Uniselector wanted

[treefrog]

Dumb question, but if the voltages so the main issue is it not easier to change the voltage supply?

I did pick a few uniselectors recently , with 5 lines, but no idea of brand, as there are no labels, one is brand new old stock

[garythegolfer]

I did consider using a 50v Power supply. You could get a laptop-type 50 volt PSU from EBay for about £10 and use BOTH 24 and 50 volt power supplies and "steering diodes". Alternatively, you could try to use ONLY a replacement 50v PSU. I did consider this but it also seemed quite an inelegant solution that scraps the original transformer/rectifier. I was willing to play with a low voltage uniselector coil but less willing to chance my hand with rewinding a mains transformer.

Given the choice of a) 50v machine conversion, b) mixed 24/50v operation and c) conversion of the (new) uniselector from 50v to 24v, I chose the latter. It is of course a personal choice but for me the coil rewind was the least worst option. Originally I was considering how to keep the machine more "authentic".....but couldn't find a replacement 24v (Siemens) uniselector. It would perhaps have been better to change BOTH uniselectors to GEC version since my mods will make my machine an odd hybrid. I suppose it depends what the objectives are. If you want to sell at a maximum price, authentic (or the appearance of authentic) has much value. I have had this scrapped machine for about 50 years and my objective has become simply to make it work! (My rewiring solution is also quite unorthodox.....so I really have already given up on "authentic".)

I can't claim my solution is "right": it just suits me. I started from a scrapped and cannibalised machine. There may well be better solutions..... Who knows, if I ever get this working, I may consider replacing the other Siemens uniselector with a GEC 50v version (and rewinding its coil).....at least I know how to do it and have the coil wire to do it!

It's amazing how defensive I have become for even a sub-optimal solution.....!

I suppose my main intention was just to show that rewinding the uniselector coil was a possible/practical solution. And give enough technical help for anyone else who wanted to try it.

Oh and by the way....it was never a dumb question.

Regards,
G

[pennymachines]

Your approach sounds good to me and may have saved me figuring it all out for myself.

I may consider replacing the other Siemens uniselector with a GEC 50v version

If you do, you'll know who to sell the Siemens uniselector to...

[treefrog]

No idea what type this is, but is brand new.

[operator bell]

Tip 'o the hat to Gary The Golfer for rewinding the coil. I was going to suggest it before I saw that you'd done it. I've done it myself. I once bought a job lot of uniselectors on Ebay and when they showed up they turned out to be SIX VOLTS. I had no idea that six volt uniselectors even existed. The coil resistance was about 10 ohms. I rewound them for 24V.

I encourage anyone needing a different voltage coil to have a go at rewinding. It's tedious to do it by hand, and it needs to be neat, but it's not difficult. For half the voltage, you need wire that's twice the cross sectional area, which you can find out from a copper wire table. Being twice the area means you can only get half the number of turns on compared to the original, so the final resistance will be a quarter of the original (half the length times half the ohms per foot). Then at half the original voltage, you get twice the original current. The magnetic strength of a solenoid is given by ampere-turns, so now you have twice the current times half the turns, or the same number of ampere-turns (and thus the same magnetic strength) as the original.