Heathkit v7a vtvm battery

There aren't satisfactory zeners with voltages under 5V, so a bandgap reference is more usual. Rectifier, capacitor, and an LM317 regulator (which uses a bandgap reference and handles 1.5V output) will do just fine.

The accuracy of a Zener is not good enough for your application. What you need is a very low noise, high stability, low source resistance precision power supply to replace the 1.5 V Battery. I would use a 1.5 V battery, and change it once every year.

Jerry G.

There is an absoute reason for never ever using ziner references in ohms measurement.

The voltage must be stable and totally free of ripple or other electronic noise. There s no ziner driven source that can do this cheaply.

Jist install a quality battery they last the normal shelf life of the battery and are cheap. Nix on using ziners.

Gnack

I performed similar surgery on an old Knight kit VTVM years ago, and it worked great. Actually, I used a diode/transistor circuit to make a simple 1.5V regulator. No more dead batteries at the exact moment you don't have any spares and all the stores are closed. Just make sure that the ohmmeter circuit doesn't need to be isolated from the heater circuit. If the heater(s) isn't tied into the tube's DC circuit somehow, it should be fine.

If you want to go the Zener route, just make sure that you do the math and get a Zener that can handle the current (assuming that you can find a 1.5V Zener). Put a new battery into the unit and measure the current from it when on the lowest ohms range, with leads open and shorted. Use the highest current that you measured to calculate the needed Zener current and wattage. Remember that the Zener will have to absorb the full current when no current is being drawn by the ohmmeter circuit if it's not switched in with the meter's function switch.

Here's a quickie regulator that should give you something close to 1.5VDC.

View in a fixed-width font such as Courier.

+---+ 1N4001 TIP29 | | +-----++ C E 1.5VDC | +---+ |+---+-----+-----\\ /---------+-------o | | +-----++ | | \\ / | 6.3VAC| |+ +++ --+-- | | | -+- | | B | | | | 50uF -+- | | +-----+ | | +---+ | | |1k | | |+ | | | -+- +++ | | -+- +---+ | Gnd | | | -+- -+- +--------+ | | 10uF Gnd | |+ | | -+- | +++ -+- | | | | 10uF | 1N4001 | | | | +-+ | -+- +++ | Gnd | | +-+ | | | -+- 1N4001 | | Gnd +-+ +++ | -+- Gnd

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Dave M
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On Sat, 29 Nov 2008 16:15:50 -0500, "DaveM" put finger to keyboard and composed:

AISI, that will give you an output of only 0.6V, ie two diode drops minus the Vbe junction voltage. Replacing one of the 1N4001 diodes with a red LED should get you closer to the mark.

- Franc Zabkar

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Thanks to all for the guidance and amazing ASCII art! This is high on my list of low-budget winter projects now that leaf-raking season endeth.

I should admit that it had been a VERY long time since I had even seen my first Heathkit. I opened it expecting to see serious damage from battery acid. Happily, the "Radio Shack New Formula Steel Clad, Custom MFD in Japan" was intact (although totally dead). It had been in there for at least ten years; maybe twice that. My vtvm serves my needs but my old friend deserves some attention.

Bryce

Thanks for catching that error, Frank. I was going to copy & paste two more diodes but the phone rang and I forgot. I hit the send button while the phone was hung out of my ear. Never try to multitask while one of the tasks is talking on the phone.. never works!!!

Using an LED will work too... thanks for the suggestion.

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Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
address)

Life is like a roll of toilet paper; the closer it gets to the end, the faster 
it goes.

Must be more to it than that - the voltage from a dry cell varies by a large amount over its life, hence the set zero facility.

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I found the Heathkit assembly manual, which includes scrawled notes by the builder to the distant future and a schematic.

Resistance measurements were made by measuring the voltage at the probe which is fed through a precision resistor string from the battery. The lowest range resistor is 9.1 ohms, so the battery must supply as much as 165 ma. Heath does not sense battery terminal voltage directly, so internal resistance of the source must be much smaller than 9 ohms to maintain calibration. Looks like a good candidate for voltage regulator.

My archives also yielded a 1957 Radio Amateur's Handbook. I probably oughta clean the basement. It lists a page of germanium crystal diodes, a couple of were rated at 300 ma. Also a page of (mostly PNP) transistors. So the stuff needed to leave out the battery existed in 1957.

I'm going to call Heathkit and ask them why they didn't do that.

Bryce

On Sat, 29 Nov 2008 11:12:45 -0500, Bryce put finger to keyboard and composed:

I was curious, so I had a look at the manual:

Here is the circuit:

It seems that you first set the FSD by selecting your particular ohms range (with no resistor connected) and tweaking the OHMS ADJUST pot. The meter should then reflect the OC voltage of the 1.5V battery. If you then measure a resistor whose value is equal to the internal range setting resistance, the meter reading should be exactly mid scale. However, the lowest range setting resistor is 9.1 ohms (instead of

10R) which suggests to me that the designer may have allowed 0.9 ohms for the ESR of a typical battery of that time. If you were to now use a precision 1.5V supply with an ESR of zero ohms, then your scale would read 10 ohms for a 9.1 ohm resistor.

- Franc Zabkar

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Half-scale is 10 ohms and I agree with your analysis. I have always thought of an aging battery as having a larger than usual internal resistance; i.e., open circuit voltage may still be about nominal but the output voltages collapses under load. Doesn't that mean this sort of ohmmeter is inherently inaccurate without a "nominal" battery regardless of the ohms adjust setting? Not a big error for an analog readout though.

Bryce

On Mon, 01 Dec 2008 16:01:58 -0500, Bryce put finger to keyboard and composed:

I've never thought about multimeters much until your post, but it does seem to me that anything that relies on an unregulated battery supply cannot possibly lay claims to any sort of precision.

In your case I would try a precision voltage reference for the 1.5V supply and a 10R 1% resistor in the lowest ohms range.

- Franc Zabkar

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