Posted on April 15, 2010 - by Vintage Tube Admin
The Hallicrafters SX-100 is a 14 tube Double Conversion receiver that was made from 1955-1962. This receiver is a Mark 1B. I don’t know what year this one was manufactured.
This receiver is in excellent condition. The front panel and paint are in excellent original condition. The Bandspread knob has a small chip. The line on the Bandspread glass is wavy. I suspect that someone over the years tried to clean the glass and got the paint too wet and it slid on the glass. I have had the glass silk screened back into original condition.
The receiver worked when I got it, but not very well. I replaced all the paper capacitors. All electrolytic capacitors were also replaced. I do not trust any of the old capacitors to last, even if the radio works well. Electrolytics are known to fail catastrophically. There were some out of spec resistors, but nothing out of the ordinary for a receiver of this vintage. Generally resistors in the audio section and the B+ circuitry are out of spec.
The second IF section is a sub-chassis mounted on the main chassis. There aren’t any paper capacitors in this sub-chassis so there is no reason to remove that chassis. The capacitors are mica and ceramic which are very reliable and not prone to failure like paper capacitors.
The 6C4 in the notch filter circuit, the 6C4 first converter oscillator, the 12AT7 in the second IF, and the 6SC7 audio amp and BFO were all bad and replaced. The other tubes with the exception of the 6K6 audio output are original Hallicrafters tubes. It’s common for the audio output tubes to go bad before any others.
I replaced the power cord, installed an inline fuse, and an inrush current limiter. The inrush current limiter will help extend the life of the vacuum tubes by powering them up slowly. The white tube in the rectifier socket is a 5Y3 solid state replacement. This helps provide stable DC power and cuts down on the load on the transformer. The 5 volt filament current is eliminated by the SSR replacement.
Alignment of the SX-100 was not particularly difficult. Both IF stages were out of alignment. The RF section just needed some minor adjustments. I used a Heathkit audio frequency generator that produces 50 kHz output for aligning the 50.5 kHz IF stage. I used an EICO AM modulated RF generator for the 1650 kHz IF stage.
Once I aligned this radio, it turned out to be extremely sensitive. There is so much gain in the RF section, that I can only turn the volume up to 1 or 1.5 before it becomes too loud. This is a really hot receiver!
I’m impressed with how well the notch filter works in a receiver of this age.
The Main and Bandspread tuning are gear driven with flywheels. The tuning is very precise and incredibly smooth. The only dial cord in the receiver is for the bandspread ganged capacitor and is a metal cable. I don’t ever expect to have to replace this dial cord.
The SX-100 has SSB capability and seems to work quite well. I am still learning about the SSB capability on this SX-100. You can receive standard AM broadcasts on either Upper or Lower side band, which can eliminate interference that affects one of the sidebands but not the other.
There is a built in 100 kHz crystal marker to calibrate the main tuning.
The audio output is single ended (6K6) so it is not hi-fidelity, but sounds quite good.
This is an extremely nice receiver and is definitely a keeper.