Sencore LC102 troubles - Page 1
Forum section: electronics / repair

author: valley001
author_rank: Frequent Contributor
timestamp: May 27, 2021, 04:03:02 pm
content: Hello all,I have this LC102 here, it seems like it could be a very useful machine and would be my most advanced cap analyzer on hand...if it worked. The schematics are available on the Bama site, but I can not locate a service manual.It is not the usual failed zero that I have seen posted about, though I have removed and cleaned the input fuse. It will zero and it will read small capacitors mostly fine, for instance 220pf will read spot on 220, 680pf will read 663, 82pf will read 75 and so on, fairly reasonable readings. 22uf will read something crazy like 775uf. 47uf will read 1600uf.I have checked L2 and it seems fine, in the closed position reads less than an ohm. I assume the other sealed relays are used to control the ranging and they may be suspect, but I am not sure which should be closing when.Other things I have checked are some of the voltage rails, 5v to the sealed relays and 12v at TR34 seem OK. I have not checked all of the electrolytic caps as I think it is a ranging issue but I am open to ideas.Any assistance/input is most appreciated. I would love to get this thing running.

author: valley001
author_rank: Frequent Contributor
timestamp: May 27, 2021, 06:02:42 pm
content: OK so just going through the diodes I found CR33 and CR44 antiparallel diodes are a dead short. Also CR39 reads .182 volts in both directions.CR33 and 34 are UF4002 is it ok to use 1N4002 in their place?

author: mzacharias
author_rank: Frequent Contributor
timestamp: May 27, 2021, 07:49:19 pm
content: I have the "LC-102 Operation, Application & Maintenance Manual"which includes schematics, although in smaller pieces one might have to print out and tape together.111 pages in all - PDF format.I can email it to you.my email is mark_zacharias@sbcglobal.net

author: valley001
author_rank: Frequent Contributor
timestamp: May 27, 2021, 08:50:01 pm
content: I actually have that one, the schematics are nice but there is no troubleshooting help in it unfortunately. Did sencore produce service manuals for their gear like HP, Fluke, etc?Quote from: mzacharias on May 27, 2021, 07:49:19 pm

author: valley001
author_rank: Frequent Contributor
timestamp: May 27, 2021, 09:12:56 pm
content: So when I disconnect the 2000 board (the board with CR33 and 34 on it) the short goes away. Is there any normal circumstance where diodes in this arrangement would read a short? It is the ground connections between the boards that cause the short to appear. I have checked for shorted caps etc on the 3000 board and nothing...worry I may be chasing wild geese here.

author: valley001
author_rank: Frequent Contributor
timestamp: May 27, 2021, 10:33:16 pm
content: Well, now its hanging up when I try to zero it, completely unresponsive until I cycle power. There must be more serious problems with this than I hoped. Putting it back on the shelf for now.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 27, 2021, 11:27:28 pm
content: the service manual i've patched together is at ko4bb a 53 meg zipthe lc102 lcr have a problem with the big relay in it, do a proper cleanup with deoxit ... there is some threads about this lcr here, some of the small relays can create some problems too.http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=06_Misc_Test_Equipment/Sencore/Sencore_LC102For the 2 diodes in reverse of each other, i think the 1n4002 4003 and up can do the job, sometimes a circuit like this is to create / force an 0.6v kinda voltage drop ...Check the internals connectors, they have crimped wires in them, it's been reported in the past some wires in them are getting loose

author: fastguido
author_rank: Regular Contributor
timestamp: May 28, 2021, 03:03:47 pm
content: Also check the PAD20 diodes in the range section(silver transistor looking devices), I have found those shorted or leaky.

author: valley001
author_rank: Frequent Contributor
timestamp: May 28, 2021, 04:04:08 pm
content: Thanks everyone for the input.The first thing I did was clean contacts, re seat everything. I think I damaged it either while probing IC27 dual comparator, stupid mistake with the test probes slipping, or relays L5-L7. It stopped working after that, freezes if I try to zero for open circuit. It will also no longer take any readings at all, the display remains at 0000.This morning I decided to get the hot air station out and removed IC27 dual comparator but I am not sure how to test it and my IC tester does not have it in its library. I ordered a couple just to see if that fixes the hanging up issue. Observing the 2000 board through thermal I noticed TR14 will begin to heat up when the unit hangs up, so I removed that and it tests fine out of circuit.I'm stubborn so I cant give up on this thing, any input is appreciated. I have a scope but without any sort of trouble shooting manual I am not sure how to proceed. There are some clues in the circuit description but much of that is above my knowledge level.Score so far is 0 steps forward 11 steps backward.

author: valley001
author_rank: Frequent Contributor
timestamp: June 01, 2021, 03:22:54 pm
content: Found a bad zener CR5 on the DA converter IC14 MC14433, replacing it has not improved the situation.Could someone who owns one of these read the resistance between the output leads on your unit? I am getting 100 ohms, wondering if this is correct for the unit. I cant see in the schematic where I should have 100 ohms between the positive output and ground.

author: fastguido
author_rank: Regular Contributor
timestamp: June 01, 2021, 04:13:51 pm
content: Just the leads themselves or with it hooked to the unit?

author: valley001
author_rank: Frequent Contributor
timestamp: June 01, 2021, 08:02:41 pm
content: Quote from: fastguido on June 01, 2021, 04:13:51 pmThe resistance between the positive output and ground on the unit.

author: fastguido
author_rank: Regular Contributor
timestamp: June 01, 2021, 11:11:02 pm
content: Quote from: valley001 on June 01, 2021, 08:02:41 pmi'll try and do that tomorrow.

author: coromonadalix
author_rank: Super Contributor
timestamp: June 01, 2021, 11:54:56 pm
content: Check cr1 on the 2000 board, re-take the ohms / impedance measurements with the high voltage board unplugged with the sencore not powered upYou can have minimum 50 ohms on the output with r34 r35 in parallels on the DA2000 boardMaybe the relays switching logic may have some problems ??

author: valley001
author_rank: Frequent Contributor
timestamp: June 02, 2021, 02:01:31 am
content: I checked CR1 and it is good. I think I blew some logic somewhere, and the schematic is not detailed enough to troubleshoot it (or I lack the necessary education lol). Where can I probe to see if the capacitance logic is working?

author: valley001
author_rank: Frequent Contributor
timestamp: June 02, 2021, 10:33:16 pm
content: I decided to step back and start over on the basics. Probing the power supply rails everything seems fine except the 12v rail, (P4 pin 6) I am reading 13.3v and it is very noisy like 100mv p-p noise on this rail (118khz, sawtooth shaped noise). +5 and -5 rails seem fine. What might cause the 12v rail to be so high?

author: Smoky
author_rank: Frequent Contributor
timestamp: June 03, 2021, 01:38:28 am
content: My LC102 is powered by an external 3-pin Sencore wall transformer, PA251, which supplies 17 volts at 1 amp.I measured my transformer unloaded and my DMM reads 22.8 VDC.Looking at the block diagram, the power heads straight to a +14.5 voltage regulator.The red dots mark connector P4 pin 6 which reads +12v:My LC102 runs fine so I'll lift the cover of my LC102 and scope pin 6 and see what I get.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 03, 2021, 02:09:39 am
content: And looking at the schematic, I put a yellow dot where the power comes in from the external wall transformer.The red dot is the +12v rail heading to connector P4.Capacitor C19 looks like an electrolytic that I would test.I haven't touched the insides of my meter but I do plan to re-cap it in the near future.

author: valley001
author_rank: Frequent Contributor
timestamp: June 03, 2021, 03:30:38 am
content: Thank you for your input on this. Let me know if yours is dead on 12v at the P4 connector. I believe I did check C19 and IIRC it was dead on 470uf in circuit. Ill de solder it and check it out in more detail.

author: valley001
author_rank: Frequent Contributor
timestamp: June 03, 2021, 03:52:29 am
content: OK, there is something wonky going on with the 14.5 volt regulator IC5 LM317T . When I try to adjust it to 14.5v it will jump all over the place, up to 16v then down to 13v, sometimes settling somewhere between 14-14.5v. I'm guessing this regular is likely faulty?

author: Smoky
author_rank: Frequent Contributor
timestamp: June 03, 2021, 04:01:50 am
content: Here are the results from my +12v rail and I apologize for the lousy pictures:IMO, this is total nonsense for a 12v rail.I re-capped my LC53 a little over year ago and its accuracy can walk all over this LC102 when it comes to capacitance measurements.My LC102 will get some high quality electrolytics before it's turned on again because I believe these meters get killed by a slow death due to the aging caps.

author: valley001
author_rank: Frequent Contributor
timestamp: June 03, 2021, 04:16:10 am
content: Right, probably a good idea. Mine is really noisy also I am wondering if it is cap related or a combination of things. I think I may replace IC5 and see if that fixes the 14.5 volt output but not sure if there could be other contributing factors. I am hesitant to recap mine until I can get some life out of it.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 03, 2021, 04:27:24 am
content: No. You start with C14, C18, C19, ...

author: valley001
author_rank: Frequent Contributor
timestamp: June 03, 2021, 05:54:45 am
content: I removed C14, 18, and 19 and they all seem fine vloss around 1.2v .5 ohm esr and within spec capacity. I am going to order new caps anyway and go from there, at least then ill know the problem is not with the caps.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 03, 2021, 06:08:32 am
content: I'll pull C14, C18, and C19 too and experiment a little. I'm confident that I'll drop that ripple substantially.As for the voltage regulators, they may need to be cleaned or replaced with multi-turn versions.It may be a day or so but I'll post updated scope shots.

author: valley001
author_rank: Frequent Contributor
timestamp: June 03, 2021, 06:26:22 am
content: I ordered new caps for the power supply board, and an LM317T just because they are cheap. Ill update when I have them installed. This thing really has been a headache but I cant give up on it lol.Oh, incidentally, as I was probing around the power supply board I discovered the junction of CR8 to the trace that connects to T1 was open. The part went through the hole and was bonded to the pad on the other side but not the trace on top, and the connection was open. Must have been like that from the factory.

author: fastguido
author_rank: Regular Contributor
timestamp: June 03, 2021, 02:10:10 pm
content: Measured the input resistance on my LC102, 49.7 ohms.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 04, 2021, 01:39:29 am
content: I just finished replacing C14, C18, and C19:The change in the level of ripple was minimal. It dropped maybe 5mVRMS:The voltage rose .01 volts to 12.63:I followed that with a test using a 1% 100pf Mica and it's off slightly as always:My LC53 will nail all of my test capacitors and be dead stable while the LC102 dances around the number as you hold the button. Maybe that's normalHey Valley001, you said that you see ~100mV PPK of noise @ ~120Hz on your scope while I see ~100mVRMS @ ~60Hz while on Pin 6. Was that a typo?

author: valley001
author_rank: Frequent Contributor
timestamp: June 04, 2021, 02:31:40 am
content: Yeah, I should have taken a snap of that. Ill post some images of scope readings once I recap mine, caps arrive next week. When you adjust your 12v rail is it smooth or does the voltage jump around? I watched this on the scope, a small bump of R40 and the voltage would snap from 14volts to around 16 or something like that. If I zoomed in on the rail I could see the 118khz noise following the ripple (does that make sense).

author: Smoky
author_rank: Frequent Contributor
timestamp: June 04, 2021, 03:10:27 am
content: I have never made any adjustments to this LC102. It sits most of the time since I like using the LC53 more.If you need any voltage measurements or signals read on the scope just let me know

author: valley001
author_rank: Frequent Contributor
timestamp: June 04, 2021, 03:00:43 pm
content: Thank you very much for offering that. I will definitely need that assistance.

author: coromonadalix
author_rank: Super Contributor
timestamp: June 04, 2021, 04:30:28 pm
content: Always do an open short calibration on each usage ...

author: Smoky
author_rank: Frequent Contributor
timestamp: June 05, 2021, 12:54:06 am
content: Hey Valley001, there are a few voltages that are checked and adjusted prior to the calibration procedure.I'm going to continue on replacing electrolytics and then go back and make adjustments to the vr's if needed.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 06, 2021, 07:15:18 pm
content: I turned back around to the Sencore wall transformer to see if it's adding to the noise we see on the 12v rail.Well, lookie here...I attached the oscilloscope probe to the power adapter and the ripple looks acceptable.Just farting around, I mounted two 2700uf 35v capacitors in parallel (the board's traces are printed to run capacitors that way too). I only needed to relocate the 1K Ohm bleeder resistor.The original 4700uf 35v capacitor measured exactly 4600uf. The two new caps both measure 2519uf (5038uf total).The ripple dropped to 9.2mVRMS (~10%).The output is rock-steady at 22.80 VDC with no load.*Update*The capacitor job is done. All of the electrolytics within the LC102 have been replaced with fresh ones including the 10uf on the front panel board:I followed that with a voltage check/adjustment from the calibration instructions above. What's nice is that Sencore installed test point loops for three of the four locations.CR16 was adjusted to 14.5v on the button and it's rock-steady. It originally read 13.9v:IC6 was next and it was adjusted to 18v and it is rock-steady too. It read ~17.88v originally:IC14 was next and it required no adjustment. It was measured right at .20v. No picture since it had no test point and I didn't want to risk shorting something out if my probe slipped while holding the camera.IC27 was last and it was adjusted to a super-stable 1.7V. No flutter whatsoever:The VR's appear to be multi-turn pots since adjustments were broad and fluid.When it comes to checking leakage, I find that when you set the LC102 to 500v, the actual voltage reads 490v. And when it's set to 990v, the actual voltage reads 973v:I hope to get 20 years of service out of this LC102. I just starting reading the manual to this thing and I didn't realize all of the amazing tests that can be done with it

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 06, 2021, 08:35:32 pm
content: Wow! a power brick that is actually SCREWED together, normally they are glued or welded together, nice!

author: coromonadalix
author_rank: Super Contributor
timestamp: June 07, 2021, 01:37:49 am
content: I ditched this psu and put an switching power supply brick of the same voltage but 2 amps of output, just transfered the sencore wiring, and fitted perfectlyThe original sencore is an half bridge transformer, some noise will occur, you could patch an 0.1uf 250v in parallel of the main capacitor, and could update the main capacitor say with 30% more value not more than that, it should help a little.