Sinclair PDM35 multimeter teardown

Another Hamfest find is the Sinclair Radionics PDM 35 digital multimeter. Having grown up with ZX Sinclair Spectrum clone, and reading a lot about Sir Clive Sinclair and his creation I wasn’t about to walk by this. If the look of it is a bit calculator-like, that’s because they actually reused a calculator enclosure! This was a  cheap model, selling for 33 pounds in the 70-s and 80s

It's a calcu-err-meter

My bounty included the meter itself, a user manual (A scan is here), and a set of original probes

Original probes and a user manual with a schematic

All ranges are selected by a slider switch

One slider to rule them all

After shaking out bits of battery compartment foam, and cleaning the stickers off the case. it looks pretty good. A fresh 9V battery and the thing is running.It's alive. Missing one segment

Top right segment is missing on all digits, but otherwise it seems alive. I fed it a few voltage levels and compared the results to my Fluke 8842A. Both read pretty close. Good enough at this stage! Time to take it apart

Not the cleanest assembly job

A bit messy here- lots of flux, questionable sodler joint. No comparison to the Fluke 8000A I just wrote about in another post. I imagine they served rather different price target markets. Desoldering the four tabs we can now  take the board out. The front panel confirms the calculator ancestry of the case:

It was indeed a calculator case

Did anybody hear of flux removers?

The front of the board is an even bigger mess- I am honestly surprised the thing even powers up! We have AY-5-3507 display driver, LM3900N quad opamp,CD4007AE dual complementary pair with inverter, and an unknown RCA 17898 part. The most interesting part though is the display. It’s an NSA1540A.

It's nice when built-in lenses magnify the die

Looks like the display has lead frame for more digits, but only the four got the actual die. Pins 1 and 2 looks shorted.Pin14 solder joint is suspect. I suppose this is IPC class -1 assembly?   And here is a magnified scan of the schematic:

 Conclusion:

The meter is quirky enough to keep in my collection, but is not something I’d end up actually using for anything useful.

P.S. There is a discussion of this post over at EEVblog Forum 

 

 

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