Rebuilding R-390A Power Supply Caps. Step by Step With Photos

Discussion in '"Boat Anchor" & Classic Equipment' started by KC8VWM, Apr 16, 2016.

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  1. N3DT

    N3DT Ham Member QRZ Page

    I got the meters today. Mucho thanks. I've already got one apart and cracked the glass but it's under the cover so it doesn't show/matter. I think I need to make a spanner of sorts to get it back on, it's real tight, but I'm not going to put it back together until I modify at least one for the level meter. You've really got to press down on the back to get the ring back on. Need 4 hands that are real small (so to speak). I'll get the loot off to you in the morning.

    The PO mis-sent it, that's why it took so long. So much for PO speed.

    Did you remove the movement from the back cover or did you just take the front scale screws off and slide it under the needle? It looks tight. Did you glue your new face on the metal face plate? I only want to do this once. There's too many areas for easy mistakes.
    KC8VWM likes this.
  2. KC8VWM

    KC8VWM Ham Member QRZ Page

    Basically to remove the ring you have to spray WD40 on it first, then place a small screwdriver against one of the edges on the ring. Ensure the screwdriver remains in place and tap the end of the screwdriver with something.

    Use a little WD40 before tapping on the ring otherwise it will be very difficult to move the ring because the "dry" rubber seal will be tightly pressed against the locking ring making it nearly impossible to start moving it in any direction. The lubricant will make it much easier.

    Don't whack it too hard, the object here is to get the ring to start turning (sliding) like you are unscrewing the lid from a jar.

    When the keys on the round ring line up with the 4 "corners" on the outer meter housing, this will release the locking ring and permit the front cover to separate away from the meter housing.



    If you have difficulty putting the ring back on, you might consider cutting the ring up into separate pieces and locking the meter housing in position to the front cover using "individual keys" as opposed to attempting to "locking the entire ring" in place at the same time.

    The movement inside can be removed from the housing by disassembling the two nuts from the two screws located at the back of the meter housing. (see photo above) With the nuts removed, press on those two bolts on the back of the housing and this will push the entire meter movement forward sliding it out of the front of the meter housing. When it's removed, handle it carefully as it's delicate like the parts inside a watch.

    When the housing is removed locate the red and black wires leading from the meter movement and simply bypass the internal diode and resistor circuitry and connect the meter movement wires directly to the two bolts used to connect the meter to the housing.

    The two screws located at the front on the meter scale are only used for holding the meter scale in position and they do nothing else.
    Last edited: Dec 4, 2017
  3. KC8VWM

    KC8VWM Ham Member QRZ Page

    No I did not glue the new scale on top of the old scale.

    I cut the new scales from card stock paper and then used the old meter scale as a template to find the meter screw holes on the card stock paper. I used a sharp "awl" to press through the holes and this created the correct locations for the meter screw holes on the card stock meter scale.

    Then I placed the two tiny screws through the card stock meter scale paper and then carefully positioned the scale behind the needle movement and lined up the screws with the holes in the meter movement which holds the scale in place and then screwed down the card stock meter scales in place.

    Just to be clear, I removed the old scales completely and replaced them with printed card stock meter scales. The card stock is not steel like the original meter scales are, but they are rigid and stiff enough to work on their own anyways.

    Some people glue a new scale over the old one, but I avoid doing that because there's a tight clearance between the needle pointer and the scale. If the meter scale you are using is too thick, then the needle might rub against or get hung up on the meter scale.

    I hope this explanation makes sense, if parts of it don't then please ask me to clarify and state it again. :)
    Last edited: Jun 4, 2016
  4. N3DT

    N3DT Ham Member QRZ Page

    Got it. I'll have to get someone to print the scales for me, I only have a laser printer and I don't think it will print on card stock, but I'll check it out. Like I say, I have a friend that has an archival printer and am sure he can do it. I did see that there's very little clearance between the needle and the scale. I figured those 2 nuts on the rear would loosen the innards like most meters, I was just trying to avoid that if I could.

    Yeah, I got the ring out fine with a small screwdriver like you said, it was a bit tight but I should have used some WD40 like you did say(and I read). Funny, it didn't feel like it broke, and I'm not sure I did it, but am supposing I did, but it's no problem. It's not visible.

    Someone here has to be on the R390 list. I'm not subscribed any more, this thread should be mentioned on that list so others can benefit. I think replacing the meters was one of the biggest issues. Most all the other stuff has mostly been rehashed before. I did have a couple guys on the list help me out with parts like the swinging chokes that are nearly uuobtanium. I bought a bunch of SS nuts, washers and machine screws and sold as a kit for the front panel at cost to a lot of them.

    One of the mods I did that I'm not completely happy with is adding the thermistor in the AC circuit. It makes too much heat for my liking.
  5. KC8VWM

    KC8VWM Ham Member QRZ Page

    If you need me to print some meter scales and send them, I sure can! :)

    I agree, getting the meters right on the R-390A is probably a large percentage of the entire restoration! :eek:

    Besides many of them missing their original meters after getting the yellow stripe treatment, many of the original R390A meters are just getting plain old. I have heard stories where many of them are starting to fail. People are often hesitant to take them apart and repair them because they are radioactive.

    Since these brand spanking new meters from their original boxes are of equal quality and ruggedness, they should provide a lifetime of service in the R-390A. I am extremely pleased with the results and how they turned out.


    Last edited: Dec 28, 2017
  6. KC8VWM

    KC8VWM Ham Member QRZ Page

    I looked at some of the mods indicated on the pearls R-390 list before and I don't agree with many of them. Some of them are what I call "tinker mods" people are doing just because they can. They often don't serve any practical or useful benefit over the originally intended design and sometimes they cause unforeseen and unintended problems with other circuits.

    However, If you want to reduce the voltage in the power supply due to the increase in AC voltages in modern day electrical outlets, it might be a more effective approach to install a 15 watt 220 ohm dropping resistor inside the audio module. This is actually an approved military depot mod and I have details of that mod if you're interested.

    There are a few mods that make sense but my general approach when restoring is to avoid mods unless they are absolutely beneficial out of necessity. Take the meter resistor mod in the IF module I did for example. When reading the "pearls" R-390A list, it's clearly apparent everyone has been struggling for years with trying to get the carrier level meter working.

    We all know the idea of locating a 17 ohm replacement meter is simply just not a reality for many people. But if you follow the pearls R-390A list, it seems no one has come up with any practical solution to this problem for a very long time with the possible exception of adding an external op-amp circuit to the meter but that's really not a practical endeavor for most people.

    On the other hand, we know suitable 100 ohm meters are plentiful and are available everywhere, but they simply don't work as a direct replacement in the R-390A because they are not 17 ohm meters.

    In this case, I think the meter resistor mod is absolutely necessary if you're going to be using any kind of replacement meter. However, I researched and studied the IF schematic and gained a complete understanding of how the circuit in question works before doing this mod to ensure it won't mess up the AGC circuit.

    Sometimes I see original R-390A meters on sale on eBay going for over $150 and they are snapped up pretty quickly too because they are so rare to encounter. But I think a 25 cent 44 ohm resistor and a $7 NOS meter can resolve the R-390A "meter problem" once and for all.
    Last edited: Jun 4, 2016
  7. N3DT

    N3DT Ham Member QRZ Page

    I've got a bunch of meters I picked up, but they're not very accessible and the originals I got with one were not working. I'm sure they're radio active (the ones I made the pictures from). I keep them on the other end of the room in a metal can which probably doesn't help. A couple of the other meters I have, have the same visible format, are 500µA and 500Ω DC but measure 300Ω but I haven't been able to get them apart, they probably have radium on them too. These meters you found seem to be the cat's meow.

    Shoot, I've got a Rolex watch that I wore for years that had that radium painted on it. I even used to sleep with it next to my head. I think the radio active issue is over rated, unless you're painting the stuff on the meters and licking the paint brush. We're probably more prone to leukemia from HF radiation than the meters. Or radon from the quartz in the ground around here.

    I probably won't get back on my 390 until the mowing season is over for the spring. At least I've got the New Holland 451 mower working pretty good now, 5 acres per hour. That's 7' at a whack and 5.5 mph.
    KC8VWM likes this.
  8. KC8VWM

    KC8VWM Ham Member QRZ Page

    Yup. :) It rained here all week here and needless to say I am probably going to get lost in my backyard wilderness while driving around on my cub cadet today. So if you don't hear from me in the future, send some helicopters and a search party team to find me. :D It's the first sunny day of opportunity we had here in a while, so I better get to it before the lawn mowing police show up at my door.

    I gave everyone a sneak preview of freshly striped knobs a few postings ago, but I will detail with actual live photos showing how that's done soon. ;)
  9. KC8VWM

    KC8VWM Ham Member QRZ Page

    I am just going to send you a replacement glass.

    I have one of these meters that isn't working because I accidentally dropped the internal movement on the floor when I had it apart and broke it, so I am now just using it for parts anyways. :)

    I will also send some printed meter scales just like the ones seen in the photos in this thread at the same time.
    Last edited: Jun 4, 2016
  10. N3DT

    N3DT Ham Member QRZ Page

    That means I need to get your loot to you asap.

    I actually got the old meters I have apart, the rubber was glued to the back and it took some prying to get it apart. Not sure I'm going to use it now that I have yours and they work so good. I'd have to reinvent something for these meters. They'll be good for backups though and they do look appropriate except for the scale.

    You're too kind, I'll need to repay the kindness somehow. Do you need any Motorola Spectras? I've got a couple 50W VHF and a 110W VHF with HH control heads. They'll program in the 2M band. I've been trying to figure out what to do with them.

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