Simpson Model 360-2 Digital Multimeter

This meter is a good example of why I now seldom buy any vintage electronics from a certain online auction site. Advertised as being "collector quality", outside it was in decent shape but inside it was a trashed rat's nest. Some hack had miswired or disconnected the connections to the internal fuses, the external power connector and the batteries, and had completely destroyed the delicate analog meter movement. No wonder the seller made sure to say he had "forgotten" to take any pictures of the inside of the unit.

The meter is from the mid 1970's and is kind of a digitized version of the venerable Simpson 260. I like that it is auto polarity, and for resistance measurements has a low power 200 ohm range and also a 20 Megohm range. The small analog meter would have been useful for checking capacitor leakage or peaking up alignments, things a digital readout fails miserably at.
The 360 is powered by 4 NiCd C-cells or the wall wart charger, if connected. It can also run off 4 Alkaline C-cells, but the wall wart must be disconnected so it doesn't attempt to charge them. ​I salvaged 4 C-cell NiCd batteries for the meter from an old Tektronix 326 oscilloscope battery pack I replaced.

The case is in pretty good shape other than some scratches on the face and the trim of both knobs.
Internally, some of the battery contacts were ruined by leaky batteries, and the wiring to the battery compartment was completely miswired. New battery contacts were made from sheet brass to replace the contacts that couldn't be cleaned. I found a schematic online and sorted all the wiring to the batteries, charger connector and the two fuses for the low ohms and low current scales. Plus there were numerous wires broken off at the PCB from being flexed too much and several capacitors with broken leads or corrosion that needed replaced.

When first powered up the meter would zero but trying to measure anything caused it to immediately go over range. This was traced to an LM2901 quad comparator with three bad sections. It's in the upper left corner in the IC socket. I added the socket to the PCB.
​After finishing up with a good cleaning and a calibration it's all working except the small analog meter. That was totally destroyed internally and is unobtanium now days. My plan is to replace it with an LED bargraph display unless I can think up something better.
Here's the parts I replaced which includes corroded capacitors, caps with broken leads, and the bad LM2901.
In this photo the meter is connected to my signature tracer's meter output. It's measuring the breakdown voltage of a zener diode.

Replacing the Damaged Analog Meter

This is the guts of the destroyed meter movement. It's made specifically for the 360-2, so not much chance of finding another one just laying around. Whoever hacked it up obviously didn't have a clue what they were doing.
I don't want to physically modify the meter, so whatever replacement I come up with needs to fit in the same opening as the original meter movement. And I want to emulate an analog readout. Here I'm breadboarding a zero center LED bargraph meter that has a full scale range of +/- 1VDC. That was the voltage range of the original meter.
I decided to place the LEDs inside the front face of the old meter to keep the looks as close to original as possible.

I need 21 LEDs total, 10 postive and 10 negative plus a center LED that is always on. I chose green LEDs for the meter readout and a red LED for the center.

The problem is that 21 rectangular LEDs mounted side-by-side are wider than the meter by almost half. To make them fit inside the meter face I had to grind down the sides of each LED to get them as thin as I could. Then they were superglued together.
The LEDs were mounted on a piece of prototyping PCB to provide support and a place to connect wiring. I forgot to take a photo but it can be seen through the opening in the PCB board.
With the meter face reinstalled the ribbon cables come through the hole in the PCB that the meter movement previously occupied. The driver board will mount against the side of the battery compartment.
The driver board is built on prototyping PCB and the circuit uses two LM3914 driver ICs and a LM337 for the negative reference. It's basically the same circuit as in the LM3914 spec sheets, with a couple of changes. I trimmed R-Lo and R-Hi to adjust the range to +/- 1 volt. And I altered some resistor values to minimize LED current, plus added the center, always on LED. Now I'm waiting on an order of header strips to connect the ribbon cables.
This project has been sidelined by other more pressing projects, but I'll get back to it when I can.
Page Created 2/22/2020
Last Updated 5/11/2020