[Milsurplus] Prc-6 frequency stability

[John Vendely]

Hi Robert,

Transmit modulation and frequency control problems are very common in 
the PRC-6, but it's almost never the crystal--which is unfortunate, 
since this would be such an easy fix.  I've fixed a buncha PRC-6s over 
the years, and I can count the number of bad crystals I've seen on one 
finger.  But checking the crystal frequency is easy, so it's worth doing 
anyway.

The PRC-6 uses a simple frequency locked loop to lock the transmit 
modulated oscillator frequency to the receiver via the receiver 
discriminator.  It's virtually always discriminator and AFC problems 
that cause drifty PRC-6 transmitters.  In relatively few cases, the 
transmit oscillator is the problem, but this is usually accompanied by 
modulation problems, which you probably would already have noticed. 
Modulation problems are more frequently caused simply by bad carbon mike 
elements.  Some of these elements can be rejuvenated, but that's another 
story.

There are several paper dielectric capacitors in the discriminator and 
AFC loop filter which often become leaky and mess up AFC action.  Early 
PRC-6s use an antiquated type of carbon composition resistor which are 
notorious for aging problems.  These resistors have a "pebbly" or rough 
appearance, as opposed to the later (and much better) smooth, 
shiny-looking types.  Check all paper caps and resistors in the 
discriminator, loop filter, and transmit modulator areas for value, and 
for leakage in the case of capacitors.  Even a few tens of microamps of 
leakage can cause problems in certain circuit locations.  Of course, the 
discriminator itself must be functioning and properly aligned.  I've 
seen one or two PRC-6s with one bad discriminator diode.  There's a 
-4.5V DC bias taken from the battery which serves as the discriminator 
center frequency "zero" voltage and thus the AFC reference voltage--the 
AFC voltage is developed with respect to this bias.  Make sure this bias 
voltage is present and correct, and that it doesn't shift in value from 
the time of discriminator alignment to the time the transmit modulated 
oscillator center frequency is set.

Getting a PRC-6 to work really well takes a bit of effort.  It's best to 
start with the receiver and get it working really well before proceeding 
on to the transmitter.  You'll probably have to weed out a buncha leaky 
capacitors and aged-out resistors before you're done.  Careful 
discriminator alignment is really important.  Once you've got this under 
control, the transmitter is usually pretty easy. A correctly repaired 
and aligned PRC-6 can actually have pretty good frequency stability and 
good transmit audio quality.  Good luck.

73,

John K9WT


On 8/3/2012 12:26 PM, Robert Newberry wrote:
> I have a prc-6 that no matter how many times I bring it on freq it likes to
> drift. Today testing it I had it drift up to 51.16. Also it would drift
> down to 51.1. Also radio is rocked for 51.000. I know these were used with
> radios that had front ends that were "50 wide" but it seems like it drifts
> quite a bit. Is this normal?
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