[Hallicrafters] SX-42 Garbbled CW and SSB

[John Meyer]

Hello List,

I recently dug into a sick SX-42 who's main tuning dial  would turn but the
capacitor cluster remained stationary, and all CW and SSB signals sounded
garbled, AM signals sounded okay.  I thought I would pass on how the
problems I encounted was resolved.

The tuning problem turned out to be the small gear between the two main
plates of the gear cluster had split open so when the large gear that was
attached to the shaft of the main tuning capacitor came in contact with it,
the teeth didn't line up and would move no further. I had to disassemble the
entire gear cluster which proved to be a challenge. It seems the other small
gear that is mounted on the outside of the gear cluster that needs to come
off first did not want to be removed. I didn't want to get too rough with it
because as you all know we just can't pick up the phone and order a new one
from Hallicrafters, so I enlisted the help of a local machine shop, whose
owner thought it was going to be a piece of cake to remove but soon realized
different. He ended up making a special tool to extract the little beast.
Once I had the broken gear removed I placed the gear into a vise and
compressed it to close the gap where it was split. Then I took a small round
file and filed out a small area on the inside at the crack, then filled it
with solder and filed again to round out the hole making it slightly
larger.This was originally a press fit which I'm sure contributed to it's
demise. When I got done the gear could be pushed on by hand. To keep it in
place I used a very small amount of JB weld which holds it from slipping on
the shaft. If anyone else on the list has a need to do some repair work on
the gear cluster and has the same problem extracting the little gear drop me
a line and I'll loan you the puller I had made.

The garbled sound when listening to CW or SSB didn't prove to be as big a
challenge. It was apparent the previous owner was bugged by the problem
because all the components around the BFO had been changed. I injected a 455
signal into the IF string and recovered a very clean beat note so I knew the
problem had to be someplace other than the BFO. Next I listened to the local
oscillator with my Yaesu FT767 in the CW mode and got a real rough sounding
note. The problem traced to a corroded common ground at the 7F7 oscillator
tube socket where the filament and cathode share the same ground connection.
Hallicrafters used rivets to hold the tube socket ring and solder lug to
chassis which over time became corroded and created a slight voltage drop
which elevated the cathode above ground with a trace of filament voltage
sitting on it. I took my 300 watt soldering gun and soldered the rivets
which resolved the problem completely and while I was at it did the rest of
the other rivet points around the receiver for good measure.

Hope I didn't bore you all!!

Regards,

John Meyer K8IHY
jomeyer at msn.com
www.dipple.com/jmeyer/