Hallicrafters SX-100 mark 1A

The HT-30, the first SSB transmitter by Hallicrafters, came out in 1954 was replaced by the HT-32 in 1957. The HT-30 was initially advertised with the SX-96 receiver, but when the SX-100 came out in 1956, its styling was updated to match the new receiver. The SR-500 console comprised of the pair and a HT-31 linear amp also hit the market. The SX-100 was massively popular, bu the transmitter not as much.

There are mainly two versions of the SX-100, mark 1A and mark 2A. The SSB reception is markedly better with a mark 2A.

The overall condition of the receiver was very good. Even the 60+ year old filter cap seemed working fine. I hear almost no hum. Did not find any out of tolerance resistors.

A literally leaky cap. There is a power resister nearby. Years of exposure to high heat shortened it life. Others near the resistor were also replaced.

The paper caps in the 2nd IF selectivity switch were replaced with polypropylene types. At the 5kc position, there were two humps and deep valley in the middle in the response. Replacing caps made it more flat. Also the skirts feel more steeper.

The pwoer cord was replaced. A fuse was added internally.

In order to bring it to the Mark 2A level, a relay was added to swith the AVC release time and the BFO at the same time. Mark 2A uses a dual circuit toggle switch to achieve this, but I could not find one that matches the other toggle switches. The relay is powered by a half wave voltage doubler on the heater supply.

After all the cleaning and modification, I started alignment. Everything was fine until I tried the 4.8 - 12MHz band. The HFO trim cap only allowed about 10kc of variation. The dial was way off and could not be corrected by adjusting the inductor alone. It turned out that the trim cap is faulty. Luckily I had a similar part from the same manufacturer, Erie. NP0 3-12pF was close enough for NP0 2.5-13pF.

All tubes were original with a Hallicrafters logo except for a couple. The date code on them were for either 1955 or 1956. The 6BA6 pictured below was maufactured in the 8th week of 1956 by Raytheon.

I sucessfully made a couple of contacts on 40m after pairing it with a HT-30.