Electronics > Repair

Repair of a voltage/current calibrator

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128er:
Hey!

A few months ago I bought a broken calibrator on ebay. When I opened it the first time, it was like "OMFG, what a bunch of crap". The whole thing looked like it was hand assambled or something, witch botches all over the place (as you can see in the pics).

Ok, however . . .
Powered the thing up to see what it does. Display was ok, switches too. But it was always sourcing 55 mA. No matter what I've tried.

I opened it and immediatly noticed that the 5V linear regulator was boiling hot. I disconnected the power, scratched my head and measured the resistance at the 5V regulator output against ground. ~20 Ohms, tadaaaa. Traced the short circuit down to a DG444 quad analog switch. I desoldered it and the short was gone.

Then a few weeks passed away and I ordered a new DG444. Today it arrived and I soldered it in. And did it works? NO!  :palm:

Frustration took hold for a couple of minutes  ;D

Then I measured the supply voltages here and there, traced around with the scope (incidentally I found a row of solder bridges were you can select different frequencies for whatever, ~7,1 kHz, 14,2 kHz, 28,4 kHz . . . .).

After some more measurements I figured out that the supply of the feedback ADC was faulty. The voltage comes from an ICL7660 chargepump converter, fed from 5V, the output was at ~4,7V.
No idea if my guess was right. But I tried to measured the internal 10 kHz oscillator at pin 7 and there was nothing. But the datasheet says, you can overwrite the pin 7 with an external oscillator. Hmm, there was this row of solder bridges! Tapped off the 7 kHz signal and connected it to pin 7.

It worked!  :-+

It seems that the internal RC oscillator of the ICL7660 is dead. No idea if the DG444 has killed parts of the ICL7660.

Happy as I was, I built the thing back together, to do some measurements. And it seems fairly accurate.

As you can see in the pictures below, if you have the wrong footprint for an IC, no problem. mount it at 45° angle  :-DD

The last pic in this post shows the ICL7660 with the external clock signal fed in by enamelled copper wire.

radhaz:
Wow, that thing is a mess! What brand/model is it?

128er:
And here are some readings I've done with my Agilent DMM. I only checked the sourcing function.

128er:

--- Quote from: radhaz on November 30, 2014, 11:02:27 pm ---Wow, that thing is a mess!

--- End quote ---

Yes, it definitely is!


--- Quote from: radhaz on November 30, 2014, 11:02:27 pm ---What brand/model is it?

--- End quote ---

It seems that the manufacturer is "Intelligent Display Engineering Ltd". And IV-CAL is the model. The calibration certificate is from 1993.

m100:
This equipment must hold the record for most bodges per unit area! 

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