I contributed to a recent sidetracking of a NC-109 thread with some comments about the Hallicrafters SX140. So I thought it would be better to give the SX140 its own.

On Christmas Eve, 1964, my dad bought me a Hallicrafters HT40 transmitter as a reward for figuring out how to get to NYC from NJ by myself, find the FCC office, and take the General class exam (and pass it). (Btw, I took the train, the ferry and subway). We selected the HT40 because that was the only transmitter left at Federated Electronics in Springfield, NJ. The HT40 worked fine, but since then I wanted the matching SX140 receiver (I was using a National NC105). Finally got it about a year ago.

Here's my comment from the NC109 thread and then Carl's comments about how he modified it:



I've wondered why Hallicrafters 'wasted' a triode as an S-meter amplifier in the SX140, yet omitted a BFO. After all, it was a novice receiver, it would be used first of all for CW. CW should have had priority. The S-meter doesn't even work on CW.

Anyway, the SX140's regenerative IF amplifier is slightly more sophisticated than usual. It has a diode from the suppressor to ground which I presume is to clamp the voltage at about 0.6 volts. It can work OK, but that IF stage is very easily overloaded, just like a regenerative detector would be (after all, it's got an RF amp and a mixer ahead of it). The trick is to keep the RF gain way down. Way, way down. Then the selectivity is not bad, but certainly not single signal. An RF attenuator greatly helps. And while I'm kicking the SX140, how about the single tuned RF circuit between the antenna and mixer? The RF amp is simply impedance coupled to the mixer. The RF tank is not even ganged to the oscillator VC. Jeez.

Rob


All so true but it is a wide open radio begging for mods. On the one that was given to me some years ago along with a Knight T-150A I decided to make them useable at our summer cottage in Maine. Originally I lugged a Viking I, VFO, and HQ-129X back and forth and then it stayed there a few years as it was such a PITA to keep moving them. OTOH they took up a lot of room and with grandkids and family they had to go.

The SX-140 received a few changes. The 6BA6 IF became a 6BE6 converter to 455 and a small vertical subchassis on top held a pair of 6BJ6 IF stages with a Murata AM and CW (actually a sharp SSB one) filter and a Schottky diode for AM. The ANL was also SS. The other sockets held a 12AT7 BFO/Product detector (lifted from Heathkit), and the 6AW8 became both audio stages. I could care less about an S meter.

While still far from a high end radio it does OK with a HB 4el 6M yagi on a gable end,10-40M vertical on the middle of the roof with elevated radials and a haywire loop in the pine trees for 80M. Being 100 yards from the ocean helps and the rather basic front end was tolerable so I didnt bother with drastic changes there. The Murata filters helped a lot.

The T-150A received several changes and it is also now tolerable. A small three section shelf cut down in 2 dimensions from a yard sale find houses the SX-140 on the bottom, the T-150A in the middle, and a speaker, plus a HB SWR meter/ATU/antenna switch, and a B&W 381C electronic T/R switch on top

Carl


First of all, I'm in awe of Carl's modifications, casually mentioning it's now a dual conversion receiver with ceramic filters and a product detector. (Btw, one of my minor modifications was to add a separate, dedicated 470 ohm resistor and 47uF cap to better filter the B+ for the audio output tube plate: that greatly reduced the hum).

I wonder if anyone knows what Hallicrafters original intentions were for the SX140. The chassis is really too large for what it is. As I wrote above, it's odd to include an S-meter amp, yet omit a separate BFO. My guess is Hallicrafters had grander thoughts: two IF stages with a crystal filter at 1.650kHz, perhaps a ganged LC tank for the mixer stage, a dedicated BFO. But somewhere along the planning stage, they realized it was getting to be an expensive yet still basic, single conversion receiver and so they retreated: just one IF stage with regeneration to make up for the loss of the BFO and a crystal filter. Again, I'm just guessing, I don't have any hard information. Any former Hallicrafters employees out there?

Anyway, I'm very fond of this receiver. And it only took me 50 years to get one.

Rob

I can't explain a lot of Hallicrafters' mysteries. So many things they did made no sense. Maybe they kept a stash of good stash of peyote in the lunchroom. I digress...
Why the dumb T/R switching arrangement for CW on the HT-46 that requires manually rotating the knob whereas phone offers VOX and PTT? Thousands of CW receive-to-transmit cycles has made it necessary to replace this PITA function switch at least once. Why did they include 160 meters on the SX-101 but the matching HT-32 didn't have it? Why the RF gain pot is not controllable in AM on the S-72, only when the BFO is turned on? And am I the only one that thinks the HA-5 VFO sucks? I've owned 3 or 4 of them and they were all equally bad. Why all the effort to design a complicated heterodyning scheme for this VFO? There were many other VFOs of the day that beat the HA-5 out in stability and they were cheaper and didn't use near as exotic of a design. Bill got a lot of mileage out of boasting about the HA-5's "heterodyning VFO" but, other than a marketing ploy, I still can't see what the reason was. My goofy little Knight Kit and WRL VFOs oscillated circles around the HA-5.
Back to the SX-140...
You're right about the S-meter. It's as effective and as smooth as the meter on my battery charger out in the garage. So why the amp? Weed. It HAD to be the weed.
But I must give the li'l SX-140 is just due...it is a sensitive critter and fun to use.
By the way, adding the resistor and capacitor to reduce the hum is a terrific idea and I plan to do that tonight. In fact I might try the same thing in the B+ of my S-72 to see if the hum can be minimized. And I would love to see an article from you delineating your improvements in a magazine like, perhaps, Electric Radio.

My modifications are very minor. I found that to keep the regenerative IF amplifier from overloading it was necessary to turn the RF gain way down. This created two problems: first, sometimes the gain was turned down so much that the signal began to be lost in the noise. The solution was an external RF attenuator, The second problem was I had to turn the volume up to compensate and that exposed a lot of hum and noise in the audio. Normally I would rewire to keep the heater current from returning through the chassis, but instead I did a few easy things. I reduced the audio coupling caps and the power output cathode bypass cap to 1/4 their original values. Then I ran that separate filter line for the audio output stage plate (pentode plates are supposed to be relatively insensitive to PS ripple but that extra filtering did make a big difference). At this point, the audio is mostly hum-free. Finally, I connected the audio output pentode as a triode: 100 ohm resistor screen to plate. Why? I don't like the sound of the 6AW8 pentode. I only use headphones, hi-fi headphones at that, so I don't need much audio power. It's sounds great now, I can listen for hours without any fatigue. If I started to use a speaker I think I'd add a triode-pentode mode switch.

Anyway, you can see these modifications are very simple, and no holes were drilled during their installation. My fondness for the SX140 is mostly nostalgia for something I didn't have a long time ago. But I now often use it to keep up my CW, and for the International broadcasters on the 41, 22 and 15 meter bands. Btw, I'm happy to say my old HT40 still exists, still works fine: my nephew is now using it.

Rob