The LT1088 is available on Ebay.
If you have an oscilloscope, does it have a 5V square output for calibration of the probes? That output can be 1% accurate on some scopes. I would put a small RC (1 kohm and 0.01 uf) to filter the edges of the square wave so the energy is inside the bandwidth of your voltmeter. If your meter errors are the same with the HP651A and with the filtered scope output, then use the HP651A to calibrate the meters.
If the Voltmeter error is constant on all ranges and frequencies they may just need the meter movement gain adjusted.
I'll look at that, I have Tektronix 2465 that may have a good calibrator.
Thanks, Mikek
Edit:
I tried that, I have not seen a calibrator like the 2465 has, it changes frequency with the horizontal time base.
Anyway it is designed to be able to drive 50 ohms at 1/2 voltage or 1M ohm at full voltage. (0.2vpp or 0.4vPP) New info to me.
So I set it up without you filter because that was easy. With a 50 ohm termination the meter read 0.119V RMS
with supposedly 0.2Vpp signal in. I thought, well that messed up, but, this is a squarewave and the HP400E is an average responding, RMS calibrated meter, Making me think, the average of a squarewave is 50% so 50% of 0.2V is 0.1V.
Can I get a confirmation?
So it reads 19% high. The other unit reads 0.109 or 9% high.
I put the filter in and (I removed the 50 ohm termination) the reading dropped low, so I decreased the cap by a factor of 100, this just makes me think I can get any value by picking the right cap. So I dropped the filter idea. FWIW, the scope reads 4.3% high as far as my eye can tell.
Not sure I learned anything, Mikek
I have sent out for calibration quotes.