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Member postings for Roger Hart

Here is a list of all the postings Roger Hart has made in our forums. Click on a thread name to jump to the thread.

Thread: What Did You Do Today 2020
29/03/2020 11:47:58

Did a bodge repair on a fan motor. This seized the bearing bush after a sintered bronze bearing had cooked its oil. But motor had kept going and rotated bush inside the self aligning housing. Worked for a while then gone sloppy and finally stalled.

My usual source of spare parts - boot fair/junk yard is closed off due to CV and I sold off my lathe last year. So a rummage in junk box for a brass bush. A few minutes work with a sharp new file and calipers took it down to size and after some plain and fancy cussing back in business.

Doubt it is a permanent fix but long enough to procure a replacement. Message - don't get rid of your lathe and do buy new files now and again.

Thread: EMI from electric shavers etc
25/03/2020 11:20:05

Long ago a colleague was sent to a small island in a big ocean a long way from anywhere. His job, to solder the seams on copper sheets glued to the inside walls of a room. The room was to hold teleprinters, code machines and computers. Officer in charge declared that when the last seam was soldered the radio in the middle of the room would stop playing - it didn't All to do with near fields and far fields and sensitivity to the magnetic and electric component of radio waves.

As with others I doubt a bit of al foil will make much difference. Complicated business screening and EMI. Personally I would not worry about a shaver but I would not be waving it over my ticker either.

Thread: Fixing Eye bolt to old Lead longcase weight
26/01/2020 11:11:46

TBH this looks a job that is not worth going, leave it alone, the nail 'shows a bit of age'.

Not that easy to drill down with a hollow drill. Even it the drill does not snap off you are still left with how to get the core out. Very unlikely to drop out.

So unless this thing is a Tompion or a Knibb, leave it alone.

Thread: rolling brass strip in the lathe
15/01/2020 09:37:27

I once made a bezel for an old French mantle clock, about 150mm dia with bevelled glass set in. This was not an easy job and I would not try it again!

Cut 3/8 square brass to length, anneal, bend into ring and silver solder. Use oak former to make round. Set into grooved oak plank on faceplate, clean up front and back and use form tools to make nice rounded bezel shape. Cut groove for glass and clean up inside. Then silver solder on hinge and catch and re-true up and cut snap groove on inside rear. Then cut glass and bevel and polish.

This worked but was 'difficult', good luck with your bezel.

Thread: Fixing the AVO
08/12/2019 10:30:57

Thanks for the responses. I looked at VintageRadio, very handy info. The bottom line is that I used 50 SWG wire which is 0.001 dia and requires 78,750mm for 2608 ohms resistance. This leads to 1078 turns approx. This IMHO turned out to be too few for the magnet block. Ampere-turns is what sets the sensitivity.

Had I used 49 SWG .0012 dia and requires 113,550 mm for 2608 ohms which leads to 1554 turns approx I would probably had the right result. But being a bit dim, although the wire sample I measured did measure a bit over 0.001 I took the extra to be the enamel. That 0.0002 makes a lot of difference - cunning chaps those AVO people.

07/12/2019 16:35:47

p1040337.jpgAfter deciding to press on and rewind my AVO coil (see last posting) I took the coil apart and stripped off the wire and measured. Then built very simple coil winder for 50swg wire (a bit fine). Put on 1200 turns from calculated value plus 10%. Then made jig to re-glue bearing pivot plates and re-assembled. Disappointed to find read about 25%. Didn't fancy winding again so fitted some bigger neodymium magnets plus a bit of jiggy-pokery with shunts and swamp resistors. Now reads correctly. Was it worth it - no, cheaper and easier to visit eBay. But I've started so I'll finish...... Sorry about order of photos, not got the hang of that.p1040339.jpgp1040336.jpgp1040335.jpgp1040333.jpg

07/12/2019 16:26:17


Thread: AVO 8 Mk2 meter wire size
31/10/2019 07:01:59

Thanks for the replies and advice. I figured out in the end that although 50 gauge wire has an od of 0.025mm it has a coat of enamel as well. This adds around 0.003mm depending on the class of enamelling. The upshot is that 50 gauge wire will mike a little over 0.001 inch, so I will go with that.

I avoided disassembling the magnet block and put the central core back in, so I hope to avoid problems from that area.

This chap seems to have done something similar, quite interesting, don't know how to do links, so

restauration eines US-Roehrenpruefgerates TV-7 A/U

should find it.

30/10/2019 17:06:09

Having decided no more weird projects I am now tackling the repair of a Test Set Multirange No1, basically an AVO8 Mk2. Very nice instrument that tempted me into thinking all that was wrong was a loose glass.....

Not so simple, this one has an open circuit meter coil. I have tried all the simple fixes and am looking at a rewind. This has a 37.5 microamp movement which should be 3333 ohms including the swamp. This one should be 2608 ohms for the coil alone.

However having unwound part of the coil I am having trouble deciding whether this wire is 49SWG or 50SWG. It measures just over 0.001 inch. Now one step in gauge makes a lot of difference to the number of turns. So does anyone have an idea what wire AVO used or the correct number of turns?

Too nice for the bin but I don't want to wind the coil twice....

Thread: Bushing for clock arbor
29/08/2019 10:26:09

I was taught to use your thumb to press the 5 sided clockmaker's broach over toward the lesser worn side (not too much). Then broach out whilst keeping upright then bush in the usual way.

That is the French technique and very good it is. A fair number of old English clocks show an older technique. Use a round ended punch and anvil to belly the brass on the worn side. A few dents around the hole on the worn side and then use the broach and then burnish with a smooth broach. This technique is also seen on cheap old clocks where in former times a bush would not be worth the bother. Nowadays mending a clock is an expensive job so the French technique it is.

Thread: Grid Frequency [mains electricity]
13/08/2019 14:37:08

Sorry if this is a bit simple minded, but do wind generators generate 3 phase mains that is synced to the grid or do they generate dc which then feeds a big inverter synced to the grid?

The reason I ask is that if directly synced to the grid then each wind generator has a complex problem matching wind speed to its output. Indeed keeping a few hundred all synced up and matching output looks a bit difficult.

One the other hand if they actually generate dc then I can't see that wind speed would affect the overall output frequency, the inverter(s) sort that out.

So how do they work?

Thread: Little experience so advise needed
30/07/2019 07:15:09

As said blue pivot steel is the stuff to use. The problem is that you can't easily harden the staff after making - you burn the pivots. So it has to be made from a good grade of steel hardened to spring temper from the get go.

A read of some of Donald de Carle's books might be an idea. The Web has a pdf. Also getting a pair of pivot files left and right (and they are not cheap if new). To check the size I have made a temporary staff from brass, easy and quick.

Usually hand turning with a lozenge graver is the traditional way but I could never get on with that tool and used engraving tools. Experiment, you will surely end up making a few before you settle on a technique that works for you.

You 'can' polish the pivots in the lathe but it is a bit risky. Some sort of support like a Jacot tool makes it less likely to bust the pivot at the last moment (v annoying).

You can also make a simple turns and do the job between centres. Make a little pulley, some cotton thread and a motor and you are good to go.


Thread: Quicksilver
04/06/2019 14:10:16

Just checked my (UK) local council waste disposal website and it says that the usual dump site takes mercury and other chemical stuff. Seems pretty sensible because I guess no-one wants that sort of stuff chucked in the nearest hedge or pond.

Rather leaving themselves open though, I suspect the denizens of this site might know of many nasty substances lurking in attics and basements.

Way back telephone exchanges used mercury arc octopuses. Lots of fun when one broke and about 1 pint of Hg spilled on the floor..... More fun when someone went to change one of them new fangled germanium rectifiers and diligently turned off the mains but forgot to isolate the battery. Much very quick movement.....

Thread: Another Workshop lighting problem
04/06/2019 13:58:09

Time to dredge up this topic?

Now repainting the workshop and looking to replace the 6 foot 85Watt fluorescent tube with some sort of led fitting. There are what look to be led striplights about 4 to 5 foot long but it looks to need two to get up to or over the old 85Watts. I am just a bit concerrned that 'led watts' do not amount to 'proper fluorescent tube watts'.

Workshop is only 2.5metres by 3.5 metres with normal height ceiling. Any advice?

Thread: Overload trip
09/02/2019 10:35:52

You might like to try the small synchronous motors used in central heating diverter valves. They are designed to operate stalled (and get a bit hot!) without burning out. In the stalled mode there is usually a resistor and a diode in series with the motor. They are internally geared down a lot so operate at a civilised speed.

These motors are pretty cheap in the usual place and come with a small pinion and have a useful torque.

Thread: Is it a drone?
04/02/2019 12:39:48


I bought a flapping bird toy from China. About £10 and includes radio, motor flapping mechanism etc. Works but not as good as the Japanese effort. Thought I might have a go at using the bits to make a better one. The Dellfly project looks interesting and includes links to code and control system hardware.

so many interesting things to do and so little time.

Thread: Mystery optical device
03/02/2019 09:56:35

I found something like that in a box of old vidicons, pcbs etc. Inside the bigger triangular box are a couple of front surface mirrors and a dichroic block like one shown. I got it for the input output lends units (nice eyepieces). The block shown is 50mm on a side and carries two pretty coloured bits of glass. Basically it takes in an image, splits up the colours and shoves out 3 beams to the vidicons.

If anyone wants the dichroic mirror block shown you can have it, just a PM.


Thread: Dial Gauge Advice
31/10/2018 11:09:06

I agree that the DTI is a good first choice. I got a 0.001 inch one at a boot sale many years ago and it is the most useful I have. It is robust, useful travel and not too nit-picky. For most jobs this is the one.

I have a couple of much better DTIs and a finger type gauge, but they are a d%^n nuisance to use with the needle going round several times until you have got the work nearly central. In this case posh is not necessarily better unless you want real accuracy and that is whole ball of wax on its own.

Thread: Astro Compass back into use
07/10/2018 09:35:15

Recently dug the top half of this astro compass out of the junk box and made it functional once more. Handy for finding out if some planet or other is visible between the trees that mostly block my south view.

I always thought you would have to be in pretty dire straits to need to use one of these.

By way of confession, the bottom half got made into a harmonograph about 55 years ago.

07/10/2018 09:30:15


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