Hi all,

A friend gave me this Collins 75A4A receiver today, along with a nice Altec Lansing A333A monobloc 6L6 amp. I took the stuff in trade for building him a few basic guitar amps. The equipment belonged to his late grandfather.
Anyway, I've found lots of information about the 75A-4, but not about the one I have, with the "A" suffix. Does anyone know the difference?
I'm not sure what I'm going to do with the set. It's a top shelf radio that is pretty far afield of my collecting interests but impressive from an engineering standpoint.

Ted

The 75A4 with an A suffix was modified after it left Collins and the new plate was added by the modifier. The Collins factory never produced an A suffix 75A4 but at least one of the aftermarket modifiers had his own replacement label plates made up.

For some the 75A4 was considered the best ham receiver made and to have one better than every else who merely had the best some modifiers made changes, often to the mixer circuits, and once you had the new model label you were no longer just part of the crowd

It is a good receiver and assuming the modification was done well it should still be a very good receiver.

Depending upon what sort of restoration it has had prior to you it probably has some capacitors that need to be addressed.

Rodger WQ9E

Interesting, thanks for clearing that up.
Gibes with what I see with the set. It has its original 75A-4 manual along with notes, receipts and schematics pertaining to the modifications. Thanks.
It's in great shape, the only thing that doesn't look stock to me is the rectifier. 5Y3's gone, replaced with an octal base and diodes. All original bumblebee caps underneath, looks pretty much untouched.

The bumblebees absolutely MUST be replaced before you apply power.

That is likely true of the electrolytics also.

- Leigh

Absolutely, if I decide to keep and restore it, they are gone. They'd be coming out for sure, filter caps as well. Rsistors,etc....
Learning how to use this receiver is going to take me some time ...

That "75A-4A" plate was likely added in the late 1970s by a ham / technician who was commercially modifying these receivers. I believe his name was Robert Stankus. Over the years, the consensus evaluation of his modifications by users is that they actually reduced the performance of the receiver's overall S/N ratio and AVC. Your radio may have those and more, or less (if someone attempted a reversal).

There are many published modifications that did improve performance. As has been noted, change out the bumblebee capacitors, for openers. See here for some helpful hints:

http://www.collinsradio.org/cca-collins ... t-manuals/
for versions of manuals and schematics

You'll find that Collins updated circuitry throughout the production run, so it's important to use the schematic / manual that applies to the serial number of your set.

http://www.collinsradio.org/rx/
Scroll to the bottom of the page for 75A4 information

Seems from the photo your radio is not an early one, as it displays a "DIAL LOCK" instead of "DIAL DRAG" adjustment.

Wonder what two IF filters are in the sockets?

Take your time working with this receiver, and you will be rewarded with outstanding performance.

Best of luck!

-O.B.

Thanks for the great information, OB.
Serial number is 5791. What was the total run?
Below is one of the copies of the schematic changes that were with the radio when I got it. It appears you are correct: Mr. Stankus did the modifications to this set.
I will have a look at the filters tomorrow.

Again, many thanks for the insight.

Ted

Serial number groupings here:

http://www.collinsradio.org/wp-content/ ... -4-TDS.pdf

Just about at the end of the run...

The 6GM6 RF amplifier will increase input gain when compared to the stock 6DC6 circuit. Just not sure if his circuit constants were appropriate for the distributed AGC.

Mixer mod replacing 6BA7 stages was very popular, but generally not with the tubes used in your radio. In that era, 6BA7s really weren't noisy or troublesome as mixing devices, especially when used with a separate crystal controlled oscillator as in the 75A4. An internet search on "Pullen Mixer" or "Pullen Mixer+Collins 75A4" will provide more background for those mixer stage changes.

All the best,

-O.B.

There are also number of mica capacitors that typically are bad by now. Chuck Rippell has a list of these on the web somewhere.

I must have been lucky. I ran my 75A-2 for a couple of years before I did a recap. I didn't quite finish the recap and have been running the radio for a few more years.

i bought the modification compendium sold by Electric Radio to make it a little easier to track down articles with mods. There's a disclaimer up front about the efficacy of most of the mods, and some with descriptions in the table of contents that some don't work very well. After study, if appears that very few published mods are worth the effort.

I find that completely believable, given the extremely high proficiency of the Collins engineers.

- Leigh

And that is true of the majority of modifications. The original engineer(s) took a systems approach because what they designed is a system. Amateur modifiers generally focus upon one or two "benefits" from their modification and then ignore all of the negative consequences that are created when focus is upon the limited instead of total impact of the proposed changes.

It isn't just hams that do this and the Northern Radio modified NC-183D I worked on a few years ago had a modification induced issue. The additional tubes added in their changes increased B+ current draw which caused the power supply circuit used in this receiver to develop more bias than it should throwing off the gain control system. Since I didn't need the functionality of some of the added circuitry I pulled one of the added tubes which dropped the current draw sufficiently to bring the developed bias back into a near normal range. Alternatively I could have changed the resistor value so that it would have developed the stock bias level with the full new tube complement just as Northern should have done when they made the receiver changes. Perhaps this set of modifications was designed by a summer intern.

It is the same approach I took when doing risk management consulting; pretending that linkages to other parts of the operating environment don't exist is a high probability method of reaching the wrong answer to a problem.

Rodger WQ9E

Just looked at my 75A-4. It's serial number is 2739.

We are both in danger of becoming members of the Collins Cult. Once you have one, and study the history of the company and its products, read a few interviews and personal letters from Art Collins, well, the die is cast.
Art, from what I can tell, was a perfectionist and his products and the way he ran his business shows it.

Sorry all,

Meant to get back to the group with info on the two mechanical filters in the set, but it took me a few days:

They are:

Type F455J-31
Type F455J-21

I was poking through the set last night, having a look. Though the alignment of this receiver is going to be WAY beyond me, I was surprised to see that most if not all of the caps and resistors in it are very accessible and should be simple to swap out when the time comes. Really looking forward to listening to this set. Of course, I have zero idea at the moment how to use it, but I will read through the manual and go from there.

I disagree. The engineers were designing to purpose. At the time of the predecessors
of the 390A, and to some extent the 390A itself, SSB was not in the list of purposes.
AM, MCW, CW, RTTY were on the list. Thus the plethora of mods about that area,
mine included (and mine are, of course, the best).

For instance, the way the limiter and the first audio stages work, and overload,
just screams "RTTY", but works OK for CW. Without mods the limiter simply does
not work for voice, be it AM or SSB, without serious distortion, not matter
what the RF gain setting with the AGC off.

The Lankford AGC mod does not fix that. The whole use of the tube amplifier for
slow AGC is predicated on slow fading, assuming that AGC will work on that
with a continuously on signal ... not SSB nor any but the fastest possible ham CW,
where you need a very very fast rise time. The parameters in the AGC in the slow
setting are just wrong for that.

My mods fix all that with just one (Edit: two) downside. Ultimate sensitivity and S/N are not changed
one bit, with very careful real checking. The downside is a 10 dB reduction in audio
output at full gain. This is necessary to avoid clipping in the cathode follower stage.
This stage was clearly designed for no overload margin.

The second downside is that very large, very short, noise bursts activate the AGC.
With the original they don't. With SSB and CW that problem is in general incurable
without very sophisticated SDR with a huge dynamic range..

Fixing this would require a major change: replacing the 2.5k audio gain pots with 10K ones.
The cathode bias current of the follower is inadequate for the needed AC voltage across 1.25k.
Its not clear why this was designed as it was. The pots are not driving a low impedance load,
and I doubt that capacitance of the cables is enough to matter.

One thing:

Does the radio need any filters to be installed in the three slots, to operate? Or do the filters just expand the range/options? A bit unclear on this. Thanks!

Ted

Yes, it does, and you have one filter in the B position according to your eBay pictures....

Yep. I pulled that one.
I am going to pass the set on. It's just not my area. I was reading the alignment instructions (or trying to) and it's so far out of my range of interest that getting the set up and running properly would probably be very difficult for me. I have other fish to fry and as a result would likely never get to it