Need Schematic for a Precision battery VTVM

Discussion in 'The Projects Forum' started by asimov, Dec 24, 2007.

  1. asimov

    Thread Starter New Member

    Jun 25, 2007
    I am working on trying to restore an old, possibly 1956, VTVM. It is a Precision model 78 made by Precision Apparatus Co., It is battery powered, not A.C.

    It appears to take 2 "D" cells and than another type battery of which I have no idea. It has 1 tube, 1U4.

    I got the meter on ebay and looks complete. Has all probes. It's very clean inside and out and would like to put it back into use.

    A schematic, manual, anything at this point would be useful.

    I have tried I also emailed a few online vintage manual sources. No luck. One response was that it is a rare unit and would be extremely lucky to find any info.
  2. SgtWookie


    Jul 17, 2007
    You can find some data on the 1U4 tube here:

    I suggest that you simply leave the VTVM as it is, and use a modern device instead. You can get cheap DVM's that are pretty accurate just about anywhere for under 10 bucks.

    If your Model 78 is in as good condition as you say, it belongs in a museum. Handling such a delicate instrument would not be good for it. The acids in your finger oils will eat away at the test leads and other components, making them hard and brittle. To preserve them, you could wash them in a mild soap and water solution, dry them, and place them in a sealed plastic bag.

    All of the resistors and capacitors would need to be replaced. After a half-century, such devices would not likely be anywhere near their original values. However, that would ruin it as a museum piece.

    It's unlikely that you'll be able to find one of your "mystery batteries" in working condition; I haven't seen one in years. Seems to me they used to make a variety of high voltage carbon-zinc batteries for use in tube equipment to supply voltage to the plate; available voltages ran from the low 20's up to 90+ or so. But to use the wrong voltage battery would either give you inaccurate readings, or more likely destroy the 1U4 tube due to overcurrent.

    A quick glance at the datasheet tells me that your 1U4 tube uses a plate voltage of 90. You could get that by stringing 10 9V batteries together ... but doing so without verifying every part of the circuit could destroy your 1U4 tube.

    A little story about antique resistors:
    In the 1970's, I built a number of Sparrow missile simulators for use on my squadron's F-4J Phantom II's. I had a number of mil spec 680 Ohm carbon resistors left over from this project, which were in hermetically sealed aluminized plastic bags. One would assume that mil spec carbon resistors in sealed packages like that would remain good practically indefinitely. I was surprised to discover a year or so ago that after 30-some years, they measured between 400 Ohms and 1.4K Ohms when I opened up a couple of them; not even close to their original specifications.
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    That other battery would probably be a radio "B" battery, 45 volts. They were hard to find 15 years ago. Even with one in hand, there is no great possibility of the VOM still functioning. Put it on display.
  4. mrmeval

    Distinguished Member

    Jun 30, 2006

    The D battery is commonly available

    The naysayers may not like it but I say return it to working condition.

    First though you'll need to learn how to read the parts. There have been some changes so look online for how to identify old parts.

    Then you'll have to draw your own schematic, it's not too hard and I've done it a few times for old radios. You may need it to figure out what the value of some of the parts are.

    Then you'll have to either find working old parts which is possible for some parts, new parts that are fully compatible or fabricate a part to replicate the function of the bad part.

    I'd suggest finding a ham radio forum and asking the fellows there for help.

    For resistors and capacitors you should be have no problem finding new replacements if you can read the value or determine the value using your schematic. Simple coils can be recreated if you can determine the value.

    Have fun and be safe. I restored a Philco Predicta television using that method up to the point I was stumped and then I found a schematic which gave me the information I needed on a coil. I had to recreate the coil. On a Zenith radio it was much worse off but I had a schematic and none of the unobtainable and difficult to recreate parts were damaged.

    Failing all of that if the meter still works you can feed a the extremely tiny voltage it needs which represents some real world data such as room temperature. Not a trivial task but learning how to do it without destroying the meter will teach you a great deal about electronics.
  5. SgtWookie


    Jul 17, 2007
    Happened to stumble across a photo of old "B"-type radio batteries; thought you'd like to see what they looked like. As you can read, the type 479's were 90 volts.
  6. SgtWookie


    Jul 17, 2007
  7. asimov

    Thread Starter New Member

    Jun 25, 2007
    Thank you for the Battery Pix. The 479 does look like the right fit.

    I enjoy working on old tube radios. I am restoring to working condition a Hallicrafters S-38C. I like to use the old test equipment on these. I have plenty of modern test equipment I could use but I rather use my Simpson 260 series 3 or my SICO 670-A meter.

    The new digital meters can be a hinderance when looking at old readings on the data sheets and then compareing than to modern, high impedepance, meter readings.

    I've already started drawing a schematic out, would just have been simplier if I could have found one online.

    At some point I will hook up my power supply and based on the tube data, will slowly bring up the voltage to the approximate rating.

    Thanks thus far for all the support.
  8. SgtWookie


    Jul 17, 2007
    Gee, I wish you'd said that beforehand. Now I see your logic ;)

    Once you get it working, you might consider duplicating the Wheatstone bridge so when the restored meter eventually goes kaput, you can duplicate the functionality using a modern DVM - or at least chart the readings between the two meters.

    I love the Simpson 260 as well ;) Great old meters. Back in the 70's, 260 was not only the model but the price as well! :eek:

    The antiqueradio link I posted earlier also has a link to a battery substitution schematic, but I don't recommend using that particular schematic. When certain components fail, you and your equipment will be exposed to line voltage.

    I suggest that instead, you use the ten 9v batteries as I previously mentioned (which is also mentioned on that site) or else get a 120 to 18VAC transformer and build a voltage quadrupling circuit using 1N4004 (or better) diodes and 100V (or better) caps. Theoretical output would be close to 97V, but you can reduce that by using non-polarized caps in series with the transformer output; preferable to using resistors in this case.

    Since the plate current of the 1U4 tube is 0.1mA, you could simulate that load by using a 0.9M resistor.

    Speaking of caps - have a look at this page:
    Although the gent is talking mainly about capacitor response in audio circuitry, you'll probably find that your caps are oil & paper construction. You would want to use a modern equivalent to obtain a similar response curve; otherwise your meter's readings will be off by a considerable margin.
    In any event, you'll find it informative in your radio restoration efforts.