Collins 75A-4 Receiver

Rats had made a comfortable home in various parts of this 60+ year old receiver.  It's unfortunate that Collins designed this radio with an easy-to-open top cover, with a mouse-sized hole for a finger-hole.  Because rodents of some kind made themselves very much at home inside of this receiver.  After removing one desiccated rat carcass, lots of fur, rodent nests, and other foreign items, I was ready to start fixing replacing components and such.

Looking at the top of the receiver with the debris cleaned.

The cabling running across the top of the radio was almost completely chewed apart by rodents!  Not just the insulation, but the stranded tinned copper wire underneath was also chewed away, judging by the tooth marks in evidence!   The wiring and circuitry on the back of the "S" meter was also damaged.  Thanks to good wire color coding, good Collins schematics, and photos from the web, I was able to repair the damage.

Looking at the bottom of the radio chassis

The round yellow capacitors are new 0.1uF 400v capacitors, replacements for most of the old "bumblebee" or "Black Beauty" capacitors found in some older tube radio equipment.  The old caps were rated at 250v.

The aluminum can style 3 section 40uF electrolytic B+ filter capacitor was completely bad (open).  This was responsible for a loud 60 cycle hum from the speaker whenever the radio was powered up.

Numerous other capacitors were replaced, as per what is called the "killer capacitor" list of capacitors that typically go bad in 75A-4 receivers.

This radio appears to have been all-original, without any mods.  None of the changes recommended by Collins Service Bulletins had been made, so I implemented some of those changes as well.

One important change involved bypassing B+ voltage that originally was connected directly to the mechanical filters.  One capacitor failing could wipe out the now rare and expensive mechanical filters.   I used a compression mica adjustable capacitor in series with the capacitor named in the "B+ Filter" mod, so that I could fine-tune the additional circuit to the 455KHz I.F.

Dave Curry custom mechanical filters for the 75A-4.

I made this change before installing my newly purchased Dave Curry 500cps CW filter.  (This radio originally had only one filter, a 2.5kc SSB filter).  I paid more for just this filter alone than I did for some of my modern day radios!  But the satisfaction of using a receiver that was built before I was born, and hearing how well it can dig weak signals out of the noise is worthwhile.

The 75A-4 was beautifully designed, and deserving of some time, effort, and expense to restore it back to its former glory.  It was designed with a number of features that were advanced or leading edge for the time, such as a  product detector,  Q-multiplier notch filter, passband tuning, and mechanical filters.  All of these features serve to improve the quality and readability of signals in noisy band conditions.  The Rockwell/Collins mechanical filters make for startling, rock-solid selectivity.  Tuning CW signals with the 500cps filter feels almost surgical in precision.

Once I finished with replacing capacitors,  making service bulletin modifications to the radio, and aligning it, it was time to try it out.  I connected about 30 ft of wire to the antenna jack (I have no HF antennas in place yet), and I started tuning around on 20 Meters.  I easily copied a number of SSB and CW QSOs, and found the receiver a pleasure to use.  Gone of course was the 60 cycle hum.

This receiver was built around 1956, and apart from rodent damage and some easily replaced components, it has aged remarkably well.