The Viz WD-750A Voltohmyst Digital Multimeter

Viz is the test and measurement division of RCA.

This is a fairly old (late 1970s/early 1980s), LED 3½ ("1999") digit portable LED button-select non-autorange multimeter. It's a standalone unit - there are no connections on the back other than for power input (uses a 3 circular prong oval connector) and a slide switch to change between 115V or 230V. The case is made completely of ferrous metal, probably steel. I wouldn't say it's built like a tank however, but it's something most of my other meters do not have. A flip up stand is part of the front legs to keep the meter up and at an angle. Four screws along the sides towards the bottom loosens the top cover.

It was meant to be a portable with its internal NiCd batteries in a battery holder, I measured around 400mA current draw. When on battery power, just don't forget to turn it off before the batteries get depleted - that will kill the batteries. When the multiplexed refresh LEDs stop refreshing (meaning, only one of the four 7-segment displays stay on, usually brigher than normal), it's time to plug in right away and recharge.

When plugged in, it will charge the internal batteries. It has a charging current of around 110mA (though less when multimeter is being used), so with 1.2Ah cells, it will take around 15 hours to fully charge. Since the current is fairly low compared to capacity, it's OK to let it keep charging, but yes it will 'overcharge' them. A #47 lamp 6V lamp is the current limiting device for charging, and you can see it lit through the analog meter in the front.

The analog meter seems to be a kind of indication how hard something is being driven, so that despite the meter appears to be digital, there is an analog meter that one can look at to see when the measured voltage is fluctuating.
I finally ended up figuring out what the original intent of the little analog meter. This analog meter was to emulate what old VTVMs could do - you could center the current voltage in the middle of the meter using the zeroing knob. On the WD-750A, the "Analog Ref Set" knob on the front (it's actually a potentiometer) works like the zeroing pot on a VTVM specifically for the little analog meter. Now the old die hard VTVM users have no excuse for not using a DVM!

The six white buttons on the left are a radio button select to select AC voltage, AC current, DC voltage, DC current, Resistance (200Ω through 2MΩ) and a separate mode for 20MΩ resistance (the range select is ignored when using 20MΩ). The center push in/out button is independent and only selects the voltage used to measure resistance, specifically for whether or not it can turn on semiconductor junctions. The five white buttons to the right is radio buttons for range selection, from 0.2 through 2000 (5 ranges). Apparently the meter's isolation insulation is probably not good enough for 2000V so it writes no more than 1200V AC.

Power Source115/230VAC switch selectable
Internal "not easy to replace" 4 x C-sized NiCd, approx 3 hours
TypeIntegrating, Intersil PMOS LD110/LD111 based
Input Impedence10MΩ in 20V range
Display3½ digit LED multiplexed
common anode, driven by 7446 (wish I had a 74246, like that "font" better)
approx 5 readings per second (a lot faster than my B+K 2833)
AC/DC V ranges200mV, 2V, 20V, 200V, 1000V
AC/DC A ranges200µA, 2mA, 20mA, 200mA, 2000mA
Resistance200Ω, 2KΩ, 20KΩ, 200KΩ, 2MΩ, 20MΩ - Also Ω/LPΩ selectable

I hacked it internally a bit (but easily revertable) to use NiMH AA cells since I didn't have NiCd C's on hand, by removing the 4C holder and using an 4AA holder. Harbor Freight Tools occasionally has two C sized NiCd cells (1500mAh) for $4 on sale ($8 for a full replacement). However I had the AA's and a holder from a trashed 286 computer, didn't have any other use for them anyway. Besides, Harbor Freight sells a pocket multimeter for $3 on sale too (or less, sometimes free! PN 90899), granted it doesn't have LED display or have low voltage AC modes, but it can fit in a pocket, longer battery life (usually 200 hours or so from an ICL7106 based meter with PP3 battery), can measure up to 10A DC, measure hFE, has diode check, ... times change... Granted, it doesn't have a display that can be read in the dark and can't be plugged in and continuously monitor voltage, nor can it measure AC current.

I also added a fuseholder for the current fuse after burning out that fuse. The current mode fuse was originally solderred from one of the current measurement binding post to the board. BE CAREFUL: Due to the lip on the top half of the case, it's easy to overlook that your probes are connected to the current flow posts versus the voltage/resistance mode posts. ZAP! There goes the fuse... Of course if you work with it every day, you'll get used to the fact left is for voltage, right is for current/milliamperes... Or near the white binding post is for voltage...

Eh. Not sure that's always easy to remember. Maybe that's why Viz doesn't make meters anymore. My Tenma 3½ and my Mastech 3¾ meters will beep if I set the meter to voltage with the probes connected to the current measurement ports. Granted, the 'cheap' meters I have do not beep either, but they are a bit easier to tell when they're in the wrong mode... since the mode selector knob is right next to the probe inputs! My Fluke, however, is an abomination - the current port is a bit further away from the selector.

Also over the years, the switches on my Viz WD750A have become dirty and sometimes don't connect properly leading to improper behavior. Its buttons are sealed on the inside and getting cleaning fluid in would make a mess.I just found that the switches are opened in the back of the switches and getting some contact cleaner in there was possible. I put some contact cleaner in there and the switches work a bit better now.