SX-43 compared with SX-42
The SX-43 is a lower cost (and easier-to-lift) version of the SX-42 with 11 tubes compared to 15 in the SX-42. Introduced in 1947, a year after the SX-42, the SX-43 uses a ratio detector in place of the SX-42's limiter-discriminator and single-ended audio in place of push-pull audio. It also eliminates the voltage regulator and has a number of changes in tube complement for otherwise similar functions. Like the SX-42, the SX-43 is a general coverage receiver for AM, CW and FM. The SX-43 has a gap in coverage from 55 to 86 MHz which the SX-42 covers fully.
Bandspread for the SX-43 is calibrated for the 80, 40, 20, and 10 meter ham bands. AM broadcast and shortwave coverage is continuous from 540 KHz to 44 MHz in four bands. A separate bandswitch setting for the 20 meter ham band is provided covering only 14 to 14.3 MHz. Both AM and FM are covered from 44 to 55 MHZ. The SX-43 covers the 86 to 109 MHz for band FM only while the SX-42 has both AM and FM coverage for that band. Like the SX-42, the FM broadcast band uses an IF of 10.7 MHz, and the shortwave and AM broadcast bands use an IF of 455 KHz. However the SX-43 is dual conversion for AM in the 44-55 MHz band which includes the 6 meter band. The second conversion to the lower IF improves selectivity.
Like all other Hallicrafters with the "X" in the model name, the SX-43 is equipped with a crystal filter. Price of the SX-43 was $169.50 compared to the SX-42 at $275.
Like the SX-42 , the SX-43 needs a matching transformer for its 500 or 5000 ohm impedance audio output. The R-44 speaker was designed as a match for the SX-43, but the larger R-42 which matches the SX-42 is also a proper choice according to the SX-43 instruction manual.
Preliminary checks for safety and continuity using an ohmmeter showed that the On-Off switch was open. A bit of contact cleaner and working the switch helped solve that problem. Two capacitors were obviously split open. One was a Micamold that connected the chassis to the power line. Both were immediately replaced.
One small section of the chassis had considerable rust on it. I scraped, sanded, cleaned, and scrubbed those spots to remove the rust. The rust was probably caused by a corrosive chemical, possibly urine from a mouse. The chemical had corroded the 6SQ7 tube socket pin connectors to some extent as well as several of the pins of the 6SQ7 tube itself. Similar problems were found in the 6J5 BFO oscillator socket. Cleaning the sockets did not completely solve the problem. I thought about replacing the sockets themselves but opted to remove the socket pin connectors one at a time, cleaning the openings and then transplanting the pin connectors from a matching donor socket from the junk box. The coupling capacitors to and from the 6SQ7 as well as the 15 megohm grid resistor were replaced at the same time.
The electrolytics reformed readily. With the 5Y3 removed, the set was tested and began working to some extent with B+ supplied from my external monitored power supply (Heathkit PS-4). Current draw for B+ was as expected per the manual which I had downloaded from BAMA. After re-inserting the rectifier and powering the set, I checked the voltages for several of the tubes. I found at least one error in the Sams Photofact schematic. The pin locations for the 6V6 grids are reversed. The schematic in the Hallicrafters manual is correct.
Some lack of sensitivity was traced to weak IF amp tubes. Replacing those brought the sensitivity of the set up to par. I also replaced a weak 7F8 oscillator/ mixer. The 44 to 55 MHz FM band worked after its replacement, but the 86 to 109 MHz band did not. Checking with a frequency counter confirmed that the oscillator was not properly functioning on the highest band. I checked the bandswitch. Cleaning it, especially the oscillator switch wafer section, helped somewhat. However, thoroughly cleaning the loctal socket and the pins of the replacement 7F8 brought the oscillator/ mixer to full operation. Since the 44 to 55 MHz band was already working on FM (but not AM) , I was surprised by this. Apparently just a little resistance in the bandswitch or tube pins can cause the oscillator to drop out at the highest frequencies. After these repairs and a bit of alignment tweaking, I listened to some favorite FM broadcast stations and to a variety of SSB transmissions on the lower bands. I noticed that I had plenty of RF signal but that the AM volume levels seemed a bit low. This was especially noticeable on SSB where the audio volume control is set to maximum and the sensitivity and volume level are controlled by the RF gain (sensitivity) control.
Band 5, dead on AM
While the 44 to 55 MHz band was working well on FM, it was dead on AM. The 6J5 oscillator that is used for the BFO is also used as the 11.155 MHz fixed oscillator when AM is selected and band 5 is switched in. That 11.155 MHz signal is then injected into the 2nd IF amp tube which does double duty as a mixer and feeds the difference between it and the 10.7 MHz IF signal into the 455 KHz chain. The BFO was working fine. Checking the oscillator with a frequency counter showed that it was not operative at anywhere near 11 MHz. The 270 pF mica capacitor was checked and circuit parameters were OK. However, the counter showed it was oscillating at about 160 MHz. Oops! I'll bet this thing wants a metal 6J5 not the glass GT version of the tube. Sure enough. Replacing the 6J5GT with the metal version caused the oscillator to settle down, and I was able to tune it to 11.155 at which time band 5 started to work properly on AM. However, the volume level was still low when compared to the same band on FM.
Follow-up on low AM volume
I tested the 6V6 audio output. It was slightly weak but replacing it made no noticeable difference. Testing the 6H6 AM detector tube revealed the real problem, one side was extremely weak. A NOS (new old stock) 6H6 solved the problem. The result was plenty of volume on AM and SSB. The SX-43 was finally acting like the rather hot receiver I had expected.
Performance after repairs
SSB and CW reception on the lower ham bands was excellent especially with the crystal filter options to narrow the selectivity. AM and shortwave broadcast reception was also very good. The narrow AM selectivity position was very effective at eliminating interference from adjacent local signals on the AM broadcast band. Even the 6 meter ham band reception was quite good. The FM broadcast band reception was up to par for a 1947 set which should not, of course, be expected to compare to the performance of a modern solid state receiver. However, on AM broadcast, the SX-43 easily outperforms the same typical top-of-the-line solid state AM-FM receiver.
A Hewlett-Packard HP-400D AC VTVM was the previous item on the bench.