Collins 75A-4

What follows is a posting for a 75A-4 that I sold on the internet. I've included it because it went through an evaluation on the test bench and the specifications are noted. Further, there are some interesting photographs of the 75A-4 from the inside. Perhaps you've never seen inside - here's your chance. So, enjoy this quick tour of a very nice 75A-4.

CCA overall rating is VG

Serial Number: 1650+

Includes standard 3.1 KHz filter, no 4:1 knob, but one is available.

The cabinet is in very good condition and has only very minor cosmetic flaws. The two very minor scuffs are visible in this photo. One is to the upper right of the RF gain control. the other is a small white discoloration just below the "0" mark of the AF gain control.
Underside view of the receiver front end. It is very clean and appears to be free of modifications.
In the photograph below, you see the same part of the receiver from the top. This is the RF section which includes the calibratior, RF amplifier, two mixer stages and the crystal hetrodyne oscillator.
This is the underchassis view of the i.f. side of the chassis. One power supply choke has been replaced, as has the chassis mounted electrolytic. There is some evidence of minor repairs. Obviously, this receiver was used and did not just sit on a shelf.
Most of the original "Black Beauty" and "silver mica" capacitors are still in place. Obviously, this receiver is a good candidate for a capacitor modification.
This is the left side of the chassis. It is quite clean, though some dusting wouldn't hurt.

Here you see from the front to rear, the mechanical filters, the slug tuning rack inside the rectangular box and the calibrator, mixer and oscillator stages. The PTO is the box with two tubes on the right side of this photo. That's the dial drum in front.
The same goes for the lright side of the chassis. All components appear to be original and in good condition. The big "can" in the center of the photo with the shaft and shaft coupler is the BFO tuned circuit. The BFO shaft is mechanically coupled to the PTO. When the BFO shaft is turned, the PTO "rocks" back and forth is near perfect frequency sync with the BFO. The result is mechanical passband tuning. The BFO control is labeled "Passband Tuning" on the front panel. This is a very effective system and seems to work as well and perhaps even bettern than current electronic passband tuning schemes.

Before listing this A-4, I put it on the test bench to determine if it would meet specs electrically and to evaluate its overall cosmetic condition. (Things look different under the bright light of scrutiny. In a typical operating position, one overlooks most of the flaws I will detail. However, I offer the following information to give you the most accurate description I can without your actually seeing the rig.)

Overall Rating - Electrical


Receiver meets published specs. I have not aligned this radio, but it is an excellent performer. Audio quality is rich and full. Sensitivity is outstanding and its mechanical filter is excellent. I did replace the mechancal filter because the one which came with this receiver was bad. It had a lot of signal loss which is becoming quite typical as these filters age. While I had it on the bench, I subbed several tubes to make sure the tubes were OK and found a few that needed replacement. Thus, this A-4 has a selected very "hot" 6DC6 RF amp. Also, especially good 6BA7's were selected as the mixers. This contributes significantly to the outstanding sensitivity of this receiver.
With a cursory inspection, it appears that one choke in the power supply has been replaced and the chassis mounted electrolytic capacitor has been replaced. None of the usual mods have been installed and all of the original capacitors are still in place.
While I was inside the radio, I did touch up the tracking on the bandpass tuning.
Operating on all bands Yes
Sensitivity Within Spec. Measured on 20 & 10 meters. MDS (minimum detectable signal) <0.1 uV on both bands. Note the S-meter reading in the photo at the top of this page. That S-meter reading represents a 100 uV signal. This receiver is hot!
VFO/PTO Calibration When calibrated to the nearest 100 KHz point, the PTO is within 200 Hz. If calibrated at the middle of the dial scale, the end points (at +/- 500 KHz) are -250 Hz at the bottom and +750 Hz at the high end of the scale.
Stability Meets or exceeds specs.
Filter/selectivity I replaced the original 3.1 KHz filter which had the high loss characteristic of many 3.1 KHz mechanical filter. The new filter decreased signal path loss by over 20 dB.
Notch Filter S9+20 signal reduced to S1
AGC Operating normally
Product detector and audio system Very clean SSB and CW audio
Alignment Not measured, appear OK in light of the sensitivity and PTO accuracy of the receiver.
Switches move crisply Yes
Controls (including pots) work properly Yes
No obvious problem areas None noted
My recommendations for further restoration include such standard items as replacing the "7 black-death caps" and the three "heartbreaker" micas. I'd also put in the AGC, product detector, and audio mods. These are standard mods for all 75A-4's.

Overall Rating - Mechanical


No major damage. Overall, there is nothing wrong mechanically. Everything seems to work smoothly and properly.

Cosmetic Rating - Front Panel


A pretty face

This 75A-4 needs a little cleaning. It's been sitting around for awhile. There's some dust, etc.
The dial drum has some minor flaking. The KHz dial is nice and white. Overall all the front panel is quite good.

Personality and experience marks

  1. 2 very minor nicks - one to the top right of the RF gain control, one below and left of the AF gain control. Easy touch-up.
  2. The front panel looks like it may have been repainted, if that is the case, the job was excellent and the finish has an original appearance.
  3. Lettering is excellent
  4. All original knobs including proper passband tuning knob.

Overall Rating - Cosmetic, Cabinet

7, potential 8, 9 or better

Overall Generally better than a typical unrestored 75A-4
Top Looks original.
Sides Look original. Very minor discoloration and very minor scraps at panel edges from where receiver has been moved around.

Cosmetic Rating - Inside


Inside -
chassis top
Very nice. A little dusting will give you a "eat-off-of-it" appearance.
Chassis -
component side
Back panel All connectors are original and work properly.

The cabinet cosmetics are fine as they are. However, in the future you might want to bring this radio back to its former glory with a very light overspray of the cabinet sides and top. I would do little to the front panel other than change the dial drum overlay (about $8). There are no dents or major problem areas.

Further Comments:

I was delightfully surprised when testing this receiver. After changing a few tubes and little else, it is a really "hot" receiver. It sounds especially good when listening through a quality speaker. Combine great sensitivity, good looks, excellent audio with an accurate dial and this 75A-4 is a real winner. I'd give it a bit of a cleaning, and at some later date, replace the aging paper caps and failure prone micas. However, that does not appear to be immediately necessary. In the few hours I had it on the test bench, it was a delight to listen to.