Randall RH200SC G2 Reverb switching issue

[The Dude]

^^^^^^ Good question. I was also wondering if maybe it's the wrong footswitch for the amp. Is there a model number on the footswitch or some way you can verify that it's the correct one? Did it ever work? Or, yes it worked and then malfunctioned?

[g1]

Also, Q24 voltages, when reverb switched on, and off.

[Delta362]

I can't find a model number on the 5 button footswitch, but it's definitely the correct one. One of the complaints was that the pedal's reverb switch isn't working. I don't know if that meant he bought it used and just got it with it not functioning, or if it was working and recently stopped. I did test the reverb button as I moved the wiring on it to control the chorus and that button/LED controlled the chorus, but in retrospect, I didn't move the reverb switch wiring to another button to test that. I did ohm out each switch and checked each LED.

Voltages on Q24 in clean channel mode are as follows:
Reverb switch off - E 0, B 0.7, C 5.5 slowly decays does to 5.2
Reverb switch on - E 0, B 0.7, C 2.4

Reverb switch off for the rest of these: The E was 0 for all, B was 0.7 for all.
The collector changed from 5.5v to the below with each button pressed individually.
Drive on, C 0.03
High gain on, C 0.02
Clean Boost on, C 0.03
Chorus on, C 0.03

I don't have a schematic for the footswitch, but it goes as follows:
SCN3 pinout top to bottom on the schematic is pins 1 - 7
Channel
Gain
Boost
REverb
Ground
Common signal (not ground)
Chorus

Each signal pin that comes in goes to one side of a pushbutton. The normally open side goes to the anode of the LED. Cathode of the LED goes to Common. All LED cathodes are tied together to that common pin 6.

[Enzo]

Looks like the footswitch cares about the settings of the panel switches. ANy chance we have to set a panel switch the other way to enable the FS function?

[Delta362]

All of the panel pushbuttons must be in the out (open) position for the footswitch to work. When I pressed the channel switch on the amp to go dirty, the amp switched to dirty mode. The footswitch LED did not light up and I was not able to switch between clean/dirty using the footswitch.

One thing I did notice is that when on the clean channel, the LED is mostly bright. When I turn another function on (say chorus or the clean boost), the clean reverb LED goes full bright. I think this is because of the pin 4 of the footswitch voltage dropping from 5.5 to 2.4 pulling more current across the clean channel reverb LED. The dirty channel reverb LED is full bright and unaffected by any other pedal position when in dirty reverb mode.

[Enzo]

Do all functions work without the footswitch?

[Delta362]

Yes, though there is no switch on the amp to turn off the reverb.

[g1]

Q9 over by IC5B is your reverb kill. It should read low resistance from source to drain when power is off.
Positive voltage at the gate should turn Q9 off, which enables reverb.
Sounds like you must have the positive voltage there all the time, or the fet is open.

[Delta362]

Q9 seems to have +13V on the gate all the time, keeping the FET off, thus keeping the reverb on. The lack of voltage change (also stuck with high positive) on Q18 base also seems to prevent the reverb from turning.

[g1]

High positive doesn't sound right for Q18 base, what are Q18 voltages with rev. switch on and off?

[Delta362]

Q18 E 14.70, C 14.64, B 14.04 clean or dirty, no change with any other footswitch toggling.

I also measured resistance from the 15V rail to R136/R135 junction and measured 15k. Then I also measured from Q18 base pin to the same junction and got 100k. Then I verified from +15v rail to Q18 base and had 115k so the resistances were good and had a proper path for those at least.

The R135/R136 junction is also the same as pin 4 of SCN3. I am seeing the 5.5V to 2.4V to 0V there, depending on reverb pushbutton position or other footswitch button.

[g1]

I have a sneaky suspicion that if you disconnect Q24 collector, it will be fixed. Is there a jumper link to it anywhere?
See how all the panel switches are but the reverb line doesn't have one? And the other functions don't work if the panel switches are closed.
What doesn't make sense is that they couldn't figure a way to have it work with or without the footswitch, so my logic is probably flawed.

[Delta362]

No related jumpers found, but I have confirmed your suspicion. I desoldered the collector of Q24. Reverb footswitch LED and reverb turns on and off appropriately. Thank you!

I guess next step I'll go back and ask if they want it to only function with the footswitch, or to be always on. Strange that the design didn't account for one more pushbutton on the panel to engage/disengage the reverb. One other option is to use a push pull pot that has a switch on it. Use that to switch Q18 collector in and out.

Edit: Unless of course, there's still something going on with it preventing it from functioning...

[g1]

Edit: Unless of course, there's still something going on with it preventing it from functioning...

Yes, I would think this is just a workaround. The fault is probably still there. If someone forgets their footswitch at home, the reverb should default to 'on'.