[Hallicrafters] SX-42 BFO glitch

[Roy Morgan]

At 02:55 PM 7/29/2005, Craig Roberts wrote:
>...SX-42 ...  BFO is "twitchy".  The note is unsually unstable and prone 
>to shifting with mechanical disturbance -- kind of like an old regen 
>receiver. ...
>Any ideas?

Craig,

1) Did you try a different tube?  There seems to be only one 7A4 in the 
radio, so swapping with another one will mean having a spare.

2) Any ground points such as tube socket mounting rings should be tested 
for bad connections - stuffing a sharp screwdriver at the joint between the 
mounting ring and the chassis can sometimes reveal a bad contact.  Same 
with terminal strip grounding tie points.

3) The schematic shows a shielded wire between the variable cap and the BFO 
Coil.. replace it. Old wires can be "microphonic" or behave strangely in 
other ways.   I think this is a very likely culpret.  This wire may benefit 
from being grounded at only one end, by the way. The schematic is no help 
with this fine point - you may need to experiment. Try a new wire in place 
of the old one.

4) There are three caps inside the BFO can.. You say you replaced them all, 
but a friend (who is working on an SX-42 as we speak) got some bad new caps 
recently (and put them in a Johnson 500, with disastrous results to a 
driver tube or two.)

4a) C 104, 0.02uF is the bypass cap at the grid resistor.  It is suspect, too.

5) The coil itself may be mechanical adrift; either one or more of coils 
(there seem to be three) or the core may be loose on the adjustment 
rod.  Removing the coil can and probing with a plastic rod or tuning tool 
would reveal any looseness in the coils.

A further note here: on the SX-42 I recently saw with its coil cans off for 
polishing and cap replacement, the I saw that the caps are fastened under 
metal tabs of the frame of the coil assembly, AND that the cap leads form 
the tie points for the leads exiting the can and the coil windings, 
too.  What a lashup!  In any case, if you forced a new cap under a tight 
fitting tab, you may have a microphone in addition to a mounting method.  I 
wonder if you used "new old stock" rectangular red silver mica caps as were 
the original.

6) The function switch may be loose or making bad contact.. touch, prod and 
poke it to see if you can make the thing warble. There's a hundred ohm 
resistor from the tuning cap to the function swtich.. prod it to see if it 
is the cause.

7) The (few) resistors in that circuit may be causing trouble: tap them to 
see how they behave.

8) It's not clear to me, but the tube used (a 7A4) is supposed to be both 
the BFO and the S-meter Amplifier. (A cute trick!)  Shorting out the 
s-meter signals may remove the symptom: if so the source of the trouble may 
not be in the BFO at all but rather in the AVC or S-meter signal source (or 
load).


GOODness, what a lot of possibilities. I hope one of them works for you.

Roy
Whose SX-42 awaits overhaul.



- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Cell 301-928-7794
Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan at nist.gov --