Sencore LC102

My downstairs neighbor Ben loaned me his Sencore LC102 so I could start troubleshooting my newly acquired Tektronix 576 Curve Tracer that I suspected had some bad caps (spoiler alert, it had many bad caps). I’ve typically slummed it with my Heathkit IT-28 to check PS caps, but that’s not really as useful for lower voltage caps, nor will it tell you anything about ESR.
I’ve tended to ignore Sencore equipment on the count of it’s membrane buttons and non-backlit LCD displays, but damn this LC102 is useful. Ben just so happened to have both a working and non-working unit, and let me hang on to both for a bit.

The bad unit will fire up, but won’t test a cap. It will also lock up if you hit the ‘lead zero open’ switch. The other telltale symptom that something is amiss is a distinctive whine coming from the unit’s power supply, specifically this transformer:

We thought at first the noise was coming from the SMPS responsible for generating the test voltages, but this one handles the +/- 5V rails. The rails measure OK, but this circuit is clearly working overtime to make that happen:

For reference, here’s the good unit:

Here’s the output of the PWM driver on the bad unit:

And here’s the PWM output on the good unit:

Note the frequency readout on my 7854 is being generated by my new-to-me 7D15 plug-in, sitting in the ‘B’ horizontal slot, getting the signal from the trigger pick-off. I know any basic scope can do that these days, but that was cutting edge back in the early 80s.

Also note that the Circuit description says this frequency should be around 27KHz, but this is closer to 16KHz.

So there’s one of two things going on:

  1. Something in the rest of the circuit that uses the +/-5V rails is taxing the power supply, causing it to work harder to maintain these voltages.
  2. Something is awry in the power supply circuit itself.

Given how hot the circuit gets, I don’t want to run the bad unit for too long. I think the best place to start is to measure DC resistances from ground to the voltage inputs on the main board.

Another thing I learned while making these measurements:

  1. tables in wordpress is wet, hot garbage (at least WPTableBuilder is)
  2. The chassis is 100k away from ground (?!) there’s actually 3 different grounds – chassis, circuit & input.

Rail

pin

good unit

bad unit

+5V

P4 - 5

1.37k

1.16k

-5V

P4 - 4

9k

1.33k

+12V

P4 - 6

1.38k

1.54k

+18V

P4 - 7

540k

870k

So right away, that discrepancy on the -5V rail is a big red flag. Lets investigate. Where does the -5V rail show up? A lot of places:

  • It’s on the supply of every opamp
  • It’s a part of the offset null circuit on some opamps (a feature I’m not familiar with)
  • It feeds a pair of 4051 multiplexers
  • It feeds a MC14433 A/D converter
  • It feeds something called the “Ringer” circuit.

Here are all those locations in the schematic (click to get the PDF):

I’m going to rule out that ringer circuit for now since the first thing that the -5V rail hits is a 10k resistor, and I’d be real surprised if that was shorted.

Actually, scratch that – the next thing to do is look at the main board with a thermal camera.

Here’s the good one

and here’s the bad one

A clue! What’s getting really hot is IC27, an LM319 in the capacitance measuring portion. IC22, a TL084 opamp that’s a part of the current source circuit is also getting warmer than expected. The other half of that dual opamp is in the ESR measuring circuit.

I measured the output of IC22 into the LM317, and it’s -1.8V. On the bad unit, it was +3.3V. And here’s where I’m going to say a bunch of things that are probably wrong: With -1.8V on the regulator, it’s essentially shut off – I would expect this since it’s a higher range; and you wouldn’t have a higher range enabled during power up.
Something is causing the regulator to be on in the bad unit. Is it an erroneous signal from the microcontroller, a faulty opamp, something with it’s surrounding analog circuitry, or a problem with the LM317?
I expect pin 12, the input from the microcontroller, to be low. It is on the good unit, but on the bad unit, it’s at like 1V, which is odd, and wrong. I wonder if the next move is to yank the LM317.

Ok, yanked the LM317. With that out of the picture, The SMPS is no longer angry; it’s not buzzing, nor is it getting super hot. The LM319, however, is still getting getting very, hot, theres now +2V coming out of IC22 pin 14, and .4V on the input terminal pin 12. I can’t make that make sense to me.

I mean, fuck-it do I socket IC22 & IC27 next? I feel like they may not be the problems and are just collateral damage, but I’m not sure what to do next.

— The Next Day —

IC22 & IC27 have been socketed. With both removed, and the LM317 reinstalled, the power supply doesn’t whine anymore. With IC22, the TL084 quad opamp reinstalled, the whine returns. I threw in a TL074, and the whine stopped. I measured the resistance between the positive & negative terminals on both devices; on the TL084 I pulled out of the unit, I measured 340 ohms. On the new TL074, I measured 3 Meg, which is closer to what I’d expect. Reinstalling the LM319 Comparator made the whine come back. Measuring between the rails on that chip yields a similarly low value. I’m waiting on replacements. Fingers crossed this is it.

— Parts are in —

I replaced IC27, the LM319 Dual Comparator with a new part I got off eBay. Thermal camera looks good, but I’m still getting the same issue:

  • Open Test, with input open: I get one progress dash on the screen and the unit hangs.
  • Open Test, with input shorted: same thing.
  • Short Test, with input shorted: Error 4
  • Short Test, with input open: Test runs and says ‘Open’ (is this correct?)

Note that the chip labeling looked a little suspect at first, so I whipped up breadboard set-up just to check it out. The Power Designs triple output supply is new to me, and I’m a fan.

Testing some suspicious looking chips from eBay

Something is still fishy in this neighborhood though –

Here’s a few measurements from the good unit.

Pin 10, the input of the Comparator IC27
Pin 9 input of the comparator IC27 – the high limit

I’m going to post a few more pics of signals on that comparator, but I think it might be a red herring. The input signals on pin 5 of IC27 are driven by a pair of MOSFETS, which in turn are driven by two pins on the main processor. Those signals are different on the good unit vs the bad unit, so either there’s EPROM bit-rot (possible but unlikely) or much more plausible, the CPU isn’t getting the right signal from somewhere else to set those pins correctly.

Looking for a new lead, I’m turning again to the thermal camera.

Here’s the bad unit, on for about 45 minutes. IC23, another dual op-amp, looks unusually hot.

The bad one

Here’s the good one, on for maybe 25 minutes (yes yes, I know, it’s not the same amount of time, bla bla bla…). IC23 is warm but a good 20 degrees F cooler. The hottest of the lot though is IC3, the regulator that’s responsible for the 12V reference.

The good one

There’s already a 12V line from the power supply, so we’ll need to do some sleuthing in the schematic to see where this reference line goes. The fact that it’s dead cold in the bad unit could be another clue. I may also desolder IC23, though I think I’m fresh out of quad op-amps at the moment.

I checked resistance between the positive & negative supply inputs of that opamp in circuit (oddly +12 & -5) on both good & bad units, and they both read 3.3k, so I’m no longer suspecting a shorted IC.

Well, things are getting weirder. I double-checked the rails on both the good one and the bad one – they’re all as expected (but note that +12V means more like +13.8V). The 12V reference on the bad unit is bang on at 12.00V. The 12V reference on the good unit is only like 11.7V, so I wonder if even the good unit is harboring some gremlins. Also, another dead lead.

I’m not admitting outright defeat, but I’m going to retreat to other endeavors for now.