Sencore LC102 Repair and Better Schematics

[pcwrangler]

Hi Everyone! I finally obtained an LC102 for repair and immediately wanted to chew on glass trying to read the schematics at the bottom of the service manual. To save my sanity I assembled them as best as my skill allows. I'm posting them all here in hopes that it saves someone else their time and sanity.

Now I can continue my repair without turning my head or jumping pages. Here is my progress so far...
I observed intermittent success booting and scrambling display upon testing each function. I also noticed capacitors and regulators on the leakage supply board(3000) getting way too hot.
Replaced all electrolytics and the dead 7905 at IC7 to get back the -5v supply and solve the overheating when machine is idle. I followed the calibration procedures for the power supply successfully up to step 7. I can only adjust from 3.5v-4.1v range when .975v is expected. I did not move past that step.
The Capacitor Leakage function looks like it works but the other functions just lock everything up while the heat on caps, regulators and coil skyrocket. I have to turn it off to prevent fire.

I would appreciate any hints. Thanks for reading!

[pcwrangler]

Split posts for file size limits.

[pcwrangler]

Update: I found a shorted switching diode (1N4148) at CR15 on the Main board (2000) when running tests. It would shoot over 120C. Replaced that and the unit booted faster, lowering temperatures even on the 3000 board. I can now run Cap Value and ESR tests successfully.
The Dielectric Absorption test shows 0% on every cap I test, regardless of what capacitor I use.
The Capacitor Leakage Ohms test looks like it is working but the Current test shows a weird value that looks like 49.99mA but a missing upper left segment on the 4. See pics
When I try to zero the leads I start with OPEN and it immediately returns ERROR 7. When I do the SHORT it works perfectly. I am using Pomona Electronics which I have read others have used successfully.

The R69 adjustment procedure (step 7 on cal document) still only results in a range of 3.2v-4.1v instead of the expected 0.975v

[coromonadalix]

check the big relay contacts, it was reported  having oxydation can cause some zeroing issues and or other problems

[wn1fju]

I feel your pain with regards to some Sencore schematics.  I have the LC53 which is a junior version of the LC102.  The schematic is only two pages, but contains nearly 100 lines that terminate in a number in seemingly random order.  One then has to search through the schematics to find the matching number.  Identifying the connections to just one integrated circuit took me about 15 minutes.  Worst schematic I have ever seen (and I have seen a lot)!

[pcwrangler]

I saw a discussion in another thread with the same ERROR 7 symptoms (open lead zero and caps <2uF) as my LC102 and their solution was to replace the PADs at CR29 and CR30. I pulled those and they both had the expected voltage drop according to the datasheet. The only difference from that thread was he calibrated the voltage section fine and I can't get step 7 within expected range. I also noticed discrepancies between the service manual and the calibration instructions. The service manual listed R69 on board 2000 instead of board 3000 like the cal document describes but I can't find R69 pot anywhere on board 2000. The board layout picture in the service manual is missing the R69 pot entirely. Are those calibration instructions universal for all board revisions?

Maybe I should still replace the PADs but I just don't see how that would effect the tests in the lower capacitance sections. See pic. Also they are $15-$20 each so an expensive shot in the dark.

[pcwrangler]

Also, I just found the assembled schematics coromonadalix uploaded to K04BB site. Wish I saw those sooner! 

[coromonadalix]

it was a nightmare to patch the sheets

but on one schematic  you had some problems around one ic ....

[pcwrangler]

I found the relay at L10 was shorted closed so I replaced it with a K model I found on that one auction site. Luckily they were all good. After replacing the relay I was able to zero the leads successfully. The cryptic screen seen above is also gone and replaced with flashing "88.88 mA".

I still can't adjust R69 down to .975v.  One step closer.

Edit: I forgot to mention I can now measure capacitors below 2uF

[Robert Grieb]

Hello,

    Not sure where to post this.   I recently spent some time figuring out and commenting some of the LC102 V17 firmware.   Probably not helpful for repairing a unit, but perhaps of interest to some assembly language programmers.   Here is a link to my page on the LC77/102. 

http://www.tauntek.com/LC77.htm

Bob Grieb