[Hallicrafters] SX-130 Assistance

[Wj5mh at aol.com]

Hello to all...

I'm just about finished refurbishing an SX-130 receiver...  I had one like it 
when I was 16.  The only problem I have yet to resolve has to do with voltage 
regulation on CW/SSB.  I'm only guessing, but I suspect the AVC isn't the 
greatest to begin with.  As best I can tell from the schematic, the AVC is 
developed at the 6AL5 AM Det/Noise Limiter, and fed back to the grids of the 
RF, 1st IF, and 2nd IF amps.  But there isn't a large capacitor in the path.  
Consequently, the AVC is too fast, following the RF too closely.

Since the power source is a cheap half-wave, unregulated supply, the B+ 
changes on strong CW/SSB signals, causing instability.  CW signal slightly 
chirp, and SSB signals FM.  (The same B+ is fed to the oscillator.)  I can 
compensate by reducing the RF gain control (which controls the cathodes on 
the RF and 2nd IF amps).

One would think that if there was a large cap in the feedback path, the AVC 
would keep the gain reduced between voice peaks or CW pulses reducing the B+ 
variations.  (You may hear a change when the signal first triggers the AVC, 
but you could live with that...)

Since this wasn't really meant to be a top-of-the-line CW/SSB receiver, maybe 
it's supposed to work the way it's working.  It could be that you were 
expected to ride the RF gain control, or at least not keep it wide open all 
the time.  It's been so long since I owned one of these, I can't remember how 
it sounded on CW/SSB in 1965.

Anyway, the radio sounds great on AM, and good on weak CW/SSB, or when I 
reduce the RF gain control on strong signals.

I've aligned the unit.  The IF was right on, but I had to touch up the 
front-end and oscillator tanks slightly.  Of course, I replace all the 
electrolytics, changed the power cord to a 3-wire type, and added a fuse.  
I've swapped the tubes, but no change.  Voltages are very close to the 
schematic notes...until I receive strong signals.

I see the AVC working, but maybe the AVC isn't working quite the way it 
should.

Any comments from users with SX-130 experience would be greatly appreciated.

73,
Joe ( WJ5MH )