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Hallicrafters SX-71

November 5, 2012

This SX-71 has serial number 457954. The SX-71 was made from 1949 to 1954. The dial on units from 1949 production have black markings on a white background (that I think looks better) and I read somewhere that if a 15M band is marked on the bandspread dial that it would have been made after May 1952. This one has white markings on a black background and does not have the 15M marking, so it looks as though it may have been made between early 1950 and May 1952. The X in the model number indicates that it has a crystal filter.

It is a dual superhet conversion communications receiver that predates the Single Side Band (SSB) technology that has become so popular for its ability to transmit voice communications using a very narrow bandwidth by using just one side (upper or lower) of the passband.
(A normal tuned circuit has a symmetrical bell-curve passband centered on the pass frequency.) Furthermore, the transmit efficiency is high since no carrier wave is transmitted.

It has 5 bands that can receive in AM, narrow band FM (NBFM) and CW modes. While not designed for SSB it is possible to inject the SSB carrier using the tunable BFO. The bands are:
1/ 560KHz to 1.6MHz
2/ 1.65MHz to 4.7MHz
3/ 4.7MHz to 13.4MHz
4/ 12.8MHz to 34MHz
5/ 46MHz to 54MHz
Two dials are provided, one for general coverage over the 560kHz to 34MHz range and the other for electrical bandspread. The bandspread dial is also used to tune the 46MHz to 54MHz range. The two conversion frequencies are 2.075MHz and 455KHz; it operates as a single conversion receiver on bands 1 and 2. (The 455KHz IF frequency is nominal, the receiver should be aligned at the actual crystal frequency. My unit is at 452.5KHz.) There are many other features that at this point in my radio education I do not feel qualified to discuss, since while I understand them theoretically, I really have no feel for them in operation. They include:
Crystal filter, Crystal phasing control, BFO frequency (CW pitch), Noise limiter (ANL), Tone control, Sensitivity control and an “S” meter.
The valve line-up is comprised of 13 valves as follows:
RF, 6BA6
Oscillator, 6C4; Voltage regulator for the oscillator, 0D3
1st Converter, 6AU6; 2nd Converter, 6BE6
1st IF, 6SK7; 2nd IF, 6SK7; 3rd IF, 6SH7
Detector, 6AL5
BFO/1st Audio, 6SC7 (6SN7-GT)
Audio Output, 6K6-GT
Rectifier, 5Y3-GT

The general condition is quite good though the chassis is dull and a bit rusty in places. I am used to the build quality of Tektronix, General Radio and Hewlett Packard so to me, its construction is frankly rather nasty but I suppose fairly typical for domestic electronic equipment of that era; frankly it has so little internal visual appeal to me that I can’t be bothered to clean it up properly. There are some restorers out there who have done a beautiful job with the SX-71. Externally, it is quite clean and impressive in appearance. Having a steel chassis it is heavy though!

I spent a lot of time sorting it out, starting with re-stringing the dial. All the paper in oil capacitors needed to be replaced also I padded critical value resistors back into tolerance. (Carbon composition resistors tend to increase in value with age quite significantly.) A particular area of attention was the AVC line resistors and associated screen grid resistors; these were all high resulting in low sensitivity. Somebody had done a horrible job of replacing the chassis mounted twist-lock power supply electrolytic capacitor, hanging the replacement under the chassis with appropriately ugly wiring. I found a replacement twist-lock in my stock, installed it and cleaned the wiring up. I took advantage of the line voltage being higher these days (greater than 120V vs 115V or less when it was built) to use the increased B+ voltage to accommodate a series mosfet regulator bring the B+ to a stable and correct value of 280V. In particular, I found that the antenna core for band 4 (14MHz to 30MHz) had broken away from the adjusting screw, which explains why the man who sold it to me had told me that he could not get it to track on the band 4; it was effectively deaf on that band! After that, performing alignment and tracking was fairly straightforward, the manual is quite clear.

I am not strongly knowledgeable about radio or well experienced with radio (although I have am licensed as KK4BAX) and I do not have an antenna worthy of the description so I took it to a local Ham for him to try it out. He seemed to feel that its performance was fully up to expectations, meaning quite good. He was able to inject the SSB carrier using the BFO for quite acceptable audibility.


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