Thurlby 1905a Intelligent Digital Multimeter

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5 1/2 DMM from the 80ies. It has a hidden 6 1/2 digit mode. My meter does not have the True RMS module for AC. But it has a connector that emits the measurements via RS232 (not tested).

While all date codes from the digital board are from 1984, I found also some chips from 1988. So the meter is from 1988 or later.

It looks like the meter was in “official” use until 2023, because there is a safety sticker from this year.

Back side of meter. Two fuses, RS232 connector. [

The device is controlled by a 6502 CPU, located on a small PCB, together with all other digital parts of the device. It uses a 6522 (VIA) for IO. It has 2 EPROMS (I assume 2732 alike) for firmware. Firmware version is “CD00”, as can be seen by the stickers. This is also displayed when switching on the meter. There is 2K RAM (HM6116P) on the digital board. CPU runs with 4 Mhz.

The analogue board is below the digital board, access requires removal of front panel. So I decided to not tear down further, because the meter is in very good condition and seems to be well calibrated. No work left to me this time :-)

Operating Manual

Service Manual