Cohu 324 Voltage Calibrator...looking for part or ideas, maybe??

[3dclint]

I happened to find part of a cohu 324. The whole front panel. Someone turned this into just a really accurate decade box for some reason I'm sure isn't worth contemplating...
I'm only missing from just behind the front panel...back...lol

I did throw it in a box( NOT an easy size to find there was basically One option to get little tight but fits)and filled the holes up that were missing for the time being.

So in the end i have the entire front panel of a cohu 324 voltage calibrator.
I would love to get the voltage section of it working again to atleast some degree...
 either by some miracle finding someone that has an entire unit ( a 10"×16"×5" case...case was actually made of a shielding metal i believe so was custom to the unit) missing a front panel laying around.
This one doesn't seem particularly possible..

 Or.... I did find a 1000v module online but it was supposed to put out 500-1000v 2-20ma and instead is current limited at 500-1000v 100ma. (I actually got two of these HIA4V1 4 modules and they both seem to be current limited at 100ma...)
Haven't figured out if or how that could be changed into what I was looking for...  But it's atleast something to start with instead of air...lol

 Or maybe someone around here has a good idea??
 I'm by no means trying to get 6.5 digit accuracy but 1 or 2 digits past the decimal at 1000v would be nice if possible.


Apologies if this was wrong place/section for this but being the unit was a calibration source I thought it would be appropriate and the best place for ideas.

[Kleinstein]

The switches and precision resistors to make it an accurate resistor decade are a major part for the accuracy. So there may still be much of the critical part still there. So how many resistors are still there at the switches. The likely missing parts are a stable reference (originally likely based on 1N829A) and the amplifier stage, exspecially the power stage to get up to some 1000 V. The precision part of the amplifier is today quite a bit easier than it original was.

[3dclint]

I would say the only parts missing that may have been there are 3 resistors for the voltage selection....a 6k 60k and 600k
I do wanna put in a metal black button instead of the toggle.
Also need to decide on an indicator light...just too many options and no idea what power source is yet either...lol

[3dclint]

After repainting the white indicators it seemed in such good shape was a shame to have no power.

[Kleinstein]

Not having the 6 K 60K 600K for the range selection is a pitty, as ideally they should be matched to the other resistors. For stability (low TC) it would really help to have resistors from the same series as in the decade switches, especially the upper decade. Different resistors could limit the TC. Ideally the resistors from the decade should be low TC, to also avoid much heating effect, when used with a high voltage.

How much current the rebuild circuit could deliver is still to decide. A low current could be a plus for safety, though it would only be a current limit by electronic means. Transistor tend to fail short and thus not that much safety from such a limit.

[3dclint]

This is based on I wanna say 15ma by the little math I've done. I think it says it goes between 2-25ma for it's rating?..I would have to check the manual again

[3dclint]

And ya the 3 resistor s missing really upset me and is why I said someone turned this into a decadebox instead of just finding a front panel unfortunately....ugh would love to "insert something here" to the guy/gal that did this

[3dclint]

Was doing some odd thinking and realized...
This unit reminds me of a "Rune Goldberg Machine"...lol

Oh also checked manual and it says 0-25ma

[3dclint]

Did anyone have any ideas on how to transform a 998-1001v 100ma source(it just flips between those 3 volts when set)  into 1000.0v or 1000.00v and 2-25ma?

[3dclint]

Would something simple like a cap and resistor filter work?....I mean I am trying to limit 80ma out of 100ma which is alot at 1000v. But also doesn't seem like a ridiculous amount either. I believe it works out to about 80w to dissipate.

[Kleinstein]

To control the output voltage to a defined voltage also at in between values one would need some kind of a linear power supply circuit. The circuit of some of the reference (e.g. Fluke343) is in principle not that different from a lab supply with floating regulator (Harrison topology). There are transistors (today possibly also a high voltage MOSFET) to control the voltage and limit the current.

It is still not ideal to start with a rather powerfull raw supply. I would prefer a lower power one to limit the worst case current already there to a safe level. Here a SMPS has some advantage, as less capacitance is needed for filtering compared to a mains transformer. Also a SMPS usually fails off if it breaks down.


P.s.: The questions ask suggest a more beginner level. With this knowledge one should not use high voltages anyway. I would really consider limiting the voltage to a safer level, like 40 V or 50 V.

[scopeman]

I put the complete manual with schematics up on ko4bb years ago. Hopefully it's still there. I have one of these stashed somewhere....

Sam
W3OHM