Nordmende DiVo 3362 Digital Voltmeter

This German DVM by Nordmende has a quite unique design. It uses a mix of TTL and PMOS circuitry for the digital parts (control, counter, etc). It has a dual-slope converter and should be very accurate.

A the front, it has four range pushbuttons and four function pushbuttons:

• 1, 10, 100 and 1000
• V, Ω and mA
• and an “AC/DC” button

I bought this digital voltmeter on the quarterly NVHR radio swapmeet in 2009. A manual came with it, thin but containing enough information. I was really happy to have the circuit diagram, because it had some really unusual components and circuits.

The display consists of two Sperry Panaplex displays: a SP-351 display having 1.5 digit and + and - signs and a SP-352 display having two full digits. Panaplex displays are flat, 7-segment neon displays. Like Nixie tubes, they need a 180-200V supply voltage. The displays are driven in multiplex mode, the anode drivers each have two BF458 transistors.

The DiVo-3362 uses a number of rare IC's:

• Sperry DD-700 7-segment decoder-driver,
• SAH161 control circuit (PMOS).
• SAH171-2 counter (PMOS).
• SAH181 display scanning circuit (PMOS).

All the IC's carry production dates in 1973, so the instrument must be from that year. In the manual, the copyright notice is dated 1974.

When I switched on my DiVo, nothing happened except for the “AC” input light coming up. I switched it off and removed the top and bottom of the case to be able to measure things. Using a scope, I found the clock oscillator was working and the displays received their multiplex signals. When I looked again, I noticed the rightmost display had lit. It stubbornly kept showing “00” whatever I did with the range buttons and inputs. Apparently, the A/D converter wasn't working.

I hoped the SP-351 display on the left would wake up after a while, too, but unfortunately, it didn't. I did some in-circuit tests, but had to conclude the SP-351 display had to be replaced.

Finding Panaplex displays

I searched on the Web for SP-351 displays. I did find some data on collectors sites such as Dieter's Nixie World, but nobody had a spare one for sale. Only one Canadese site had some, but they were asking ludicrous prices. Every now and then, I did a search on Ebay and such sites. But to no avail, until end 2012. Somebody in Italy was selling some on Ebay. I decided to take the plunge and buy one. Unfortunately, something went wrong in the mail. And suddenly, there was a second Ebay seller in Italy who was offering some Panaplex displays. After 5 days, the second shipment arrived duly. I opened the package and was in for a start. It looked like the glass tube on the back of all the displays was broken. I tested them... phew, they were working really fine. a week later, the delayed first shipment finally arrived, too. So I now have spare SP-351's.

The photograph on Dieter's Nixie World inspired me to connect all segments together using aluminium foil to test all segments simultaneously and show the display in all its glory. Testing them, I learned some more things about Panaplex displays. They are sealed by some kind of cement, not by molten glass. This makes them more sensitive to leakage than a nixie tube, I presume. I also noticed that the anode, instead of a mesh as in nixies or a mask as in the Philips ZM1500, is an ultra thin transparent metal layer at the back of the front glass pane. There is a brass or bronze contact spring that connects the layer to the anode connection pin.

The SP351 datasheet also mentions a “keep alive” electrode per digit. While I was fixing the meter, I noticed these were glowing continuously. They were hidden by a black metal mask so that they were not visible. I realised that the black PVC tape that covers the lower 15% of the Panaplex displays on the display boards of the Berkel 176SG scale must have had the same function, to hide these orange dots under the digits.

Fixing the display

I started out to replace the display and removed the front plate. A black metal mask was covering both displays, apperently to hide the keep-alive electrodes and the spaces between the digits.

This mask was glued to the displays. I used a thin knife to detach the mask. It had been stuck by a piece of double-sided sticky tape. Then I was able to unsolder the faulty display using a solder pump. After removing the tin solder, I wriggled the pins in their holes to detach them from the through-plating in the holes. I also unsoldered two wires sticking out from the bottom edge of the display, connected to a narrow circuit board running along the lower edge of the displays. After I extracted the display, I slapped my forehead. Why hadn't I seen that the display was sitting in a socket? I resoldered the socket. I bent the two pins of the keep-alive electrodes and soldered a short piece of bare wire to them. These were the wires sticking out at the bottom edge. I carefully pushed the display in its socket and threaded the two wires through their holes in the narrow circuit board.

I checked everything and switched the meter on. The display worked. It was looking beautiful. Now the original 2-digit display on the right stuck out as old and worn. It was flickering a bit and it was less bright than the new display. You can see the difference in color in this picture.

I decided to also replace the right display. I scavenged one SP-352 display from a display board of the Berkel 176SG scale. This one was soldered in place, without a socket, so I had to clean the pins very carefully after unsoldering. After I replaced this one, the displays were about equal in brightness and color, although the older display had slightly fuzzier segments.

After this, I started testing the meter. No matter what I did, the display always said "000". I tested all the signals of the SAH161 control circuit and was in for a disappointment: no signals were coming out of it. So nothing was contolling the A/D converter or making the counters count. Now this IC is even harder to replace than the SP-351 display. It has (almost?) exclusively been made by ITT in Germany. A very specialised circuit, so not very widely produced or distributed. I will have to search and wait for a few more years...