Thunder 1 A bass pick-ups

Hi, 
One of the pick-ups on my Thunder 1 A bass is not working in passive mode, the other one seems fine. Any advice on what pick-up/model number to replace it with/search for on the internet? The guitar has a 1984 serial number. Is  "Hard Puncher I pickup"  a model type?
Thanks,
Gabriel

Does it work in active mode?

Well, it is a bit of an oddity. It kind of works in active mode when the volume pot is in a certain position, close to max volume, but otherwise the top two strings (E and A) are very loud and there is little noise from the D an G strings. I have tried changing the vol pot but same problem. That is why I said "not working in passive mode", as this possibly rules out the problem being with the active circuit. It may be a wiring problem, something not grounded somewhere, but I have checked the wires and they seem fine.

In case you have not seen this already, here are two wiring diagrams for the T-IA so you can double check the circuit:
LINK

And, at the risk of being Captain Obvious, have you also checked the batteries?

A squirt of switch cleaner. Good start.

I may have fixed it. The wire from the active/passive switch to main volume was to the wrong connection, as was one of the pick-up cables. So hopefully it has sorted it now without the need for new pick-ups. Thanks for advice!