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 Post subject: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Mon 26, 2020 6:12 am 
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Location: Ventura, CA 93004
Working on a Heathkit V-7A.

First problem: While unpowered, I could not mechanically set the meter to zero. The closest to zero I could get was 2V on the 50V scale. The front adjustment worked, just didn't have enough throw. I ended up removing the meter scale and moved the equivalent 'lever' on the rear of the meter to center the front 'lever' at the zero point. Put the meter back together and put it back in the case.

Second problem: Bad electrolytic. This was replaced with a 22uf 160V electrolytic. I also replaced the selenium diode with 2 1n4004's in series (I already have lots of them on hand), replaced the battery, measured all the resistors on the scaling switches (they measured close enough), then fired it up and adjusted the DC Cal, AC bal and AC cal towards the upper part of the meter scale. Both tubes measure 'good' on my emissions only TW-11 tester.

Measuring DC voltages on the lower part of the meter is quite off though. And it's not just a scaling/gain error, but varies across the meter face.

Here's an example on the 5V scale.
DMM vs V-7A VTVM
0.50V 0.7V
1.00V 1.05V
2.00V 1.65V
3.00V 2.82V
4.00V 3.95V
5.00V 4.98V

50V scale
15.0V 13.8V
20.0V 16.8V
24.0V 23.0V
28.0V 26.5V
35.0V 33.0V
40.0V 39.0V
48.0V 47.0V

AC measurements are better, but I only have a limited selection of voltages to measure
15 VAC scale
DMM vs VTVM
10.70V 10.5V
14.09V 13.6V

50VAC scale
10.7V 11V
14.09 13.5V
24.77V 24.0V


Is the linearity supposed to be this bad on the V-7A? If not, what should I be looking at to correct this problem?

Thanks for your help,
Dave


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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Mon 26, 2020 6:42 am 
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Posts: 1797
I think your problem is related to the first thing you did. The meter movement is nonlinear either due to some damage or from new. If this was a kit, all bets are off but if not, the movement has been damaged. I can't imagine such a great error passing any kind of factory inspection.

There may be some foreign matter in the works of the meter or something is bent, etc. You can verify this by substituting a different movement of similar sensitivity.


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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Mon 26, 2020 2:44 pm 
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Location: Sunbury, Ohio 43074
I agree with bob. The meter is out of adjustment or bad. Verify the meter itself before doing anything else. Here's a very simple way to do it quickly, and to the accuracy of your DVM : https://www.bbtvtestequipment.com/testing-a-panel-meter

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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Mon 26, 2020 3:25 pm 
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Joined: Jul Sat 11, 2020 6:58 am
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Location: Ventura, CA 93004
Thanks Bob and Barry. I'll test the meter and see how it performs.

It's weird how non-linear the meter is. I read some stuff about VTVM's not being linear on the lower part of the meter, but that thread had to do with measuring AC voltages, not DC. Then I found a post about the 12AU7 not being used in it's 'linear' region for a guy repairing a VTVM, but I can't find the post again to verify if was AC or DC being measured.

Regardless, I'll test the meter tonight.

Dave


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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Mon 26, 2020 3:31 pm 
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Location: Sunbury, Ohio 43074
I suppose it's possible that the meter movement was designed to be non linear, but it would be an outlier if so. Only way to know for sure today would be to compare it to a known working good one. I'd think it should be linear however. DC volts, Ohms certainly would require a linear meter movement unless there was some very strange compensation in there for those type measurements such as weird values of resistors for example. Doubtful.

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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Mon 26, 2020 4:34 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
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According to the manual, accuracy on DC was stated as +/- 3% of full scale; on AC it's +/- 5%. So 3% of 5 volts is +/- 0.15 volts and it looks like you're okay above 2 volts DC. On the 50 volt DC scale the accuracy is +/- 1.5 volts, so it's okay above 35 volts. On the AC ranges where the accuracy is +/- 0.75 volts or 2.5 volts, you are well within spec. Quite frankly, these are typical results for many two-tube bench VTVMs. That may seem alarming to somebody used to digital multimeters, but VTVMs were designed to troubleshoot radio and TV circuits that had 20% tolerance components and voltages that varied as much in practice. These VTVMs were service tools, not high accuracy lab instruments.

What changes when you switch from DC to AC? Well, for one thing you no longer have the 11-megohm input resistance of the probe and DC divider resistor chain. The input grid of the 12AU7 is "looking at" a much lower resistance. So if the tube is even the slightest bit leaky or gassy, it will tend to be nonlinear itself, especially at low input voltages, and more so on DC than on AC. This is the kind of problem most tube testers have no hope of catching; the only way to find out is to try a few different 12AU7s (preferably NOS) and keep the one that works best.

Another consequence of the high input resistance on DC, that does not exist on AC, is the possibility of "parasitic batteries" forming on connectors, PC boards. and switch wafers which may have been soaked with chemicals from batteries that leaked in the past. Such things can be really pernicious and difficult to neutralize.

You mentioned having to reposition the rear spring anchor to get the meter to zero properly. This should not have affected accuracy much. The deflection of a meter is the balance point between the force created by the current in the coil between the magnet poles, against the torque of the control springs. If the springs weaken, the meter will deflect further than it should and read high. All of your measurements are lower than expected. If the permanent magnet weakens, less force is produced and the meter will read lower than it should. However, the problem would be worse at the high end of the scale than the low end, since it takes more force to overcome the control springs at the high end of the scale.

Finally, it looks like you are comparing the voltages on the VTVM to the voltages read by a DMM--hopefully both connected to the voltage source simultaneously. How do we know the DMM is accurate? Many of them fall down on the job near the low ends of their ranges because only a few of the least significant bits in their ADCs are used. DMMs are also susceptible to giving inaccurate readings when their batteries get weak--long before the "Battery" warning symbol comes on.

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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Mon 26, 2020 4:47 pm 
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Chris108 wrote:
What changes when you switch from DC to AC? Well, for one thing you no longer have the 11-megohm input resistance of the probe and DC divider resistor chain. The input grid of the 12AU7 is "looking at" a much lower resistance. So if the tube is even the slightest bit leaky or gassy, it will tend to be nonlinear itself, especially at low input voltages, and more so on DC than on AC. This is the kind of problem most tube testers have no hope of catching; the only way to find out is to try a few different 12AU7s (preferably NOS) and keep the one that works best.

One clue to excessive grid current in the 12AU7 is that the zero shifts more than few minor scale divisions on the 0-5 scale when you switch ranges from 50 VDC down to 1.5 VDC.

How long did you burn in the VTVM before calibrating it? I find it often takes two to three days for a tube to come down in grid current after being unused for many years. I'm guessing this is from the getter scavenging whatever gas has leaked in over the years.

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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Mon 26, 2020 5:12 pm 
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Joined: Jul Sat 11, 2020 6:58 am
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Location: Ventura, CA 93004
Thanks Chris and Steve!

I did have my AC powered benchtop Keithley 175 DMM connected to the variable power supply at the same time as the V-7A. The Keithley hasn't been calibrated in ages but it still compares well with my handheld DMM's which were calibrated more recently (though still a long time ago).

The VTVM has sat around for a very long time. My dad's friend gave it to me easily over 20 years ago, and that friend was retired for a long time before I got it. The 6L5 tube looks original, the 12AU7 is a Tungsol from the 40's with the 'black glass' getter. Maybe it's bad? Just in case, I did leave the unit on overnight to see if baking the VTVM will help. That said, checking the meter with a variable power supply and 100K resistor seems easy enough that I'll give that a try tonight. If the meter tests good, I'll let the VTVM 'bake' some more while I look around for replacement tubes.

Appreciate the help!

Dave


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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Tue 27, 2020 2:52 am 
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Beware of that link above for how to test a meter. The resistor values given are for a 50mA meter, the one in a VTVM will be MUCH more sensitive so the resistor values will have to be increased considerably to avoid damaging the meter. Since you mentioned using a 100K resistor, it looks like you know this but someone else reading this thread might not. So here is the warning for them.

Another point that you may know that others might not is that the accuracy of analog meters is generally stated in percent of full scale. Chris108 used this method in his calculations without stating why. So here is how it works. You look at the full scale reading of the meter (according to it's scale) and calculate what that percentage is. Then this much error can occur anywhere within the range of the meter. So, to repeat what Chris108 did, if the full scale is 5V and the accuracy is rated at +/-3%FS (full scale), multiply 5V by 0.03 (3%) to get 0.15V. Then you add and subtract this number from any point on the scale and the meter should read between these values. Back to the example, 5V +/-0.15V means that the actual voltage being measured is between 4.85V and 5.15V. Now, on the same range, if the pointer stops at 4V, the actual voltage will be between 3.85V and 4.15V. If it says 3V the actual value will be between 2.85V and 3.15V. At 2V the actual voltage will be 1.85V to 2.15V and at 1V the actual value will be 0.85V to 1.15V.

Ohmmeters have other accuracy specs.

The accuracy of digital meters is usually specified as a percent of full scale (like above), +/- a percent of reading, +/- a certain number of counts (the variation in the reading the right most one or two digits of the display).

Since a VTVM is usually calibrated at full scale on one range, this one point should be accurate. However, the accuracy of the resistor on the range switch is usually 1% so this will add an additional error on other ranges. Presumably the 3% spec takes this into account. But if your meter is pretty close on all scales, it might be possible to tweak the calibration point slightly so that it meets spec on all scales even though none is "right on".

If the meter movement is non-linear by design or if something in the circuit affects linearity, the scale would be printed in a manner to correct for this. Most VTVMs have a separate scale for the 1.5VAC range to correct for the non-linearity of the 6AL5 tube.

Is your meter movement balanced? With power off, lay the meter on it's back and adjust the pointer to zero with the adjuster on the front. Then stand the meter up to it's normal position, the pointer should still be on zero or close to it. Now tilt the meter so that the pointer is vertical, it should still be zero or close to it. Then tilt it so the pointer is horizontal. Again, it should show zero or close. If it doesn't, the pointer is out of balance and will affect the linearity of the meter. It can probably be rebalanced but you can look that up elsewhere.

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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Tue 27, 2020 3:33 am 
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Location: Ventura, CA 93004
Hi Jim. The meter definitely doesn't maintain 0 position when it's flat, facing up. I haven't tried tilting it to the side but I'm sure it will be off too. Thank you for the explanation on accuracy - I didn't know that.

For everyone - yeah, it's the analog meter that's messed up. I drove the meter in 10uA steps and took readings from the 0-15V scale. Here's how it measured.

Attachment:
V-7A error.JPG
V-7A error.JPG [ 157.14 KiB | Viewed 395 times ]


At no time during the measurement did the meter appear to be sticking - the needle moved very smoothly.

FWIW, I do have a Heathkit IM-32 also. Actually bought that one a few weeks ago and fixed it, though I didn't check it for linearity but nothing stood out that I could remember so it must be decent. It's the reason I dug out the old V-7A to repair.

So I'll put a sticker on the V-7A to warn it's not accurate and to only use for comparative measurements - like peaking circuits. At least now I know the source, and I appreciate all your help on this!


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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Tue 27, 2020 3:44 am 
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That actually doesn’t look too bad. You should express the error as percent of actual measured full scale. (The absolute accuracy of full scale doesn’t matter, because that is compensated for when calibrating the VTVM.) when expressed as percent of full-scale, the meter error should be under 2%. I bet your meter meets this spec, or at least isn’t very far out.

If you have a function generator, it might help that bump at 70 uA to exercise the meter with a slow triangle wave for a few hours. See my thread on restoring an RCA WV-98C VTVM for details.

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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Tue 27, 2020 10:43 pm 
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I have a Heathkit IM-13 VTVM that had cheap wafer sockets, so I replaced them with molded phenolic sockets for better contact with the tube pins.


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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Wed 28, 2020 3:32 pm 
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Location: Ventura, CA 93004
Jim Mueller wrote:
Is your meter movement balanced? With power off, lay the meter on it's back and adjust the pointer to zero with the adjuster on the front. Then stand the meter up to it's normal position, the pointer should still be on zero or close to it. Now tilt the meter so that the pointer is vertical, it should still be zero or close to it. Then tilt it so the pointer is horizontal. Again, it should show zero or close. If it doesn't, the pointer is out of balance and will affect the linearity of the meter. It can probably be rebalanced but you can look that up elsewhere.


SteveByan - I did exercise the meter with a function generator for a few hours - no change.

Jim - my meter balance is definitely aggravating the problem. I re-took the data with the meter lying on it back so the needle balance doesn't come into play. There is still deviation from ideal in 40-80uA region but it's much smaller.

I'll read up on how to balance the meter, however in trying to move one of the static weights, it wouldn't budge at all. The posts look square in cross section and the winding reminds me of wire wrap where the post digs into the wire.

I'll post the graph of the meter on its back tonight.

Dave


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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Wed 28, 2020 4:05 pm 
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ai6vx wrote:
SteveByan - I did exercise the meter with a function generator for a few hours - no change.

That's too bad. It's really far out of whack. I'm afraid you're going to have to find a replacement.

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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Wed 28, 2020 4:57 pm 
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Location: Lexington, KY USA
I wonder how many old meters, or even new ones, are just as bad, but have never been checked.

The high end instrument makers were known to sometimes individually calibrate their meter movements. The scales might be slightly different on each unit. So even expensive analog movements were not always super linear.

The problem might be solved with a microcontroller. Even the mechanical imbalance might be corrected. The firmware could look at the signal level and the tilt angle, then provide a correction current to make the meter read right. This assumes that the non-linearity is consistent over time, of course.

This could work really well, and might make the old VTVM almost as accurate as a "free" meter from HF.

Not completely serious, here, but this would be a fun project, providing many hours of software enjoyment.

Ted


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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Wed 28, 2020 6:08 pm 
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I think you are trying to make a silk purse from a sows ear. The upper half of the meter reads very good and that is where you are supposed to do your measuring at. This is an entry level tester working as designed.

DM


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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Wed 28, 2020 7:10 pm 
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devilsmist wrote:
The upper half of the meter reads very good and that is where you are supposed to do your measuring at. This is an entry level tester working as designed.

The Heathkit spec for DC voltage accuracy is 3% of full scale. The range resistors are 1%, so that leaves 2% for the meter. This meter exceeds that error for most points below 11 on the 0-15 scale. I haven't worked on a V-7A, but my IM-13 meets Heathkit's 3% of FS spec over the full scale.

Attachment:
V-7A meter.png
V-7A meter.png [ 36.84 KiB | Viewed 243 times ]

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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Wed 28, 2020 7:36 pm 
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That's why HP began to calibrate meter movements individually for a number of years. It's cheaper than trying harder to make a linear magnetic field.


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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Thu 29, 2020 2:58 am 
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Back in the day, many higher end analog meters had hand drawn scales for the utmost accuracy. As mentioned, HP did this. They even invented a machine to replace the line of ladies with pen and ink. They did a very nice writeup on this machine in an old issue of The HP Journal. General Instruments and Boonton, to name a few, also made hand drawn scales.
Rick


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 Post subject: Re: Heathkit V-7A VTVM, is non-linearity really this bad?
PostPosted: Oct Thu 29, 2020 4:44 am 
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Location: Ventura, CA 93004
Here's the % FS error with the meter laying horizontally so needle imbalance is removed.

Attachment:
V-7A error Horiz.JPG
V-7A error Horiz.JPG [ 176.31 KiB | Viewed 205 times ]


Much better. Now I just have to figure out how to add some weight to the right spots. I'll either have to use solder or glue as the existing weights are immovable - or at least immovable for as much as I wanted to try and not break the meter.

I googled looking for tips on balancing meters but came up empty handed. Does anyone have any links they can forward me?

Thanks guys!

Dave


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