[Milsurplus] PRC-74B low RX sensitivity

[Mark]

> From: roy wildermuth 

> 
> Hello
> 
> 
>  I have a PRC-74B that has a very heard of hearing
> receiver.
> 
> I would like to start with an attempt to align it but the
> Bama manual only
> shows overall operation and use. I am looking for a more
> detailed manual.
> 
> Has anyone experienced this problem with a PRC-74b. My
> PRC-74 has good
> sensitivity.
> 
> I would also like to know if any on the modules are
> swappable in these
> radios. I might find it easier to locate the fault down to
> a specif module.
> 
> Roy Wildermuth
> AWA Museum


Roy,

The I.F. Module will swap between radios.  The R.F. Module will 
NOT swap (the 74 and 74A are 2-12mc, the B and C are 2-18mc).  I 
believe the P.A. will swap (74/74A P.A. will run out of gas a little 
above 12mc in a 74B/C).  Power supply will swap (probably not your 
problem, but always check supply voltages first thing).  The Frequency 
Standard Module will swap.  The panel/chassis will NOT swap (obvious 
mechanical differences).

Is the receive function dead or just very low gain?  The 74 gets its 
front end selectivity using a series of cascaded tuned radio frequency 
(TRF) stages.  Since one tuned stage feeds the next, their correct 
alignment is absolutely critical.  Over the years one or more of the 
transformers in the R.F. module may age and change their inductance 
slightly.  That can be enough to foul up the critical tuning between 
stages.  DO NOT attempt to adjust any of the R.F. Module transformer slugs 
or trimmer caps without the tech manual, and UNDERSTANDING the procedure. 

Does the set transmit at all?  Both the transmit and receiver path go 
through the R.F. Module.  Any misalignment or fault in the R.F. Module TRF 
stages will also show up as low power out.

Are symptoms any different on different bands?  The break between bands 
on the 74B is 

3.999/4.000
6.999/7.000
11.999/12.000

If TX power out or RX sensitivity is noticeably different either side 
of these breakpoints, the R.F. Module alignment becomes more suspicious.

You could try swapping the 74 I.F. module into the 74B and see what 
happens.  If no change, R.F. module is still suspect.  Put the 74B 
I.F. module in the 74 for a last check.  A fault in an I.F. module would 
affect all four bands (2-3.999, 4-6.999, 7-11.999, 12-17.999).

Do as much troubleshooting as you can using the radio's own controls. 
Get the manual and read up on how the set works.  That will help you 
understand and decide what to do next.  For instance, the Synthesizer 
Module is simply a variable oscillator that puts out a sine wave signal from 3.75 to 19.749mc in 1kc steps.  The I.F. is at 1750kc.

Aligning the R.F. module is a real pain in the backside.  All the slugs 
have to be worked loose with MEK (methyl ethyl ketone) solvent.  If you 
break one, the transformer must be replaced, a job in itself.  All the stages must be adjusted repetitively.  The job can take a couple of 
days altogether, easy.

BTW, make sure the RF module is bandswitching correctly.  You should 
see the bandswitch Oldham coupler move to the next position each time 
you move across the band boundaries listed above.

Also, depending on the problem at hand, and my prior knowledge, I often 
use an attempt to align a circuit as a troubleshooting method.  This is a 
good example.  I've seen so many 74s with R.F. modules out of kilter that 
I use an alignment attempt for troubleshooting - once I've eliminated 
all the obvious possibilities of course.  Aligning the R.F. module is a 
major undertaking.

--Mark Francis
  KI0PF

Author of "Mil Spec Radio Gear Volume 2"
available direct from the author,
$29 cash/check/MO including postage, 
$30.17 via Paypal, both within U.S. 
(foreign buyers please contact me).
Signed by request.

Excerpt here:
http://www.mmfrancis.com/MSRG2/MSRG2.pdf

Also available from:
Universal Radio
http://www.universal-radio.com/catalog/books/3495.html
Fair Radio Sales
https://www.fairradio.com/catalog.php?mode=viewitem&item=6569
Electric Radio
http://www.ermag.com/index.cfm?v_link=product_detail&v_key=415

****

Volume 1 is still available direct from
CQ Magazine,
http://store.cq-amateur-radio.com/Detail.bok?no=113
Universal Radio,
http://www.universal-radio.com/catalog/books/0415.html
and 
Amazon.com