Started my 75A-4 Work

Discussion in 'Amplitude Modulation' started by KA4KOE, Oct 11, 2019.

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  1. KA4KOE

    KA4KOE Ham Member QRZ Page

    I purchased my 75A-4 from Liz K4GHT about a year and a half ago. At the time I did not realize she was dying and in retrospect, I think she liked me as a friend and wanted to do something nice for me, given the low price of this largely unrestored radio. It had all the filters and worked as well and looks nice. We'll, I've started on the capacitor recap job....about 26 to install. Also going to do the filter shunt mod to protect those little cans.

    All of this is significant because I'm getting back in the groove after a lot of health problems. Next I have to repair my R390 Collins and finish restoration of a HQ-140X. Finally, restring a Zenith B600.

    Then.....the Bauer 707 repair.

    This week I removed some things from my upstairs shack and relocated the work bench so I can actually have room to do things in here. The futon simply had to go.

    Be happy for me!

    my75A4.jpg bench.jpg
  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    You are going to replace the silver mica coupling capacitors? Those capacitors are just as bad as the "black beauty" capacitors. If you have not found the list (probably have found it), the list can be found at

    Of course, the 2-wire power cord has to go being replaced with a 3-wire power cord.

    The "stock" AM filter is a little too narrow for most AM operators tastes. It does work well but a wider filter is better. The 8 kHz Collins mechanical filter, for the R-390- series, works very well. They do have to have a 9-pin miniature plug adapted. Unfortunately, these filters are getting fairly hard to find. Fair Radio used to sell them but then stopped this practice. Now, at least the last time I looked, Fair Radio is only selling these filters to persons purchasing an R-390- series from them.

    I do have one of the 8 kHz filters in my 75A-4. Since I don't use the receiver for CW, I also have a 2.1 kHz filter, as well as the "stock" 3.1 kHz filter, for SSB. The 2.1 kHz filter, at least in my experience, works much better.

    Glen, K9STH
  3. KA4KOE

    KA4KOE Ham Member QRZ Page

    All of the 7 deadly caps except C-52 which was not in my stash of parts. I am going to do the B+ filter mod next. All black beauties gone. New multisection.
  4. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    You don't have to replace the coupling capacitors with new silver-mica or dipped mica capacitors. The primary reason that Collins used sliver-mica capacitors was that, in the time frame when the receiver was designed, small value ceramic capacitors were not readily available but silver-mica capacitors were available. I just use ceramic capacitors when replacing those coupling / bypass capacitors.

    Now, in certain tuned circuits, where excellent stability is required, ceramic capacitors are not usually a good choice. Otherwise, modern ceramic capacitors work fine where the smaller values are concerned.

    Glen, K9STH
    KA4KOE likes this.
  5. W8KHK

    W8KHK Ham Member QRZ Page

    Fleep, Good to see you are back at it after all the health problems. I am still down from the chemo and radiation, and it is hard to do things that were very easy in the past. Chemo after the age of 70 is a very difficult thing to endure, I can tell you from personal experience.

    I have so many projects I want to finish, including the restore of my 75A3, 75A4, 51J4, SX28, etc. I have two RME-69s and an NC-300 to go as well. Not to mention all the transmitter restorations and the Class-E / PDM build in progress.

    I often wonder if I will be around long enough to see these things completed. I appreciate you sharing your progress on the forum. It gives me hope. If nothing else, I want to finish the legal-limit series-modulated 3CX3000 and have a few 75M QSOs with it before moving from vertical to horizontal polarization.
  6. K5UJ

    K5UJ Ham Member QRZ Page

    No it doesn't need to be replaced with a three wire cord. It needs to be replaced if the old cord is cracked and rotting and dangerous, but it can be replaced with a new two wire cord under one condition: The operator wishes to isolate the receiver on transmit as much as possible and the transmit antenna is in close proximity to the shack and the rig is QRO. If a three wire cord is used, the entire grounded cage of the shack location is an antenna and the chassis and cabinet will have RF on them. This makes it impossible to lower the signal level in the set if you don't want to completely mute the audio. The only way to prevent that and shield the set's circuitry (assuming the rx feedline is completely relay disconnected also) is by floating the chassis and cabinet with only a hot and neutral return. Note that an antenna relay on the set such as the famous dow key won't work because the unbalanced feed shield is always connected.

    If none of this matters, then go ahead and run a three wire cord which I use as much as possible. But there are times when a grounded plug is not wanted, and it is perfectly safe if the operator knows what he's doing, despite the propaganda from the safety obsessed hand wringing nanny state that will have you believe that all power cords must be three wire.
    KA4KOE and K4KYV like this.
  7. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    I strongly disagree! Unless the chassis is grounded, as was standard operating procedure when 2-wire AC wiring was used in every house, there is about 60 VAC on the cabinet / chassis although at a fairly low current that causes a "tickle". This is due to the bypass capacitors that are from the hot side, and neutral side, of the AC line cord connected directly to the chassis.

    Using a polarized 2-wire cord is still very common with consumer devices. However, all of those devices have insulated, almost always plastic, cases and not metal cases.

    There is nothing stopping you from not replacing 2-wire cords with 3-wire cords in "boat anchor" equipment. But, if there is some weird condition that causes problems with a 3-wire cord, you really need to eliminate that condition rather than undertake "band aide" solutions that often result in unsafe conditions.

    I absolutely refuse to allow any "boat anchor" equipment, that I service, restore, etc., for others, to go out the door with a 2-wire AC line cord. If, for some reason, the client does not want this, then I refuse to work on the item and it goes back to the client with him / her paying the return shipping. In many decades of doing this work, there has been exactly 1-person who refused to have a 3-wire cord installed.

    Glen, K9STH
    W1BR and KE4OH like this.
  8. K5UJ

    K5UJ Ham Member QRZ Page

    I knew you would. Get apoplectic all you want but take something for blood pressure.
    Why, thank you for your permission.
    Who said I was doing anything for someone else?
    Probably someone who wanted to keep RF out of his receiver. What do you know, my CR88 has been working just fine with a two wire cord. I don't see how it hasn't exploded or something.
  9. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    My blood pressure normally is in the 120 / 60 range so I don't need to take a blood pressure pill!

    Looking at the schematic for the CR-88 I do notice that there are no bypass capacitors on the AC line. Therefore, the chances of having a "tickle" from the cabinet / chassis is eliminated. However, the CR-88 is among a relatively few receivers that do not have such capacitors installed.

    The Collins 75A-4 definitely has these AC line bypass capacitors installed. Therefore, to eliminate any chance of "tickle", the chassis needs to be connected to ground. In the "goode olde dayes" this was accomplished by connecting the ground connection, on the rear of the chassis, to an external ground. Going to a 3-wire cord eliminates the need for the external ground connection because the green wire, in the cord, connects the chassis to the grounding circuit provided in the now standard NFPA NEC (National Electrical Code) 120 VAC wiring.

    As I said before, you are definitely free to do whatever you want. However, please don't recommend such to others especially where safety is concerned. There are definitely very good reasons that the "nanny state" NFPA NEC now requires 3-wire cords in units that do not have complete insulation between the power cord and the AC line.

    Glen, K9STH
  10. K4KYV

    K4KYV Subscriber QRZ Page

    The 3-wire cord doesn't guarantee that the chassis is connected to ground if the house wasn't fairly recently constructed. An enormous percentage of older homes are wired with two-wire/no ground cable and equipped with two-prong outlets. Knob-and-tube wiring may even still be in use. Owners are reluctant to spend thousands of dollars to have walls, floors and ceilings ripped open and restored, to pull older wiring that has safely performed without a glitch for decades, and rewire the whole house with 3-wire cable. Even adding an outboard ground conductor to existing wiring would likely be expensive even if feasible without the same demolition/restoration. So what to do? Go to the nearest Dollar Store and purchase a handful of 3-to-2 prong adaptors for less than $10. Or else purchase a bag full of three-prong outlets from the Big Box store and perform a DIY replacement, leaving the green ground screws unconnected.

    It is not uncommon when selling an older house to invest tens of thousands of dollars to bring old wiring up to present-day code, just to get the bank to finance it for the buyer. As long as the current owner lives there, he probably isn't going to make such an investment if the existing wiring hasn't become obviously defective.

    Any ham station assembled with minimal care will have a station ground running to an outdoor rod, or better still, a mini-radial system, for lightning protection and RF grounding. Running a wire from an external station ground to the receiver's ground terminal will guarantee that the chassis is safely grounded regardless. The 75A-4 has a well marked chassis ground terminal specifically for this purpose. Another solution is to feed the receiver and other station equipment with an isolation transformer.
    K5UJ likes this.

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