Help wanted to test blood pressure monitor

[ianr]

My trusty BP monitor, bought years ago at Lidl I think, has suddenly begun to fail after fairly intermittent use.  On starting the test it starts pumping up the arm cuff to what seems like close to peak pressure, and then gives up and deflates.

I'd like to find a remedy, but first need to know whether it's the cuff or the monitor internals that's at fault.  That could easily be found  by trying the cuff on a known working monitor on which the cuff fits well, and/or trying a known ok cuff on mine.    The cuff attachment nozzle diameter is 2mm internal, 4mm external.  and its insertion length is 1.1mm.  The socket is 5mm wide at its opening.   Photo attached.  It's branded Medisana, but I don't think a name match is vital.  It's the fit that matters.  I'll be glad to hear from anyone who thinks they might be able to help.