75A-4 Receiver
November 1, 2008
This late serial number receiver with three mechanical filters and four to one dial was obtained from an ebay auction in like-new condition. A slight hum was fixed by replacing C94. A few other capacitors had already been replaced.
I pulled out the noise limiter, V12, and plugged in a capacitor across pins 2 and 7 to eliminate the possibility of distortion caused by the limiter. (You can put the tube back any time.)
This also allows access to the receiver's audio input at V12 pin 2. I applied a Hewlett Packard 200CD audio oscillator set to one kilohertz with a comfortable audio level. I connected a Hewlett Packard 332A distortion analyzer to the speaker and measured 1.10 percent distortion. I swept the audio oscillator frequency up and down to find that the frequency response drops off below 200 Hz and above 10 kHz. These are not spectacular numbers, but not what I would consider a problem for a communications receiver.
I eventually bought a six kHz wide AM mechanical filter for this receiver. I still prefer to use the 3.2 kHz SSB filter for AM when noise and interference is a problem. It works beautifully, but you can't count on everyone to be on the same frequency, so some tuning is required.
The knob in the receiver's upper right corner (mode switch) is too large. I can't explain this.
A few Hallicrafters SX-28 receivers were available from an estate sale, and I am sure more will appear in the future. This individual set was an early version from 1941. Later sets had progressive modifications for cost reduction that may make them less desirable As you may know, replacement of almost all capacitors and some of the resistors is required to restore this top-line pre war radio. The first link is a very detailed step-by-step instruction with illustrations that shows what is required. It looks like a nightmare because of the design of this radio, but can be done in a day if you keep at it.
[SX-28 Instructions]
The restoration starts with replacement of all the paper capacitors shown above.
I ran into an unexpected problem with the SX-28. This could be a problem with any radio, but is related to an unusual type of hardware that is used as both a solder lug and cable clamp.
After several days of use, I plugged the SX-28 in to the AC power, and heard a pop and saw a puff of smoke from under the chassis by the rectifier. After a careful check I plugged it in again and turned it on. I got more smoke and the radio was dead. One of those goofy long-tailed solder lugs was wrapped around a wire bundle under the power transformer. It was soldered to two shielded cables at the top of the bundle. Deep under the bottom of the bundle the hot wire coming in from the AC power line had been cut by the solder lug that was used as a cable clamp. The cotton insulation on the wire failed, shorting the hot wire to ground. The wire was burned open since there is no fuse in the SX-28 civilian version. After pulling the wire bundle up I could then see the half inch burn mark on the chassis where the black power wire separated. Fortunately, the first thing I did when I got the radio was cut off the old power cord and replace it with a three wire cord with a safety ground. The safety ground on the power cord prevented the chassis from being hot to ground, and saved me from an early grave. This is a hazard when you are working on something on the bench and it is not connected to the station ground, antenna, or other permanent equipment. Gosh, I remember an old time technician who would never put anything on his bench without first connecting a big ground wire with a huge alligator clip to it!
A similar short in the second RF amplifier plate coil caused loss of gain on one of the bandswitch positions. You must be very suspicious of this old insulation sleeving when things are pressed together.
The S-meter didn't work, and the cause is usually a partial short to ground in the S-meter adjustment pot. This is covered in the first link above.
The S-meter looked funny, so I took it apart. I can't explain why the back end of the meter case was flared out as if it had exploded. The plastic window was warped as if it had been hot. I removed and discarded the plastic window and the metal ring holding it. The meter looks normal now without the warped plastic in there. The Hallicrafters bezel on the panel, of course, has a glass window in it. The magnet and other internal parts of the meter movement are the negative meter connection, and if shorted to ground, will cause the meter to pin to the right regardless of terminal connections. I put a hose clamp around the meter case. I couldn't find another case the same size. I guess these things just explode now and then.
Most consider the HRO-5 to be the best of the series. It retains the original beauty of the "brute force" engineering design and the look of the original cabinet. This HRO receiver was found in an old barn in North Carolina where it had sat for many years. It had been "recapped" with new capacitors sometime in the late 1960's. Original power supplies and speakers are very rare and fetch a good price at auction. Like many of the original owners, I made my own. After I applied power and had it working a while I noticed that one of the old capacitors had ejected itself, and was found on the bench under the radio.
The outside of the cabinet cleaned up very nicely with washing and a light spray of Krylon semi-gloss black. The inside was rusty and dirty and didn't look worth restoring. I decided to modify this receiver to meet modern requirements, rather than to try a cosmetic restoration.
I added a product detector using the two triodes in a 6SN7 and a crystal controlled BFO using another 6SN7. The BFO knob now is a rotary switch selecting the two oscillators. I removed an IF transformer and replaced it with a nine pin socket for a mechanical filter from the Collins 75A-4. Used filters are still available for reasonable cost, and I obtained filters for SSB and CW. The original crystal filter still works, and I happened to obtain a new unused crystal filter assembly. The phasing control capacitor can be used as a notch filter.
The most important modification is a good AGC for CW and SSB. The best method is to use RF derived signal from the last IF amplifier. This gives quickest response without clicks or pops. Use of audio derived AGC was popular, because the BFO signal would get into the front of the IF amplifier and ride on through stronger than any signal. RF (IF) derived AGC would be "full on" and keep the S-Meter pinned all the time. I discovered that the BFO signal was getting into the mixer plate circuit through the B+ power. Substantial filtering was required here, because you must consider the low impedance of the power lines and the low frequency of the IF. Once this was fixed, I just used a single 1N34 diode AGC rectifier similar to what Ted Crosby used in his famous HBR receiver series in QST.
I never did find the HRO frequency stability to be very good on the higher frequencies, so I quickly built a two tube crystal controlled converter for forty meters. Using the 160 meter plug in coils, the receiver works very nicely. I still haven't figured out how to get straight line frequency calibration on the dial for more than 100 kHz, however.
Click on the above image for a larger view.
[Western Historic RADIO MUSEUM] [Rockwell/Collins Mechanical Filters] [NU9N] [California AM] [The AM Window] [SX-28 Instructions] [HRO Senior Instructions] [HRO-5 Instructions] [SX-28 History] [East Coast Sound] [AMfone.net Index] [Mr. Beaky]
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