Stiff dial on HRO-M

[Keith]

I have just started using a 1942 HRO-M in earnest and have come to the conclusion that the dial is too stiff. There's just a bit too much "stiction" to allow sufficiently fine tuning of SSB/CW signals. I recently got my hands on another HRO dial and this seemed much easier. I've removed the top of the gearbox assembly and re-distributed the grease on the split gear but this has had no effect. I have also lightly lubricated the bearings of the tuning gang with similar lack of results. I am beginning to suspect that the problem lies in the micrometer dial itself. Having found some advice on the web I attempted to remove the dial (having set it to 250 as recommended) but, even with the grub screw completely removed the dial refuses to detach itself from the shaft. For the moment I have squirted some WD40 down the grub screw hole and am hoping this will help. Any advice would be appreciated.

[MichaelR]

Hi ,

I would recommend that you strip the gearbox and thoroughly clean the gears of old grease and then regrease.

Likewise with the dial itself which may need disassembling and the old grease cleared.

Be careful with the dial as it must be reassembled correctly , if you are not aware of this I can send you instructions on how to reassemble and align the dial correctly.

Most of the stiffness is usually the 70 year old grease having gone hard with age ,temp and muck.

Very satisfying when it all goes back and works smoothly

Mike

[Keith]

Hi Mike,

Thanks for the reply. I rather suspected that this would be the advice. I did find some information on the dial dismantling/ re-assembling at http://www.io.com/~nielw/hro_dial.htm so not sure if this is what you are referring to. As I said in the post the first problem is to persuade the dial to detach from the shaft. Hopefully then I can tell whether it's the gearbox or the dial itself that's the cause of the stiffness.

[Keith]

For any future HRO restorers who may encounter this problem the following is an account of how it was resolved.

I eventually managed to remove the dial (first setting it to 250 as advised on the website linked above). The WD40 must have worked its magic as the dial finally responded to finger force applied with a slight rocking motion. I refrained from the use of tools as a precaution against damage. The sticking was due to a slight build up of corrosion on the shaft over the years.

Although the stiffness appeared to be on the gearbox side rather than the dial I decided to dismantle the latter anyway for cleaning/re-lubrication. This is fairly straightforward although getting the two springs back on is something of a challenge - the trick is to allow gravity to keep them in place whilst offering up the inner plate and then using a wire hook to pull them down onto their lugs. The cleaning and re-lubricating with a small quantity of standard LM grease made a slight improvement but it was clearly not the real problem.

With the top removed from the gearbox the massive split gear is clearly visible but the worm itself cannot be seen. The only answer was to bite the bullet and remove it. The four screws in the shaft housing need to be taken out carefully as the worm drive is spring loaded - inward pressure on the shaft is needed when the last screw is removed. At this point the shaft and worm drive and it's spring, bearing, etc., can be eased out. Note that this will alter the postion of the moving vanes slightly but re-setting after re-assembly is not difficult. The position and orientation of the spring, bearing and various washers on the shaft should be noted before dismantling further. This is more like car mechanics than radio - but without a Haynes manual!

Brushing the worm drive in paraffin got rid of most of the old grease but a residue of hardened "gak" in the bottom of the thread required the use of a blunt screwdriver to assist removal! The whole assembly was then re-greased and re-installed in the receiver. The result - no change!

Having dismantled it all again I had a close look at the assembly and decided that something didn't look right. There was a fibre(?) washer between the bearing race and the worm/shaft collar which meant that the bearing was not able to rotate freely. I moved this washer to the other side of the bearing (i.e. as a spring retainer similar to that at the other end) and re-assembled. Eureka - silky smooth operation restored!

My suspicion is that either the drive had been incorrectly assembled at some time in the past or that possibly this was a deliberate modification to prevent unwanted dial movement under vibration conditions (the HRO has no other dial locking provision).

The HRO dial is now just about usable for SSB/CW reception on the lower amateur bands without the luxury of bandspread coils. I still think it could benefit from further loosening, but I believe this would require the use of a weaker spring on the worm drive shaft.

[MichaelR]

Originally Posted by Keith

This is fairly straightforward although getting the two springs back on is something of a challenge - the trick is to allow gravity to keep them in place whilst offering up the inner plate and then using a wire hook to pull them down onto their lugs.

The trick is to remove the Dial Finger Knob, three screws hold this in place and this is the reason that there are various holes in the back plate. Once the Knob is removed it is very easy to use just your fingers to replace the springs.Once done replace the Knob and screws then reset the dial and backplate as per HRO instructions.This is important, when at the correct postion the two springs should be vertical.

[Keith]

Thanks Mike,

Hopefully I'll remember that useful info next time I take the dial apart - should be about 2027 .

I don't suppose you have a diagram showing the correct assembly of the shaft components - I'd love to be able to confirm my hypothesis (or otherwise, of course!).

Keith

[MichaelR]

Hi Keith,

No I do not have any diagrams, I strongly suspect that the gearbox strip down and clean is what gave you the smoothness. I have done a couple of these HRO sets as complete rebuilds starting from bare chassis. The gearboxes have grease that has turned into cloggy lumps ( would do after 70 years).

Did you reset the backlash gears before refitting into the gearbox , this is what I find to be most tricky.

great sets to restore though, I love em !

Mike

[Keith]

Hi Mike,

I don't think the problem was just down to the need for cleaning and re-greasing. It did help but there was still stiffness present after this. I believe the washer was defeating the action of the bearing.

Yes I did offset the anti-backlash by one tooth before I inserted the worm thread. However, I can now detect a very small amount of backlash so I will probably dismantle it again and see if the springs will allow a two tooth offset. If I do I'll try and post a photo of the assembly for any comments.

[MichaelR]

Ok keith, two teeth offset is what I normally put in.

[Keith]

I had another go at my HRO stiff dial problem. I tried using an offset of two teeth but, perhaps unsurprisingly, this made the dial even stiffer! In fact a one tooth offset seems to be sufficient - the backlash that I had introduced was due to the fact that I hadn't "screwed" the worm in on reassembly and had thereby lost all the offset.

I still think the dial is too stiff, however, and find it hard to believe that it could have been used as is by female intercept operators for many hours at a time during the war.

The only way I can quantify the problem is to set the dial at 100 and flick it clockwise as hard as possible. The furthest I can get to is ~250. I would be interested in any comparative results!

As promised I took some pictures of the HRO worm drive/shaft assembly and one is attached.

[MichaelR]

Keith,

I have spun the dial on my unit and I can get easily 450 from 0.

From the picture can I remind you that the drive shaft bush housing has the word "TOP" embossed on it and hence should be at the top when you refit the drive shaft housing. This is essential to make sure that the eccentric bearing is correctly positioned to support the dial plate.

Mike

[Keith]

Mike,

Thanks for that. I guess that proves that I still have a problem. I think I shall have to remove the complete tuning gang assembly and do a proper strip down and cleaning job (only four wires to disconnect, apparently).

Yes, I did note the orientation of the drive shaft bush so it's not that, but thanks for the reminder.

Happy New Year

[MichaelR]

OK Keith,

One last point just check that there is no slight bend in the shaft, I do remember on one unit that I did that it had slight damage of this nature.

Finally as a sanity check double check you have fitted the springs in the dial correctly. You can fit them in such a way that they do not move as intended in the hole of the back plate as you use the dial. If incorrectly fited they in fact move the opposite way as intended and foul the edge of the hole in the backplate, this could cause the symptons that you have got.


And a Happy New Year to you as well

Thank you
Mike