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Old 11th Oct 2019, 1:04 pm   #1
WaveyDipole
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Join Date: Mar 2013
Location: Leicester, UK
Posts: 2,505
Default Heathkit IM-28 VTVM

I have just received a Heathkit IM-28 VTVM from a member. I am very pleased with what I have received. It came in a wooden case and complete with an original probe. The unit itself is in very good and clean condition. No dirt or grime. The inside looks like it had just been manufactured!

Except for the one across the secondary winding which looks like it may have started to leak at the positive terminal, the paper type caps look like new. As is usually recommended, I will be replacing all of them as a precaution.

One thing that puzzles me at the moment is the transformer and I would like to confirm my understanding before applying any power (once the caps have been replaced). The unit has a rather stiff mains cable (it would be a stretch to call it a flex) terminated with a USA style plug. The transformer primary seems to have two windings which are connected in series. The numbers printed on the winding correspond to the list of part in the build instructions (54-2-24) which the schematic and build instructions would appear to suggest can be connected in series for 240v operation, or in parallel for 110/120v operation. Since this example has the two windings connected in series, that would mean that it is configured for 240v use, but if so, then why does it still have a USA plug? I am considering replacing this cable with a proper flex and UK plug.

The total resistance of the two windings in series is about 730 ohms, but there is a disparity between the two windings of about 30 ohms (measured approximately 350Ω and 380Ω). Should I be concerned about that?

The other omission seems to be an internal fuse. Would it be advisable to add one?

My last question has to do with one of the capacitors. It is shown as C1 on the diagram, it is rated as 0.047uF at 1600v. It does appear to be a de-coupling cap so am I OK to use the nearest rated part that I have to hand, which is rated 0.1uF @ 2000v ? With the original having +/-10% tolerance, I can't imagine it being measurement critical, but I'm not sure whether the additional capacitance might have any undesired effect. I don't have 2 to place in series (although I do have two 0.1uF @ 630v). There seems to be no indication of the actual voltages present so I'm not sure whether the two lower voltage ones (possibly allowing for 1260v when in series) would be adequate?
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Attached Files
File Type: pdf IM-28-schematic.pdf (367.2 KB, 125 views)

Last edited by WaveyDipole; 11th Oct 2019 at 1:14 pm.
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