[Hallicrafters] SX-42 BFO problem

[Edward B Richards]

Hi Craig;

Maybe troubleshooting it would be good for you. You would learn
something. You can't go through life expecting other people to solve your
problems for you. That said, I will do what I can to help. I have had to
ask for help too, but only after extensive troubleshooting. The SX-42 has
a BFO can with two slugs. If I remember correctly, the top slug is for
455 kc and the bottom slug is for 10.7 mc.  SW-1KK shorts out part of the
coil (yellow) for 10.7 mc. My SX-42 had a shorted mica cap in the BFO
can. I don't remember which one. It killed the BFO and the FM tuning
meter, as the 7A4 tube is both the BFO oscillator and a DC amplifier for
the tuning meter. The tuning indication is taken from the 6H6
discriminator, pin 4, through R65 (150k), through R66 (470K), SW-3AA, R71
(47K),  to pin 6 (grid) of the 7A4.  C100 (.02) bypasses this line. Plate
voltage comes from VR tube (0D3) pin 7 through SW-3BB, to the +side of
the meter. The -side of the meter goes back through SW-3BB to R67 (1.2K),
then through the BFO coil (in red, out blue), then to the 7A4 pin 2
(plate). C99 (.05) bypasses this line. The 7A4 cathode (pin 7) goes
through  R68 (5.1K) to ground. It also goes through R69 (10K), R70 (1K),
to SW-3AA where it ends for FM. The junction of R69 and R70 is bypassed
by C100 (,02) to ground. It also goes into the BFO can to two caps
(black). The cathode also goes through a shielded wire to C80 (7pf),
where it goes to the 3rd I.F. transformer for BFO injection. Normal
voltages for the 7A4 are: 1= ground, 2=+135, 3,4,5=0, 6=+20, 7=+23, 8=6.3
AC. This should enable you to find the problem. Based on your description
and my experience, I suggest you check the mica caps inside the BFO can
first thing. Good luck.

73, Ed Richards K6UUZ
 


On Thu, 03 Mar 2005 11:20:25 -0500 Craig Roberts <crgrbrts at verizon.net>
writes:
> Hmmm.  The BFO isn't working on the top two bands of my newly 
> refurbished SX-42 (they're the bands using the 10.7 MHz IF). For 
> that 
> matter, the S-Meter isn't responding in FM mode, in which it works 
> as a 
> tuning meter. The meter rests at the FM tuning "0" mark as it 
> should, 
> but does not respond to signals. In all other respects, the receiver 
> 
> operates normally -- and very well.
> 
> My gut tells me that these two problems are related and I'm sure I 
> can 
> trace the cause in time. I am appealing, however, to this wonderful 
> team 
> of experienced experts -- one of whom may have had this same 
> situation 
> -- to guide me quickly to a solution.
> 
> In management circles we call this sort of plea "wise utilization of 
> 
> resources".  The truth is, I'd like a quick and simple answer 
> without 
> having to wrap my brain around imagining how the complicated switch 
> positions connect in the top two bands.  That is to say, I'm a lazy 
> bum.
> 
> Many thanks and 73,
> 
> Craig
> W3CRR
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