I found it difficult to source fender pots at a reasonable price for theses amps so I bought a load of different values from China. If you remove the mounting legs from the original pots and with a little bit of basic engineering you can fit the legs to the new pots. Job done for little money....

To clarify.
I had replaced the reverb potentiometer but it was marked with the incorrect value. Replaced with the correct value and reverb worked again. I also tried the original tank which is working also. It doesn't seem to make any difference which way the new tank is installed, ie in/out rca plugs....

UPDATE.
New tank arrived and fitted but alas still no reverb so I had a look at the o/p side and found that the reverb pot was 50k and not 250k, so a man made fault. Now working.
For the life of me I cannot tell the difference in sound reverb no matter what way I connect the tank. The original unit has a red RCA on the side marked IN, whereas the new one has a red RCA marked as out. The original ones markings are obvious as the red RCA has signal coming from the preamp stages so I would call it i/p also.
Puzzling.

First thing I checked.
Both ends are closed circuit which I measured on the rca sockets....

Hi Capleaker,
Thanks for your reply and schematic.
I have a schematic and went through the normal process of signal tracing. I replaced U4 and U5 as I thought this would be a quick and cheap fix as these handle the i/p ond o/p of the tank reverb. I injected a tone on the o/p tank socket and this gets through to the speaker ok. I tried injecting a tone (experiment) on the tanks i/p but nothing appears at the other side. The i/p and o/p on these tanks have different impedances so I realise that my test may not be very scientific or conclusive. I found some DC resistance figures for the...