LC102 Lead zeroing!!. was zeroing perfect the other day now will not!.. what pot can i adjust?

I have a NOS fc221 I would like to use to calibrate my newly acquired lc102... I do not have a schematic package. I am looking to know which pots to turn for which calibration property. Thanks!

You will only make any existing problems worse (or create new ones) by changing the internal service/calibration adjustments.

You will need to attain the proper service literature from Sencore.

There are other reasons that the Lead Zero produces inconsistent results. The BNC connector on the test lead, or the front panel may be oxidized or worn out (the front panel BNC is a threaded part, it needs to be tightened snuggly). The fuse in the input/test connector needs to be gripped securely by the adapter. There could be a broken/frayed conductor, or other loose connection in the test lead. If you're not using the genuine Sencore test lead, you shouldn't expect good results. The original part is a length of RG-62 B/U IIRC, and not common coaxial cable. The center conductor in the Sencore test lead isn't surrounded by dielectric material the way most coax is.

Having the Calibrator accessory isn't entirely error-proof if the value drift hasn't been measured by a traceably accurate reference recently (within about 1 year). The values listed on the Calibrator are probably close, but if the accessory is old, the values have drifted. Attempting to use the Calibrator without the Sencore test lead isn't going to result in accurate or consistent readings.

The Sencore equipment that I've used and serviced have all been very well constructed, and reliable instruments. Tampering with internal settings should be avoided. Removing the instrument from it's case can result in calibrated settings being changed if handled carelessly. Without some other reliable test equipment, and the Sencore service information, you aren't likely to be able to troubleshoot any problems or faults this unit might have.

-- Cheers, WB .............

The schematics and parts list package doesn't include the calibration information. You will need to attain that info from Sencore by specifically requesting it.

If the instrument doesn't zero or give consistent readings using only the operator controls, you will very likely need the calibration info. If you can't achieve proper readings after setting the front and rear externally accessable adjustments, the instrument most likely needs to be serviced.

-- Cheers, WB .............

Hi, thank you for the detailed and informative response... I cleaned the bnc plug, took about a centimeter off of the black lead where the cable meets the solder point. Now all works perfect on morning starts. Zeros out and great reads! Thank you. Must have been a frayed lead.

Boy someone how my profile name changed.. testing to see if back to my name.. thanks again!

That's great. You may have noticed that the short leads that attach to the mini-clips can rotate in use, causing more broken strands. I've put small amounts of hot glue at the entry point of the small leads to prevent them from rotating in use. This can prevent future surprises, like not being able to perform the Lead Zero again.

The small lead for the red clip is a section of RG-174 IIRC, and fairly fragile. Putting a small amount of hot glue at the clip ends of the short leads will also extend their useful life.

You'll no doubt find out that attempting to attach the mini-clips to the solder side of a board for in-circuit teast will generally be an exercise in frustration.

I had a couple of extra probes that are type recommended to be used with the Cricket transistor testers, so I got some of that RG-62 B/U (I think) coax and made up a test cable with the probe on the end, instead of the test clips. Only two of the sharp pointed pins are needed, of course, and the pins need to be marked negative and positive.

I'm fairly certain that all of the Sencore LC/Zmeters back to the LC-77 will give incorrect measurements if the original type, low capacitance coax is not connected to the input connector, but a probe-type test cable made up with the correct coax type works very well. Generally, this leaves the other hand free to select the test functions of the instrument (the temporary in-circuit connection of the probe is not suitable for capacitor leakage tests, as leakage testing isn't done in-circuit).

-- Cheers, WB .............