OK. Have you checked with varying the settings of the 5k pot in the amp
section? - have you checked that the battery volatge is correct (ie,
measured it?). If that doesnt work, connect your CRO across the mic.
capsule and see if its the cause of the problem..... If its the amp,
shouldnt be too hard to troubleshoot - even if you shotgun replace
EVERY component on the board its a simple job. For schematic, just
Google for it - for some reason, I cant copy the link to this reply.

Andrew VK3BFA.