The following are extracts from messages on various reflectors concerning the updating of the 75A4 receiver. It is particularly concerned with recapping. For whatever this information is worth, it should only be considered supplemental to the excellent Compendium published by Electric Radio Magazine.
I installed the N0DMS mods in an early 75A4. The end result was the most amazing radio I ever laid ears on. Dead quiet, incredible sensitivity. I only have one generator so could not measure IMD performance,etc. but I would say that I have heard no evidence of strong signal handling problems. In fact, it seems a lot more crunchproof than a stock 75A4. The mods are pretty extensive and require a decent knowledge of how-to-do-it skills. I would recommend the mods highly, though, if you think you could definitely get everything working right. Have to be careful about marginal grounds and so forth, but overcoming that sort of problem is job one in any 75A4 refurbishment, in my experience. Undertaking an extensive mod job like this is not for the squeamish collector type. My radio needed some pretty serious work to get all the bugs out and it was no mint specimen to begin with. But even if it was mint in a sealed box, I wouldn't have regretted doing the N0DMS modifications. It's that good.
If I read you right...Concerning the AGC...is that the modern receivers hang the AGC time a bit more than the 75A-4's etc. i agree. N0DMS is his AGC mod, adds a 5.0meg resistor in lieu of a 3.3meg on the SLOW AGC...i.e. that is for the SSB is hang about 1 second..I believe it is necessary our mutual A-4 needs that one part. I am sure the S-line equipment hangs the AGC more than the 75A-4; but less than the modern gear; which to their credit is (now a bit) easier to tune in on.
Does you're A-4 have the AGC mod installed? If not, it's a must. What happens is that the AGC is much to slow to react and the receiver overloads before the AGC brings the gain down to the proper level. Every 75A-4 suffers from this problem if the AGC has not been modified. If your receiver has the AGC mod, I'd go with your guess about the capacitors. The problem will probably be one of the mica caps. What happens is that the impedance of the grid circuits is so high that slight leakage of those caps drives the affected stage's grid more positive than normal. The resulting improper bias places the tube outside of its linear operating range. Of course, other nasty little gremlins can cause some problems, too. That's why some of us just change all of the "black beauty" and several of the mica caps as a matter of course. There are also three popular mods which are usually installed at the same time. One is the AGC mod, the second a change in the signal level delivered to the product detector and the last is a change in the feedback circuits in the audio stages. All of these mods are easy to install and I think all three are very worthwhile. There are many other mods which can have a profound effect on the performance of the 75A-4, too. For those who asked more about the usual bad caps in a 75A-4. Chuck Ripple called the "seven deadly caps". C-34 100pfd V-3 pin 3 C-52 4 pfd V-5 pin 7 C-68 470pfd V-7 pin 2 C-75 470pfd V-8 pin 8 C-81 470pfd V-9 Pin 1 C-104 470pfd V-21 pin 1 "Butch, K0BS"-- Later Added to the list 3 more troublesome micas: C-71 1000pfd V-7 pin 6 a high failure item C-95 .01uf V-11 pin 6 C-96 .01uf V-12 pin 2 Jerry Solomon added: C-147 V-12 Pin 1 C-148 V-12 pin 2/5 Also: C-113 50ufd 150vdc Bias Supply to much higher voltages C-137 50ufd 150vdc Bias Supply to much higher voltages and of course any source of heavy hum puts it right back to the three section Electrolytic: C-94 a,b, c, 40ufd/40ufd/40ufd 300vdc. High Voltage supply--get higher working voltages. These tend to go out fast after a set has not been used for a long time. Reform carefully. C-124 B+ Bypass Capacitor .5ufd 200vdc. Collins lists it incorrectly as a .5mmf. Might have to use a very stout .47ufd 300+ volts. Also: On the Black Beauties--they are a real controversy--some sets shouwed them perfect; others showed them leaking; when disconnected from the circuit; they made very little difference "In my Opinion". They are all .1mfd. They are usually in they way of these others so I generally change them out. However, they are in no way any where near the problems of any of the other above mentioned caps; again in my opinion; I think Butch might agree. And they are very Pretty but.....
I have gotten a lot of replys with regard to replacing the wax paper, "black beauty" capacitors in 75A-4's. Every one I HAVE EVER tested leaked like a sieve. To my knowledge, there are two mfgs of "Orange Drop" type capacitors: Sprague and Mallory. They have orange bodies and axial leads. I can't remember all the numbers but the Mallory number for a .01 ufd is a PVC411 (400v) or PVC611 (600V). Resource Electronics in SC stocks the Mallorys and has a well known ham who works there, Paul Marsha, K4AVU who can help you. The "800" directory can get you the number. Also, Antique Radio Supply in Tempe, AZ also stocks a selection of several kinds of Orange Drops. They are billed out as "major mfg" caps but I don't know who the "major mfg" is. Thus far, I have not been successful in deciphering the markings on the cap body but they seem to work just fine. Orange Drops are not cheap selling in the .75 - $1 range for a .01 600V and about $7 for a 600V .47 but work well at audio frequencies. 400 or 600V caps are ok in all the "black line" Collins but I prefer the 600V as they are more resistant to voltage and you typically have the space to install the larger bodied components in these large radios with lots of open space. All Orange Drops should be used in areas where they can be effective. They are NOT efective in things like tank coils and are not especially advantagious at RF frequencies which is normal. Why? Go read up on the capacitor section in your handbook and study the matter. There are many different types of capacitors whose medium is broadly effective under certain conditions. I wouldn't want to count the number of "how come" notes I received after posting the R390A audio mod where I specified Orange Drops v/s ceramic discs as interstage audio coupling caps. A simple read through the ARRL handbook capacitor section will also yield broad understanding about the best applications of polystyrene, ceramic, polyproplyene, etc... Wanna make your 75A-4 really play? Replace all the resistors NOT EXPOSED TO RF with modern, metal film 1/2W units. They are VERY quiet. That's quite a job and unless the user is very experienced in the application of proper component lead dress, would not be attempted. Again, here is a post I made a year or so regarding the "7 deadly caps" in the 75A-4. These are mica coupling caps and most couple the plate of one stage to the grid of the next. With age and exposure to comparitively high plate voltage for all their lives, they will tend to leak and thus change the grid bias of the affected stage or even bias it off. The best way to check these is with a megger or a cap checker capable of putting about 400V across them and measuring leakage. I find that a megger works just fine and is a cheap piece of test equipment. Sometimes you can hit a suspect micas with freeze mist but that is not a 100% reliable wya of ferretting a bad out. Unless the capacitor is REALLY bad (shorted), a digital VOM WILL NOT REVEAL ANY LEAKAGE. The 9v it uses to measure resistance is not enough to coax the offending cap to leak. Here's information I gathered from John Bess, Chuck Ripple and Butch Schartau: 1. The black beauties are C20, C62, C70, C97, C98, C108, C111, C112, C115, C116 and C146. 2. The 7 deadly plus some added by Butch are: C34, C52, C54, C68, C71, C75, C81, C95, C96, C101, C104. 3. My adds to the list were: C37, C92, C93, C100. I replaced them all in my 75A-4. Also did the K7CMS mods as explained on page four of CCA Technical Data Sheet A97-001 Radio works like a million $$. Oh, make sure all the ground connections are good. One problem I had that escaped me for three weeks turned out to be a bad ground on the BFO can. Other useful info: Replace all the 470 pf mica caps in the IF amps and AGC input. I think there are five of them. Dipped Micas are fine. This is the one thing you can do to an A4 to get the most improvements. These caps become leaky and positive bias the control grids of the IF amps and AGC amp. Second replace the blocking caps in the audio amps. Black Beauties. at 01. You should replace all those black .01 caps. But really the micas are worse and should go first. Then replace the input cap to the mech filters also very important.
(following are responses to my question about replacing old capacitors and whether to completely remove the old leads) I recapped my SP-600 "black beauty wonder" with about 50 new caps. What I did was clip out the old cap wire as close to the terminal as possible. Then wrap the new wire around this terminal and solder real well being sure to cover the connection well. I agree that unsoldering and completely pulling leads off can become a mess if there are many other leads soldered to it.. I would NOT solder to old mid air leads. That DOES look sloppy. However, the compromise method described above is just as strong and cannot be detected once soldered well. I know of others who insist on pulling old leads completely off, and appreciate their extra patience.... but each his own.
I recapped my SX28 by clipping the old caps and leaving about 1/4" on the old leads. I then scraped these leads with an Exacto knife a bit. Then I took the leads of the new cap and made little pig-tail spirals on them. It's not hard if you make a jig from some stiff wire and once you get the hang of it. Then slip the pig tails over the old cap leads and solder. Very strong electrical and mechanical connection, and looks good. I used one lead of an old 5-watt resistor as the "coil" form. Wind 3-5 turns closely spaced, and the lead from the old cap fits inside it like a glove.
I find it is very easy to remove old leads from connections if the solder is wicked or sucked off. Usually you can easily unwrap the wire with needle nose or zercon incrusted tools. most people heat the jount then try to wiggle the lead out and then break the terminal. Many hackers still think the bigger the blob the better the job. Removing old solder insures the reflow makes contact. I've seen old tube sockets with very poor connections. Flux cleans the connection and replaces the old solder. Added flux makes the removal and installation of new leads/solder much easier. Rat Shack has some nifty tubes of rosin flux in a tube (not the handiest dispenser). It's well worth the 2$. Get a toothpick to apply it with and you're golden.
My input is, if the rig is a collectors piece and you want top buck for it, don't touch it(re-cap). Put it on the shelf for all to view and admire the workmanship of days gone by. If you want to use the piece and enjoy it on the air, and the components are under the chassis, cut the leads near the component, and put small loops in the leads with minature needle nose pliers and solder. Shrink sleeve can be put over the solder joint and make it look nice. Taking the leads off sockets and other components may result in more repairs than needed in the first place. I recapped a Hallicrafters receiver like above and it worked great. Being able to use the vintage gear and have it sound good is where it is at. There have been good articles in the Antique Wireless Assoc's Old Timers Bulletin (OTB) about restoring vintage gear.N1EU Home