Help Sought Identifying IF Trimmer Screw

[IsquaredR]

I am restoring a 1939 Hallicrafters SX-25 and each IF can needed work due to extensive corrosion and other damage. Each trimmer capacitor is fitted with an adjusting screw, presumably brass. I need to replace two of them but so far have been unable to identify the screw from cross reference charts. The screw itself is 0.5095-inch long fitted with a pan head on the top and a very short unthreaded stud on the bottom. When counting the turns there are 27 which equates to a 57.6 "turns per inch" fastener. The major thread diameter is 0.1065-inch and the minor thread diameter is 0.1000-inch. Although the screw is close to a No. 4 US screw, it fits none of the common threading patterns I have been able to locate for No. 4s. Does anyone have any clue as to what this screw might be known as commonly?

[Silicon]

It may be a special screw based on the Unified Extra Fine series.
Possibly a No. 4 with 56 or 60 TPI?

The thread diameter seems to be between the No.3 and No.4 screws.

UNF No. 3 used 56 TPI. Could the Hallicrafters screw be a No.3 screw with a thick nickel plating?

Do you have a 'Thread Guage' or any screws with 60 TPI threads with which to compare?

[dtvmcdonald]

Good luck with the screws. I have an SX-25 and it is one of my best radios.

A couple of hints: this radio is exceedingly sensitive to part placement and
lead attachment points in the RF parts: some leads are attached to the center
of other wires, including some ground leads. If these are not exactly right,
to 1/4 inch, the RF simply cannot be aligned.

The values of the padder caps in the oscillator are critical, and may be out of range.
Tweek to get the oscillator alignment to the dial correct. It can be done.

The adjustment of the crystal filter is very critical and the instructions vague.
It too can be gotten perfect. The adjustment of TXS (or is it TX3 or TX8) is critical ...
if wrong, the width is much too narrow even for CW. If right, its perfect for CW and
also for SSB .... for SSB adjust the phasing to null at about 300 hZ in the
unused sideband. My SX-25 sits on the shelf underneath my SX-88, and any signal
copyable on the SX88 is copyable on the SX-25, its just somewhat harder to adjust. This is quite remarkable. Only R390As and RA17-C12s actually beat the SX-25, and they
are years later.

[IsquaredR]

Thank you Silicon...very helpful information. Although I do not own a thread gauge I certainly will be buying one today; don't know how this has been overlooked for my tool box all these years. I might just throw caution to the wind and buy some No. 3s and No. 4s with 56 and 60 TPI, see what happens. Who knows? I might just hit it lucky.

Dtvmcdonald, you are absolutely correct on all points. This is the fourth SX-25 I have rebuilt and am quite aware of its idiocyncracies. I've needed to add capacity to a number of padders to bring them to within trimming range. The crystal phasing is wonderful although I discovered the key is knowing the resonant frequency of the crystal. I have yet to find one that is resonant at 455 kc. This SX-25 was actually a cadaver that was being parted-out so it's a little more of a challenge but I am making good progress and been enjoying listening to it while its on the bench.

[Radio1950]

Hi there,

Standard metric M2.5x0.45 is remarkably close.
It is a common size, and should be easily available in pan head.

Model shops often carry small sizes.

I think your chances of finding originals are very small.


As you have also found, when aligning crystal filters such as these, you first need to determine the crystal resonant frequency. Some digital sig gens have a minimum frequency increment of 10 Hz, which may be too big a jump.
If you are sweep aligning the IF, I find it better to use a very slow sweep speed for the crystal bandwidth alignment, and normal speed for the other BW settings.

Good luck.

[emeritus]

The almost contemporary 1942 edition of the American edition of "Machinery's Handbook" has extensive tables of screw threads, but this size does not appear in any of them. The SAE extra fine thread series on which the UNEF series was later based, do not extend to screws this small. Other than the M2.5 screw size, the closest pitch/diameter combination in my screw thread book seems to be the 56tpi British Standard Cycle thread, used for spokes. Spoke sizes 14, 13 and 12 have diameters of 0.905", 0.1025 " and 0.1145" respectively.

[IsquaredR]

Thanks so much Emeritus and Radio 1950. You've both provided me with some splendid information from which I can begin my search! I've considered re-tapping the capacitor flange to accomodate a more easily-obtainable screw but
am always reluctant to do things like that because my "window of vulnerability" as far as accidental damage is opened wider. I'd hate to end up destroying the assembly while re-tapping. Instead I'm going to procure some of those sizes you've suggested and will report back here if any of the sizes work. I really appreciate you gentlemen digging into your early machinists' libraries for me.

[emeritus]

I posted a PDF the relevant extract from the 1942 catalogue in #8 of this closed thread.

https://www.vintage-radio.net/forum/...10#post1125210

[IsquaredR]

Many, many thanks, Emeritus. That is a gold mine of information...so much so that I printed it out, will comb bind the six pages and keep it in my reference library. I appreciate your sharing that.

[Radio1950]

Hi there again.

Spoiler – boring detail follows.

I remembered that I had a few mica trimmers, spares for an Australian (Codan) HF SSB transceiver from the early 80’s.
Rummage, rummage, eureka!

I recalled that a company Arco from the USA made these “type” of trimmer.
I downloaded an Arco catalogue from RF Parts of California (USA), who incidentally sell some Arco trimmers.

My trimmer, marked A4607/0X, is probably a type Arco 46 variant, as all physical measurements are the same.

The screw thread diameter on mine is 0.109 inch measured, (2.75 mm measured), 11 mm long overall, with 10 mm thread length, and 1.0 mm unthreaded and reduced diameter nose, and pan head 5.4 mm diameter, and 1.4 mm thick.

The thread pitch is about 63TPI by count, and about 61 TPI against my Whitworth thread gauge; my Inch thread gauge goes only to 42 TPI. Using a metric thread gauge, it is about 0.4 mm.

This is not a standard thread.
The closest is probably an old German metric M2.6 x 0.45, (about 57 TPI), but it might bind due to a coarser pitch, and in any case would be almost impossible to buy off the shelf.

I cannot suggest any commonly available alternative.

I tried various alternative screws in the trimmer brass threaded boss of my trimmer.
M2.5 x 0.45 is too loose, diameter wise.
6BA is too large in diameter and of a lower TPI.

The Arco catalogue says that their Type 46 trimmer screws are 4-62 NS2.
This means that is USA 4 gauge with 62 TPI, and the NS means that it is a “National Special” thread, insofar that the thread form follows USA 4 gauge dimensions and thread angles, but with a special TPI to suit the individual end purpose.

There is no mention of any NS dimensions in my Machinery Handbook, and I didn’t expect any.

These Arco specs correlate with my trimmer screw dimensions.
I am not saying that your trimmer screw is the same size as mine, but it is a possibility, as different trimmer manufacturers often use similar parts or parts standards, as the fastener suppliers could offer cheaper prices for similar items.

If you wanted to replace all your screws, I suggest that you buy a sample Arco 46 trimmer from RF Parts California and see if the screw fits your IF trimmer, with a view to purchasing more afterwards.

If you want to replace one or two screws, then I can post you one or two of my trimmers free of any charge.

As a very last resort, and if it were my call, I would drill out the trimmer brass boss and tap with maybe 3.00 mm, but being prepared to lose some “fine” trimmer adjustment.

And lastly, I am curious as to why you need to replace a trimmer screw, as these normally last a lifetime??

Hope this helps, and good luck.

Photos are of my trimmer.

[Radio1950]

My error.

The Arco catalogue says that the Type 46 trimmer screws are 4-64 NS2, which are 64 TPI.
.

[Radio Wrangler]

Not an impossible job for a lathe owner with a good selection of change-wheels. If you only need one, do you know any model engineers?

David