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Old 2nd Nov 2019, 1:17 am   #21
usradcoll1
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Default Re: Heathkit IM-28 VTVM

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Originally Posted by WaveyDipole View Post
The meter is marked "BENTON HARBOR MICH." and there is no "U", just "Heathkit IM-28", so it would seem to be the US model. I have already PM'ed the member regarding the USA plug. I also noticed that the schematic shows a mains selector and fuse, but this example does not seem to have either. The mains cable is soldered directly on to a tagboard which connects the transformer and elsewhere. The black connector that joins the two windings in series is evidently not original so its seems to have been adapted at some point.

Thanks for the heads up on the 6AL5 and you mention of the EB91 as an alternative. Both the 6AL5 and the 12AU7A are genuine Mullard and look in pristine condition. The only thing I have been able to determine for now is that the heaters look OK. In any case, I will be very careful, although it is good to have a backup option of the EB91 just in case!

BTW, the seller was aware of the US heritage of the instrument because of the Nema5 plug, but was also unsure as to whether it would run on 240v.
Maybe the thing came from the Philipines, where the mains voltage was changed to 240 volts, but they're still using the US type 120 volt wiring devices. Our wiring methods were used because of the strong US presence there. No exactly the best way to do it.
This was stated in the various websites regarding the US electrical practices.
Dave, USradcoll1, opinionated as usual.
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Old 2nd Nov 2019, 2:32 am   #22
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Default Re: Heathkit IM-28 VTVM

Phillipines is 220v 60Hz
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Old 2nd Nov 2019, 11:31 am   #23
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Default Re: Heathkit IM-28 VTVM

New tag strip is now in. I found one in my parts bin so this seemed the safest option.

I notice there is a quite bit of zeroing adjustment required when the range is switched particularly between the lowest three ranges. Switching between the top three ranges does not seem to be a problem. Is this normal?
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Old 2nd Nov 2019, 5:34 pm   #24
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Default Re: Heathkit IM-28 VTVM

I realised that since this is a VTVM, it will have a high input impedance so might be prone to picking up residual noise like a DMM. A little more exploration seems to confirm this. Shorting the probes in the DC setting I get a steady zero on all ranges, except the 5v range which produces a very slight reading of a minor mark or so. One thing I noted was that the the probe input wire was originally routed almost across the back of the neon and over the tag board in between the mains connections to the neon. The insulation had slightly chaffed against the adjacent tag. I replaced the wire and re-routed it underneath the tag board away from the neon and in close proximity to the chassis. This seemed to reduce the 'noise' a little, but there is still around 100ma of noise on the 1.5v DC range. My Fluke 187 picks up around 60ma AC noise, but no DC noise.

Further probing the with oscilloscope does show that this is mains bourne noise, which is present even with the unit turned off, so long as it is plugged into the mains socket! Unplug from the socket and the noise is gone. A 0.047 or 0.01 cap across the input seems to eliminate the problem, but I'm not sure how that is likely to affect the AC readings? I would have thought that the 0.005 grid to ground cap should be eliminating parasitic noise?
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Old 3rd Nov 2019, 12:44 pm   #25
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Default Re: Heathkit IM-28 VTVM

I think I am going to need a little help with this as I have come to the conclusion that it is not possible to follow the calibration steps as stated in the Heathkit manual. I have made no changes to the circuit, only replaced paper capacitors and some resistors that were out of tolerance. I am now trying to follow the procedure in the Test and Calibration section.

I can complete the Zero Adjust step with probe removed as described. Connecting the probe has minimal effect on zeroing and I can then proceed to complete the DC Calibrate step. However, when I start taking readings on other scales, the readings are well adrift. For example, 5v measures 4.1v!

I have been watching this video from Mr Carlson's lab:

https://uk.video.search.yahoo.com/yh...c4&action=view

In it he performs a repair and then a calibration (1h16min mark) of a very similar Knight KG-625 VTVM. Here, the zeroing step is performed with the probe ends shorted. There seems to be no mention of this in the Heathkit manual.

I tried shorting the probe ends to zero and then the readings did then make sense and are pretty accurate which is encouraging, however, he did not seem to get the zeroing variations when switching between ranges that I get on the Heathkit. Some minor tweaks were still required, but not adjustments along the line of 1 or more major divisions as I am sometimes experiencing. For the present, this has me a bit stumped.

Incidentally, in the video he also shows how to adjust the VTVM to dial out the probe resistance. I tried that and found the resistance readings were quite accurate as well.

I have now also cleaned the contacts inside the probe and checked the resistor. It measures around 1.2 MΩ, or 20% adrift from the 1 MΩ that it is supposed to be, so that will be replaced.
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Old 3rd Nov 2019, 4:51 pm   #26
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Default Re: Heathkit IM-28 VTVM

Sanity at last!

I had replaced 2 of the 22MΩ resistors that were obviously adrift of their proper value, but I had left the remaining ones that were "about right" in circuit. However, I decided that since I had 8 left, I might as well complete the job and replace the 4 remaining ones. When the resistors were removed they were found to be all over the place, ranging from 30MΩ down to 19.8MΩ.

At this point another undetected fault surfaced. There was a dodgy connection to the alligator clip on the probe. This had not been obvious until the clip just dropped off! The strands did not appear to be frayed, but had become loose in the solder and came adrift. The solder blob protruding from the inside of the crimping point looked solid, but the problem was hidden inside the barrel of the crimping point itself, which had never been crimped. I re-worked the joint and lightly crimped the clip onto the lead.

Upon reassembly, things started looking much better and there was far less variation when zeroing and switching between ranges. There is still a calibration problem between the 1.5v range and the other ranges. If I set calibration on the 1.5v range as described, then the other ranges (5v, 15v, 50v) read on the low side. If I set the calibration on the 5v range, then the upper ranges are fine but the 1.5v range is inaccurate and reads high. Other than that, things are looking much better.
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Old 5th Nov 2019, 6:45 pm   #27
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Default Re: Heathkit IM-28 VTVM

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Phillipines is 220v 60Hz
I'm well aware of the fact!
I don't know when it was changed, but the way it was implemented was very poor. When South Korea changed, all the wiring devices were changed as well.
Dave, retired US electrician as well as a radio collector.
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Old 9th Nov 2019, 5:19 pm   #28
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Default Re: Heathkit IM-28 VTVM

I was feeling a little better today (have been quite poorly for a few days) so I had another look at this Heathkit VTVM with a view to tracing that 100ma or so "phantom voltage" that I mentioned in #24 and #25.

The voltage at the grids of the 12AU7 was measured and pin 2 was found to have approximately -0.1vDC when the meter was zeroed. This decreased to around 0.05vDC on the 5v range but was almost insignificant on the higher ranges. This negative voltage tracked all the way back to the probe input. This, then, was the reason why such a big adjustment was required on the 1.5v scale. The 12AU7 valve was removed and when the unit was turned on again, the negative voltage was still present on pin 2. The rectifier valve was also removed. Still the same. Since the probe was not plugged in and the circuit consisted of a chain of passive components only, what was the source of this voltage?

The input wire was disconnected at point no. 6, just before the grid series resistor and filter capacitor on pin 2. The voltage then disappeared, but here's the weird thing: it wasn't present on the input wire either! The input wire was then re-soldered to the tag and pin 2 of the valve base still read zero volts. Both valves were replaced and the meter zeroed, and still read pretty much zero volts.

Could the tag board or valve base have become slightly conductive? Might there have been some piece of dirt or hair or fibre or something that got dislodged during the de-soldering of the wire? It seems unlikely that re-flowing the joint will have fixed the problem as the 3.3MΩ resistor which was also connected to that tag had been replaced so the joint had already been re-flowed. Whatever the case, after re-assembly when the meter is zeroed, there is now no significant deviation of the pointer between ranges and only a very minor correction is occasionally required, which is as it should be.

As previously mentioned, there is still some variation of accuracy between ranges, but this is probably down to the ageing of the precision resistors in the chain which have drifted somewhat from their expected value. Given that there is no additional per range adjustment, I imagine this is to be expected. Given the unusual values, at some point I may do a few calculations and see whether I can match the values that have drifted the most using standard resistance values.

Last edited by WaveyDipole; 9th Nov 2019 at 5:29 pm.
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Old 9th Nov 2019, 6:44 pm   #29
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Default Re: Heathkit IM-28 VTVM

Glad to read your making some progress with this, sounds like its been giving you the run around!
I still have an HP 3400A that i want to fix up and use, that's one project i would like to try and get done.
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Old 11th Nov 2019, 12:24 am   #30
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Default Re: Heathkit IM-28 VTVM

The link is a good tech info resource.
http://www.angelfire.com/electronic/...ubes/VTVM.html

I have a B&K 'dynamatic' 375 that I'm part way through restoring. It uses Welwyn panclimatic 1% resistors for the divider, and the largest 3 (7M, 2M, 700k) are now about 2-3% high - perhaps due to the level of voltage applied to those particular resistors, but also possibly due to their construction (as all the lower valued panclimatics are still spot-on). But my hassle has been an open-circuit HT secondary, so to jump that hurdle I've got a small pcb ebay isolated dc/dc module set up to generate 140V at circa 2mA which is powered off the 1A heater winding. I was also not going to fit a 1.5V battery (due to some internal rust from past leakage and finding a dead battery soldered in place), and use another dc/dc module to run the ohmmeter section. I did notice the input AC range cap is rated at 400V, but the device has a 1500V capability - the cap at least gets a clean bill of health for leakage with a 500Vdc IR test.
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Old 11th Nov 2019, 6:23 pm   #31
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Default Re: Heathkit IM-28 VTVM

Thanks for that link. The Heathkit in that image has an almost identical scale on the meter to mine.

Perhaps that cap was replaced by someone in the past who didn't take the trouble to use one of the same rating? The AC input cap in this Heathkit is rated at 1600vDC. The specifications in the Heathkit manual say that this Heathlit IM-28 VTVM is rated to 1500vAC RMS and up to 30,000vDC with accessory probe, although I'm not sure I would dare put that probe anywhere near 30kV! However, there is a 1MΩ resistor in series when switched to AC mode, so if the cap is original then perhaps the designers felt that was sufficient to allow a lower voltage rated cap to be used. Of course if the probe is left switched to DC the resistor is bypassed so that might become a problem....
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Old 15th Nov 2019, 1:03 pm   #32
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Default Re: Heathkit IM-28 VTVM

I think that when Heathkit say the meter can measure up to 30kV with accessory probe, they do not mean the standard probe that came with the instrument.

The 1M resistor in the probe tip is for DC measurements, not AC.

The blocking capacitor for AC ranges would have been rated at 1600V to allow the measurement of AC signals superimposed on DC voltages up to 1600V.

Hope this helps.

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Old 15th Nov 2019, 3:04 pm   #33
WaveyDipole
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Default Re: Heathkit IM-28 VTVM

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I think that when Heathkit say the meter can measure up to 30kV with accessory probe, they do not mean the standard probe that came with the instrument.
Agreed, and thanks for making that clear. The supplied standard probe is ABSOLUTELY NOT suitable for measuring voltages in the multiple kV range. Probably OK for up to 1kV or so.

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Originally Posted by stuarth View Post
The 1M resistor in the probe tip is for DC measurements, not AC.
Yes, I got this wrong, for some reason thinking it was switched IN for AC measurements. Since the AC setting is also shared with the resistance measurement function, it would make sense that the resistor would be switched OUT of circuit on this setting.

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Originally Posted by stuarth View Post
The blocking capacitor for AC ranges would have been rated at 1600V to allow the measurement of AC signals superimposed on DC voltages up to 1600V.

Hope this helps.
Thank you. That makes sense especially since the capacitor is rated at 1600vDC NOT AC. I'm not quite sure how the safe AC handling capacity is worked out, but 1600v p2p equates to around 565vAC RMS, while 400v p2p AC would equate to around 140vAC RMS, so it seems to me that the max safe AC handling capability for a 1600vDC cap would be no more than around 560vAC, and for a 400vDC cap would be around 140vAC RMS. However, I'm not sure whether there are other factors to consider such as the dielectric type perhaps? Still, one definitely must not assume that a cap rated for a certain DC voltage can handle a similar AC voltage.

Last edited by WaveyDipole; 15th Nov 2019 at 3:09 pm.
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Old 15th Nov 2019, 4:22 pm   #34
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Default Re: Heathkit IM-28 VTVM

The impedance of the input blocking capacitor at the lowest frequencies the instrument is rated for will be much less than the impedance of the input attenuator resistors. Hence if you are measuring AC voltages, most of the voltage will appear across the resistors, and very little across the capacitor. You don’t need a capacitor with a high AC voltage rating.

It’s not the same case where a filter capacitor is connected directly across the mains. A mains filter capacitor, unlike your input blocking capacitor, will have high AC voltages across it, and must have a high AC voltage rating.

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Old 26th Nov 2019, 3:40 pm   #35
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Default Re: Heathkit IM-28 VTVM

Once again, thank you for the clarification.
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