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.

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

Larry Gagnon VE7EA

"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, 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.

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

Larry Gagnon VE7EA

IS the crystal in the PLL "ON FREQUENCY"? IF it is way off - but still
oscillating - due to some defect, that could cause the problem. The voltages
and such may "appear" ok, but the crystal won't allow "ON CHANNEL" signals
to come or go. Check your frequencies and such. The other thing common - is
some of the audio stages. Are they working? How about the function switches,
maybe not making full contact - especially if there is a radio/pa switch or
something similar. They could create similar problems. Those are the other
common things aside from the Power Supply legs. Microphone wiring is
sometimes common also, a broken wire or two, won't allow it to transmit OR
audio to come through to the speaker. I don't have any info on this radio to
be more precise.

Lots of luck.

DM

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

On Mon, 13 Dec 2004 13:28:24 GMT, Joe McElvenney
wrote:

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.

Good advice. Also check the LO(s) is/are working properly. It should
be a piece of cake to track down a fault like that, certainly if you
have a schematic.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

On Sun, 12 Dec 2004 09:46:01 -0800, 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, 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.

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

Larry Gagnon VE7EA

Check the microphone. A broken ground wire may cause this.

On Sun, 12 Dec 2004 09:46:01 -0800, Larry Gagnon wrote:


....thanks to all who responded. I have now narrowed the problem down to
the VCO, no LO signal to the mixers. Unfortunately the VCO is in a 1.5"
square tin-shielded box, all nicely sealed with beeswax. Wonderful,
Murphy's Law ain't it that the prob is in the most hard to get at part of
the radio! Wish me luck....

Larry VE7EA

watch ebay for a radio or parts..

--

73
Hank WD5JFR
"Larry Gagnon" wrote in message
news
On Sun, 12 Dec 2004 09:46:01 -0800, Larry Gagnon wrote:


...thanks to all who responded. I have now narrowed the problem down to
the VCO, no LO signal to the mixers. Unfortunately the VCO is in a 1.5"
square tin-shielded box, all nicely sealed with beeswax. Wonderful,
Murphy's Law ain't it that the prob is in the most hard to get at part of
the radio! Wish me luck....

Larry VE7EA

On Wed, 15 Dec 2004 14:40:22 +0000, Henry Kolesnik wrote:

watch ebay for a radio or parts..

You're right on there - it might come down to that! I'll make sure I don't
sell it to a ham. ;-)