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 Post subject: Heathkit IM-28 VTVM Restoration
PostPosted: Sep Mon 23, 2019 6:03 am 
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Location: Littleton, MA
I recently restored a Heathkit IM-28 VTVM.

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I had always wanted Heathkit’s IM-13 bench model VTVM. The IM-28 is the “new look” version of the IM-13, and I couldn’t pass it by when I saw it at the MIT Flea.

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Unlike the RCA WV-97A VTVM that I recently restored, most of the precision range resistors were still within tolerance.

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I replace all that were badly out of tolerance, but left in a few that were close, such as R17, the 6.838M Ω resistor in the DC voltage divider,

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and R2 and R4 in the AC voltage divider.

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The film capacitors were all modern plastic dielectric, not paper, so I left them in place.

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Of course, I replaced the electrolytic filter capacitor. The rectifier was a silicon device, so I left it in place.

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I replaced the battery holder with a full-wave bridge rectifier feeding an LM317 regulator set to 1.55 volts.

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I cut the wire that grounded one side of the filament circuit, since the negative side of the full wave bridge has to be grounded. If the ground on the AC side isn’t lifted, it will short out the full wave bridge.

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Here’s another view of the battery eliminator circuit.

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All back together.

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I calibrated it using my HP 6920B meter calibrator. The DC and Ohms ranges are pretty good, usually better than the specified 3% accuracy, but a few of the spots on the AC ranges are out of the 5% spec. That’s probably due to the slightly out of tolerance resistors in the AC voltage divider that I didn’t replace.

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The unit came without the mounting bracket, so I drilled out a pair of angle irons to fit the mounting thumbscrews on the ends of the IM-28 cabinet.

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They are a workable substitute.

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The unit came without a probe, so I built a homebrew switchable DC/AC-Ohms probe for it.

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 Post subject: Re: Heathkit IM-28 VTVM Restoration
PostPosted: Sep Mon 23, 2019 7:08 pm 
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Joined: Jul Wed 11, 2018 7:14 pm
Posts: 60
Location: Calgary, Canada
You have a very nice meter there. I have a Heathkit IM-13 which I think is almost identical to your IM-28.
My brother built the IM-13 in 1964, and he recently donated it to me. I replaced the rectifier with a IN4007, and I also replaced 3 resistors and 2 capacitors. I also replaced the power cord with a 3 wire/3 pin cord to ground the chassis.

I love the look of these meters, and they work quite well.


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Heathkit IM-13.jpg
Heathkit IM-13.jpg [ 239.73 KiB | Viewed 578 times ]
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 Post subject: Re: Heathkit IM-28 VTVM Restoration
PostPosted: Sep Mon 23, 2019 7:18 pm 
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That's a beautiful example of an IM-13. It looks like new!

I've never seen a Heathkit with the Daystrom logo on it. That's an interesting bit of history.

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 Post subject: Re: Heathkit IM-28 VTVM Restoration
PostPosted: Sep Mon 23, 2019 10:56 pm 
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Location: Florida
stevebyan wrote:
I recently restored a Heathkit IM-28 VTVM. .................. ..............I replace all that were badly out of tolerance, but left in a few that were close, such as R17, the 6.838M Ω resistor in the DC voltage divider,
.


That weird value is the result of the way they laid out the scales. Most others that use 5,15 type scales align them to allow values that are whole numbers (1,2,7 series) for the multipliers. Always kind of wondered why Heath did what they did.

I have an IM-13 that had fallen into the hands of one of the geniuses who apparently think Heath and other manufacturers forgot to put the 1 Meg probe resistor inside the unit so they not only moved it inside but added a banana jack for it. Luckily the hole isn't all that noticeable. I need to find a plug for it.

RRM


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 Post subject: Re: Heathkit IM-28 VTVM Restoration
PostPosted: Sep Tue 24, 2019 12:39 am 
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Retired Radio Man wrote:
That weird value is the result of the way they laid out the scales. Most others that use 5,15 type scales align them to allow values that are whole numbers (1,2,7 series) for the multipliers. Always kind of wondered why Heath did what they did.

I recall doing the calculation and finding that the odd values in the IM-13/IM-28/etc gave slightly more accurate ratios for the ranges.

It's interesting that, aside from the range resistors, the IM-11 and the IM-13 have the same circuit.

Retired Radio Man wrote:
I have an IM-13 that had fallen into the hands of one of the geniuses who apparently think Heath and other manufacturers forgot to put the 1 Meg probe resistor inside the unit so they not only moved it inside but added a banana jack for it. Luckily the hole isn't all that noticeable. I need to find a plug for it.

Tape some plastic wrap over the outside of the hole, then fill it from the back with J-B Weld. The color ought to be about right.

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 Post subject: Re: Heathkit IM-28 VTVM Restoration
PostPosted: Sep Tue 24, 2019 4:24 pm 
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Joined: Jul Wed 11, 2018 7:14 pm
Posts: 60
Location: Calgary, Canada
stevebyan wrote:
That's a beautiful example of an IM-13. It looks like new!

I've never seen a Heathkit with the Daystrom logo on it. That's an interesting bit of history.


The IM-13 was bought in Canada, so maybe that has something to do with having the Daystrom logo.
The plastic meter face had quite a few scratches on it. I used automotive rubbing compound and the result was a meter face that looks like new. I use the same rubbing compound to remove the haze on my car headlights.

Your battery eliminator circuit is interesting. I don't use a battery because of fear of corrosion damage. I may consider using that circuit in the future.


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 Post subject: Re: Heathkit IM-28 VTVM Restoration
PostPosted: Sep Tue 24, 2019 4:54 pm 
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VA6JRC wrote:
Your battery eliminator circuit is interesting. I don't use a battery because of fear of corrosion damage. I may consider using that circuit in the future.

It's basically Mike Yancey's circuit, but with a cheap Chinese built-up 317 regulator board instead of his 317 circuit with all the frills from the LM317 data sheet.
https://mikeyancey.com/an-improved-vtvm ... liminator/

I believe Peter Bertini first pointed out the desirability of using a full-wave bridge rectifier for VTVM battery eliminators. If you search on ARF you should find a thread about it.

For the particular 317 regulator board I use, I put a 51 ohm resistor across the trim-pot to reduce its range, so that the voltage setting is more stable.

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Last edited by stevebyan on Sep Tue 24, 2019 5:02 pm, edited 1 time in total.

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 Post subject: Re: Heathkit IM-28 VTVM Restoration
PostPosted: Sep Tue 24, 2019 4:55 pm 
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VA6JRC wrote:
The IM-13 was bought in Canada, so maybe that has something to do with having the Daystrom logo.

Yes, upon closer examination I noticed the Canadian address for Daystrom on the meter card.

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 Post subject: Re: Heathkit IM-28 VTVM Restoration
PostPosted: Sep Wed 25, 2019 1:57 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 6382
Location: Sunbury, Ohio 43074
Nice restoration Steve, and a nice little meter. I had one on my bench when I first started fixing TV's before the earth cooled. I've got a few waiting in the que in my legendary out of the way steel file drawers .... not high on the list at the moment. Hope to get them into users' hands before I'm gone .....

But geez man, you only included 1,277 photos. We need at least 2,000 to see what you did. ;-)

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 Post subject: Re: Heathkit IM-28 VTVM Restoration
PostPosted: Sep Wed 25, 2019 3:00 pm 
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Barry H Bennett wrote:
But geez man, you only included 1,277 photos. We need at least 2,000 to see what you did. ;-)

I ran out of gas after the WV-97A restoration :-)

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