Orange Rockerverb 100W Mk3

[dmeek]

This one has a fairly loud hum on the clean channel. All volumes are down, not affected by any tone controls, When I switch to the OD channel, hum fades away completely
as if something is powering down. Pulling any preamp tube has no effect. Hum is only on the speaker out, not effects send. No ripple on the power supplies.

Other than the hum, both channels are working and sound OK.

This is a strange one. A schematic would make it easy.

[nevetslab]

Here are the Schematics I have on the Orange Rockerverb 100.

Rockerverb 100W Power Board.pdf

Rockerverb Preamp Sheet 1.pdf

Rockerverb Preamp Sheet 2.pdf

[dmeek]

Thanks nevetslab, I believe that schematic is for the Mk1. I don't know how much difference there is with the Mk3. The Mk3 does not have any ICs or transitors, but the pcb is painted dark both sides so it's hard to see the traces.
The hum is caused by the channel switching circuitry and originates in the power supply or output stage. It happens even with the phase inverter pulled. Audio is switched with relays same as the MK1.

[dmeek]

I'm getting closer to figuring this out. The hum is present when any of the 3 relays is energized, as is the case when the clean channel is selected. I noticed the power supply for the relays is 10V. The relay coils are 5V.
When a relay is on the voltage drops to 6V and hum ensues.
Here is the schematic:

Power is taken from the filament winding which is balanced by 2 resistors. The Rockerverb Mk1 used a grounded center tap instead of resistors.
It seems to me the hum is caused by filament supply unbalance when current is drawn from the 10V.

Not sure why they used 5V relays, 9V would make more sense.
If anyone here has a Rockerverb open, could you tell me if it uses 5V, 9V or 12V relays, the schematic doesn't say.

Lifting the PCB is a real PITA so I tried paralleling caps across the 470/25v - no change.

[x-pro]

Check two caps 470/25V and 10000/16V

[dmeek]

The mystery is solved. It turned out to be a bad tube. A filament-to-cathode short in one output tube.

[g1]

It turned out to be a bad tube. A filament-to-cathode short in one output tube.

I guess no relay coils are engaged when using OD channel?

[dmeek]

The other two relays are for Reverb and Attenuation. Both are normally not engaged. There is a separate footswitch jack for each relay.