Larry Gagnon wrote:
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
i assume that you have acknowledge that the unit is actually attempting
to RX/TX via the mic button?
like for exampl a TX light coming on etc..
in any case, you may have a mixer/IF stage in the frequency section that
is common to both RX and TX.