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

Philip Pemberton philpem at philpem.me.uk
Mon Dec 24 14:06:54 UTC 2012

Right then, time for a writeup :)

Long story short - the 7150Plus is just about fixed. Parts were sourced
as follows:
  * Three 3N163 "analog switch" FETs at £11 each from Littlediode on
Ebay. These turned out not to be required...

  * Analog Devices AD637 (SOIC16) RMS-to-DC converted with SOIC breakout
board. About £30 of silicon and plastic... also not used (though may be
needed later if the 637 in the meter doesn't stop drifting).

  * Three LH0062H opamps. $45 US plus shipping for all three, or £27.81
in the Queen's currency - also obtained from ebay

To recap, the problem was that the AC ranges were drifting badly and
calibration reported an error condition after warm-up. After applying
freeze spray to the opamp/FET section of the AC stage, the whole AC
stage failed and the meter reported an input overload.

Initial investigations suggested that IC15 (LH0062C, National
Semiconductor) was running open loop (output was clipped to within a few
volts of the negative-15V rail). No reason for this could be readily
ascertained and the feedback loop was producing erroneous results when
probed with a DMM (due to the opamp feeding back into the circuit).

Initially I suspected IC18 (DG211, Siliconix) and replaced it. This had
no effect on the failure symptoms.

This morning I desoldered the LH0062C and replaced it with an 8-pin IC
socket (machined-pin type of course). Probing the opamp support
circuitry with the opamp out of circuit suggested that the attenuator
and feedback loop were now operating as per the service manual description.

For "Gain of 10" ranges, resistance between IC15 pin 6 and IC15 pin 2
was measured at 1M-ohm after a settling delay of several tens of
seconds. The settling delay can be attributed to the charging of C29
(3u3); the resistance is from R72 (1Meg). TR12 (WN1001) appears to
prevent the multimeter from measuring the R21a/R21b divider chain.

For "Gain of 1" (unity) ranges, IC18 SW3 (pins 9, 10, 11) shorts over
C24 and puts the opamp into a unity gain state. Resistance between IC15
pins 6 and 2 is approximately 30 Ohms in this mode.

The function of TR12 and the circuitry around it still elude me. I've
never been much good with JFET circuits!

After replacing the opamp, the AC range was selected with a 2V range.
This sets the attenuator to 1:1 mode and IC15's gain to unity. The whole
AC chain now acts as a buffer. Applying a 2V pk-pk (~0.7V RMS) 400Hz
sine to the input with a signal generator and probing the opamp output
at TP3 suggested that the opamp was now functioning to spec. Testing the
AC ranges suggests that aside from some expected calibration issues, the
meter is functioning normally.

I'm hoping that trimming the offset is as simple as grounding the input,
selecting the lowest AC voltage range and adjusting for zero volts at TP3.

After that, I need to figure out how to calibrate the AC voltage and
current ranges! I'm hoping the signal generator will work for the lowest
ranges, but the 20V, 200V and 750V ranges (not to mention the AC current
range) may prove somewhat tricky...

Then finally I need to remove the front panel and re-glue it to the
plastic base. Unfortunately the old glue (actually double-sided tape)
has perished and the front panel is now looking a little sorry for itself...

philpem at philpem.me.uk

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