{"id":1632,"date":"2024-03-25T20:16:51","date_gmt":"2024-03-26T01:16:51","guid":{"rendered":"https:\/\/paulcarbone.com\/blog\/?p=1632"},"modified":"2024-07-20T09:06:06","modified_gmt":"2024-07-20T14:06:06","slug":"sencore-lc102","status":"publish","type":"post","link":"https:\/\/paulcarbone.com\/blog\/sencore-lc102\/","title":{"rendered":"Sencore LC102"},"content":{"rendered":"\n
\"\"<\/figure>\n\n\n\n

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.<\/p>\n\n\n\n

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: <\/p>\n\n\n\n

\"\"<\/a><\/figure>\n\n\n\n

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: <\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

For reference, here’s the good unit: <\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Here’s the output of the PWM driver on the bad unit:<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

And here’s the PWM output on the good unit: <\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

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.<\/p>\n\n\n\n

Also note that the Circuit description says this frequency should be around 27KHz, but this is closer to 16KHz. <\/p>\n\n\n\n

So there’s one of two things going on: <\/p>\n\n\n\n

  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. <\/li>
  2. Something is awry in the power supply circuit itself. <\/li><\/ol>\n\n\n\n

    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. <\/p>\n\n\n\n

    Another thing I learned while making these measurements:<\/p>\n\n\n\n

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

      Rail<\/p><\/div><\/div><\/td>

      pin<\/p><\/div><\/div><\/td>

      good unit<\/p><\/div><\/div><\/td>

      bad unit<\/p><\/div><\/div><\/td><\/tr>

      +5V<\/p><\/div><\/div><\/td>

      P4 - 5

      <\/p><\/div><\/div><\/td>

      1.37k

      <\/p><\/div><\/div><\/td>

      1.16k<\/div>

      <\/p><\/div><\/div><\/td><\/tr>

      -5V

      <\/p><\/div><\/div><\/td>

      P4 - 4

      <\/p><\/div><\/div><\/td>

      9k

      <\/p><\/div><\/div><\/td>

      1.33k

      <\/p><\/div><\/div><\/td><\/tr>

      +12V

      <\/p><\/div><\/div><\/td>

      P4 - 6

      <\/p><\/div><\/div><\/td>

      1.38k

      <\/p><\/div><\/div><\/td>

      1.54k

      <\/p><\/div><\/div><\/td><\/tr>

      +18V

      <\/p><\/div><\/div><\/td>

      P4 - 7

      <\/p><\/div><\/div><\/td>

      540k

      <\/p><\/div><\/div><\/td>

      870k

      <\/p><\/div><\/div><\/td><\/tr>

      <\/p><\/div><\/div><\/td>

      <\/p><\/div><\/div><\/td>

      <\/p><\/div><\/div><\/td>

      <\/p><\/div><\/div><\/td><\/tr><\/tbody><\/table><\/div><\/div>\n\n\n\n

      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:<\/p>\n\n\n\n