Sencore LC75 Help Needed - Page 1
Forum section: electronics / repair

author: techymechy
author_rank: Newbie
timestamp: March 24, 2024, 04:35:15 am
content: I bought a Sencore LC75 at an estate sale. I have the proper Sencore lead cable. The leakage, ESR, Inductance, and Ringer seems to work "close enough." The unit could be calibrated. The capacitance test works for capacitors above 100uF. below 100uF, the meter reads 000 and the uF indicator LED remains on uF.I am a mechanical engineer and I'm trying to learn how to repair vintage test equipment as a hobby. This is not a money making venture for me. I have almost zero knowledge of digital circuits and limited knowledge of analog circuits. I have repaired tube based test gear by Eico, Heathkit, etc. The Sencore data in BAMA is riddled with errors, I think. THe schematic is not totally readable and the material list is loosely correlated with the schematic. There is virtually zero information on how the capacitance circuit works.Could someone get me pointed in the right direction and potentially help coach me through this repair? This tester is really close to being a working unit. I would really like to get it working and learn something.

author: techymechy
author_rank: Newbie
timestamp: March 25, 2024, 06:04:33 pm
content: Sencore just sent me a much better copy of the schematics and parts list for the LC75 (see attached). Can someone help point me to where I could find the issue with the auto ranging on the capacitance measurements? Also, is it OK to use my Fluke 83 DMM to check for continuity of all of the devices on the the boards? Is there anything sensitive I should stay away from?

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 26, 2024, 12:05:35 am
content: Nice Schematic....I do not think checking any semiconductors with your Fluke Multimeter will hurt anything. When you are done checking the transistors with the diode function then...Check your voltages also.If the ICs are in sockets, (the pictures look like they are NOT in sockets) I would clean the pins with a pencil eraser and the socket with a toothpick with a little isopropyl alcohol.Check and see if the switch is actually working on the cap rangeCapacitor charge and discharge is on page 6 of the PDF and is IC 11 and the surrounding transistors.LM 317 is an adjustable voltage regulator that is adjusted by the circuit.I am a dummy, but I think the volt regulator delivers a different voltage for the different ranges. Make sure it is working, these things do blow..

author: techymechy
author_rank: Newbie
timestamp: March 26, 2024, 08:16:05 pm
content: Thanks for the help Wallace,The IC's are all soldered directly on the board but I will check for cracked joints, etc. I'll double check the switch as well. The only reason why I think it's working is that it does work for greater than 100uF capacitors (I think I only tested up to 680uF caps though).I need to clean off my work area and start to look carefully at all of the joints and start to test the diodes and transistors and look for broken solder joints/broken resistors, etc. It will just take a careful look and diligence. I'll start where you suggested.Thanks again,Dave