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

[Philip Pemberton]

On 13/12/12 17:32, Rob Klein wrote:
> Phil,
> 
> However contradictory it may sound, that is actually a good thing.
> Continuous
> failures are always easier to chase down than your gremlins.
> 
> Tracing front to back should bring you to Mount Doom relatively quickly ;-)

Indeed!

Well, here's the sum total of this evening's testing:

* Disconnecting LK2 is not a good idea. It appears to connect the A/D
converter up to the input loop. Disconnecting it kills all the
measurement ranges (they'll read a constant low voltage).
  I thought it was for disconnecting the drift-correction circuitry from
the input loop... c'est la vie.

* Lifting one leg of R82 (the 75R input resistor for the RMS-DC
converter) and injecting 2V pk-pk 400Hz from a sine generator (Zo=50R)
results in a stable reading of ~0.65V RMS on the 7150 display, the same
as my Tek DSO reports. The reading is still drifting around a bit.

* All the resistors in the AC input attenuator and opamp feedback loop
are fine.

* At the junction of RLE, C2 and R10 I'm seeing the input signal with a
negative-6V DC offset. The output from the opamp is stuck at negative 15V.

This leaves the following suspects:
  Relay RLF (an SDS-Relais RS12 metal can relay)
  IC18 (DG211 analog switch)
  IC15 (LH0062C opamp)
  TR12 (WN1001 FET)
  TR1 and TR2 (3N163 FET)

I have some LH0062H opamps on order -- the tin can variety, not the DIL
part. Incidentally the 7150Plus PCB is padded out for the DIL part but a
tin-can part has been installed by Solartron and had its leads bent to
fit the DIL footprint. Whatever works, I guess.

In summary: I'm getting closer but I'm not quite there yet.

Adjusting the opamp "offset trim" may be quite interesting -- I don't
think the adjustment procedure is in the service manual!


I'm wondering if it's possible to swap the 3N163s for something else if
they prove difficult to locate. A 3-pin FET wouldn't work (it wouldn't
allow negative Vds like the 3N163 does)... that means it'd probably have
to be an analog switch and some form of level translator.


As for the WN1001 - I can't even find a datasheet for it!
For that matter, the WD460 dual matched FET in the DC loop is also
proving difficult to find even a spec sheet.
I think it's safe to say I'd be pretty well stuffed if the '460 in the
DC amp ever failed!


I did notice a mildly amusing easter-egg on the schematic: the
star-ground on the first floating board schematic is labelled "ROME". As
in "All roads lead to Rome"... Reminds me of the "washer lady" and
"wizard" who were often found in Tektronix service manuals, back when
the schematics were lovingly (?) hand drawn.

Cheers,
-- 
Phil.
philpem at philpem.me.uk
http://www.philpem.me.uk/