I am trying to clarify why i get a 0.5% low FS displayed measurement in a vintage Systron Donner 7205 DMM. This DMM has a front-end analog section which appears to be fine, as it sends an accurate 1V FS signal for voltage, current and resistance processed levels onto a subsequent analog section that prepares signals for a final dual-slope integration based measurement/display digital section that uses a Fairchild 3814 IC.
The relevant schematics of the front end sections are on pages 83-4 (of 89) of the manual at
https://www.dalmura.com.au/static/Systron-Donner%207205%205.5%20Digit%20Digital%20Multimeter.pdf, and the digital section schematic is on p.80.
Before starting a FS cal check, I was able to confirm ok levels for shorted inputs on all ranges, and confirmed supply rails are ok, and exercised the range selection switches (with judiciously applied contact cleaner spray where appropriate), to remove the typical switch contact related gremlins, as well as removing the BCD option board (which was a contributor to noisy readings).
During FS cal checking, the front end switching and dc zeroing circuitry on p.83 generates a 1V FS level at the 'Amp Out' TP1 node, and I have confirmed that for voltage, current and resistance references of various decades applied to the DMM inputs, I do get an accurate and processed 1.0000 Vdc level being passed on to the next section (p.84 of that doc).
But that is where my understanding of the analog and subsequent digital processing circuitry functions stop as to identifying why and where there may be a 0.5% low FS error subsequently coming in. I have confirmed the +10Vref and -10Vref levels generated on the p.84 schematic are acceptable (error and balance within 2mV).
I'd appreciate any guidance on what to assess on the p.84 analog circuitry, as I haven't yet got the scope out to confirm digital control lines are appropriately applying the +10 and -10V refs and the zeroing, or the operation of the integrator and zero detector analog circuitry, prior to being passed to the digital board.
Ciao, Tim