Sencore LC102 troubles

[valley001]

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. 

[valley001]

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?

[mzacharias]

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

[valley001]

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?

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

[valley001]

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.

[valley001]

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.

[coromonadalix]

the service manual  i've patched together is at ko4bb  a 53 meg zip

the 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_LC102

For 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

[fastguido]

Also check the PAD20 diodes in the range section(silver transistor looking devices), I have found those shorted or leaky.

[valley001]

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. 

[valley001]

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. 

[fastguido]

Just the leads themselves or with it hooked to the unit?

[valley001]

Just the leads themselves or with it hooked to the unit?

The resistance between the positive output and ground on the unit. 

[fastguido]

Just the leads themselves or with it hooked to the unit?

The resistance between the positive output and ground on the unit. 

i'll try and do that tomorrow.

[coromonadalix]

Check cr1 on the 2000 board,  re-take the ohms / impedance measurements with the high voltage board unplugged      with the sencore not powered up

You can have minimum 50 ohms on the output  with r34 r35 in parallels  on the DA2000 board

Maybe the relays switching logic may have some problems ??

[valley001]

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?

[valley001]

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? 

[Smoky]

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.

[Smoky]

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.

[valley001]

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. 

[valley001]

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?

[Smoky]

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.

[valley001]

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. 

[Smoky]

No. You start with C14, C18, C19, ... 

[valley001]

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.

[Smoky]

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.