Avo EM272 now identifies as Zener Diode Tester!

[Chrispy57]

I picked up an Avo EM272 a couple of months ago, bought a B154 battery - testing just over 15V, did a quick check and found it was basically working, but reading around 90% of the applied voltage, so put it to one side awaiting further attention. Six weks later, switched it on again and the meter just sat at about half scale, and barely moved when voltage was applied, and the new battery was now down to about 12V.
There is a circuit diagram in the small user manual available on-line, but despite having spent countless hours testing and replacing components and cleaning switch contacts I was unable to identify the actual fault or repair the amplifier circuit.
I decided to make some use of the meter by incorporating it into my planned Zener Diode Tester project, but that would require a correctly working DC voltmeter. I thought I'd build an alternative amplifier circuit, since the meter movement was functional and the switching and divider chain all checked out OK.
By stripping out the old amplifier circuit I was able to create sufficient room to accommodate a small VeroBoard (1 7/8in by 1in) under the PCB, which was based around an 8 pin Instrumentation Amplifier (IN114) that I had on the shelf. I opted for a simple potential divider chain to give a "0V" reference for the single-ended power supply and an adjustable zero offset function, and a potentiometer to allow a gain setting to immitate that of the original discrete amplifier circuit.
Removing the internal battery contacts and using a Dremmel left just enough space for a PP3 and battery connector in the old battery compartment area. The old AC/DC switch was repurposed to become the On/Off switch.

Following on from my experiments with a ULN2068B Darlington Array chip and surplus 11V transformers that also lay on the shelf, and as utilised in my earlier Mains Voltage Prover Unit project, I made an adjustable high voltage DC supply on VeroBoard, cut to fit in the old test lead compartment of the EM272 case. This runs off a separate PP3 battery, due to the higher current demand than the meter amplifier, and its more easily accessible location.
Mimicing the 30V and 100V ranges of the Avo I have incorporated low voltage and high voltage options for testing Zener diodes up to 91V, reflecting my stocks.

The combination of these two elements has produced a handy quick-check facility for Zener diodes, that seems to have coincided with renewed interest in these devices in recent Forum posts.

[The Philpott]

I see you described this as steampunked in the AVO survey- but no, not being a lamp or a clock (or just tat) you've actually made a 'thing'. I've been inside one of these meters that i found in a skip (not working properly) and i was so uninspired with it that i disposed of it, and sold the leather case for a couple of quid.

Once the circuit board's started to rot or peel or the meter's playing up, i don't think repurposing is a crime!

Dave

[Chrispy57]

Hi Dave,
thanks for the alibi. In my defence,I did use period-appropriate Dymo labels!

Cheers
Chris