FIXED Sencore LC102 Capacitor testing problem low values

[fastguido]

I recently purchased a Sencore LC102 that won’t measure any capacitor value below 2uf, just shows Error 7, it’s fine at any capacitor value above this. It also won’t complete the OPEN test, again shows Error 7. The error comes up right away, doesn’t even think about it, (no moving dash across the display like the short test) The SHORT test works properly though, which would rule out any of the common relay problems that these unit have, (I think? ) I replaced the test cable with a new one from KK4HXJ just to be sure that wasn’t it. I’m thinking that it must be something with the two lower current sources in the capacitor test section? Does anyone know how the circuit works any far as the autoranging or the OPEN test? Of coarse there is no theory of operation, so I’m not sure how the circuit is supposed to work. Any ideas on what to look for?

[bob91343]

I don't know enough to attempt to answer your question.  I do know that for the most part I don't trust Sencore products.  Perhaps it's their unpleasant advertising that gets me, but I have never liked their stuff.

It would be nice if you knew what method it uses to measure.  It can be one of several, and if you know that, you at least have a starting point for solving the problem.  It could be a bridge or simply an impedance measurement such as used by Hickok under the guise of measuring capacitance.

[Johnny10]

Lots of information on the Sencore LC102 on the forum.
Manual, Calibration, Pics.

It is a very nice unit, I have had about 6 of them.

They are very sought after.

Open and short test problems usually have to do with relays.
Also problems with the input connector, the fuse is contained within the input assembly.
Sometimes reseating cables is a good first check.

[Smoky]

I'm using a Sencore LC102 as I type this. I'm matching Nichicon UKL electrolytic capacitors in the .33uf to the 1.5uf range with no issues.

The only time I see the "Error 7" is when I attempt to measure ESR on a capacitor below 1uf, which is normal.

I would make sure that your probes pass the "open" and "short" tests and to go ahead and unscrew and clean the BNC adapter/fuse assembly very well.

[fastguido]

Lots of information on the Sencore LC102 on the forum.
Manual, Calibration, Pics.

It is a very nice unit, I have had about 6 of them.

They are very sought after.

Open and short test problems usually have to do with relays.
Also problems with the input connector, the fuse is contained within the input assembly.
Sometimes reseating cables is a good first check.

I have already cleaned relays L2, L3, removed and checked out of circuit the reed relay L4. Cleaned and Deoxided the input BNC assy, replaced the fuse. Cleaned and deoxided all the PCB cables/connectors. Kinda don't know what to check next, since I don't know the theory of operation. I guy years past had the same problem with the open test and low capacitance test, but he disappeared from the forum, so I don't know what is found or if he ever found a solution.

[fastguido]

I'm using a Sencore LC102 as I type this. I'm matching Nichicon UKL electrolytic capacitors in the .33uf to the 1.5uf range with no issues.

The only time I see the "Error 7" is when I attempt to measure ESR on a capacitor below 1uf, which is normal.

I would make sure that your probes pass the "open" and "short" tests and to go ahead and unscrew and clean the BNC adapter/fuse assembly very well.

Passes the short test just fine, just won't pass the open test, I guess I will clean the  BNC adapter/fuse assembly again for the third time, but I don't think that is the problem. Thanks for the input.

[coromonadalix]

the error 7 could be caused by bad test probes and some relays who can be stuck in it, theres one who can give some trouble.

you have to do an open and close test   to see if your test probes are okay, the original cable has a special impedance .... you can buy an compatible cable on Ebay as i did and it worked perfectly

Sadly i  dont own an lc102 anymore    sold it   1 month ago

On ko4bb website, you have schematics, user manual and calibration and test sheets,  i had uploaded the joined images schematics in the Zipped file

[Quarlo Klobrigney]

If the output leads at P3, P3, P3 mean the input circuit, then check all these diodes below.
It's not the easiest schematic to follow as it is broken up so badly.
All you have to do is print the original out in a pdf software suite and use the split pages function if it was originally a full size schematic.
If it wasn't, then it's a poor scan to start.

[Johnny10]

There is specific circuitry for 0- .002uF Capacitance and .002- 2uF
It is on the 2000 Board schematic page.

Shown in attachment  coromonadalix stated Ko4bb website

[Johnny10]

coromonadalix
Thanks for uploading the joined schematics.
Back when I first started working on LC102 and your joined schematics were unavailable, I printed out, taped together and laminated those separate pages. Took forever!

[Quarlo Klobrigney]

I see those PAD 20's in there CR29 & 30.
They gave me a fit with my various pre-LC101 testers.
In fact they were even circled on the PDF.
I tested them and they read strangely.
I found some replacement$ at Mouser. TO-39 or was it 72?

[fastguido]

There is specific circuitry for 0- .002uF Capacitance and .002- 2uF
It is on the 2000 Board schematic page.

Shown in attachment  coromonadalix stated Ko4bb website

TR11 and TR12 never turn on because the timing isn't right in the 74c374 which controls these two current sources (in both the open test and the < 2uf cap test). I think that the
processor isn't telling these to turn on for some reason.

[fastguido]

I see those PAD 20's in there CR29 & 30.
They gave me a fit with my various pre-LC101 testers.
In fact they were even circled on the PDF.
I tested them and they read strangely.
I found some replacement$ at Mouser. TO-39 or was it 72?

I will check out those diodes, thanks.

[coromonadalix]

coromonadalix
Thanks for uploading the joined schematics.
Back when I first started working on LC102 and your joined schematics were unavailable, I printed out, taped together and laminated those separate pages. Took forever!

I did my best to join them  on Ms Paint,  not perfect tough,  better than nothing  loll

[coromonadalix]

Op   you should check every voltages given in the calibration document, you have to do some verifications before changing parts, as i wrote, some lc102 were prone to have a relay going bad on the contacts (the biggest one with a clear shell)  and other small relays ... mine was slightly different with metalised  / shielded relays  straight form sencore.

[fastguido]

I started doing the calibration per the document starting with the voltage calibration which was fine, then to the capacitor cal, but couldn't do that because it won't read the 1500pf cap or the open test. On the two large relays, I burnished and deoxided both.

[fastguido]

I see those PAD 20's in there CR29 & 30.
They gave me a fit with my various pre-LC101 testers.
In fact they were even circled on the PDF.
I tested them and they read strangely.
I found some replacement$ at Mouser. TO-39 or was it 72?

Great lead! CR-30 is bad, but they appear to be no longer available . Do you remember what you replaced them with? It seems the they are only rated at 50ma, so they put two in parallel for the 60ma current source, The newer JPAD's are only rated at 10ma so that's a no go.

[fastguido]

On ko4bb website, you have schematics, user manual and calibration and test sheets,  i had uploaded the joined images schematics in the Zipped file

Thanks for doing the schematics! I had them printed onto blueprint sized sheets, they came out great. My old eyes can now see the pin #'s.

[coromonadalix]

the  LCR102   i had never developped any problems,  it was a good horse

[Quarlo Klobrigney]

I will search the archive, the dusty shelves of my mind. I'll let you know later today.

Quote from: fastguido on Today at 02:14:29

Great lead! CR-30 is bad, but they appear to be no longer available . Do you remember what you replaced them with?

[fastguido]

I will search the archive, the dusty shelves of my mind. I'll let you know later today.

Quote from: fastguido on Today at 02:14:29

Great lead! CR-30 is bad, but they appear to be no longer available . Do you remember what you replaced them with?

Thanks

[Quarlo Klobrigney]

It used to be a Fairchild product now it's off to InterFET. Try Mouser with 106-DPAD5.
That's what I ordered and used. It all comes back to me now... I called the original device's manufactures and found someone knowledgeable who steered me in the right direction for a prahpaah (Boston) sub.
She's an expensive little thing that.
And I thought that it was expensive then!

[fastguido]

Very good, thanks for the time searching for it! So I see that it is a dual package, so you must of replaced both CR29,30 with one of these? $17.53 each wow!

[Quarlo Klobrigney]

Double what I paid !
But the pA leakage rating is better than the PAD20.
I needed 2 to do the job on my LC 53 & 75.

[fastguido]

Fixed, replaced both CR29, CR 30 PAD diodes, only one was bad, but replaced both from the same lot #. They are in parallel with each other , so they share current. Did the calibration on the capacitor section and is good to go. I am noticing that measuring in the pico farad range that reading do wander a little bit, compared to my DE-5000, but it has kevin leads on it.

But a question, On top of the unit there is a sticker that mentions, that there will be poor readings in the low capacitance range unless the unit is grounded. Any ideas on why that would be the case, ( maybe something to do with the switch mode power supply in the unit not being shielded?). I noticed that it appears the chassis/case is only grounded through the external power supply ground.