Sears Roadtalker 40--anybody have the mod to "clip"?
Steve Eklund
Hi all,
I was at a flea market on Sunday and picked up an old Sears Roadtalker 40
mobile radio for 5 bucks. It still had the original box and looked like
it has not been used much. It is a fun radio to use and will make a nice
back-up radio for me, but the problem is that it sounds awful on
transmit. My station tester reads only 25 percent modulation, and it
sounds like it. The radio does not have an internal adjustment pot for
the Automatic Modulation Control (AMC). It probably uses a fixed
resistance. Since I don't have a schematic for this unit, I was wondering
if anybody knows which resistor/diode combination makes up the circuit,
or if anybody has the mod to "clip" this radio?
Thanks in advance!
Steve
Codys PC
In article <3252AC...@compuserve.com>, Steve Eklund
<72527...@compuserve.com> writes:
I would say it was already clipped and the driver/final pair may need to
be replaced.
Or...
ASSUMPTION=On...
Radio does TX at about 3.5 to 4 watts, audio to optional PA seems to be
ok, and RX shows PGP [Pretty Good Poundage] for receive. Get out that
Magnifying Glass from the drawer and get a flashlight with some good
batteries - you may be at this for a while...
Some of use a lighted magnifying glass - not only to burn ants on the
sidewalks or light our cigarettes - we also use it in the third office -
which is what it was REALLY intended for...
The Audio section with the diode that taps off the modulation transformer,
[you didn't mention if it had SSB - so my guess is for AM only] the diode
may be bad - possible short or blown open. Or the person that had it
before you possibly clipped the wrong part and got rid of it.
The limiter conventions follow; typically using a 1N60 diode, hooked to
the feedback path, from the audio transformer and to a band pass, bias
control pair of transistors by the Mic amp section.
What it sounds like from your description, is they've removed the limiter
or one of the bias control transistors [clipped lead] and the audio is not
being either amplified, or the feedback path is shorted.
Check the caps in this area, one may have failed - or has a short on the
circuit trace side.
If the recieve portion seems to work ok, then suspect the Mic amp curcuit
or mike plug for shorts or opens. This includes wires, solder joints,
resistors and caps, along with the transistors. The RX portion of the
signal is piped in past that point, through a cap, into the audio chip.
If nothing else works, check to make sure the Mod transformer doesn't have
a short in it's wiring.
When you Tx, you should have close to 12 volts on the board following the
transformer over to the driver/final section when the mike breaks contact
with the speaker to switch over. The cap used to block the DC to the
speaker may be shorted to ground through a protruding screw striking
against the caps' body. OR the diode used to rectify the RF from getting
back into the audio transformer and messing things up, may be an
open/short condition.
HINT: This kind of a problem can be attributed to just a construction
problem, a bolt or screw was put in the wrong place and tightened a little
too far - and bam! What happened?
The audio section is pretty complex to try and map out, take your time.
You'll need to look at the TX side and audio chip. Finding a short or
open in these areas can be time consuming and very frustrating. You know
in time which diode to clip - if needed - I've already spelled it out for
you - you just need to correct the problem and then do what you want.
Sorry to hear about the radio, but a simple fix may be all that is needed.
I hope you can find your way around inside that thing - should be a good
radio for anyone to have if you get it working. I think it would be a
keeper.
There's a lot of ORs' in this message, lots of things can go wrong - let
us know how you've made out with the darn thing.
:+> Andy <+:
Scannr
what is the model number?
Carlos Avellanet
>I was at a flea market on Sunday and picked up an old Sears Roadtalker 40
>mobile radio for 5 bucks. It still had the original box and looked like
What's the model number? I may be able to help but I have them listed
by model numbers, not names.
Carlos.
OZARKS 333
This radio is an award winner in my book. If it has sideband you have
a real find. The SWR alert light is a great plus and with a jumper to
ground and a switch this radio will go up to .615 or so. I talked all over
the USA and into Europe in 1987-1988. Great radio!
Bill Eitner
There were a number of Road Talker models. The mods are model
specific. Look on the plate on the back to find the model number.
If the plate is gone, you can go by which PLL chip is in it.
I wish I could be of more help....BILL
