What I did to get my 75S-1 back into operating condition

The Collins 75S-1 Receiver as I received it.

When I got the receiver, it appeared to be in pretty good shape. After opening the top, I saw it could use a good cleaning. It isn't the worst I've seen, but enough to annoy me.

It was after I cleand the chassis I noticed the cabinet, knobs, and front panel needed a good cleaning as well. While it "looked" pretty good, I did find a fair amount of "smoker" residue still on various surfaces.

Once I cleaned the chassis, knobs, and front panel, I took the cabinet up the the kitchen sink and gave it a good scrubbing with a soft bristle brush and dish washing detergent.

The underside of the chassis was pretty clean because it had been protected by the cabinet. However, there was still some "smoker" residue to be removed. Here is a picture after the first round of cleaning:

The Collins 75S-1 Receiver after a good cleaning and the removal of modifications.

Here is a picture of the chassisas I started. It wasn't too bad:

Here is a better picture showing the contrast between the original and cleaned chassis:

Once everything was clean, I started to look into a couple of things which I thought looked a little out of place. Here are the things I spotted:

• In the back center of the chassis, a previoud owner replaced the 1N1084 with a couple of more modern silicon diodes. At least the leads were dressed with insulating tubing. Here is a picture.

  I removed these diodes and replaced them with the correct 1N1084.

• In the middle of the left side of the chassis, a trim pot was inserted along with a cap. These were in the S-Meter circuit, and their purpose was not clear to me. Here is a picture.

  These components were removed.

• In the left front center of the chassis, there was a second filter installed. Here are a couple of pictures: left side and right side. Unfortunately, the label could not be read so the purpose was not immediately obvious.

  After removing the filter it became clear it was a replacement for the original SSB filter; which had been left in place, but disconnected.

• To the right of that, right front center of the chassis, some resistors and a cap had been replaced. In addition, these new components were wired up with a push-on-push-off switch on the back of the RF Gain control. Following the wiring, it became clear this was amodification to the AVC circuitry to give two different time constants. Here is a picture.

  Because I did not have a replacement control, I left it in place, but change the wiring to duplicate that of the 75S-3 receiver... mostly. There are normal and fast positions, but no OFF.

• There were a couple of incorrect crystals installed in what should have been the 10 meter band. One was just for an odd portion of the band, and the other looks as though it was for the 11 meter band.

  No matter, the incorrect crystals were removed and the correct crystals inserted.

The one remaining item is to disconnect the seized power connector. To date I have tried different methods of gentle force, but the connector is still stuck in place. I really don't want to have to replace the socket in the chassis if I can avoid it.

Listening results

Once the receiver was all clean, unmodified, and put back together it was time for an alignment. Using the instructions in the Collins 75S-1 manual, it look me about two hours to go through a complete alignment.

After that it was time for the listening test. Connecting the receiver up to my antenna (a Cushcraft R7) I was rewarded by very nice sounding signals which ever band I chose. I was very pleased with the sound. To me, much easier on the ears than many of the current receivers.

One issue I did discover occurred as I was using the 75S-1 connected to the 32S-1 for a local 10 meter net. I needed to add an additional crystal (15777.500 KHz in the 2E position) and it only worked when I "jiggled" V2, the oscillator tube. The curious thing is all the other bands worked fine, and switching the band switch had no effect. As a first test, I'm going to try substituting a new tube.

UPDATE: After replacing the tube, the oscillator has no problems starting up on any of the bands. So, for the moment, I will put the problem down to a bad (or weak) tube.