Potentiometer Replacement, Leitz 050250/1172 PWR Source

[Element 56]

Hello,

I need to replace the potentiometer in this power source and I am not sure which potentiometer I need. There is no information on the potentiometer itself indicating size. It is simply marked "Alpha A C".

I cant ohm it out because it's completely shot. I tried cleaning it several times which will bring it back to life for a bout 5 seconds then nothing.

The input is 115 VAC 60HZ, 0.5 amp, output is variable to 5 amp 8 volts A/C. Currently it powers a 15w 6V bulb on my Diavert.

I know it's a great time to convert to LED but I have too many irons in the fire! Any help with selection would be greatly appreciated.

Thanks
Kirby

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[Zuul]

If it is truly dead, and you don’t mind a “no return” option, you could open it up and meter the contact disk directly. The fact that cleaning helps, even briefly, says to me that the wipers are the problem. If that’s true, getting an accurate measurement should even possible. The resistance from one end of the strip to the far end should match the pot’s rating.

[Element 56]

I tried and tried but cant get anything out of it anymore. It's a wonder that it worked at all!

Is there anyway to make an educated guess based on the input/output?

Thanks

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[Zuul]

Can you get a measurement across the outer 2 contacts? That will tell you the value of the pot.

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The dark grey area in the outer ring is the resistor material. When the wiper bridges from the middle ring to the outer near a contact, it doesn't travel through the resister at all. Bridging from the far end makes the electricity travel through all of the resistive material giving the maximum value.

[PeteM]

A few thoughts:

- As Zuul says, if you can get a measure across those two end points it will give the spec. Even test probes poked further in on the traces will get you close.

- Yours looks to be the type that also switches the power supply. If so, it compounds the difficulty of finding a replacement. There's room to add a separate on-off switch.

- There's almost always some marking on the case which can give an indication of the resistance. Often embedded in a product number.

- You could likely buy a cheap modern (switching) variable DC power supply for about the same price as a switching potentiometer and either use it separately or tuck it inside the Leitz case.

- If asked to guess -- and you have some spare pots around to try -- I'd guess around 5K ohms.

[Element 56]

Zuul, this is the best I could do. Two measurements from either end showing almost the same resistance. Does this help at all? Sorry I don't know a lot about electronics but I am very much enjoying this! I am able to do the work and some diagnostics but I have a lot to learn on the technical side.

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Peter,

Yes it is a switch + Pot.

The only other info on the unit is the fuse, 0.8A Slo-Blo and the serial # 116.587. I did look briefly for a manual without success but I will look again. This is a logical rout!

I could certainly add an on/off switch however, I'm trying to keep it as original as I can. Down the road I may upgrade to one of Stanton's LED kits!

While looking for parts in my basement I found a Zeiss power supply that I was able to hook up and it worked!! No more emergency but I still want to fix it. (I did of course first check to see if the Zeiss unit used the same or similar switch, but it did not)

Thank you both for taking time to help me with this. I really do appreciate it!

Kirby

[Zuul]

Can you measure with one probe on the right-most contact and the other probe on the left-most contact? That is the value you are after. X to X in your diagram.

[Zuul]

15k Ohm is a decent guess based on your partial measurements.

[Element 56]

Zuul,

The circuit between x and x is open. The best I could do is getting a measurement on each end.

Based on the suggestions I will hunt for a 5-15 Ohm. Worst case I end up with some spares for the next project.

Thanks again for the help!

Kirby

[MichaelG.]

The circuit between x and x is open. The best I could do is getting a measurement on each end.

It looks from your photo, that there might be a break in the track
[ at roughly 2 o’clock in the picture ]
... Could you try moving the test probe until you find that break ?
... might give you a better estimate of the total resistance.

MichaelG.

[Element 56]

Hi Michael,

I did thoroughly prob the surface while working under the microscope but couldn't get a value. The area between the two red dots is a dead zone. I couldn't even get a value with the probs side by side in this dead zone.

If you look closely the area I'm calling the dead zone is actually darker then the small area between 12 and 11 o'clock and 6 and 7 o'clock. It doesn't look worn but made that way. This makes me wonder if it's supposed to be insulated and I am not understanding how the switch functions.

Looking at this link on how to convert a regular pot to a switching pot kind of confirms my thoughts. In other words I should not get a value with the probs connected to the two outer terminals because it's supposed to be insulated. Of course I could be wrong but this is just my thinking at the moment.

https://www.instructables.com/id/Super- ... itch-hack/

Please note, this is a 5 terminal Pot and there is a resister between the center terminal and what I think is the neutral wire (kind of hard to see this in my pics). Maybe that adds a new element to testing?

Kirby

[MichaelG.]

Hi Michael,

I did thoroughly prob the surface while working under the microscope but couldn't get a value. The area between the two red dots is a dead zone. I couldn't even get a value with the probs side by side in this dead zone.

[…]
Kirby

.

Ah, sorry Kirby ... I hadn’t realised that

Must have a think, see if I can remember any similar-looking pots.

MichaelG.

[Element 56]

Dumb question, is it possible my multimeter can't measure the resistance if it's too low? My meter is marked 2K and I noticed if I check continuity on some resisters that are new and unused that I don't get a reading. Could this be a equipment limitation? It's a pretty basic non-auto-ranging meter.
Kirby

[PeteM]

Might show a picture of your meter. It isn't clear if the "2K" is its impedance (that would be pretty low) or the full scale reading (also pretty low). Aren't there multiple ohm ranges?

[Hobbyst46]

Dumb question, is it possible my multimeter can't measure the resistance if it's too low? My meter is marked 2K and I noticed if I check continuity on some resisters that are new and unused that I don't get a reading. Could this be a equipment limitation? It's a pretty basic non-auto-ranging meter.
Kirby

Some multimeters have an audio alarm feature in the resistance mode. A shortcut - very low resistance - is indicated by a high pitch sound.
If there is such a feature, try to touch the to probes to each other (shortcut). If there is no sound, probably the battery of the multimeter should be replaced.
usually multimeters DO show even low resistances of 1-10 ohm, or at least show an "L". Unlikely that the resistance of that variable resistor is lower than that.
To measure resistance, the circuit must be cutoff from the supply.

[Element 56]

The battery is good and the leads ohm out with a beeeep!

This is my meter,
http://www.snapon-bluepoint.com.sg/cate ... al-Ranging

Thanks,
Kirby

[PeteM]

Kirby, Looks like your meter is only designed for continuity and a bit more. Believe that even the cheap import (e.g. Harbor Freight "free with coupon") meters will get you into a useful ohms range.

[Hobbyst46]

Perhaps you could connect a known resistor in series and measure the total. Say, connect a resistor of 100 or 1000 ohm, to be far from the end of 0-2000ohm range.

[Element 56]

Thank you both for the help! I will try the second resistor and consider upgrading the multi-meter. A Fluke is overdue!

Best Regards,
Kirby

[PeteM]

Perhaps you could connect a known resistor in series and measure the total. Say, connect a resistor of 100 or 1000 ohm, to be far from the end of 0-2000ohm range.

In parallel?

[Hobbyst46]

A Fluke DMM is something that will reliably and accurately serve you and your children and grandchildren and probably beyond that. Even the battery drain is tiny.

Perhaps you could connect a known resistor in series and measure the total. Say, connect a resistor of 100 or 1000 ohm, to be far from the end of 0-2000ohm range.

In parallel?

No, in series. Just in the case that the multimeter will not read anything less than 100 ohm (say) on its 0-2000 ohm scale, and it being just a continuity indicator plus some (as you suggested), if I connect a known 100 ohm resistor in series and read 100-110 ohm (say) I verify that the resistance of the variable resistor is about 10 ohms. At least, a definite reading of the meter. Just something. An accurate reading of the variable resistor with a full-featured multimeter would be better, clearly.

[PeteM]

I see your logic; as a test that the meter is good within its 0-2K max. range (per the spec provided earlier).

The pot, however, is likely designed to be above 2K ohms. Only way to actually measure that, with a VOM that maxes out at 2K, would be the resistor in parallel.

[Zuul]

There is almost zero risk in experimenting with whatever pot you have handy. All (normal?) pots go to zero Ohms, so you can’t overdrive the circuit beyond it’s designed voltage/current. Worst case scenario if you have one rated too high is the adjustment will be extremely sensitive (and most of the range will do nothing obvious). Too low and you won’t get a full range of adjustment.

[Element 56]

First I used four 2.2k +/- 5% resisters (they were not marked but the colors are 3 red / space / gold ) and connected them in parallel and series to see what happens. My meter says the resistance in one of them is 1.997, however it struggled to get this measurement at first. The value bounced around (not an auto ranging meter) and sometimes show an open circuit. I changed the battery and it did the same thing for a while before settling down. I think I need a new meter!

Anyway for what its worth this is what I got in parallel,

+1, 1.997
+2, .986
+3, .658
+4, .497

In series
+1, 1.997
+2, open
+3, open
+4, open

Repeat with four resisters marked 1/4 watt, (red, purple, brown, gold = 270 ohm +/-5%)

Parallel
+1, .233
+2, .116
+3, .078 (beep)
+4, .058 (beep)

Series
+1, .232
+2, .465
+3, .695
+4, .926

Does this show that my meter is up to the task or is it only complicating things?

I am learning a lot and I really appreciate the time everyone is taking to help. Thank you very much!

Kirby

[Zuul]

4 x 270 Ohm resisters in series should read over 1k Ohm. Your meter is not working. You should be able to read any one of the resisters within a couple percent of the nominal resistance, so quite easy to test.

[Hobbyst46]

The results from the parallel and serial combinations indicate to me that:
1. The meter shows correct values for the combinations, aside from miscalibration (see note 2).
2. It is not calibrated and the readings deviate by about 10% from the true value. Hence the readings are ~ 2K instead of 2.2K and 233 instead of 270.
I believe that the true values are as indicated by the color codes.
3. Since the range is up to 2K, the series combination of the 2.2K resistors yields values outside the range.
4. I would proceed to measure the resistance of the potentiometer in series with a "233 ohm" resistor. It might give a reading that is incorrect by, say, 10-20%.

(the meter readings are expressed in Kohms rather than ohms).

5. For future work I would invest in a quality multimeter - regardless of the current task... Fluke... or equivalent... I wish Keithley were still in the market...

[Zuul]

.926 is 85% of 1.080. For 5% tolerance resisters, that is well out of spec. Maybe I’m too picky, but that level of error indicates a faulty meter in my mind.

[Element 56]

4. I would proceed to measure the resistance of the potentiometer in series with a "233 ohm" resistor. It might give a reading that is incorrect by, say, 10-20%.

I tried this several ways with several resisters but I still do not get a value. The "dead zone" between the red dots didn't get any smaller.

However, I actually found what I believe may have been the problem with the switch. I am going to try to fix it and retest.

I will report back!

Thanks again for the help!

Kirby

[Element 56]

Got it working! As you can see from the photo it had a bad connection at one of the terminals. After tearing apart the switch I did all my testing from the resistive material not the terminal so this had no bearing on the test. When I was wiring the resister in series as Hobbiest46 suggested I noticed I didn't have continuity at that terminal.

Even though its working now I still can't get a reading from the resistor material!

Since I had taken this switch completely apart I still need to find a way to identify it. I will get another meter and test again.

Thanks again for all the help!!

Kirby

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