Heathkit Model IM-11 VTVM

[Smoky]

Hey folks!

I just purchased a used Heathkit IM-11 VTVM for $40. I bought it primarily to help in the alignment of older shortwave/AM radios. I have a Knight-Kit radio waiting for the proper testing tools and cables to do it.

This VTVM doesn't need to be super-accurate but I'm going to open it up to see if there's any funny-business going on inside of it before I power it up. It didn't come with a probe so I'll need to make one. The needle doesn't look to be stuck, so that's a good thing.

I did read a thread where Med6753 says not to touch/swap the vacuum tubes because it'll affect its accuracy.

Anywho, I'll post some pictures of what I find inside

[Smoky]

So far, so good!

The VTVM doesn't appear to have been repaired. The capacitors look original:



I don't see any corrosion in or around the battery area and the color of the copper spring looks great. The getter compound in the Mullard tubes looks good too

[WattsThat]

$40? Without a probe? 

Nothing special about the tubes, absolutely nothing at all. At least you got a nice, vintage Eveready D cell. That’s worth something, right?

The probe will be a bit of a challenge. You need a 1 meg resistor in series for dc measurements, switched out for ac and ohms. At least it uses a standard 1/4” phone plug.

The manual is available all over the web.

[Smoky]

I guess, I look at these projects in a different way. To me, $40 is a good deal. A nice lady packaged it up for me six States away and someone from the U.S. Postal Service drove two days in a truck to Raleigh and then placed it on my doorstep.

Anyway, most of the projects I look to do are ones that desire to be pulled back from the cliff

The switches look like they'll need a good cleaning, but I'm up for it!

[Per Hansson]

If you like to hang out with others that repair tube gear I can recommend Mr Carlson's Lab Patreon channel.
The content is great, the only sad part is the Patreon part because they don't have a search function for community posts.
So basically anything older than a few weeks gets buried, but what can you expect from a platform made for the 2020's? 

[bd139]

That’s pretty tidy. Good deal for the money.

They’re still pretty good meters. Quite accurate and obviously high impedance. Not to modern safety standards of course so watch it if you accidentally ground it to something live. Also the bottom of the ohms scale resistor likes to get toasted

You can buy a replacement probe kit on eBay for it as well.

https://www.ebay.co.uk/itm/Custom-VTVM-DUAL-Probes-Heathkit-IM-11-im13-im-28-im-18-im-5218-im-5228-/254127385482?mkcid=16&mkevt=1&_trksid=p2349624.m46890.l6249&mkrid=711-127632-2357-0

Tubes are cheap. Med is right about don’t change them. If you do you have to burn them in for a few hours then feck around with the calibration which is a pain.

I picked up another one ages ago with a slightly newer battery than yours which hadn’t leaked and was surprised! https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg1775570/#msg1775570

[BrokenYugo]

Looks like you won the Heathkit lottery and got one that was built and maintained by somebody competent. Should be pretty easy to bring back to life with the usual tube gear restoration procedure, check that all resistors are in tolerance, replace any electrolytic or paper capacitors.

[floobydust]

OP you have a knack for finding gear in amazing condition.

I think I used an IM-11 around 1980, just to try a VTVM because the techs had all the other multimeters in use in the shop.
I thought the meter was susceptible to mains fluctuations so I didn't like constantly re-zeroing everything and having the needle move. The probe was too big and the metal cigar tube was not something comfy. Mr. Carlson seems OK with them though.

I see a battery eliminator for sale on eBay, looks like a chinese LM317 board outputting 1.5VDC from the 6.3VAC filament winding.
Might be an option for the Ohms power source although 165mA (1.5V/9R1) on Rx1 scale is great for troubleshooting bad connections, switches, rectifier diodes etc like a Simpson 260, it might be too hard on IC's and small semi's.

How does the Eveready 9-Lives carbon-zinc keep it together after all the years? They are always dead, even as a kid I hate those batteries my toy does not work lol.

[WattsThat]

The metal cigar tube probes were the RF probes, the very early model VTVM's and on the signal tracer. The most common VTVM's have a plastic body probe with a switch for dc/ac-ohms that pops the 1 meg resistor in and out.

I see a battery eliminator for sale on eBay, looks like a chinese LM317 board outputting 1.5VDC from the 6.3VAC filament winding.

The official Heathkit battery eliminator was (3) 1N4005’s, a 10 ohm 2W resistor and a 1000uf 25V cap. One diode is a half wave rectifier fed off the ungrounded side of the filament voltage into the filter cap. 10 ohm resistor feeding the other two diodes in series forward biased as a 1.2 volt regulator. These parts fit on a small 5 point terminal strip all within the same space occupied by the battery.

I did hundreds of these mods back in the day when I was on the repair bench at a local Heathkit store. Here’s one in my IM-28.

[Smoky]

Mr. Carlson's Lab is a favorite stop

I plan to use this meter primarily to tune coils for peak amplitude in old radios. So, maybe, I can get by with a more "simpler" probe/leads with small grabber hooks?

I'll check all of the resistors and capacitors in the meter and fix what needs to be fixed.

As for the tubes, I brought up the getter because I see no erosion (='s not used much?). What's interesting is that the 6AL5 and the 12AU7 have the same date/build code (1022-526). Maybe, Heathkit stamped or ordered the matched-sets of vacuum tubes so calibrating the meter would be easier?



Anywho, I'm thinking of just keeping the battery and just monitoring it. Maybe there's a lithium that can be clipped in?

The 16uF/150V smoothing capacitor measured high but it shows no leakage. I ordered a few new Vishay axial capacitors:



But when I pulled the .047uf/1600V film capacitor it measured slightly higher and started showing leakage at 100V so I added a couple new 2kV CD PP axial capacitors to my Mouser order:





And no kidding FloobyDust, what's the deal with the battery? It doesn't look a day old and it even measures pretty good

[Smoky]

I made some progress on the VTVM this week

I started by replacing the old 16uf/150V main filter capacitor with a new 20uf/250V filter capacitor. I looked at the schematic for the IM-13 VTVM and later models and they upped the capacitance to 20uf so I thought it would be fine. The circuits appeared to be identical.

I read a little about the battery eliminator and how essential it was so I removed the positive (+) cup from the PC board too:



I followed WhatsThat's lead and constructed the battery eliminator on a disc of Phenolic board and a nylon standoff. It was bolted to the main PC board but the center post isn't used for the positive (+) connection anymore, that trace was not used because it was easier to move the blue wire (+) to the new board instead. Nothing was modified in the VTVM to make it fit and the battery eliminator is all "solder" joints and no "bolted" joints like the old battery connections were:



BD139, all of the carbon composite resistors were replaced with Vishay MRS25 (under 1%) metal films. Here's an old 150K and a new 10MOhm:





The resistors with the red dots are the AB carbon composite resistors that I replaced. With the humidity here in North Carolina being so high, I take no chances using/keeping carbon composite resistors in my projects:



The probe's input .047uf/1600V blocking capacitor was replaced with a 3kV Cornell Dubilier. I bought the 3kV instead of the 2kV size because the 3kV cap has a smaller diameter and it fit better. Zero "leakage" too:



I didn't read about any issues with the Selenium rectifier, but when I bring the power up on the VTVM, I'll put my oscilloscope probe on its output and on the battery eliminator output as well to see what they measure and look like:



Well, I searched through my parts and found this 800v bridge rectifier. It has round leads and a weird pinout (- ~ + ~) that I bought by mistake for a past project. But it fits the spot pretty good. The distance between the positive (+) and the negative (-) lead of the rectifier matched the spacing of the PC board. I measured the voltage drop of the old Selenium rectifier and it is 4.24v. The new Vishay bridge rectifier (part# VS-2KBB80) measured 1.21v. No worries now about the Selenium rectifier failing



Now here's the odd job. I'm reading the manual and it states that if you're unable to obtain DC balance, check capacitors C5 and C6 (.005uf/400v discs). And if the AC function is inoperative or unable to to be calibrated, check capacitors C3 and C4 (.02uf/400v discs). Since there are only four capacitors like this in the IM-11, I'll pull and test them:



So, after reading the tolerance printed on the body of the capacitors, I say to myself "wow, these are loose and should test good"

Here's a .005uf. I watched Mr. Carlson's Lab's video where he says that these capacitors rarely test bad. From here on, I'll check these type of disc capacitors just as I would an electrolytic. It appears that these will drift over time:



So I matched two 630v Murata C0G capacitors that measured 4820pf and put them in their places:



Measuring the old .02uf/400v discs went in the same direction, low, about 30% so I replaced them with new Murata X7R .022uf capacitors:





I plan to use this VTVM to peak coils and such so I made a set of test leads. I used Thermax RG316-DS coax, a Switchcraft 588 1/4" plug, and two E-Z Hook mini- grabbers:



I also read that replacing the front panel's filament bulb with an LED will shave around 135mA of current on the 6.3v windings too.

[WattsThat]

I’d check the transformer regulation before permanently removing the original pilot lamp. You don’t want the 6.3 volt supply going high on you and stressing tube filaments.

I’m not someone who says everything has to be left as is. I’m just a believer in “do no harm” when making modifications. Sometimes you have no choice, but regardless of it being a must or a want, I still want my gear functioning as original and in spec.

BTW, Excellent job on the battery eliminator!

FWIW, when repairing at Heath, we would replace the selenium rectifiers with the ubiquitous 57-27 which was a 1N4005. If the original ever fails, you’ll think you someone left an egg to rot inside the meter, it’s unmistakable and very nasty. Similar to having one of the notorious Rifa film caps fail in HP gear.