John,
The AF gain is huge but typically there has not been problems with acoustic feedback.
I guess you have tested with an external speaker. Is it ok?
Please check that the main board capacitors C4...C7 and C12...C15 are right G0G type. The G0G color is grey compared to the X7R brown color. X7R type capacitor is "a very good" condenser type microphone, C0G type capacitor is much more immune to mechanical change.
Have you already adjusted the AGC? When it is adjusted properly the AF gain is little lower.
If the caps are right type there can be some unbalance in the AF differential path. That can be checked by measuring the operating point DC voltages according to the voltage table
http://www.nikkemedia.fi/juma-trx2/rev- ... -table.pdf73 Matti OH7SV