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

"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.

Don't you need an FCC license to do that?

N

On Sun, 12 Dec 2004 19:52:26 +0000, NSM wrote:


"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.

Don't you need an FCC license to do that?

N


Oh please, do get a life you sad little man! And since when did you assume
that I live in the USA anyways and that I am subject to FCC regs?

Larry

Howdy Larry.......You didn't mention having a schematic so I'm assuming
you're troubleshooting blind. Do you have any receive audio noise and/or
does the squelch seem to work? If no noise is heard I might start with
injection AF at the volume control to see if you have an audio stage (and
make notes continuously when troubleshoot unless it's a very familiar radio
to you) Does the radio seem to switch from receive to transmit? I'd
probably start by looking at the basic transmit chain for activity starting
with PTT and looking for some low level on a known frequency....say 156.300
(CH6).....of course make sure you're dumping into a suitable dummy load
(good for 25watts at least). Use that fancy scope of yours to look for the
TX chain activity.......It wouldn't surprise me if you had something "like"
a problem with the basic T/R circuitry if you don't have any activity at
all - a typical setup might use a dedicated IC for this purpose. If you
have a schematic.........and could attach it via e-mail I'd be glad to walk
you through some troubleshooting.

Note: Having a second receiver capable of receiving these frequency's
(cheap scanner for example) can make quicker work by finding out whether
you have any low level activity when placed in the vicinity (same room
etc).

73 - Gord VE1AJF

"Larry Gagnon" wrote in message
news | On Sun, 12 Dec 2004 19:52:26 +0000, NSM wrote:
|
|
| "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.
|
| Don't you need an FCC license to do that?
|
| N
|
|
| Oh please, do get a life you sad little man! And since when did you assume
| that I live in the USA anyways and that I am subject to FCC regs?
|
| Larry

Plonk!

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.

Not sure if this is applicable to your situation, but I just repaired a
different brand radio with similar issues that had controls in the mike. There
was a small 8 pin connector on the board in the radio from the mike cable that
had broken loose from the PCB. I wrapped a small wire around the end of my
soldering iron, put a couple pairs of reading glasses on, (bad eyes),
resoldered the connector, and have a happy customer.

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

On Sun, 12 Dec 2004 09:42:40 -0800, Larry Gagnon wrote:

thanks to all who replied to my query. I found the problem in the VCO. 3
cold solder joints. It meant removing the VCO compartment shields and all
the beeswax it was sealed in. Fun that was, but it worked!

Larry