The SX-130 is a 4 band communications receiver covering AM broadcast from 535 to 1610 KHz and shortwave from 1.725 to 31.5 MHz. The small gap in coverage from 1610 to 1725 KHz is necessary because the IF is 1650 KHz. Bandspread for the SX-130 is calibrated for the 80, 40, 20, 15, and 10 meter ham bands. The long horizontal slide-rule dial is bandspread. The smaller circular dial on the left is for main tuning. Tubes are 6DC6 for RF amp, 6EA8 as mixer oscillator, another 6EA8 for first IF and crystal selector, 6BA6 as second IF, 6AL5 as AM detector/noise limiter, 6BE6 as BFO/ product detector, and 6GW8A as combination audio preamp and output.
The rectifier is half-wave using a silicon diode. An S-meter is standard in the SX-130. The S-129 is a cost-reduced version that eliminates the S-meter and crystal filter and, of course, the broad and sharp selectivity settings afforded by the crystal. Another difference is the first IF tube which for the S-129 is a 6BA6 in place of the 6EA8.
Cost of the SX-130 in 1965 was $179.95. The S-129 was priced at $164.95, only $15 cheaper. That small price difference for what I perceive as a big loss in capability would explain why there are fewer S-129 sets. Note the errors in the 1966 Lafayette catalog ad below which lists the S-129 as "SX-129". For Hallicrafters, the "X" in the model number indicates the crystal filter. The ad states that the S-129 has no product detector. That is also an error.
Speakers
Like most higher-end communications receivers, the SX-130 needs an outboard speaker. The R-50 or 51 speakers match the SX-130. The speaker output is low impedance 3.2 ohm. I do not have a matching speaker so I simply used the small speaker shown in the bottom picture which works well.
Repairs
Although the inside and the knobs were very much in need of cleaning when I purchased this set, it was in nice cosmetic condition for its age. The set had a note on it indicating a hum problem.
As usual, I used a bit of contact cleaner sparingly on all the switches and the tube pins and sockets. That solved a shift in sensitivity. In fact the culprit was poor contact of the RF amp tube that I could wiggle to induce that major shift in sensitivity. Cleaning the pins with a brass-bristle brush and applying contact cleaner solved the problem.
I noticed that the hum problem would depend upon the position of the power plug. That was my first clue. The electrolytic was in excellent condition and was not the source of the hum.
I determined that the set should have its power cord replaced with a proper 3 wire grounded cord. I wired it as usual with an added 1 amp inline fuse on the hot side and then to the power switch. The neutral was connected directly to the transformer. The major hum persisted. I did not know whether I was dealing with a ground loop by adding the safety ground. I temporarily lifted the safety ground. No change.
The set has a relatively high impedance input to the audio tube. I could stop the hum by grounding the center contact on the volume control. By grounding the input side of the volume control pot, the hum stopped at the high end of the control but persisted in the middle of the range. I immediately suspected that the hum was being induced at the volume control itself. I removed the power wire connections from the switch. Temporarily connecting the power switch wires together and taping them away from the volume control eliminated the hum, confirming my suspicions that the power line was inducing 60 hertz hum into the volume control potentiometer.
I had no choice but to switch the neutral wire rather than the hot wire with the volume control power switch. I thoroughly checked the power transformer for any residual leakage. There was none. I also swapped tubes to determine if there was any heater-to-cathode leakage. Again, there was none.
The volume control with its power switch does not seem very deep compared with some other volume controls I have seen. Perhaps Hallicrafters used one with insufficient shielding between the pot and the switch or perhaps they intended for the neutral to be switched.
With the added safety improvements of a grounded chassis and a 1 amp fuse in the hot lead, I am content at this time to wire the set with a switched neutral. Let me know if you have run into a similar problem and what you have done to solve it, if anything.
Alignment
At this stage, the set was performing quite well. I decided to align it because the BFO was a bit off. The IF was aligned to the crystal. As indicated by the peak on the S-meter, the crystal resonated at 1650.14 KHz, very close to its intended frequency. The IF transformer adjustments accessible from under the chassis were on the mark but the ones from the top of the chassis were off a bit. That is the first time I have observed such a consistent difference between one side of an IF and another for an entire set. The soft bees wax covering the slugs for alignment on the lower end of each range had to be moved a bit. A gentle tug with a screw driver on each slug loosened the wax enough that my alignment tool could adjust the slugs to the proper settings.
Performance
After alignment, the receiver proved to be a very competent performer on the ham bands and the popular shortwave bands. It is easy to tune with the large calibrated bandspread and is quite stable after warm-up. Tuning the broadcast band with a proper antenna attached was also a fun broadcast DX experience. The set is very sensitive with good selectivity provided by the crystal filter. Had I the choice of the S-129 or the SX-130 in the mid 1960's, I certainly would have paid the additional $15.
date: 4-2-11
A Hallicrafters S-72 portable receiver was the previous item on the bench.
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