[volt-nuts] Solartron 7150plus - Chasing AC gremlins

Philip Pemberton philpem at philpem.me.uk
Fri Dec 7 01:01:08 UTC 2012

Hi guys,

I've got a pair of Solartron 7150Plus 6.5-digit digital multimeters
sitting on my desk which have just come back from calibration. One unit
calibrated to standard perfectly (with the note "resistance and 0.2VAC
ranges readjusted to norm and retested"). The other one seems to have
some issues.

The calibration lab reported that "Calibration on AC V and I ranges
fails with message 'ERROR' on FP. Partial calibration performed, other
ranges fine."
DC voltage, current and resistance are reading fine and well within
acceptable norms and agree with the other meter (plus/minus a dozen or
so counts in 6-digit mode).

If I connect an AC source (my function generator set to 400Hz per
service manual spec), the FG display and calibrated meter return valid

When I connect up the faulty meter, I can calibrate it fine for about an
hour after power up. After exiting CAL mode, the indicated voltage on
all AC ranges gradually drops to the point where after three hours it's
half a volt down on nominal. Putting the meter back into CAL mode and
attempting a recalibration results in the ERROR message appearing.

The service and ops manuals suggest this means the calibration source
isn't in range or there's a hardware fault. Obviously the cal source is
in range because the calibrated meter (and my Agilent U1251A) both say
it is... which leaves a fault on the DMM.

Has anyone seen anything like this before?

I'm thinking a component has gone temperature-sensitive -- and given
that this affects both AC voltage and current, it'd have to be in the
parts common to those sections. The A/D is obviously working fine
because the DC ranges are rock solid. I've yet to remove the cover and
test things, but my suspect list is as follows:

  - True-RMS to DC converter - IC21, AD637K. (£65! I REALLY hope it
isn't this thing)
  - Opamp IC15 (AD528J / LH0062CH FET opamp, seemingly long obsolete)
  - Analog switch IC18 (DG211 - a couple of quid each from Farnell)
  - TR1 and TR2 (a pair of 3N163s - more expensive parts - about £20 each!)
  - TR12 (WN1001, seems to handle gain setting around IC15 - can't even
find a datasheet for this one so I really really *REALLY* hope it hasn't
gone bad)
  - A handful of passives (resistors and capacitors)

This is based on a look at the circuit diagrams in the service manual
and elimination of parts common to other circuit blocks which are
functioning correctly.

I'm thinking that a good starting point would be to cool each of these
parts in turn with freezer spray, then see if the meter is affected. If
it is, warm the part back up with a hair dryer and see if it drifts back

However if someone has any other ideas or suggestions then I'd love to
hear them :)

philpem at philpem.me.uk

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