Products > Test Equipment
Test Equipment Anonymous (TEA) group therapy thread
Ero-Shan:
So this is probably the last episode in the PM2421 series. You may skip it if you're easily bored.
Let's go back for a moment to the first, small problem: the knackered power switch (left one in the picture). It shouldn't be too hard to replace, should it? I have switches a plenty ...
As you can see, the key in the thread is on top. The middle switch would be almost perfect, but the lever would then point down in the ON position (this is a clear no-go for me). And, even worse, the solder lugs would point down and interfere with a PCB. So I went for the right one (it has another set of lugs on the bottom). I snapped the down-pointing lugs off and had a not exactly pretty, but working solution.
Now the PSU with that resistor-cum-lamp. To get easier access I removed the whole back panel. What I had already seen but simply ignored so far became too obvious to be ignored any longer. Something strange must have happened to that DMM. All around the mains transformer there was a deposit very much reminiscent of the corrosive spilling of batteries. But there surely never was any battery. This is even more mystifying as the transformer itself or the PCBs don't show traces of the crystalline stuff. It's also on the bar (girder?) of the case, on both sides! But nothing on the top cover immediately above it!
If anyone has a reasonable explanation, I'm all ears.
To not make matters worse, I tested the PSU with resistors as load. The service manual says the +/- 15 volts shall be loaded with 100 mA, the 5 V I loaded with about 500 mA (my own guess). The +15 volts is also the main reference voltage for the ADC, it's allowed to stray 15 mV. +14.993 V it was, -15 V were a little farther off, but both well within spec. 5 volts were fine, too.
Still wary about the overload at the 5 volts, I used my lab supply for that and the orinal PSU for the rest. About half an ampere current, the thing works as before. Switching the 5 volts also to its own PSU doesn't make a difference. So no explanation whatsoever for the overload, too. Grrr.
So I continued looking for the main problem: the display shows some random number, mostly static, sometimes changing. I replaced the comparator with that of the DIVO, and suddenly the display changed constantly, but didn't show anything resembling the voltage at its inputs.
It was then that Roger Dimwit finally had an idea (why did it take so long? you ask; first bouts of dementia I guess). The input to the ADC is a voltage between -7 V and +7 V, why not just circumvent the pre-amp and apply this directly? But that didn't help much. Still, with the error persisting, there was a chance that the pre-amp with that Photochopper made of pure unobtanium might be healthy after all. To make a very long story short, I eventually came across this:
(A) The inputs of the NAND were both high, but the output was 1.6 volts. About .3 volts diode drop - might be consistent with the small current. But I pulled the cathode out of the PCB. The NAND's output now read 0. :wtf: This made no sense at all, but I put another diode in.
(B) This looks much better. And, I just couldn't believe it, the display was stable with numbers that were consistent with the voltage I had set at the ADC input.
So I connected the pre-amp to the ADC again, and I was able to measure DC volts, DC amps and ohms (didn't try AC). Not exactly dead-on, but should be adjustable.
The exaltation didn't last long. With the next power-up, the DMM had returned to its alter ego - a random number generator. Finally I recognized the smell that should have registered from the very beginning: the stink of a case full of red herrings! There actually was an explanation for the 1.6 volts at the NAND's output: a bad ground connection. 40 years ago I had learned to knock on PCBs so they showed bad connections. Now that I did that, I could see a definite response. I really deserve no better! So I got out the board once more (not to easy, as the rigid cable harness is in the way - you have to remove the back or front first). Looking at the solder joints with my best loupe, however, revealed no obvious culprit. But I resoldered all those that looked the least bit suspect (somehow I can't get myself to just resolder all of them - doing it on an obviously perfect solder joint is ridiculous).
That seems to have done the job. I'm more than a little embarrassed about that.
Still needs the adjustment (input is 10 volts), but that can wait.
Why do I tell you all this? To show off my ignorance? I don't know.
Ero-Shan:
--- Quote from: Specmaster on May 13, 2018, 08:32:12 am ---
--- Quote from: Ero-Shan on May 13, 2018, 06:47:26 am ---
--- Quote from: Cerebus on May 10, 2018, 11:18:45 pm ---Talking of TEA things that tempt me but I'm not going to bite on, there's a FLUKE 5440B Direct Voltage Calibrator going as a buy it now for £450 on ebay UK if anybody's interested. Start your own home cal lab!
--- End quote ---
If I lived in the UK I'd made an offer already. This is the thing I'm sorely missing! £450 isn't really much if it works.
--- End quote ---
Problem being of course is that it would need it to be calibrated first once you repaired it [emoji48]
From mobile device so predictive text might have struck again [emoji83]
--- End quote ---
I was under the impression that the calibrator is in working condition.
My Knick allows me to generate up to 150 volts DC, but above that I have nothing, and I also have no stable AC source.
The Fluke may not be calibrated, but it might generate stable voltages over a wide range.
tautech:
:)
Ero-Shan
I once volunteered to fix a misbehaving MIG welder... I can fix anything, right ? :palm:
Much did I live to regret it for a good six months or more .....would work fine then NOT. :wtf:
Bloody thing drove me nuts for a good while, the initial repair was simple enough, it had been dropped ! :o
A PCB mount transformer IIRC 20VA had cheerfully ripped one of it's PCB TH mounting pins from the encapsulation and the very fine primary winding. Replace transformer....simple fix.
But get this, the bloody thing was mounted approaching the middle of the PCB and well away from the PCB fixings but looking back now that probably saved the PCB from breaking when it was dropped.
Getting back to the ongoing faults.....a couple of cracked solder fillets on SMD components was deemed to be the problem.....reflowed, so they must have been.
Well back the bloody thing came, probably for the fourth time and I WASN'T to be beat ! :rant:
Then a EE buddy said; flex the PCB and listen for clicking. Well you couldn't see any cracks in the solder no matter how good the magnification or lighting was.....but the bugger still clicked ! :rant:
Reflowed every joint by hand, probably ~200 and haven't seen the thing since. :-DD
The little tricks we learn. ;D
bd139:
Ero-Shan. That corrosion looks like typical stuff you get on steel/aluminium joints. It does it all by itself. I had a Land Rover that basically looked like that all over.
Specmaster:
--- Quote from: bd139 on May 13, 2018, 10:07:05 am ---Ero-Shan. That corrosion looks like typical stuff you get on steel/aluminium joints. It does it all by itself. I had a Land Rover that basically looked like that all over.
--- End quote ---
Yes its the reaction of 2 dissimilar metals in direct contact with each other that will cause that kind of corrosion if they become subjected to moisture so its possible that it might have had some liquid spilled on it or has been in a damp environment for a time.
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