Mitutoyo DRO type GML-3705W mains transformer failed


I've got a two axis DRO on my mill with an old Mitutoyo GML-3705W
display unit. It's not been used for a bit over a year and when I
switched it on this evening nothing lit up on the display front panel.
I've taken it apart and the primary of the mains transformer is open
circuit between 0V and 100V (the lowest voltage tap). The taps between
100V and 240V all show a reasonable resistance.
The secondaries seem OK and aren't shorted. Nor are the reservoir caps
in the DC supplies shorted. The 500mA mains fuse is intact. Maybe the
transformer just gave up the ghost all on its own?
It seems unlikely Mitu will have a spare for this obsolete device, at
least not at a price I'll want to pay, although I will phone their place
in Andover next week. I'm expecting to have to get the original rewound,
or a have a custom one made, or just throw it in the bin and console
myself with the thought that DROs are cheating anyway.
In the meantime, I wonder if anyone has worked on one of these, or has a
circuit diagram, or knows what the transformer spec is, or has a working
spare they want to sell for not much? I know it's a long shot, but...
Thanks,
Alan
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Reply to
Alan Ibbetson
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Do you have a 110v transformer or a variac available?
Connect this between the 100v and 240 volt taps and measure the secondary voltages. scale up if you are only putting 110v into 140v part of the primary. Get a general purpose transformer with similar voltages and a core of a similar cross sectional area - should be fine.
Bob
Reply to
Bob Minchin
Thanks for the suggestion, Bob. I've done the measurements and the tranny is a bit bespoke. 12VAC to drive a 1A 5V rail for the logic. 3.4-0-3.4, maybe for the RS232?? 40VAC to drive a 30V rail for the vacuum fluorescent display.
I did speak to a nice lady at Mitu in Andover but she laughed a lot when I asked if they had spares for this unit. It's 20-25 years old so they've no spares, no circuit diagram, not even a parts breakdown. I might take the transformer to my local place and see if they can rewind it or make me a replacement for not too many beer tokens. Most likely I'll just stick several junk transformers in a box on top of the Mitu case.
The fault was a dead short on the 30V DC rail. A wild stab in the dark prompted me to pull one leg of a tantalum decoupling cap off that rail. Voila, il marche! Fitting a replacement in the PTH board will be a right pig as usual. Out with the solder wick, solder sucker and sewing needle. But tomorrow.
Alan
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Reply to
Alan Ibbetson
Good to hear it's a simple to fix fault. Why would replacing the part in the PTH board be a problem. In my experience with such things you just heat one side and pull the leg to clear the hole. If not clear enough to get the new component lead in then heat one side and have then solder sucker on the other side of the hole to pull out the excess solder, job done.
Reply to
David Billington
Just one other thought, on some occasions with old solder it helps the run new solder into the partly cleared PTH hole to help it be pulled out and cleared more easily.
Reply to
David Billington
The vacuum fluorescent display will have a low voltage heater usually only 3 volts ac often centre tapped to the cathode supply (they use AC so one end of the display is not brighter than t'other. This may be your 3.4-0-3.4 but that sounds a bit high. Go carefully as if you pop the heater, then the display is buggered and replacements will be on infinite delivery from Mars! RS sell transformer kits with ready wound primaries and DIY secondaries. Might fit the bill? Good luck
Bob
Reply to
Bob Minchin
Hello Alan. I'm hoping to contact you. I came across this post. I have the same problem you had. If you see this I was hoping you found a suitable tra nsformer replacement and could share that information. Thank you in advance . My email is donald snipped-for-privacy@hotmail.com
Reply to
donald_lee99

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