Sunn beta lead - no reverb with footswitch plugged in!

[unparalleled]

I have just restored this amp and made a foot switch.
I have a good grasp of electronics, or so I thought!
The foot switch works correctly switching channels A/B & "Both" but when it is plugged in the reverb stops working.
This is puzzling because the reverb switch only switches the ground on pin 4 to pin 2 on the amp.
unplug the foot switch plug and the reverb is operational.
Has anyone struck this effect?

[unparalleled]

I have just restored this amp and made a foot switch.
I have a good grasp of electronics, or so I thought!
The foot switch works correctly switching channels A/B & "Both" but when it is plugged in the reverb stops working.
This is puzzling because the reverb switch only switches the ground on pin 4 to pin 2 on the amp.
unplug the foot switch plug and the reverb is operational.
Has anyone struck this effect?

Sorted! (after a bit of head scratching)
It turned out to be the recycled multi screened cable, leakage between conductors.