middle display dig2 stays blank

[combustionmark]

After assembling 10 of the LE, I had 2 of them with a 7 segment display that stayed blank, dig2.

After checking all solder joints for shorts and opens, I started comparing the defective boards against a good board.

What I found

SAA1064 pins
              good board          bad board
pin  2      3.1 V                    2.5 V
pin 11     1.3 V                    2.5 V
pin 14     2.0 V                    0.2 V

Black meter lead is on pin 12.

After rechecking for shorts, I removed the 2700pf cap beside the SSA1064 chip. Finding the ceramic material it is maid of, was insulating it from the board. Soldered it back in, and presto the display is now working. The second LE, the same cap is bad. A 14 cent part at mouser. I will get it with my next Mouser order.

[chrisatpsu]

kinda nice when you solve it yourself?   

[mcangeli]

I've built one so far and the issue I have is the middle 7 bar led is missing the upper right bar no matter which module I put in.  I checked my solder joints and all is good and the le works so I'm not too worried...

[combustionmark]

Is the left one working properly?

The left and middle display bars "segment" are hooked together.

If the left one is OK, check pin 13 of the bad display socket "DIG2".

If both have the same problem check pin 9 of the SAA1064 socket.

Should be wired pin 13 of the left display "DIG1" to pin 13 of the middle display "DIG2" to pin 9 of the SAA1064 socket.

                           SAA1064 PINS
DISPLAY        DIG1     DIG2     DIG3
pin 1  "a"        10       10         15
pin 2  "f"         5         5           20
pin 7  "e"        6         6           19
pin 8  "d"        7         7          18
pin 9  "DP"      3         3          22
pin 10 "c"        8         8          17
pin 11 "g"       4         4          21
pin 13 "b"       9         9          16
pin 14 "com"  11        14        11

[pokey]

Same problem with my middle display -- nothing.  I pulled the 2700pf and jumpered...then middle came on, rest off.  Resoldered the 2700pf no love.  Must have been a bad batch of them...gotta go find one, but I can live w/o the middle display for now...

[mokeefe]

Well the second LE I just put together had the exact same issue -- missing middle digit.  I chased my tail a bit checking the driver chip and the 7 segment LED's themselves, until I found a thread from last year discussing this issue.  RJ indicated a missing 2700pf capacitor would cause these symptoms.  Mine wasn't missing, but I decided to focus on that area.  I tried reflowing the joints on it, but no change.  Then I pulled the cap and temporarily held in a new one. Bingo, all 3 digits displayed.  I soldered it in place and all is good now.  Now I do wonder if there is a bad batch of caps out there. I have to place an order with Mouser for some other stuff, so I'll pickup a few spares.

-Mike

[dlhoppe]

Is it possible they're more sensitive to over-heating while soldering?

[mokeefe]

Is it possible they're more sensitive to over-heating while soldering?

I suppose it's possible.  I know I'll be paying close attention when I solder that part in the next two boards.

-Mike

[RJ]

They are more touchy than most of our parts. Need to keep an eye on it. Look at them and make sure they were not dipped to deep and the coating is covering the leads and preventing soldering from working.

RJ

[combustionmark]

I know that even if it is not dipped too far, the leads are so small that the cap can be inserted too far, causing it to loose connection. I would suggest using a discarded resistor lead trimming, place it between the 2 pins on the board, and insert the cap allowing it to rest on the shim. Now take care not to heat this cap up, solder the cap in place, and slide the resistor shim out. This cap is so small that the displays and driver chip will still protect it.

DO NOT SOLDER THE SHIM TO THE CAP

Or better yet, use a toothpick..

Just a thought after my marathon run. Amazing what some sleep can do.

[combustionmark]

Oh and I did find that this symptom may be reversed, IE center display lit outside displays out. This seams to happen after the LE has been running a while, and may still change back.

Still the best dimmer kit available.

Thanks RJ and all you crazy volunteers. If not for you, this would not have happened.
Great job all.

Thanks

[RJ]

Ok I see, yes the capacitor is the device that controls the freqency of the multiplexing of the display. So it depends on where in the cycle it is to which set of digits work. One pair (middle display) only has a single display so you see one not working when it is that pair of the drive.

RJ

[combustionmark]

Friday my Mouser order came in. Hat to go to the post office  Saturday to pick it up. And after a quick cap change on my 2 suspect boards, every thing is working perfectly. Did a burn in overnight just to check the caps, and still going good.

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The price for a single has gone up. it is now .35 ea for under 100 get 100 and they are .13.

I must have gotten lucky they show .11 on my invoice at quantity of 10.

have fun

[JerryPlak]

Great Info Guys, 
I put togetter two boards and one has Same problem with my middle display -- nothing
will look later to see if it is a capacitor.

[wftxlites]

Me too

Same problem on my third LE build. First two were perfect. I went through the same troubleshooting and found the 0.2 Volts.  Then I remembered seeing this thread and came back to read it. Sure enough replaced the 2700pf cap and it now works.

Thanks so much!!   

Greg