Need help diagnosing cause of seemingly dead display on ROWE

[jollyjon]

My display on my CD100A appears dead and I have spent over 30 hours trying to fix. The CCC appears to work fine (with no display to see how it feels) and I checked the following things -

1.On the CCC the LEDs operate as should when the system boots up - detecting no errors.

2.On the display board I get the 8V and 28V measured with DMM

3.On the 5V regulator I get 4.95V out

4.I spent ages doing continuity checks on display board and cables to the CCC

5.The LED on the display PCB that detects 5V is very weak. Checking for 5V around the board seems to be normal measuring 4.85V in a number of spots including right before the 33k resistor to the LED. On the other side of the resistor it measures 1.5V which seems like a normal voltage drop with the 33k resistor (but not sure) and I don’t know if weak LED is supposed to be as bright as the ones on the CCC. It also only measures 2mA before the resistor and 0mA after it (so maybe 0.5 mA).

6.Clumsy me shorted the 5V regulator when probe slipped across the pins but I replaced it with new one. I replaced the zener diodes I was suspicious of and also the tantalum capacitors but none of that made any difference.

7.I’m not sure if it’s a data issue or a power issue I’ve got with the display. I get -38V from the transformer on the PCB that runs the alphanumeric display but I think it’s an AC out for the display and rest of circuit is DC so if I had set it to AC on DMM it might have read +38 instead of -38 as I had my ground probe on the logic ground part.

8.I have a logic probe but not good in using it. I don’t know if the circuit is CMOS or TTL so any advice to this will help (I think it’s CMOS but want to be sure). There are 2 inputs for the data line (+ &-) and 2 for the clock (+ & -) into the first chip on the display board and then to lines out of it to the main display IC. I’m not sure would it should measure with a logic probe (high/low/pulse) but I’m assuming it would be a pulse so help from anyone that knows what I should expect from any of the inputs and outputs would be helpful to know to compare with. There is also a display load/reset pint on the main IC but not sure what that should get with logic probe either.

9.The VSS for the main IC is 5VDC but I don’t know what it should be exactly for the VDD (maybe 10-15V).



Appreciate any easy tips on how to diagnose this problem to find out at least what has failed (eg:CCC/display PCB/wiring/alphanumeric display/transformer/main display IC/etc), so I can just focus on the section that faulty. I’m not sure if anything on the display should still light up (even a dot) so it would be handy if anyone else has a laserstar model they could pull out plug 6 (it only has the 4 data and clock wires) from the CCC and see if anything is left lit up on the display at all. Considering the 35hours I have invested in trying to fix this I feel like if I got a job delivering pizzas I could have made $800 instead to buy another one for sale else where in the world but I do want to finish what I started so thanks for any help in advance.

[Ron Rich]

Going from my great--but not too long memory--
There was a problem on them with the driver chip-- had to replace it with another chip--several PCB mods involved ? seems as if the LED was "weak"--also seems that if the plug was removed from the CCC, display was "dead"-- HTH-- If more comes to mind later, I'll post it-- on Rich

[jollyjon]

Thanks for the response Ron even though it wasn't much is was indeed at least something. Which exactly is the driver chip - I know of the largest one on display PCB that's for the alphanumeric display and the small one before that where the 2 lines of data and 2 lines of the clock go in and just one of each data and clock go out of to the main chip. Which one of the 2 could it be -or is there another located on the CCC board? The display never had a problem until I turned it on after quite some time left it storage and never display a thing on it. All plugs removed from CCC are always put back when testing. It uses plug 5, 6, 15 of the CCC to plugs 2 and 3 on display board. Not sure what you mean about the "weak" LED - it does have a 33k resistor right before it on the 5V line that makes it weak by reducing the voltage but it seems it's is there for a purpose - I just don't know what the purpose is as the type of standard LED should taken a 5V line I thought. If I new the logic signals I should be getting with a logical tester I should be able to narrow down to the chip where the problem is located or eliminate if it is a chip at all and just a faulty alphanumeric display that reached it's end of life after 15 years (but I hope it's not that coz there rare to find).

[Ron Rich]

I'll crawl up in the attic later and look for my notes on this-- Ron Rich