Not sure if this is applicable to your model, but a common problem with
marine rigs is that the antenna or coax opens up. When TX is pressed, the
transmitter being unloaded, tends to blow out the pin diodes that switch the
front end between Tx/Rx. The Rx front end winds up with 25 watts directly
into the input. The transmitter power module might cook as well.
bg
Larry Gagnon wrote in message
...
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, 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.

Any suggestions as to where I might start looking would be greatly
appreciated.

Larry Gagnon