Hi,

I am attempting to repair a Midland Model 78-100 VHF Marine Radio for
a friend. I have basic radio electronics knowledge, some
troubleshooting skills, a 275MHz scope, VHF sig generator, freq
meter, RF power meter, a working VHF radio etc, but am unable to
locate the problem.

Symptoms a completely dead on receive and transmit on all channels,
but unit powers up good, all power supply circuits and voltages are
good, display and frequency display circuitry are good, no detectable
cold solder joints nor sal****er corrosion anywhere, power to all
chips is good.

I first suspected the PLL circuitry (due to its being common to RX and
TX), but signals coming out of the PLL, prescaler and VCO chips appear
ok.

You've not said whether you have a schematic but if so, look for the
TX/RX change-over circuitry. If that is not working properly it could be
disabling both parts of the transceiver. Commonly that kind of low-level
switching is done by biassing a diode in the IF region or changing the
gate voltage on a MOSFET. An ICOM ham rig that I repaired recently had a
common 8V line switch to do just such a job.

I would ignore the transmitter and just try to get the receiver going
as the fault, whatever it is, will probably be common to both as above.
Coupling your signal generator into it stage by stage (starting with the
last IF) should narrow things down.


Best of luck - Joe