Author Topic: DC Voltage Calibrator  (Read 4305 times)

Mr Eastwood

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DC Voltage Calibrator
« on: November 01, 2015, 02:58:05 PM »


This is something I bought from Ebay for 30.  It can output a voltage between 0.01uV to 9.9999v @30mA,   It contains a precision zener diode which is used as a reference input to a FET chopper amplifier operating in feedback stabilized mode.  The gain is determined by a set of precision metal film resistors which are selected by the 5 decade thumbwheel.

It should come in handy for lots of things. :-)
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SeanB

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Re: DC Voltage Calibrator
« Reply #1 on: November 01, 2015, 11:52:31 PM »
That is beautiful, looks almost unused. I guess you will be changing the battery pack though, as it is likely to be dead by now.

Mr Eastwood

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Re: DC Voltage Calibrator
« Reply #2 on: November 02, 2015, 08:42:20 AM »
That is beautiful, looks almost unused. I guess you will be changing the battery pack though, as it is likely to be dead by now.

well;  when I first got it there seemed be some juice in the batteries - maybe it had been used;  so I left it charging - then let it run all afternoon and it seemed to run ok.

I just broke the seals and cracked it open and there are 11 cells and I measured 14.3 Volts;  the battery pack looks very nicely wrapped but overdue a replacement.




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SeanB

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Re: DC Voltage Calibrator
« Reply #3 on: November 02, 2015, 11:16:26 AM »
Just look to see if there is a white crust on the ends of the cells, if it is not there then leave them as is, as the seals are still fine. You might have to slit the outer sleeve though to see the inner cells.

Mr Eastwood

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Re: DC Voltage Calibrator
« Reply #4 on: November 02, 2015, 11:23:51 AM »
Just look to see if there is a white crust on the ends of the cells, if it is not there then leave them as is, as the seals are still fine. You might have to slit the outer sleeve though to see the inner cells.

ok - I'll do that tonight after work :-)
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SeanB

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Re: DC Voltage Calibrator
« Reply #5 on: November 02, 2015, 01:07:04 PM »
At that size they are probably Asize NiCd cells likely 2Ah, so replacement will have to be NImH cells, preferably 2200mAh cells with tags. Smaller, and will need a slight modification to the charger to drop the charge current down.

Mr Eastwood

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Re: DC Voltage Calibrator
« Reply #6 on: November 02, 2015, 05:15:51 PM »
At that size they are probably Asize NiCd cells likely 2Ah, so replacement will have to be NImH cells, preferably 2200mAh cells with tags. Smaller, and will need a slight modification to the charger to drop the charge current down.

Nice call Sean!







I've just taken them out and ripped them apart them dumped them,  I can live with this not being portable.
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SeanB

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Re: DC Voltage Calibrator
« Reply #7 on: November 03, 2015, 12:02:52 PM »
White and crusty, they have done well, probably sat there all the time it was powered with 70ma through them, toasting them nicely. If you want replace with 2200mAh cells and do nothing to the charger, they will charge at C/40, so will never get enough pressure inside to vent.

Mr Eastwood

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Re: DC Voltage Calibrator
« Reply #8 on: November 03, 2015, 01:31:45 PM »
White and crusty, they have done well, probably sat there all the time it was powered with 70ma through them, toasting them nicely. If you want replace with 2200mAh cells and do nothing to the charger, they will charge at C/40, so will never get enough pressure inside to vent.

They were fairly warm to the touch :-)
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SeanB

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Re: DC Voltage Calibrator
« Reply #9 on: November 03, 2015, 01:48:59 PM »
Would have been fun to see what the charge current was, you might have to replace them as they might be the voltage regulator for the unit. Care to post a nice big photo of the charge board looking down, we might be able to reverse engineer what they did as voltage regulation. Are those power leads going off the board PM3 battery snaps, they look like it.

Mr Eastwood

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Re: DC Voltage Calibrator
« Reply #10 on: November 03, 2015, 06:16:20 PM »
Let me take a look real quick, see if i can work it out, then I'll set up my mini tripod to get a nice pic for you to review.
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Mr Eastwood

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Re: DC Voltage Calibrator
« Reply #11 on: November 03, 2015, 07:03:06 PM »
Here's a pic.



It goes from ac mains to 21.5V, then rectified;  the output from the terminals into the main module is ~17.2v dc.

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SeanB

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Re: DC Voltage Calibrator
« Reply #12 on: November 03, 2015, 11:08:37 PM »
Made in 1986 or early 1987 from the date codes. Does not use the battery for regulation, just as a seamless backup supply then. The 741 looks like it is doing voltage regulation to turn the constant current charger off when the battery is charged, so it will be fine without it.

Motorola and RCA, both of who are no longer the same company and both no longer make these chips.

You might want to change that 1000uF 35V axial electrolytic, and in this case you would not have any issues using a regular 1000uF 50V radial one, you can even use an 85C part. If you have the board out to do that I would recommend sticking some 100n ceramic capacitors across the input and output of the 18V regulator on the bottom of the board. Regular disc ceramic or wire ended ceramic ones will do so long as they are rated for 50V. No harm in adding them as it is recommended in the datasheet, though this one has run fine without for decades.

steve30

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Re: DC Voltage Calibrator
« Reply #13 on: November 04, 2015, 02:12:57 PM »
Looks like a nice machine.

I used a similar one a few years ago. I don't know how accurate it was, but I wasn't too bothered about the accuracy at the time. It was quite nice to be able to just dial the voltage in with the thumb switches :).