[Milsurplus] Bob's PRC-74 questions

[Mark]

Bob W9RAN wrote...

"I've been able to get my newly-acquired PRC-74 working on receive at
least, after replacing a bad "varistor" in the 9 volt regulator
circuit.  Anyone know what these parts are, exactly?  With a nominal
resistance of 300 ohms, they're not what I think of when I think
varistor (i.e. MOV).

If anyone has experience working on these radios I have a couple of
questions related to the mechanical linkage between the Mhz selector and
RF unit.  The front panel is pretty cramped and complicated, and I'd
like to make sure I'm on the right track.

Also - is there any info on LSB conversion?  It seems that this was done
to my unit."

Thanks and 73,
Bob W9RAN


Bob et al,

I've had extensive experience working on PRC-74s over the years; the 74 
is one of my HF favorite packsets for a number of reasons.  Among them are 
that it was the world's first synthesized HF packset, and it was my first 
personal HF packset back in 1981.

The 74 pushed the state of the art to the limit when it was designed in
the early 60s.  There were no output devices that would operate on 12V to 
develop the specified power (about 15W PEP) over the required frequency 
range (2-12 MHz at first, later 2-18 MHz).  Even a custom-designed transistor, the 2N2887, required a 40V operating voltage, hence the 
DC-DC converter requirement.

Signal synthesis via phase-locked loops was just beginning to make an 
appearance in radios like the PRC-47, but there was no way Hughes was going 
to put a synthesizer module the size of the 47's Signal Data Translator in 
their packset -- it would've blown the weight and size requirements through 
the roof!  So they settled on a straight crystal-mixed scheme, which is 
simple in theory but can be touchy in practice. 

Then the designers (who were a bunch of hams BTW and tested prototypes 
in the Fullerton parking lot over their lunch hour) used a 
critically-tuned TRF stage to get front-end selectivity.  Again, simple 
in theory and touchy in practice. 

Lessee...on the MOV question, the way I see the difference between a MOV 
(metal oxide varistor) and the varistor used in the 9V regulator (R6) is this:

A MOV has a step function; at low or moderate voltages, only a tiny current 
flows, but when the voltage becomes high the diode junctions break down 
and a high current flows.  This effect is highly nonlinear and the device is used more like a fuse to protect circuits (like a spark gap).

I believe the varistor used in the 74 has a much more linear response -- the resistance through the device decreases as the battery voltage  decreases which helps maintain a constant current through the 9V Zener. 

The band switching mechanism moves the RF Module at three points, at the 
transition between the 3-4, 6-7, 11-12 MHz bands.  This is done with a cam 
mechanism behind the front panel.  The mechanism's components are aligned 
so they engage correctly and can be adjusted by loosening an allen screw. 
If your bandswitching function is jamming at a transition point, or slipping completely, you'll have to drop the front panel to adjust it.

On the LSB conversion, here are your comments again:

"...I haven't looked inside the frequency module yet to
confirm but it's obvious this is what someone did to this one.  To my
surprise I was able to hear QSOs on 75 and 40 meters so I figured it was
more likely the carrier osc. had been shifted than a different filter
installed.  But the guy didn't do the mod to the synthesizer, which is
why the khz knob is marked "-3".   If you have the details on this, I'd
be interested in taking a look."

Jim's (KA8TUR) LSB mod results in a switchable sideband function, allowing 
use of both sidebands.  He moves the I.F. signal so that when either
USB or LSB is selected, it will go through the original filter correctly. 
No offset of the dial frequency is needed like the "-3" baloney.
>From what you've written, it appears you may have a radio set up for LSB
only.  A giveaway would be the lack of a sideband select switch.

You didn't mention whether your 74 was a 12 meg (74/74A) or 18 meg (74B/74C) model.  An LSB-only conversion on any 74 would be a real 
waste of  the set's capability, since even the 12 meg models will allow 
60m ops.

I've put the complete text of Jim's conversion up in my web space at:

http://mmfrancis.com/prc74/PRC74_LSB_CONV3_REV1.doc

This is a fairly involved process, so you need to be familiar with the 
set to start with, and the unit should be aligned correctly (which is a whole challenge unto itself).

--Mark Francis
  KI0PF

Author of "Mil Spec Radio Gear Volume 2"
Available direct from the author, Universal Radio, 
and Fair Radio.
$29 cash/check/M.O. or $30.17 via Paypal, when ordered direct 
& shipped to the U.S. (Foreign buyers please contact me). 
See: www.mmfrancis.com/MSRG2/MSRG2.pdf

Volume 1 is still available from CQ, Universal Radio, and 
Amazon.

There are people who make things happen,
there are people who watch things happen,
and there are people who wonder what happened.
To be successful you need to be a person 
who makes things happen.
                           --Jim Lovell
                             Astronaut