Allied SX-190 receiver

Although my obvious preference is for tube-type radios, occasionally I run across an interesting transistorized set. The Allied SX-190 receiver is a solid state shortwave receiver with 11 bands, each of which covers a 500 KHz segment. An equivalent ham-bands receiver was sold as the AX-190. The bands for the SX-190 are determined by the frequencies of 9 crystals which come with the set and 2 open crystal slots for two additional 500 KHz ranges with crystals that can be added by the owner, one between 3.5 and 10 MHz and a second between 10 and 30 MHz. The AX-190 comes with 10 crystal positions occupied and only one open slot which must be in the 3.5 to 10 MHz range.

Made in Japan, the SX-190 and its AX-190 ham counterpart were offered from late 1971 to 1973. These were highly rated when introduced, considered the best-ever Allied receivers. They are still considered excellent analog-tuned receivers. Frequency read-out is quite good. A dial reading of zero to 500 is the value in kilohertz to be added to each band-switched frequency setting. The crystals for each band are 2920 KHz plus the base frequency of interest. For example, the 3.5 MHz band crystal would be 2920 + 3500 = 6420 KHz. Both the SX-190 and the AX-190 cover the 80, 40, and 20 meter ham bands, 11 meter CB, and WWV at 15 MHz in the 19 meter band. The SX-190 also covers the 49, 31, 25 and 16 meter shortwave bands while the AX-190 covers the 15 meter ham band and four segments of the 10 meter ham band.

Repairs
The receiver was purchased at a hamfest (amateur radio swap meet). The seller indicated that the radio was working but had weak sensitivity and probably needed alignment. It turns out he was right. The bands from 3.5 to 10 MHz worked somewhat but the radio was nearly deaf on the upper bands. I did not have a manual but found an excellent compendium of information on the radio including the user and maintenance manuals all together as a single PDF courtesy of Mark Rehorst.

Alignment
After determining that the set was indeed working properly except for alignment, I carefully followed the alignment procedure as detailed in the maintenance manual. That made a major difference on both the bands above and below 10 MHz which are tuned as two separate ranges by the preselector. While the set worked very well after a full alignment, I noticed that the bands in the preselector range above 10 MHz seemed to be significantly less sensitive than the bands in the range below 10MHz as determined by band noise. After studying the circuit and finding that the only difference between the two ranges are passive elements all of which readily tweaked to a proper alignment point, I was satisfied that this was the nature of the set. This difference in sensitivity has been described by other users of the SX-190 and AX-190.

Upgrading dial lights
One of the minor complaints of the original users of these sets was that the dial lights take more power than the set itself. This is an issue when operating from an external battery supply. The four little incandescent lights take over 300 mA so some owners who often used battery power modified the set with a separate switch for the lights. The lights themselves are permanently wired grain-of-wheat style and are not replaceable without soldering.

One of the four bulbs in my set had an open filament. With one light out and given the concerns about power draw for lights, I decided on a modern solution, replacing the little bulbs with white LEDs. The dial light terminal point on the circuit board supplies 14 volts of AC when powered by the AC line and about 12 volts DC unregulated when operating on batteries. I tested a set of four LEDs in series. By experimentation with an external variable DC supply, I determined that the best compromise for LED life and brightness was at a current of about 16 mA. A series diode and 200 ohm resistor were added to limit the current to that level when operating from either power source. The LEDs provide plenty of light but at 16 mA take very little power compared to the original incandescents.

Operating the set
The solid construction and the heavy tuning knob allow for effortless tuning and stability with little or no backlash. Specifications for the set allow for a drift of 500 Hz after warm-up. It took about half an hour for the set to fully stabilize from drift. This was surprising to me since solid state sets produce very little heating. I have come to expect drift in vacuum tube sets and just a bit in analog-tuned solid state sets but have been spoiled by modern solid state sets using synthesized phase-lock loop tuning that is now common even in less expensive shortwave sets. However, the crystal calibrator does allow for relatively accurate tuning on this analog set. I found it easy to pre-tune accurately to a favorite 80 meter SSB net. Having to retune every few minutes to compensate for drift while on SSB is a bit annoying but not a big deal. Only a 10 feet piece of wire was needed for an antenna for that 80 meter net. On the 80 and 40 meter ham bands, the SX-190 rivaled my Kenwood TS-850/AT in sensitivity but, as already noted, sensitivity falls off considerably at the highest frequencies.

Proper comparisons
When compared to lesser solid state analog-tuned sets of the era such as Radio Shack's Realistic DX-160 or DX-200 and a variety of others, the SX-190 simply out-shines those and is in a class by itself. Early reviewers noted this as well. Among analog tuned sets, the SX-190 is a formidable shortwave receiver even after 40 plus years when aligned and working as intended.

Date 11-5-13