Repair of a voltage/current calibrator - Page 1
Forum section: electronics / repair

author: 128er
author_rank: Regular Contributor
timestamp: November 30, 2014, 10:48:35 pm
content: 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!Frustration took hold for a couple of minutesThen 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° angleThe last pic in this post shows the ICL7660 with the external clock signal fed in by enamelled copper wire.

author: radhaz
author_rank: Regular Contributor
timestamp: November 30, 2014, 11:02:27 pm
content: Wow, that thing is a mess! What brand/model is it?

author: 128er
author_rank: Regular Contributor
timestamp: November 30, 2014, 11:05:29 pm
content: And here are some readings I've done with my Agilent DMM. I only checked the sourcing function.

author: 128er
author_rank: Regular Contributor
timestamp: November 30, 2014, 11:08:51 pm
content: Quote from: radhaz on November 30, 2014, 11:02:27 pmYes, it definitely is!Quote from: radhaz on November 30, 2014, 11:02:27 pmIt seems that the manufacturer is "Intelligent Display Engineering Ltd". And IV-CAL is the model. The calibration certificate is from 1993.

author: m100
author_rank: Regular Contributor
timestamp: December 01, 2014, 03:34:14 pm
content: This equipmentmusthold the record for most bodges per unit area!

author: Vgkid
author_rank: Super Contributor
timestamp: December 01, 2014, 04:39:49 pm
content: I love the bodged diodes??? going from U2 to U5.

author: Fraser
author_rank: Super Contributor
timestamp: December 01, 2014, 04:52:55 pm
content: I've got one of those units. Mine will not measure the voltage correctly so I bought a new ADC but never got round to fitting it. I do not recall that it was a messy design inside as I would have likely junked it. I will have to see if I can find the box it is stored in.Aurora

author: Rasz
author_rank: Super Contributor
timestamp: December 01, 2014, 06:37:22 pm
content: thanks for those great pictures, looks like Bodge City from TRONyour calibrator still needs calibrating

author: Neilm
author_rank: Super Contributor
timestamp: December 01, 2014, 06:52:46 pm
content: Wow - is there a real serial number on that? The last thing I saw with that number of bodges (dodgy looking ones at that) was an engineering demo build of an instrument.

author: SeanB
author_rank: Super Contributor
timestamp: December 01, 2014, 07:07:56 pm
content: Seeing as likely the number built was in the low dozens, and probably they were produced in batches a few years apart ( those would be a very slow mover in most cases, you do not often use a portable calibrator unless you are installing plant and need to calibrate/setup controllers) they probably made a few hundred boards and stored then in a CA store and only assembled to order, with the issues of component obsolescence and packaging making those bodges a part of manufacture.

author: J.Tax
author_rank: Contributor
timestamp: December 02, 2014, 06:03:03 pm
content: Seems fine to meWhat a mess inside, it looks like a lot of after thoughts or someone has tried to improve it.Nice solution of placing the IC on a 45 degrees angle