Junking half the Ohmite

Peter. · Sep 21, 2017

I love my 10ee but there's one niggle I am looking to overcome. It seems that my Ohmite has suffered a repair in it's lifetime which means there's a quite significant jump in the rpm as I turn the dial. On inspection it seems that a small portion of the winding has been replaced, probably by copper wire rather than resistance wire but it's in a really annoying spot that makes the motor leap from about 300rpm to about 700rpm.
Now because I use the DC panel in completely standard form I have to keep the Ohmite for field weakening and that half of it work perfectly. The repaired side I use as a simple potentiometer to provide a 0-10v to my drive to control the base speed.

What I would like to do is disconnect the troublesome half of the Ohmite and somehow attach a modern pot to the shaft to use as base speed control. The trouble is that the Ohmite only works (as a variable resistor) on half of its turn but the new pot would work on the full turn which is undesirable.

What I want to know is can I do something to force a normal 10K pot to behave like the Ohmite? I want it to give me 0-10v (as supplied from my regulated 10v supply) over half it's swept range and stay at 10v for the rest of it. It's just a control voltage so a few milliamps required.
Is this possible or must I contrive some kind of mechanical setup?

MattiJ · Sep 21, 2017

So you want 180(or 135?) degree rotation to go 0-10 volts and rest of the rotational angle to stay at 10 volts?

Ready to solder your own concoction around opamp and few resistors or rather sort out funky mechanical solution?

Monarchist · Sep 21, 2017

Peter. said:
I love my 10ee but there's one niggle I am looking to overcome. It seems that my Ohmite has suffered a repair in it's lifetime which means there's a quite significant jump in the rpm as I turn the dial. On inspection it seems that a small portion of the winding has been replaced, probably by copper wire rather than resistance wire but it's in a really annoying spot that makes the motor leap from about 300rpm to about 700rpm.
Now because I use the DC panel in completely standard form I have to keep the Ohmite for field weakening and that half of it work perfectly. The repaired side I use as a simple potentiometer to provide a 0-10v to my drive to control the base speed.

What I would like to do is disconnect the troublesome half of the Ohmite and somehow attach a modern pot to the shaft to use as base speed control. The trouble is that the Ohmite only works (as a variable resistor) on half of its turn but the new pot would work on the full turn which is undesirable.

What I want to know is can I do something to force a normal 10K pot to behave like the Ohmite? I want it to give me 0-10v (as supplied from my regulated 10v supply) over half it's swept range and stay at 10v for the rest of it. It's just a control voltage so a few milliamps required.
Is this possible or must I contrive some kind of mechanical setup?

It is easy, if tedious.

One takes a wirewound, and no other, preps the/a side where the slider does NOT run, and solders a copper bridge wire along that "half", shorting all turns along the way.

What is left is the resistance of the UN shorted turns, which ceases to change as the slider move onto what is now a near-as-dammit length of copper wire in series with them, resistance-wise, even though said "wire" is a "bunch" of 1/4 turn resistance wire away from the slider.

Easier yet, as I have TWO sets of OEM Ohmites spare at present, and a third that will be pulled-out once done recording performance curves on an original MG, is that I just test a set to make sure they are good, box it up, and ship it to you.

Or .. you could upgrade your SSD 512C to a 4Q SSD 514C and no longer need the DC panel or the Ohmites.

The OEM Ohmites are actually a rather poor match to the front-end of the 512C's Op Amp for speed set control in any case. It would prefer a new set be made from lower- wattage wire-wounds that started out as 10K ohm rather that 400 ohm-minus.

Most of the DC panel could be served from a separate, fixed supply.

I leave it to you and Cal to review what the sensitive relays need - the 4Q drive no longer needs those, and I don't know if you are still using them even with the 1Q 512C drive.

BTW - Some of us with SSD conversions have actually come to PREFER our Armature and Field controls NOT be "ganged". Not "always", anyway.

Allows for an extra measure of fine control, and is more amenable to "presets" that can be switched in. or not.

Bill

Peter. · Sep 21, 2017

MattiJ said:
So you want 180(or 135?) degree rotation to go 0-10 volts and rest of the rotational angle to stay at 10 volts?

Yes, that;s exactly what I want to achieve

Ready to solder your own concoction around opamp and few resistors or rather sort out funky mechanical solution?

I'm quite confident that I could cobble up a simple circuit from basic instructions but if I'm honest I like Bill's idea in the post below yours a lot. Shorting out half of the turns of a wire wound pot appeals to me as an excellent solution, it is after all exactly what the Ohmite does but on a much larger scale. It would be simple to mount and readily adjusted.

Peter. · Sep 21, 2017

Monarchist said:
It is easy, if tedious.

One takes a wirewound, and no other, preps the/a side where the slider does NOT run, and solders a copper bridge wire along that "half", shorting all turns along the way.

What is left is the resistance of the UN shorted turns, which ceases to change as the slider move onto what is now a near-as-dammit length of copper wire in series with them, resistance-wise, even though said "wire" is a "bunch" of 1/4 turn resistance wire away from the slider.

Easier yet, as I have TWO sets of OEM Ohmites spare at present, and a third that will be pulled-out once done recording performance curves on an original MG, is that I just test a set to make sure they are good, box it up, and ship it to you.

Or .. you could upgrade your SSD 512C to a 4Q SSD 514C and no longer need the DC panel or the Ohmites.

The OEM Ohmites are actually a rather poor match to the front-end of the 512C's Op Amp for speed set control in any case. It would prefer a new set be made from lower- wattage wire-wounds that started out as 10K ohm rather that 400 ohm-minus.

Most of the DC panel could be served from a separate, fixed supply.

I leave it to you and Cal to review what the sensitive relays need - the 4Q drive no longer needs those, and I don't know if you are still using them even with the 1Q 512C drive.

BTW - Some of us with SSD conversions have actually come to PREFER our Armature and Field controls NOT be "ganged". Not "always", anyway.

Allows for an extra measure of fine control, and is more amenable to "presets" that can be switched in. or not.

Bill

I like your idea a lot Bill. For clarity, I don't tap the panel for the signal to the drive. I have a small 10VDC power supply that I plumb into the Ohmite to give a 0-10 putput which I send to the DC drive for base speed regulation. The panel runs from it's own 115VDC supply and works well on that.

I'm off to find a nice sturdy wire wound pot.
Cheers
Pete.

Monarchist · Sep 21, 2017

Peter. said:
YShorting out half of the turns of a wire wound pot appeals to me as an excellent solution, it is after all exactly what the Ohmite does but on a much larger scale. It would be simple to mount and readily adjusted.

One can do weird - or precise work that way with wire-wounds.

Skip sections. Add sloped step-plateau's. Not limited to just two flavours.

As a kid, College lab tech, 1920's, Dad salvaged an Old Style Wheatstone bridge by laying a run of Silver solder along each "arm" of the repaired resistance wire.

Then carefully shaved it away with the edge of a razor-blade until OTHER instruments proved it was back in balance.

I cheat. Two Alps or Bournes slide-pots.. The sort used in Studio master audio mixers, and 40, 60, 80, 100, 120 mm long.

Easy-peasy to use more than two, push or pull with weed-wacker cord or banding, cams, springs... and basically arrange the 'handover' points mechanically instead of in the resistance elements.

labeeman · Sep 21, 2017

You can also drive the 10K pot with a 20 volt supply put a 2K resistor in series with the wiper going to your control add a 10 volt zenner diode from the control wire to ground.

MattiJ · Sep 21, 2017

labeeman said:
You can also drive the 10K pot with a 20 volt supply put a 2K resistor in series with the wiper going to your control add a 10 volt zenner diode from the control wire to ground.

That was also on my mind. Plenty of ways to skin a cat.
I also like Monarchist's soddered wire-wound pot solution.
Just please don't go to "carbon rod lowered to salt water(mercury?) bath with cams, springs and cables" territory

awander · Sep 21, 2017

How much power does the pot dissipate?

You can get pots with a center tap.

If you tie the center tap and one end to 10V, and the other end to ground, it will do what you need.

Access Platform - AP332 - Potentiometer-1K CENTER TAP

Monarchist · Sep 21, 2017

MattiJ said:
Just please don't go to "carbon rod lowered to salt water(mercury?) bath with cams, springs and cables" territory

Ummh... well there's a drawer full of NOS Macbeth photolithographers Arc-lamp rods around here for similar nefarious porpoises....

whateg0 · Oct 11, 2017

Peter, I have an assemebly if you are interested in a plug and play solution. I'll have to look it over again, but as far as I know it was working fine when removed. PM if interested. I've replaced the whole thing with a single pot that I made a bracket for so it fits in place of the original Ohmite dual wirewound pot to drive my VFD.

Maybe I should look at offering a modified version for those using an aftermarket DC drives. I'll have to look at it tonight and see how hard that would be.

Peter. · Oct 11, 2017

Thanks for that. I already have a wirewound pot on route, probably be a week or two from China. If I can't make that work I'll take it under consideration. FWIW Monarchist has also said he has spare units.

Cheers
Pete.

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