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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: LED lamp innards
18/10/2020 15:32:05

p1040365.jpgp1040364.jpgHad trouble with 20W led bench lamp flickering, been that way a long time, finally swapped lamp and chopped open the offending item. Cut away the grey goop potting stuff and...

Inside is a typical power supply based around a BD3216 chip. A close look shows the mains fuse ended in a dry joint with a black burn mark. Also L3 the buck inductor is missing, it came away with the potting - it too seemed a bit cooked. The mucky soldering on the led assembly is original.

From a quick trace of the circuit the transformer is part of the Vcc power supply and feedback loop.

Now to think of a use for it...

p1040363.jpg

Thread: The Apollo Guidance Computer by Frank O'Brien
11/10/2020 10:05:42

Why not build one. Try klabs.org/history/build.agc

Thread: Arduino Pendulum Clock Design - Comments Welcome
04/10/2020 10:32:11

Apologies for useless calculation. With a very thin rod this setup looks more like a Foucault pendulum and those who set up that type go to a great deal of trouble to control wobbles and uncertainties. Even then Foucault pendulums are liable to start following a figure 8 pattern unless controlled.

I do feel that some of the trouble lies in the lack of a conventional suspension constraining the degree of freedom. I also worry about interactions between an iron electromagnet coil and the iron pendulum - symmetry and residual magnetism. I would be thinking to keep the electromagnet very symmetrical and possibly air cored.

Interesting project, good luck with it.

03/10/2020 13:02:03

Whoops again stick to 3.78Hz. Blame scruffy notes.

03/10/2020 13:00:28

Whoops 0.387Hz.

03/10/2020 12:56:51

I know very little but FWIW I think this is a single ended tuning fork. Had a poke around and no convenient formula came up on the web. Lord Raleigh did one but can't find it. But Wiki has a formula for a conventional tuning fork.

Taking E as 100GPa, rho as 1.5g/cm^3 and the radius of the rod(s) as 3mm and taking the Wiki round rod option for a 240mm long unloaded rod I get 3.78Hz resonance, other options are available.

Now I don't think losing one rod makes a big difference (!!) but the loading lump on the end will drop the resonant frequency, but not smart enough to figure out how much. What bothers me about this design is the round rod - it has no favoured direction of vibration - could go anywhere. Takes us into Sturm-Liouville territory where chaos reigns.

So, a rectangular rod and much heavier stiffer mountings is my feeling - or buy a rubidium oscillator.

Thread: Vacuum Pump Advice Please!
24/09/2020 19:35:11

looks a decent pump, if a bit small. Should get to about 1mm mercury on a small tight chamber.

On the inlet side the vanes sweep oil+air round and compress the air to be exhausted. There is no valve on the inlet side. On the outlet side there is usually some sort of reed valve. The old slow pumps had a neoprene flap, I have seen a steel reed valve on faster pumps. These sometimes break and don't work.

There is sometimes a bleed valve to let air into the pump on the inlet side to help degas the oil and 'give something to chew on'. Usually this can be screwed up tight shut.

The vanes run immersed in oil but not too much - say 3/4 full. Ordinary 20/50 oil works OK, Edwards oil v pricey.

The drive shaft sometimes has a simple oil pump - waggling rod in off centre groove to pump bearing oil. Take care to put this back right. Otherwise, if the vanes are not stuck there is not much to go wrong. Gaskets not usually used but some older pumps used a lick of shellac on the mating faces.

Slightly worried about running a clock in a vacuum chamber, outgassing etc. Not at all easy to get a really good seal, not really a vac once and forget job. Still - enjoy.

As said in ATM books, high vacuum is a technique that makes figuring mirrors look easy - and I agree!

True high vacuum is in the 10^-5 and beyond region, think turbo pumps etc.

Thread: Sodium Nitrite
07/06/2020 06:57:22

I am afraid I have never used this technique. I looked into making an STM some time back and remembered the technique.

The nearest I have come to making needles is the watchmaker technique of:-

fit brass wire in pin chuck

pull out bench drawer and file shallow groove in top edge

lay wire in groove

file and twiddle to get long thin point, use very fine file at end

You 'LINK' technique looks like it should work and from a credible source - and simple too. Good luck.

06/06/2020 15:07:44

Forgive me if this is granny suck eggs time.

Google STM tip preparation, I found what looked a credible approach at Zeljkovic Lab

Basically an electrolytic etcher with some fancy electronics to cut the etch current.

I seem to remember doing the etching with the tungsten inside a plastic tube hanging in the usual NaOH or KOH type electrolyte.

Some advise a loop of s/steel wire as the other electrode - presumably to shape the current density gradient.

25/05/2020 14:43:45

If no luck elsewhere (and in no hurry) you might try adapting the time honoured 'saltpeter from urine' approach. Involves a pit. straw, chicken poo and urine and time. I seem to remember the result was boiled up and stewed with wood ash (potassium carbonate). Easy to substitute sodium carbonate (washing soda). The result is usually a mix of nitrate and nitrites. Best to wait till SWMBO is out....

Recall reading in some medieval gun history revival of some Dutch folk trying this out, the results a bit disappointing though. Perhaps some good English pee will do the trick. Other recipes are available.

Thread: Holding glass lens for grinding
27/04/2020 11:34:31

The books Amateur Telescope Making published by Gall Inglis covered this, can't remember which volume.

The idea was to chuck a brass tube, turn true to a convex or concave chamfer to suit lens then stick the lens on with a beeswax/rosin mixture applied hot. You centred the lens by some optical trickery then let it cool down. Next a sheet of brass made into a trough and grinding grit/water and slowly take off the high spots until lens was round and to the right diameter.

I have done this for a clock glass but held an emery stick in the tool holder (paper washers). The trick was not to go too quick and generous water.

All to be done on your best Cowells lathe......

I should think any adhesive that was gooey enough not to let the lens slide away would be OK.

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

p1040333.jpg

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....

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