Collins Radio 75A1 Restoration

Collins Radio History (from Wikipedia)

Arthur A. Collins founded Collins Radio Company in 1933 in Cedar Rapids, Iowa. It designed and produced both shortwave radio equipment and equipment for the burgeoning AM Broadcast industry. Collins was solicited by the military, the scientific community and by the larger AM radio stations for special equipment. Collins supplied the equipment to establish a communications link with the South Pole expedition of Rear Admiral Richard E. Byrd in 1933.

In 1936, Collins had begun production of the 12H audio console, 12X portable field announcers box, the 300E and 300F broadcast transmitters. Throughout World War II, the 212A1 and 212B1 replaced the 12H design. Collins became the principal supplier of radio and navigation equipment used in the military, where uncompromising performance was required.

In the post war years, the Collins Radio Company expanded its work in all phases of the communications field while broadening its technology. This moved Arthur Collins into a more active role as CEO guiding department leaders holding significant responsibilities. New developments such as flight control instruments, radio communication devices and satellite voice transmissions created great opportunities in the marketplace. Collins Radio Company provided communications for the United States' role in the Space Race, including equipment for astronauts to communicate with earth stations and equipment to track and communicate with spacecraft. Collins communications equipment was used for Projects Mercury, Gemini and Apollo, providing voice communication for every American astronaut traveling through space. In 1973, the U.S. Skylab Program used Collins equipment to provide communication from the astronauts to earth.

Around 1947, the company introduced their first amateur radio receiver, the 75A-1 (called the 75A). This set achieved excellent stability for the time due to high build quality and the use of a permeability tuned oscillator (PTO) in its second conversion stage. It was one of the few double conversion superheterodynes on the market and covered only the amateur bands.

With the experience gained in the design of the 75A-1, Collins released the 51J-1 receiver, a general coverage HF set covering .5 to 30 MHz. It would be produced in somewhat updated versions (51J-2, 51J-3, 51J-4) for about a decade. It was known as the R-388 and was used in multiple receiver diversity RTTY installations.

The 75A amateur line was updated throughout the early 50s, finishing with the 75A-4, which was released in 1955. The Collins mechanical filter was introduced to consumers in the 75A-3, and the 75A-4 was one of the first receivers marketed specifically as a single sideband receiver.

My Collins 75A1 Restoration Project

(Click any photo to see a larger view)

I was given a Collins 75A1 which set on one of my museum shelves for many years. Finally in 2017 I decided to pull it down and take a look at it. The front panel was in excellent condition and inside it just needed some dust and dirt cleaning except for a couple of leaky "bath tub" capacitors. When these things leak it is a mess to clean up and while I am told the smaller lower voltage capacitors do not have hazardous oil, how do you know about a 70 year old part from a time when the dangers of chemicals was either unknown or ignored. In cases like this I generally chose to get rid of the problem so as you will see in the series of photos below all of the bath tub capacitors were removed. The bottom line is that if you have an old product that has oil filled whatever in it chances are it will eventually leak and when it does it will make a mess both within the product and possibly outside. The other myth is that these capacitors do not fail. I found about three out of 33 separate sections of the eleven "bath tube" capacitors had failed. A relatively small percentage but it shows that they do fail. All of those that failed were open, reading capacitance in the low pf. area.

There were actually two different versions of the 75A1 and who knows maybe more. Changes were made in the muting and B+ circuits. This is the earlier version which brings B+ out to the rear terminal strip for receiver muting. Schematics for both versions are shown in the links at the end of this article. If you have a 75A1 be sure to compare the schematics to what you have. Here are some photos showing what the receiver under chassis looked like before restoration.

Under chassis view showing the leaky "bath tub" capacitors removed and hanging by their wiring. The brown goo on the cross rail shows the oil migration. It took awhile to clean this. At the top of the photo is a solid state product detector which was removed in the restoration
Another view of the leaky capacitors which were later removed.
A view of the "bath tub" capacitors showing leaky seals
Capacitance measurement of one section showing open circuit
Size comparison of original and replacement electrolytic capacitor
Chassis top view showing holes left by removal of "bath tub" capacitors.
Holes covered with an aluminum strip.
Chassis top view showing another row of holes left by removal of "bath tub" capacitors.
Holes covered with an aluminum strip.

Although this shows all of the capacitors removed the actual work was done systematically, removing one at a time and replacing with new capacitors. The new capacitors most of which were bypasses were placed close to the place they were bypassing unlike the original design where in many cases rather long inductive leads were necessary to connect to the "bath tubs." The following photos show the replacment capacitors.

New capacitors installed in the RF front-end section of the receiver
Note bus bars and terminal strips added for grounding. All of these capacitors replaced the row of .1 uf "bath tubs" that were removed.
New capacitors installed in the IF section of the receiver
Closeup view of the IF section. These yellow capacitors replace the row of "bath Tub" capacitors next to the IF transformers. Terminal strips were installed where required to accommodate the new capacitors.
View of the Oscillator (cover removed) with old "bath tub"
and with new capacitors installed.

In all there were eleven triple .1 uf for a total of 33 replaced as well as other single capacitors and the electrolytic filter can. I generally order from Just Radios. They offer a large selection of capacitors and resistors as well as other items.

I am usually very carefully about the diference in line voltages we often see today verses 70 years ago. Most equipment of that era was designed around a nominal 115VAC line voltage. My line voltage is often over 125V, a 10% increase. This would increase the output voltages of the receiver power supply by that amount including the filaments. Not a good thing. I intalled a 12V@2A filament transformer to buck the primary and thus reducing the 125V line voltage to about 112V. This is a kinder way to treat some of this older equipment. I especially like to see the filaments at around 6.0 volts. Even though I have a large selection of tubes I like to see them last.

Buck transformer installed to lower line voltage. A new grounded line cord and safety line bypass capacitors can be seen at lower left.
Transformer is turned on with power switch.
New dual 50/50uf 500V power filter electrolytic installed
Top view showing new power filter electrolytic
Rear panel view showing BNC antenna connector added prior to my ownership and black tape I put over the mute connections. Full B+ voltages appear at this connector for this version of the 75A1! Later versions changed the muting method eliminating B+ at this terminal.

Photos showing the parts removed during the restoration.

These were the parts comprising the solid state product detector modification that was removed.
View of the parts removed. Note only 8 of 11 "bath tub" capacitors are shown. The real leaky ones were thrown out prior to the photo. A total of 43 capacitors and one dual electrolytic were replaced.

Operation of the receiver after restoration was very good. Alignment was performed but in most all cases it was found to be almost spot on. As I usually find out in doing these restorations no tubes needed replacing except the 6H6 which had been removed to insert the product detector modification. This receiver was used during the November 2018 AWA Bruce Kelley 1929 contest and performed well.

The WA3DSP Station ready to operate in the 2018 AWA Bruce Kelley 1929 contest using the 75A1 receiver and a Hartley 75 meter oscillator with 10 watts output.

Manuals and Schematics

Parts and Chemicals

Collins Radio Links

This page last updated 1/14/2019

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