Holy Grail Attained – A National HRO NPW Gear Drive and Dial in FB Condition

Firstly, I must apologize for posting so many pictures of these National HRO gear drives and dials. Once I decided that a building a regen with one of these dials and drives as the centerpiece was a goal, it became something of a preoccupation to find both a dial and drive that were just right. Luckily, I have now found that perfect combination, and can promise that this will be the last post on the subject 🙂

An idea that has been brewing in the back of my head was to write a post on the subject of dials and reduction drives. I have purchased and looked at a number of different types over the last year or so, found a few to be slightly lacking, and others to be eminently suitable, for the purpose of tuning a home-built radio receiver. For a number of reasons (that I may go into in a future post one day), I consider the National HRO micrometer dial and gear drive to be one of the best solutions. Luckily, there are still quite a few of them knocking around, for the builder who is willing to spend a little time looking for just the right one.

When I saw this particular drive on eBay, I knew I had to have it, at almost any price (within reason). Firstly, the gear drive was in nice physical condition. Also, it had a feature I had not seen before. Although I have been aware of these drives since my teenage years, I have only been paying close attention to them in the last few months. Perhaps this is not that unusual, but it stood out to me – there was an extra feature on the front of this drive, in the form of a shaft rotation limiter. It is an ingenious mechanical device that will not allow the dial to rotate more than 10 turns in either direction. The rignt-angle drives have end-stops built into the inside of the top cap of the gear box, but the standard straight-through models have no such rotational limits.  The variable capacitor I have been planning to use with this next project does not have end stops, so to have one built-in to the drive would be great. The fact that this drive looked to be in really good condition as well (judging by the photos online) elevated it to “must have” status.

A week or two later, this little beauty was in my hands. These are the “before” pictures, so if you look at these, make sure to stick with me to the end of the post to see how things turned out. Having said that, it looked pretty darned good from the get-go though. Although you can’t really see how the shaft rotation limiter works, you can see it’s component parts that are stacked over each other on the tuning shaft, in between the metal end plate and the cast metal gear box –

On removing the top cap, I was expecting to see a moderate amount of partially dried grease on the various component parts of this drive. What I found instead was a large amount of grease, applied in generous dollops. I wonder if there had been a special directive to apply a lot of grease in order to protect this batch against an extreme climate, ot perhaps it was just down to the whimsy of the worker who had been applying the grease to these gearboxes?

Look at all that grease! You can’t see it. but there were some very large dollops underneath the gears, in the interior of the gearbox –

Before dousing the gearbox with WD-40 and then dish soap, I removed as much of the grease as possible with a toothpick, before partially disassembling it. Then I went to town on the gearbox and all the parts, with generous amounts of WD-40, a toothbrush, then dish soap and the toothbrush, followed by nice long soaks in warm dish-soapy water (and the toothbrush again). Then came a good rinse in non-soapy water, and plenty of attention from a hairdryer on the hot setting to dry it all out.

Look at the squeaky-clean result. It just amazed me, looking at this, to realize that it was something like 70 years old, and it has plenty of life left in it still –

All the parts were now clean, dry, and ready for reassembly. It is not my intent to give detailed how-to instructions here. If you read my previous posts about these drives, as well as the other supporting information online that I have linked to in the previous posts, you’ll know enough to figure things out – especially once you’ve looked at one in real life.  Although these straight-through gearboxes have more parts than the right-angle drives, they are easier to figure out. In the following shot, you can see the slip-on washers of the shaft rotation limiter that fit over the tuning shaft. Each washer has a tab with a small protrusion that prevents it from slipping over the collar next to it. There are 11 of these tabbed washers in all –

The plastic lid under the loose parts once held fruit salad from Genova Delicatessen. If you visit Oakland, CA and like East coast style deli’s, Genova’s on Telegraph and 51st is a must!

Remember to set the anti-backlash setting on the gear that is spring-loaded, before inserting the tuning spindle into the gear box. Do not over-tension it, or there will be too much friction when tuning. All you need is just enough tension to counter the backlash and no more. The sprung gear only needs to be offset by one, or two cog-teeth at the most.

Next came the lubrication stage. There needs to be grease in every place where there is metal moving over metal. The shaft rotation limiter with it’s 11 tabbed washers added a fair bit of drag, even when coated with a thin film of grease, so I squibbed a small amount of turbine oil in between each washer. While doing this, I was wondering about mixing turbine oil in with the synthetic grease. So far, it has seemed to work fine. If there are any problems in the future, it would not be hard to remove just the front plate and the washers, so they can be cleaned and re-lubricated. At this point, I am almost looking for excuses to do things like that!

This is such a good-looking gear drive. I almost can’t believe that it is something like 70 years old.  If you look closely at the 4 screws (of which you can see 3 in the following picture) holding the eccentric hub flange to the front plate, and compare with the very first picture in this blog, you’ll see that I have carefully filed them down. I did this in order to maximize clearance with the back of the micrometer dial. I am planning on using a 4mm front panel in my next regen, and need all the clearance I can get –

The dial that came with this particular drive was in good shape. However, I already had one that was in even nicer condition. This particular combination of dial and drive are the best in my small collection. They have made the cut, and with the 20:1 reduction ratio and nice heavy dial, will make a grand main tuning dial for a homebrew receiver –


10 thoughts on “Holy Grail Attained – A National HRO NPW Gear Drive and Dial in FB Condition

  1. Hi Dave, I can’t wait to see the final receiver using your great looking HRO dial. Also, I know the Deli very well. My sister lives in Berkeley and I lived in the Bay Area for 25 years before I moved to CO. 73, Jack W6VMJ

  2. Jack – me too! I already have a plan for how I want the receiver to look and am pretty close to finalizing that. I am not completely decided on the final schematic but have started to build it, and will be experimenting a little to see what works. It will be a few more months, as I like to take plenty of time when building, and there are a few custom parts to order.

    Good to hear that you’re familiar with Genova. My friend and I like to go there every now and then. The sandwiches are very reasonably priced, and good too!


  3. Dave: I have one of these beauties. Arman WA1UQO gave it to me. I cleaned out the old grease (without doing the full disassembly — you are a brave man!). My problem is that it feels a bit “loose” — I can move the knob a bit without having any effect on the variable cap that is supposed to be moving. You could say that it has too much “play.” I notice that the spring on one of the big gears seems to have been flattened compared to the very springy looking one in your picture. Any ideas?

    1. Bill – that spring is supposed to tension the gear so that there is little or no backlash (I know that you know that). I found that to minimize the backlash on mine, I had to tension those 2 gears as tight as I could manage, without hurting my fingers, before deftly slipping in the gear that is attached to the main spindle (the one that connects to the tuning knob). I’m not sure if it’s normal to have such a high degree of tension on the spring, as I only have one other of this particular drive, and haven’t used it in a project yet. Your spring sounds like maybe it’s not doing a whole lot of springing.

      Here’s the other NPW drive I have – https://aa7ee.wordpress.com/2015/04/07/the-national-hro-npw-dial-and-gear-drive/ – it’s very similar, but one or two of the parts are a bit different. Did you figure out how to put the dial on the drive? The front and rear parts of the dial have to be lined up a certain way before you slip it on the main spindle, or it won’t turn properly. I’m sure you already know this, but if the dial doesn’t want to turn, don’t force it. That’s the worst thing you can do to one of these old fellers.

      Not quite sure what you mean by the spring being flattened, but it sounds like you might need to acquire another one of these beauts, for parts 🙂 If I remember correctly, I ended up buying 4 dials and 3 drives in order to get the perfect specimen. One or two of them are pretty banged up but I keep telling myself I’ll restore ’em one day. Oh, the things we do for home-brew! I’ll be watching Soldersmoke to see how things progress on this front. Best of luck, and may the National HRO Radio Gods smile upon you!

      PS – I’m wondering if your spring is shot. Any chance of a photo?
      PPS – there’s some info on the dial in this post – https://aa7ee.wordpress.com/2015/02/09/servicing-a-national-pw-d-micrometer-dial-and-pw-gear-drive/

      1. Dave: Your excellent photos and great descriptions have, I hope, saved the day. A comment from Dr. Juliano this morning on SolderSmoke 189 also helped me understand the problem. That spring on the gear that you mention seems to be the cause of my trouble. I don’t think the spring is connected properly. Or if it is, the gear was not set to put the required amount of tension on the gears. Without any tension, we see the “loose gears” “too much play” phenomenon that I’m observing.

        Obviously I need to go in there and move that gear a few notches to get some tension in there. But how can I get that big central gear (the one that goes to the tuning cap) out without taking the whole darn thing apart?
        Thanks, 73, Bill

  4. Bill – hope you get this comment, as this blog will not let me reply directly to your reply. The spring is (if I’m remembering it correctly) trapped at one end underneath a spring clip that goes around the shaft of that gear. The clearest view of that arrangement is in the 6th photo down on this post. The other end of the spring goes into a little hole on the gear. Perhaps one of those ends has come undone? If not, the gear should just need to be re-tensioned.

    Obviously, you need to slide that central spindle out, so that the gear disengages from the other two (bigger) gears, and you can then re-tension the whole thing. I think the gearbox in this post is a bit different from the one you have. I think yours is more like the one in this post – https://aa7ee.wordpress.com/2015/04/07/the-national-hro-npw-dial-and-gear-drive/ – if it is, you’ll need to unscrew the eccentric hub from the front of the gearbox (4 screws) and gently pull the main tuning spindle out far enough so that it disengages from the other two gears. There may be another clamp on the inside of the gearbox holding that central spindle in place. You’ll need to loosen it if it’s there. The hub is eccentric – the hole through which the tuning spindle goes is circular, but it is purposely off-center. Make sure you replace the eccentric hub in the same orientation. This is important, as it’s central to how the dial works.

    Best of luck. Looking forward to SolderSmoke 189!


    1. Dave: Thanks. I got the main tuning spindle out. No problem there. But I can’t get the big central gear out. The shaft that goes to the tuning cap has to come out, right? I see it is held in place by two screws — I took them out. The shaft seems quite stuck in there. So I can’t move the big center gear and thus can’t re-tension with the spring. Any ideas? Did yours come out easily? I’ve soaked the show thing in WD-40 and am hoping that this will loosen things up. 73 Bill

      1. Bill – I think we’re having a misunderstanding due to differences in terminology. My fault for not being more precise in my descriptions. When I said “main tuning spindle”, I was referring to the shaft that is connected to the dial, and not the one that is connected to the variable capacitor. I didn’t remove the shaft that is connected to the variable capacitor at all. I did remove the shaft that is connected to the dial, however.

        That gear that has the spiral spring on it – you need to be able to turn it freely within the gear housing, so that you can see whether it is connected at both ends. I’m thinking that might be your problem. Anyway, once you’ve de-greased the whole thing with WD-40, make sure to re-lube before putting it back into service. A good automotive grease should do the trick. I have a tub of Mobil 1 synthetic lube purchased form the local automotive store, that works well. I did some reading on the various vintage radio forums, and this grease gets the seal of approval, though I imagine pretty much any similar thick grease would do the trick.

        73 for now,


  5. Fixed! Yea! Thanks Dave. I was just looking at it wrong. After I got the main tuning spindle out, I was thinking that I had to remove the big center gear in order to put tension on the system. After your last message I saw what I had to do — just move the gear with the spring on it while sliding the spindle back in. Viola! Tension! Smoothness! No more “play.” That spring is usually referred to as part of an effort to eliminate “backlash” but I also see how it prevents there from being excessive “play” or looseness in the mechanism — without tension, those gears are just sitting there, not tightly meshed, and you get the problem I was having.
    Hey, this was fun and educational. You don’t get to work on a gear box when your main tuning device is a rotary encoder!
    Thanks for all your help. 73 Bill

    1. Phew! I suspected (and was hoping) that it was a simple fix. There’s actually not much to go wrong in those gearboxes, though a worst case scenario could have conceivably required you to find another one for parts.

      I was listening to SolderSmoke 189 yesterday, and hearing you talk about how great the National HRO’s are. I forget how you put it, but we are thinking along the same lines here. I look at the National HRO’s, and those dials and gearboxes and to me, they are strongly reminiscent of an entire age in which mechanical engineering was king. I come from a town in England that used to be known for manufacturing fishing tackle and springs. My Dad began his career working for a company called Terry’s Springs, who were a big supplier of springs in Europe in the late 19th and first part of the 20th centuries. I still have his micrometer from those days, which I use to measure wire gauges when winding coils. It’s a good connection to my family history when I use it.

      Listening to Pete talking about his rigs that use Arduinos and chips like the Si5351 for frequency control, is inspiring me to get on board with the 21st century. I know I can do it, but still get wistful for the old days of grease, gears, and beautiful variable capacitors. Some of those things were works of art. That’s a gorgeous blue on his latest creation!

      73 for now,


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