Megger Insulation tester model MIT320.

[Pamphonica]

I spotted this broken Megger MIT320 insulation tester being offered here on the forum on Friday last week. It's a modern model, selling for £300++. Only £10 but it was smoking which was not good news.

I collected it from Christopher ("High Vacuum House") at Royal Wootton Bassett on Sunday.

Anyway, I had a dig around, checked that the HT transformer did have continuity both sides, checked the HT diodes were OK and carefully removed the fiddly driver FET. It was completely fried - big bulge on top and part number scorched beyond recognition.

I didn't hold out much hope but tried calling Megger directly yesterday (Monday) for advice and part number. Despite not being an "Authorised Repairer" the nice gentleman in the "Applications" department looked up the service manual, identified the part and gave me its internal and manufacturer codes.

Megger only sell these FETs in 10-offs and do charge hefty (£7) P&P on small orders (+ VAT of course). But which other manufacturer would sell someone a single, rare FET to get their testgear back working again? Big pat on the back to Megger. These FETs are not readily obtainable elsewhere, so, at only 52p each I decided it was worth a try and ordered some from Megger Sales. No formality - they set up a personal account immediately.

This afternoon FedEx delivered a reasonable sized box from Megger, with the tiny FETs in it. I carefully soldered one in, and was rewarded by all the insulation test ranges springing back to life as per the manual.

The FET is in a horrid position, hard against the HT coil, so care is needed (and a fine iron) to do a repair without melting the coil former less than a mm away.

These insulation testers are excellent for identifying transformers which leak, and for measuring insulation breakdown in cables. All a lot more safely that sticking a huge 1KV power supply across the test object and standing back.

So, job done. And if anyone needs an FET for a similar repair just drop me a PM - I have a good stock!

[Nickthedentist]

Well done, Jeremy, another nice bit of kit saved.

Nick.

[Outrun_uk]

Excellent news!

There is another FET that can be used, available from Farnell etc, would have to check my notes for the type though. They are a decent instrument but do pop the FET's occasionally for unknown reasons! Sometimes the big (zener?) and the smaller grey HV diode die too....

[Pamphonica]

Thanks Nick and Kev. I must say that Kev's helpful post on the original For Sale post steered me in the right direction.

I must confess that surface mount stuff is pretty new to me. It takes a level of care (and magnification) that I am not used to. Give me a tag-board or some nice well-spaced through-hole components and I am in my element! But it's a skill worth acquiring if I am not to just ignore opportunities to repair anything under 20 years old.

[Nickthedentist]

The worst bit about repairs to modern stuff is not necessarily the SM components, but the lead-free solder in my experience. Put the two together and you've got trouble!

[Pamphonica]

I removed as much of the existing solder as I could and used my ordinary leaded solder. It seemed to flow nicely.
I will probably get a visit from the ROHS thought police soon.

[Phil G4SPZ]

Well done, Jeremy, and worthy praise for Megger too. I have an analogue battery Megger which I repaired, and it is in almost daily use. The test current is limited to 0.82mA, and I wonder whether your digital instrument is similar?

[Pamphonica]

I think the maximum test current is limited to 1.5mA according to the spec. Fairly safe one hopes!

[McMurdo]

The FETs in the earlier (through hole) meggers used to fail the same way and also smoked. I think they were a ZVN something (ZVN110?) in a super-e-line package...maybe the SMD equivalent in yours.

[John_BS]

Originally Posted by Pamphonica

I will probably get a visit from the ROHS thought police soon.

I've just emailed them .........

J

[Pamphonica]

Thanks a bunch, John!

For future reference, the FET's original part number is Fairchild HUF75307 and the replacement was listed on the invoice as Fairchild FQT13N06L. Neither seem to be readily available except via Megger. The Megger internal part number to quote is 31000-031.

There may well be a more readily available equivalent but at least there are spares readily available via Megger.

Mods, if it's possible could you add the model number - MIT320 - to the subject line to help future searches for info on this model.

[thermionic]

An excellent result. I think other manufacturers could take a leaf out of Meggers book. I was after a circuit for a High end CD player from a UK manufacturer who just weren't interested, something about ' intellectual property rights ', apparently.

As you say, SMD repairs can be daunting, especially if you don't have access to specific rework equipment. Good eyesight, lighting and magnifying optics - essential.


SimonT

[Craig Sawyers]

Just had a look at Farnell, and that particular transistor is 71p. They do a small order surcharge of £2.xx.

Of course you would have had to find out what transistor Megger were going to supply first.

I've had a similar experience with Fluke. My 332B DC voltage standard died, and that turned out to be an FET. Fluke had replaced the part number on the transistor with an in-house number, so there was no way you could work out what it originally was. But at least they, like Megger, have no problem in supplying to an individual.

The part number thing goes into audio too. Audio Research grind the part number off every single transistor, and then paint three colour stripes on instead. They will cheerfully sell you a replacement - for a hefty cost. You have to specify Qxxx from amplifier yyy.

Craig