An Improved VTVM Battery Eliminator

One of the disheartening things about buying an older (…well, they’re ALL older, now…) VTVM is opening it up and finding either an old, leaky battery, or evidence of one. This usually damages or destroys the battery contacts and sometimes also the circuit board. The battery, usually a standard 1.5V C-cell, is necessary for measuring resistances with the ‘ohms’ scale.

A great improvement has been suggested over the past couple of years to replace the battery entirely, using a modern voltage regulator, drawing power from the filament circuit. A version of this has been available on several Heathkit lists and I’ve used it in my V-7A and IM-18. This replaces the battery and removes the risk of leakage.

More recently, Peter Bertini, Pop Comm Magazine (and others) have pointed out that the circuit used an inefficient half-wave rectifier, probably adding stress to the (already old) VTVM’s transformer.

If, in ‘ohms’ mode, the probes are touched together (0.0 ohms resistance), the entire 1.55 Volts is placed across a 9.1Ω resistor, resulting in the maximum current draw of (I=V/R), or 1.55/9.1 = 170ma. This doesn’t count diode loss and heat loss in the voltage regulator.

The following circuit attempts to repair the inefficiencies and addresses ripple filtering in the regulator, so as to provide an efficient and accurate VTVM Battery Eliminator.

An Improved VTVM Battery Eliminator

The input is from the 6.3VAC filament circuit. The output goes to the same locations as the original dry-cell battery.

In the case of a Heathkit V-7A, is grounded, with +1.55VDC at the ‘free end’ of the 9.1Ω resistor (see a schematic at http://www.heathkit.nu/heathkit_nu_V-7A.html).

VTVM Battery Eliminator, Grounded
VTVM Battery Eliminator, Grounded

Overcurrent Issues

The transformers in these VTVM’s are barely capable of lighting the two vacuum tubes. Is it possible that the addition of a ‘battery eliminator’ for the Ohms measurement could cause a problem?

While converting a Knight-Kit KG-620 for battery-free operation, I was able to measure the current draw. Instead of an LM317 (1.5A) regulator, I substituted an LM317L – 100mz, current-limited regulator.

Idle current is measured around 5 ma and peak current at around 95 ma with shorted terminals, and range switch to Rx1. Current limiting would probably curtail measurements on the low-end (below 100 ohms).

A second way to address current draw is as follows: the KG-620 has a Type #47 pilot lamp, drawing 150ma. Replacing this with a LED will reduce the lamp current by 10x from 150ma to around 15ma. This effectively ‘recovers’ any current used by the battery eliminator.

This would also work for the Heathkit V-7A and IM-18 (and variants) as the pilot lamp is also a 6.3V, Type #47 lamp across the filament supply. Added benefits: a) reduced heat, b) reduced transformer load, c) never have to replace the pilot lamp again.

 

References

  1. Peter Bertini, “Fixing Up A Vintage Heath IM-13 VTVM”, Popular Communications, March 2010.
  2. AntiqueRadios.com, forums – Search “VTVM battery draw…”
  3. LM117/LM317A/LM317 3-Terminal Adjustable Regulator (Datasheet), National Semiconductor
  4. 3-Terminal Regulator is Adjustable (Application Note 181), National Semiconductor, Figure 2. Adjustable Regulator with Improved Ripple Rejection.
  5. The V.T.V.M.: How it Works, How to Use it , Rhys Samuel, Gernsback Library 1956.
  6. Servicing Radio and Television with a Vacuum-Tube Voltmeter, an excellent, 1951 document from Sylvania Electric Products.
  7. AD5X’s method of turning Type 47 Lamps into LED Lamps, saving heat & current

5 thoughts on “An Improved VTVM Battery Eliminator”

  1. Mike,
    Is this the best way to bypass the battery? I just got an old vtvm I’m-18….yep the battery was older than dirt, white corrosion was everywhere.
    I am learning electronics on the fly and want to repair this machine, any help would be appreciated.

    Terry
    73, kk6dqc

    1. Once you clean the corrosion out, you don’t need the battery at all (unless you’d LIKE to check resistance on the ‘Ohms’ setting).

      There’s some discussion on various spots about whether the transformer can ‘handle’ the extra milliamps the little regulator draws. I’ve found that replacing the battery with this little circuit (and I use an old plastic film can to put it in) you can test everything on the resistance range except you probably shouldn’t test like ‘1 ohm’ on the lowest range. Probably not up to around 100 ohms on the lowest range. That’s the point where it pulls the MOST current.

      Or, you can put a fresh battery in it!
      I’ve found RayOVacs will leak shortly after they’re put in service. Hate ’em. But Copper-tops (Duracel) and Eveready’s seem to resist leaking better.

  2. I built the circuit as described and tried it in my VoltOhmyst.. It really struggles with the higher current requirements of low-ohms measurements. I haven’t actually put a meter on the power requirements but the readings are not very accurate with the eliminator while it was pretty much right no with a battery before I began tinkering. The with Senior VoltOhmyst, I realized I have a decent amount of space in the upper rear of the cabinet so I am now systematically disassembling wall warts and will soon setter on one (I have it narrowed down to about 3 different ones). I hope to put a shielded box (like a Bud Box) not far from the battery eliminator board and run a couple of wires up from the standard AC supply. When I restored the meter I added a fuse and made sure the correct side of the line went to the on/off switch. I plan to jumper off that to put my wall wart guts. This obviously wouldn’t work in smaller chassis with less space available but I have the space available (I think). I suppose one could also add a 3.5mm jack and use the wall wart externally but AC outlets in my lab are sometimes hard to come by.

    1. Yep – it might not serve when measuring 20 ohm resistors and such.
      Taking, say 20 ohms for a ‘test’, the current is I=V/R or 1.55 / 20 for basically 77.5 milliamps, which might be overpowering the transformer.
      The transformers on these are marginal for the existing load (two tube filaments).
      I see looking at the Senior’s schematic the ‘pilot lamp’ is on the primary side, so there’s nothing to save on the secondary side. On the Knightkit VTVM, they use a 6.3V pilot lamp – well, converting this to LED saves considerable current, about 150 ma, so low-value resistors are no problem.
      The Heathkits are similar to the Senior VoltOhmist, so – no lowest range is convenient.

      1. Yes, when I first went through the RCA I was pondering replacement of the jewel light bulb but opted to keep it original, at least for now. Next time it goes bad I’ll ponder redoing it for LED. But you’re right, no power savings there. I have another VoltOhmyst that the transformer had gone bad in (someone didn’t recap it in time, filter cap shorted and burned the transformer 🙁 Anyway, I’m on the lookout for a more heavy duty transformer.. something that had several tubes in it such that the 6.3 winding would be a bit more robust. I’m thinking something like an old oscilloscope or something. Of course size is then the most likely limiting factor.

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