Sir zamiac . . . . . . . .
A couple more probing questions while we still have you here on the 'ole Interweb . . . .
FIO . . .That unit can be used in either a conductive mode or an inductive mode.
Are you exclusively using one mode or do you switch, and if so . . . . . is your problem present on both modes ?
Are your suspect ? ? ? "transistors " in the receiver or the transmitter half ?
If you are banned from parts sales . . . forget any tech info from them . . .we will be solely dependent on good close up photos of queried sections. The analog aspect of that unit is using nothing more exotic than low frequency RF.
PLUS I am seeing analog metereing and headphones.
My first dealings with this SPECIFIC detection technique was back in the 1950's and this particular detection concept of unit was THE established gold standard for both mass and depth . . . . . plus sensitivity.
Just slide apart the two halved fiberglass clamshells . . . unless they are using hi density, ruggedized Cycolac now . . .affix to the carrying handle / balance beam, null out and then start your exploring.
73s de Edd