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 Post subject: Heath VTVM IM-13 resto
PostPosted: Sep Sat 10, 2016 4:57 pm 
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While waiting for some caps from Mouser, I noticed my neglected Heathkit VTVM sitting on my shelf, pointer sitting idly at zero, cold and quiet - but sending me a message loud and clear that she wanted to be next on the list. Soooo, here's her story: This is one my uncle also built in '64, and I have had since a youth. It never did quite work right, so I didn't use it much. Cant find the build manual, but did find schematic online. "Some" calibration instructions also located, but rather vague. I have the mounting bracket "somewhere", but never recalled having the construction book. Here's the rest:

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I was quite surprised to find this had a battery inside, and even more surprised to find it had NOT leaked!!! That battery has been in this for over 40 years!

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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Sep Sat 10, 2016 6:25 pm 
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Should be an easy to restore VTVM. The resistors that are Precision is what makes it Actuate.
The capacitors are standard values and can be replaced with modern Equivalent. You can use a known good working meter to calibrate the IM-13.


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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Sep Sat 10, 2016 6:47 pm 
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I re-did an older version of the Heath VTVM.

--replaced the selenium rectifier with a 1N4007 silicon diode

--filter caps should be replaced and I used 250 V caps instead of the 150V originals

--I built a battery eliminator from an LM317 voltage regulator. This uses the filament voltage as a source to power the OHMs ranges. Similar to this one:

Image

--Probably good to replace the series AC blocking cap with a modern film type capacitor

Rich

http://s117.photobucket.com/user/radioc ... 2.jpg.html


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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Sep Sat 10, 2016 8:30 pm 
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You can download the full manual here:
http://www.k5jwk.com/IM-13.pdf

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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Sep Sun 11, 2016 1:57 am 
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Most VTVM's that I have seen have one side of the tube filament circuit grounded. Also, one side of the Ohms measurement battery grounded. Both these observations are true for the IM-13. This requires use of a half wave rectifier in the low voltage power supply circuit, with a larger filter capacitor.

-Chuck


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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Sep Sun 11, 2016 2:13 am 
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Unlike the IM-11, the IM-13 doesn't use a printed circuit board. It is point-to-point wired. So it is easy to lift the ground on the filament circuit, thus enabling use of the full wave bridge rectifier.

I do wonder what subtle consequences might arise from lifting that ground. The filaments will then be grounded through the rectifier, alternating the side that is grounded at 60 Hz. I don't understand why the filament circuit needed to be grounded in the first place.

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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Sep Mon 12, 2016 4:19 am 
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12 parts to replace a battery? Seems like more parts than there are in the whole VTVM, once you get past the range switch! The last generation of VTVMs like the B&K 177 which had power supplies for their Ohms ranges simply took a half wave silicon diode from the 6.3-volt heater source to a 1,500-uF filter cap (the negative end of which is grounded) and put a 16 ohm, 2-W resistor in series to drop the necessary voltage. Three parts total, and no need to lift any grounds or modify the VTVM circuit in any way. One might have to use something a little higher than 16 ohms when doing a retrofit into a meter that originally had a battery.

The tube filaments are grounded for a simple reason. The cathodes of the 12AU7 are operated at -60 to -85 VDC. Consequently a DC charge could develop on the heater circuit via the leakage of the heater-cathode insulation. That wouldn't really hurt anything as far as the 12AU7 is concerned, but the 6AL5 operates at very low voltages, and if that -60 to -80 VDC gets onto its ungrounded cathode through its heater-cathode leakage, it will pretty much be the end of AC voltage measurements. Grounding one side of the filament circuit gets rid of the leakage and prevents the problem from happening.

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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Sep Mon 12, 2016 9:19 pm 
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Come to think of it, I did use a half wave rectifier circuit.

The goal was not to re-design the VTVM, but rather to simply replace a 1.5 V battery. Most folks leave batteries in VTVMs for years; they leak chemicals.

Rich


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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Sep Sat 17, 2016 3:25 am 
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Thanks for the link to get the manual and the info to eliminate the battery. I like the idea of a simple diode, cap and resistor to replace a battery that may leak. I suppose I could tweak it by adding a potentiometer in the circuit. Now, I am waiting for that .047 1600v cap to come. Actually, I ordered the .047 2000v cap to have a little more margin.

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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Dec Thu 01, 2016 12:13 am 
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After some time on different projects, I have this meter back on my bench. I replaced the blocking cap (.047 1600v) with a 2000v cap, all others have already been done. I would like to eliminate the battery, and am looking at the diode, 1500 mfd cap and 16 ohm resistor Chris mentioned. Is there anything else I should know about this before I do mod? I don't want to bang or damage the meter, so I will being up slowly on the variac while monitoring the voltage.

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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Dec Thu 01, 2016 12:39 am 
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Make sure the meter works properly on all of its functions and ranges, using an ordinary battery for the resistance ranges before doing the mod. That way if anything goes amiss, you'll at least know where you started from.

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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Dec Thu 01, 2016 2:57 am 
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I've restored one of those before. The spring holding the battery was all corroded. I just downsized to a C size battery. In place of the spring and cup assembly I installed a plastic battery holder from Mouser that looks like this:
Attachment:
12BH211GR_t.jpg
12BH211GR_t.jpg [ 3.19 KiB | Viewed 2604 times ]


Next, I tested the VTVM resistance function and it worked. While it responded proportionally to different resistors it was not accurate to the degree I've become accustomed to (no surprise). Since my daily driver for resistance measurements is a Fluke DMM , I then promptly removed the battery after the test. So now, no worries about leakage problems.

HTH,
Grid2


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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Dec Thu 01, 2016 3:33 pm 
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Last night I re-replaced the 22mfd cap after a brief power up. It was right at the 160v rating across, so I upped it to a 350v that I had on hand. Another power up with a battery in place showed me there is life in that box! The ohms scale seemed to calibrate, at least it zero'd out both ways, ac scale was way off, but calibrated in with the adjustment. DC range was off, but I shut down for the night. I will try to do a full calibration tonight and see what I have. If that pans out, then I will experiment with the circuit to eliminate the battery. I doubt I will use the ohms much, since I too am spoiled by my Fluke. But there are times I may need the voltage measuring capabilities.

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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Dec Thu 01, 2016 4:54 pm 
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Watching this thread with great interest as I have 2 of these meters, plus an IM-11. One IM-13 is in good shape and the other is for parts at best, but the IM-11 functions just fine and it used in aligning IF sections.

I like that fancy regulated battery eliminator, but agree the simple one is better. May give that a shot at some point in the future. Like others, too many projects ahead of these so I hope to get back to them eventually. :)

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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Dec Fri 02, 2016 12:02 am 
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This is what I did for my IM-13 several years ago:

http://i117.photobucket.com/albums/o46/ ... Scan32.jpg

bottom regulator

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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Dec Fri 09, 2016 6:47 pm 
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Thanks Peter for your advice! I have some regulators ordered. Found them on Ebay for a few bucks. In the mean time, I put a new battery in it and did a full calibration. Probably the most difficult was the ac balance. Other than that, things work smoothly and accurately. The ohms scale seems pretty close comparing to my Fluke.

I did a little experiment with a diode, cap and different size resistor, but didn't have any success. I needed to build a string to ground, but I am going to just wait on the regulator. Do you see any problem using the regulator with the 1/2 wave setup instead of the full wave and leaving the filament grounded?

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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Nov Fri 09, 2018 3:46 pm 
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Peter Bertini wrote:
This is what I did for my IM-13 several years ago:

http://i117.photobucket.com/albums/o46/ ... Scan32.jpg

bottom regulator



Hi Peter,

Can you possibly post the text that is referenced in your picture? It says next to the 2400 ohm resistor "see text from wireless connection march 2010". I am finally getting back around to working on a couple VTVMs I've had for years, and contemplating the various battery eliminator circuits.

Thanks in advance,
jon

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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Nov Fri 09, 2018 9:05 pm 
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Chris108 wrote:
.....The last generation of VTVMs like the B&K 177 which had power supplies for their Ohms ranges simply took a half wave silicon diode from the 6.3-volt heater source to a 1,500-uF filter cap (the negative end of which is grounded) and put a 16 ohm, 2-W resistor in series to drop the necessary voltage. .....


The I77's I've seen used the rectifier circuit and a voltage divider consisting of 36 and 12.7 Ohm resistors, with the 12.7 in the bottom of the resistance-standard tree. This gives about 1.5 volts at a source resistance of about 9.4 Ohms, which combined with lead and contact resistance, is right for the low-range center scale of 10 Ohms. The disadvantage of this circuit as opposed to the LM317 circuits is that it puts a continuous load of about 129 ma on the transformer with the probes open and about 175 when shorted. B&K would have sized their transformer for this, but it would be an additional load for other makes. I've used similar circuits without problems but it is a consideration.

I've also used the half-wave 317 circuits and they work well. I wondered about transformer loading and monitored the filament voltage when the probes were shorted. The variation was a few millivolts from open to shorted. This was with both EICO and Knight VTVMs.

RRM


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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Nov Fri 09, 2018 10:03 pm 
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RRM, thanks for that explanation of the B&K circuit. Clever to set it up with the right source impedance in place of the usual 9.1 ohm resistor.

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 Post subject: Re: Heath VTVM IM-13 resto
PostPosted: Nov Fri 09, 2018 10:56 pm 
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Is there any reason why the battery eliminator couldn’t be taken off the B+ supply? Through a diode/resistive divider feeding the regulator chip. Negligible load on anything that way just a few extra parts?

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