up to snuff.

Maybe I read this wrong, but I thought that you said that channel 1 worked fine when you are plugged into the parallel jacks. If that is so, then LDR 1 is working and it is just not turning off when you are plugged into the channel switching jacks.

If I read that wrong, then sorry for my misunderstanding.

[01redcobra]

Maybe I read this wrong, but I thought that you said that channel 1 worked fine when you are plugged into the parallel jacks. If that is so, then LDR 1 is working and it is just not turning off when you are plugged into the channel switching jacks.

If I read that wrong, then sorry for my misunderstanding.

Channel 1 works normally with the signal going into the parallel jacks. When in the switching jacks, it works with the channel 2 controls engaged, but volume dies when in the channel 1 switch position. About 1/4 the volume if that.

[01redcobra]

Have you tried removing LDR 1?
(did I say that already?)

I'm going to try that later on today. I also have a Vox AC30CC with something loose(shows up as crackle at the tail end of notes) that only shows up when it's in the cabinet. Not my week for sure.

[52 Bill]

Logically, if LDR1 turns off when using the parallel jacks, then there is probably nothing wrong with LDR1 itself. There is a switching problem that is turning on the led when it should be turned off when using switching mode. (Did I say that already?)

[01redcobra]

Logically, if LDR1 turns off when using the parallel jacks, then there is probably nothing wrong with LDR1 itself. There is a switching problem that is turning on the led when it should be turned off when using switching mode. (Did I say that already?)

With the LDR1 disconnected, channel 1 is back where it should be. Now digressing to post #32, I asked about the voltage on pins 8, 10, 4, and 6 and suspected that I should see a negative voltage there. I am getting a positive voltage on those pins. And would this be throwing my switching voltages off?

[52 Bill]

Now digressing to post #32, I asked about the voltage on pins 8, 10, 4, and 6 and suspected that I should see a negative voltage there. I am getting a positive voltage on those pins. And would this be throwing my switching voltages off?

There will only be negative voltages there if you have a footswitch plugged in to the footswitch jack. Otherwise D201 keeps the voltage there a pulsed positive dc.

The important voltage for your problem is the output of IC101a at pins 1&2. There should be about +3 vdc there unless you pull the channel switch knob out.

When you channel switch, do the lights switch correctly?

[01redcobra]

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.