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Is it just me or...

So there you are, working on something that you don't have any schematics for; (in this case a Datron 1281 8.5 digit DMM) This one wouldn't power up. Poking around I noticed some (TTL) circuits were not getting +5VDC. Traced it back through a three terminal linear regulator to it's AC input. Hmmm... Only getting about 5.5VAC, not enough there. Isolate the AC output from the transformer to make sure it wasn't a loading issue; it's not. Gets to about 5.8VAC. This unit has two toroid transformers, and the one I've traced the problem to has 8 wires on the secondary and another 8 on the primary (sheesh)

I have no idea what the primary voltages should be (As it turns out, about

16VAC. Weird!)

BUT, suddenly I hear a metallic sounding "tick!" and the unit begins running. (I quickly measure the primaries and note the voltages JIC.) sounds a bit like a relay, but there's nothing like that in any of the power supplies (there's a number of them) that I can see. Figuring an intermittent somewhere I begin jiggling connectors, tapping around with a screwdriver handle, etc. Nothing. Meter keeps working.

Cold blasts from freeze spray, Heat gun, etc. Nothing.

Turn it off for an hour and back on repeating what I did above. Still working.

Turn it off for a day and back on. Wash, rinse, repeat. still working perfectly.

Things like this seem to happen to me often. Is it just me, or do you have faults like this that drive you nuts?

I don't suppose anyone might have any schematics for this meter, would they? I think at some point Datron sold off to Wavetek then Fluke ended up with it, as the user manual is available from Fluke.

Thanks.

Reply to
JW
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Electrolytics will intermit like this. Change all that are in the power supply section.

Reply to
Jamie

cs

ut.

put

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out

a

ave

up

But 'lytics do not explain the 5.8 VAC with no load.

G=B2

Reply to
stratus46

Does this unit use an IEC type power receptable with some sort of voltage changing/fuse module that might need to be reseated or checked?

Reply to
Cydrome Leader

lets see..

~ DC = 5.8*sqrt(2) = 5.8*1.414 = 8.2;

So, with the head room required for a 5 V reg 78xx styles.. 8.2-2 = 6.2 on the average.. I put in more than the normal head room requires just to be safe on the cal's. This gives you at least

1 volt to play with...

This is all fine if you have good working caps on the device that is giving you the DC inside, mainly the rectifier..

So, putting all that in context, I'd still be checking for the caps in the power supply! Test the DC voltage on the input of the REG>>>>

Jamie.

Reply to
Jamie

Yah, i have run into wierd operation / no operation; back and forth; more than enough to drive anyone nuts (maybe that is why i am so weird?)_. At one place i worked, there was this HP programmable automatic tester, tied to (an added) keypunch for data logging. Worked for months then stopped working. Started signal tracing with a VOM starting at the keypunch. Back in those dayz that is all the was, but i would have chosen to use a VOM over a DVM so i would not miss data pulses. Got a few feet from the keypunch solenoids to input connector and then (no sound) it worked; for weeks. Next time it failed, i checked the cable from keypunch toward the ATE, but it immediately started working as i started the cable test AT the keypunch; worked for weeks. Next time, i started with the interface circuitry at the ATE; you guessed it - 15 seconds later it worked; yup! for weeks. And so on; testing creeping to the middle: the cable. Never got there; was about two inches away from the cable connector at the ATE the last time i tested the beast. Never failed again.

All you can do, is leave the damn thing alone as long as it is working, ignore the hassles, forget them and go on. But to keep Mister Justin Case mollified, do not totally forget what you did so that you can pickup the testing gauntlet where ever you previously left off.

Reply to
Robert Baer

Not the case here, problem lies in the AC somewhere. When the problem was happening, the AC voltage on the toroids primary was at about 6 volts or so, when it should have been 16.

Reply to
JW

Will check that, thanks. This meter is a bit of a rats nets of wiring, typical of other British test equipment I've worked on (Wayne Kerr for example). I hope that Fluke cleaned it up when they took it over...

It is interesting to note that some of the other 5 volt supplies were working, as well as some of the +-15 volt ones. They may be derived from the other toroid, though.

Reply to
JW

We had a coil in a reversing relay that would intermittently not work and cause the mechanical drive train to push off in never never land, thus damaging some very expensive rotor cones. It finally got bad enough at one point where we were able to fine it.

It turns out that this "GE" reversing contact assembly used pushed on spades in side to make it's wire termination to the coil instead of soldering them!>. And you had to unassembled parts not intended to be unassembled to find this. The mechanical vibration of the unit would cause a connection issue and not switch on the coil for the reverse operation when needed... The other controls would just keep cranking the motor trying to find the correct location when it was actually pushing it out of sight! And no, there were no limiting sensors in this machine ;)

Reply to
Jamie

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