Sencore LC102 Capacitor Analyzer: Need Help With Incorrect Measurements  - Page 1
Forum section: electronics / repair

author: valley001
author_rank: Frequent Contributor
timestamp: August 08, 2022, 10:57:24 pm
content: EDIT: I replaced the microprocessor and that fixed the issue detailed below. Now need help fixing the capacitance measurement ranges starting in reply 10.Hello all,This is a continuation of a previous thread below, starting a new one a year later in hope of repairing this poor LC102. I would VERY much like to get this useful bit of kit running. Note from previous thread I likely damaged something probing around the relays but the device was not functioning correctly even before this operator error damage.Prior thread:https://www.eevblog.com/forum/repair/sencore-lc102-troubles/Schematics are available here:https://bama.edebris.com/manuals/sencore/lc102/Device Behaviors:- Device will not read capacitance. Relay will click, display remains 0000- Device hangs/freezes whenopen lead zerois attempted. It WILL zero (or acts like its zeroing)shorted leads.- I get error 7 (component out of test range) when trying Dialectic Absorption and ESR tests.- Device WILL present a reading for leakage 100uf/16v cap it reports 8.4ua leakage.- Device WILL present a reading for coils/inductors both uh and rings. 220uh inductor it will present a reading of 190uh and 16 rings.Checks:I have checked and verified power supplies, including (probably un necessary) electrolytic cap replacement.I have probed all diodes and transistors and all seem ok, at least none sort or open.Clues:- I have observed the 2000 board on thermal imaging, while nothing gets super hot I note that IC46, IC23, IC22, and IC27 are warm. R84 seems warmish/hot.- When I attempt the open lead zero, TR14 gets warm fairly quickly (a clue hopefully). The device hangs/freezes once the open lead zero is initiated.Things I have done so far:- I replaced IC 27 just because it was in proximity to my clumsy probing. No improvement.- Replaced PSU electrolytic caps. No improvement.Assumptions:-I assume the micro and ROM are fine, since the device will do certain readings and seems to operate at least some of the relays.-I think I damaged a logic device somewhere. Unfortunately documentation is fairly light on this device, for instance I see NO mention of the lead zeroing process in the circuit description. In fact, the previously mentioned TR14 is not mentioned in the circuit description at all.While I have come a long way reading schematics I am not quite up to the point where I can understand how to troubleshoot the circuit without more guidance, so I am at a dead end at this point. I would greatly appreciate assistance getting this thing going.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 08, 2022, 11:19:59 pm
content: for the closed and opened zeroing, you have the big relay who could give some problems, like oxidized contacts, some members did have success with deoxit, when they made a small hole on it an shoot it insideYou must use for this sencore a specified coax wiring (capacitance) and a specified length, if theses specs are not correct you will get zeroing problems too, you have a special fuse socketAt k4obb i did patch the service manual sheets into 3 majors oneshttp://www.ko4bb.com/getsimple/index.php?id=manuals&dir=06_Misc_Test_Equipment/Sencore/Sencore_LC102read the operation manual, it will give you some insight, you have firmwares dumps toohttp://manuals.repeater-builder.com/te-files/SENCORE/SENCORE%20LC102%20Operation.pdfSorry to say: you did a few things wrong assuming this or that, you don't change parts for fun or assume they are automatically bad, you have to properly diagnose stuff before changing a partfor the psu you have many tests point to verify, and the main board too and be careful, as you know it, it can output 1kvThere was known problems for the small black relays in it tooI did own this lcr meter in the past, it had a later revision of theses small relays, they were "metal" shielded, i did bought on ebay an test lead made by a seller, it was working perfectly for open and close zeroing, before that i had errors toohttps://www.ebay.com/itm/332712610011?they are worth every penny

author: valley001
author_rank: Frequent Contributor
timestamp: August 08, 2022, 11:40:17 pm
content: Thanks much for your reply. I neglected to mention I did gain access to the large relays and cleaned the contacts. They seem to be functioning correctly.I am certain the lead zeroing and failure to read capacitance are not related to the contacts or test leads, there is something else going wrong that I have yet to identify. I did replace the IC27 and PSU caps out of frustration before putting it away for a year, though you are right to point out this was not wise.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 09, 2022, 12:23:20 am
content: follow some guides in the service manual / user manual, you will be able to check the psu / generator voltages / ranges, if not you may have logic problems

author: valley001
author_rank: Frequent Contributor
timestamp: August 09, 2022, 01:49:56 am
content: So far as I can tell all the PSU voltages are present and correct. I am fairly certain it is logic issue, but I am not sure where to start with troubleshooting the various logic ICs on the 2000 board...short of de soldering them one by one...

author: coromonadalix
author_rank: Super Contributor
timestamp: August 09, 2022, 10:19:10 am
content: sorry im not native englishi meant generate the voltages needed to test a capacitor, you start at say 1 volt and goes up to 1 kv, if i recall you had some test procedures in the manuals ... that way youll know if the voltage generator worksthat's a first test i would do,switch between different tests modes and check if the logic follow ...you may only need a good calibration and that's where you may start with some problems, because they are old equipment, and we at eevblog don't have all we need to do it properly, some software is needed, and we never had the chance to have it from a member ... search here for lc102 / lc103EDIT i think i ve left the verification sheet or its an already made copy to K4OBB website, it specifically give you the voltages at different test points .... when doing selections ...http://www.ko4bb.com/manuals/66.131.195.63/Sencore_LC102_Calibration_Procedures.pdf

author: valley001
author_rank: Frequent Contributor
timestamp: August 10, 2022, 06:06:13 pm
content: I think I have made a discovery, I am interested in thoughts from the forum whether or not I am on the right track with this.I am following the Capacitance Circuit Description attached, this is paragraph 3 on page 9.Pin 5 and 9 of IC 27 are not reading anywhere near the indicated voltages of 1v and 3.5v, im getting something very low like 20mv or so. Probing the gates of TR40 and TR41 reveals both are held high at 5v, if I am understanding correctly this means they are both turned on. According to the circuit description they should be turned on one or the other but never both simultaneously. Since these seem to be turned on by the 80c39 directly, would this indicate a fault in the 80c39 microcontroller?

author: coromonadalix
author_rank: Super Contributor
timestamp: August 11, 2022, 03:37:40 am
content: could be, one idea wold be removing the chip and check if you can manually trigger the empty( chip removed) pins with say an 1k resistor to ground or high (5v)once the chip removed, put a socket in placeif tr40 tr41 don't switch, they could be busted ... since in this case they act as a switch

author: valley001
author_rank: Frequent Contributor
timestamp: August 11, 2022, 04:28:56 pm
content: Tr40 and 41 appear to be toggled by the microprocessor directly (IC8). Luckily the microprocessor is already socketed. I managed to find a replacement 80C39 for relatively little money, but it will not arrive for a few days. I am highly suspicions of the microprocessor being the culprit of most of the issues.The next paragraph in the circuit description discusses the 6mhz clock signal, I guess basically the processor counts the 6mhz clock while the voltage of the cap under test is brought through two set points, partially determined by previously discussed TR40 and TR41.Tracing this 6mhz I can see it at pin 6 of IC25, and briefly at pin 4 when the cap test button is released , but nothing at pins 3 and 11, and nothing further down on IC13 and 31. So while the cap test button is pressed, no 6mhz signal is passed through back to the CPU for counting. (these tests were done with a cap on the leads and capacity and voltage data entered in, I also confirmed the cap received the voltage at the leads).I don't really understand the data line connections in the schematic, it looks like IC25 is connected to IC11 (74C374) with indications of "E7 RESET" on pin 2, and "E5 UNLOAD" on pin 12 of IC25.Im sorry this is rambling and potentially confusing, hopefully some sense can be made from it.

author: valley001
author_rank: Frequent Contributor
timestamp: August 13, 2022, 09:49:31 pm
content: I replaced the microprocessor and now its working, or at least now its in the condition it was in before I shorted something. Lessons learned!Now im back to troubleshooting the inaccurate readings in some ranges. I plan to start with those small black relays. Is it possible to jump them closed so I can test with my ohm meter? Im hesitant because IIRC this is where I went wrong and fried the cpu lol.

author: valley001
author_rank: Frequent Contributor
timestamp: August 15, 2022, 04:29:12 am
content: I have moved on to the original issues this unit had with capacitance measurements. These are unrelated to the two large relays, which I have cleaned, or the test lead fuse holder which was also cleaned.See the attached schematic for reference.1- Both the 3.3ua (0-.002uf measurement range) and 330ua (.002-2uf measurement range) current supplies seem to be working. 1nf ceramic cap reads near dead on 1000pf and a .047uf cap reads dead on .047uf.2- 60ma supply (2uf-2000uf measurement range) current supply iscompletely messed up.330uf cap will read13300uf3- 416ma (2000uf-2F measurement range) supply is better but not correct, a 3300uf will read low 2600ish uf.As I understand it (feel free to correct me please)... The device basically charges the cap under test and measures the time it takes for the voltage to move between two reference points. While the voltage is within this window it passes the 6mhz clock signal to be counted which is then interpreted as a measurement reading.Knowing this I probed latches F5, F0, F2, F3 to see what is happening when I pressed the test button on different size caps that should trigger within each range. I noticed the signal on F2 was longer than the others. I then probed the output leads with the 330uf cap attached and was able to view the ramp voltage. The other latches are very quick and the ramp is not easy to see, but for some reason F2 takes a lot longer (maybe 1/4 second) and I am easily able to view the ramp on the scope. I then checked the measurement window clock and I can see the device passing the clock signal (IC31 top right, pin 13) during the whole time the ramp is active, so I think that explains the crazy high measurement for the 60ma range. None of the other ranges do this.The description says the LM317 supplies the current and is controlled by the TL084. It looks like a similar arrangement of LM317 and TL084 for both the 60ma and 416ma supplies so I probed pins 12, 13, and 14 of both IC22 and IC23. See attached scope pics. Traces- bottom pin 12, middle pin 13, top pin 14. I see this ramp at the cap under test as well on the 2-2000uf range.Im wondering if someone has any insight on these observations? Is the charging voltage taking too long because the LM317 is not supplying enough current to charge the capacitor under test fast enough? Is it the LM317 that is bad, or the TL084?..or both? Maybe its a firmware issue?I would greatly appreciate any help with this.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 15, 2022, 02:29:12 pm
content: Unless mistaken, the lm317 section is supposed to generate a constant current, the capacitor measurements are time constant measured ... the ramp you see must be the result before processing, the bigger the cap, longer ramp time it should be, and this must be passed to the cpu with a gate time ...my 2 centsverify the parts around the lm317 section, this current amount should not damage it ? unless a bad solder, or some bad probing lollI think you can put an ammeter in serial with the device under tests to see what current amount goes into it ...Can't recall if you have operation(s) principle(s) in the manual(s)I don't think its an firmware issue, some firmware dumps have been made for this lcr, re-write your eeprom if you think it would help ?or something around the cpu could have been damaged ?

author: valley001
author_rank: Frequent Contributor
timestamp: August 15, 2022, 02:41:31 pm
content: Yes thats precisely the issue, the ramp is very fast on all three other ranges including the higher range 2000uf-2f. You can see this in the two scope captures, the one with the visible ramp is the 60ma supply, the one with the spikes is the 416ma supply (both measurements taken with a cap on the test leads, 330uf cap on the 60ma and 3300uf cap on the 416ma). Its only the 2uf-2000uf where I see this slow ramp, slow ramp plus longer pass through to the cpu equals the very high measurement.So I am chasing the cause for this long ramp time.So the question is, why would it produce this longer ramp on one range only?

author: commongrounder
author_rank: Frequent Contributor
timestamp: August 15, 2022, 03:51:47 pm
content: Sorry to jump in here. I own an LC102, and had a similar issue with one of the upper ranges. I could not get that range anywhere near into calibration. I checked the constant current supply and found it was way too low. It turned out to be the LM317. I discovered the devices in my unit were all plastic packages, and they develop defects over time. I replaced the two LM317s with metal tab devices, and I was able to calibrate the range again. The other range was done as a precaution, but gave me peace of mind since I depended on it for my work. This may or may not be the problem with your unit, but thought you might find my experience useful.

author: valley001
author_rank: Frequent Contributor
timestamp: August 15, 2022, 04:53:20 pm
content: Quote from: commongrounder on August 15, 2022, 03:51:47 pmThanks for this. Could you let me know how you tested the current on a particular? The output pulse is so fast my DVM can not capture it.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 15, 2022, 04:57:18 pm
content: thks commongrounder, and theses parts are not expensive

author: floobydust
author_rank: Super Contributor
timestamp: August 15, 2022, 05:28:48 pm
content: Instead of a test cap, you could put a resistor say 100R and see what current it gets. The charging ramp will never finish and I expect the meter to complain but we need to test the current-sources only. Of the F2 (60mA), F3 (410mA) sources, hopefully one is not stuck partially on.TR15 is a 350V part 2N6559, LM317's have output diodes... but I don't see any protection for the TL084, coming off the relayI would suspect it to be damaged if a charged capacitor was connected? That would zap IC23 pin 5, and/or pin 10. The voltage at pin 8 and 10 should be identical.

author: valley001
author_rank: Frequent Contributor
timestamp: August 15, 2022, 05:37:18 pm
content: Quote from: floobydust on August 15, 2022, 05:28:48 pmThanks ill give that a try, 100 ohm resistor for testing current. If one were stuck on partially how would that manifest? Would the current be additive?

author: floobydust
author_rank: Super Contributor
timestamp: August 15, 2022, 07:16:41 pm
content: There is a common bus between all four (switched) current sources running out to the relay, so I thought any one stuck on would upset things - but you say the two smallest F5, F6 work OK so that theory is wrong. Stray leakage current or extra current would upset the smallest readings.Just try to measure the larger F2, F3 current-sources when they are pulsed on. This meter changes the ramp threshold points with TR40, TR41 so if that doesn't work, measured cap values could be seen out as well.It looks like the F2 current-source is way too low and I'd see if it really is, or something else is causing the measurement error.

author: valley001
author_rank: Frequent Contributor
timestamp: August 15, 2022, 08:48:37 pm
content: Quote from: floobydust on August 15, 2022, 07:16:41 pmHmm I dont think the test works with the resistor. It does output 80ma but it looks like all the latches go into crazy mode after a moment, so not sure if I am getting a usable result... im not sure the measured 80ma is from the 60ma range exclusively.If my understanding is correct it senses the cap size and scales accordingly somehow, so im not sure how I could measure the current without using a cap sized to the range in question. Just measuring the 330uf cap with the DMM in series it shows up to about 2ma before the test pulses off, but I dont know how accurate that is. The slow update rate is pretty annoying.

author: valley001
author_rank: Frequent Contributor
timestamp: August 15, 2022, 11:08:58 pm
content: Since I had some LM317T here I replaced IC41 to see what the result would be,now it does not take readings at all in that range lol!No voltage at the test leads. The lower two ranges still work. and the upper range is unchanged. I checked over my solder work and checked connections, verified correct orientation, etc. Now im totally lost lol. I guess ill put the old LM317T back in.EDIT: I put the old LM317T back in (IC41) and its back to the original crazy readings in the 2-2000uf range... where to go from here?

author: valley001
author_rank: Frequent Contributor
timestamp: August 16, 2022, 04:18:11 am
content: I have improvement!Probing the devices around IC41, I foundR159R158 pot completely open. I didn't have any 10 ohm pots so I bodged in a 100 ohm multiturn. Now the 2-2000uf range is reporting reasonable measurements.The 2000uf-2F range is still reading low so I think ill start with the pot there also.

author: floobydust
author_rank: Super Contributor
timestamp: August 16, 2022, 04:32:45 am
content: I think you're bouncing around and the repair is not making sense - so your new LM317 was bad? An open output pot will give, well - no output so how were you getting 80mA?

author: valley001
author_rank: Frequent Contributor
timestamp: August 16, 2022, 04:48:44 am
content: Quote from: floobydust on August 16, 2022, 04:32:45 amYes...sorry. Let me say I greatly appreciate your help with this, and your patience with my amature diagnostic skills.1- I did the test with the resistor and saw the 80ma, but it looks like the latches for each range are all firing no? That is where I figured the 80ma was coming from (the other ranges), since as you point out they share a common rail. I assume the 80ma, and whatever the device was outputting with a cap in the 2uf-2000uf range, was coming from another range possibly?2- I probed the resistance between IC41 output connection middle pin hole (with the LM317 removed) and pin 13 of IC22 I had open circuit. I traced this to the R159. Now, while fiddling with it I think I got it to show something like 39meg or so, so maybe not completely open..but really flaky?3- I replaced the pot. Now resistance between output pin hole and pin 13 of IC22 is 20ish ohms.4- I installed the original LM317T5- Now I am getting very reasonable readings from the handful of 330 and 470uf caps I have on hand.I wonder now if it was a mistake to put the old LM317T in, since the new one behaved as it should (right?) IE no output with the open output resistor?

author: floobydust
author_rank: Super Contributor
timestamp: August 16, 2022, 06:29:25 am
content: How much rework do you think this piece of vintage test equipment can take?The LM317 is out, it's in, it's out, the F2 cal pot is now busted and you want to swap out the LM317 again?Just stop already.Your repair technique isn't good enough to fix this and you're going to have to have to learn and improve it or just give up now, sell it on eBay.The rule in repairing complicated circuits is to make only ONE change at a time and see where that takes you.You make 3-4 changes at a time, so it's impossible to help you.If the F2 current-source is working, not because you fudged calibration with the new pot and changed nothing else... despite the pot working a while ago... it will track capacitance across the range. So 2uF and 2,000uF will read accurate as well as your 330uF.I can't make sense of what you're doing so perhaps other people can assist.

author: valley001
author_rank: Frequent Contributor
timestamp: August 16, 2022, 06:57:13 am
content: Ok, points taken. Thanks for your help anyway. Ill carry on.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 16, 2022, 08:51:48 am
content: My thoughts too, that's why i kinda retracted myself tooAnd sadly you play with your posted threads, you change them many times, title changes, etc ... i have to go back to follow you ...And yes i think you may create more problems than solution ??sorry

author: valley001
author_rank: Frequent Contributor
timestamp: August 16, 2022, 03:18:08 pm
content: Sorry for my lack of clarity, fixing electronics is obviously not my day job but I do enjoy it and have learned a lot here. I thought I was making my thread easier to follow but obviously not!R158 potentiometer was open/faulty, I'm near certain that this has been the case this whole time. I have no idea why I was getting the readings I did earlier (80ma), or why it behaved the way it did after swapping LM317. After replacing the open pot now I get reasonable readings in the 2-2000uf range where I never did before in the time I have owned it.Thanks everyone for your patience and time.