TECH: WPC negative voltage is too low

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First let me start that saying yes, I understand that anything in the
negative going towards zero is actually going higher.

With that said I have 2 WPC DMD controllers doing the EXACT same thing!
Both are giving me -115 and -99 vdc and +62vdc. These readings are 10
volts too low on the negative side to illuminate the DMD. I've replaced
all of the high voltage components with those on a working board with
no change and the working board is still working so none of the
components are bad. I've also replaced the DMD with a brand new one and
tried a different controller and found that the display does indeed
light up if the negative voltages are -130 and -115.

If I remove Q4 completely the voltage goes to about -135vdc and if I
lower the resistance on R5 to about 30 ohms instead of the regular 120
ohms I get proper voltage.

My question is this. I have rung out all of the traces and there isn't
any high readings anywhere. Does, anyone have a suggestion of what
trace I should be looking at? Or should I just start jumpering them out
1 at a time to find where my weak point is?
 

martin

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Start by checking the voltages across D4 and D5. These are 56V Zeners,
so you should see 56V on each, 112V across the pair (or 62/124 if you
have the 62V version). Also, what transistors do you have in Q4 and
Q5? And check the value of R6.
 
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On 26 Sep 2005 19:02:27 -0700, "martin" <martin.reynolds@gmail.com> wrote:

>Start by checking the voltages across D4 and D5. These are 56V Zeners,
>so you should see 56V on each, 112V across the pair (or 62/124 if you
>have the 62V version). Also, what transistors do you have in Q4 and
>Q5? And check the value of R6.
Did all of this with the only difference at R6 where the voltage drop is 25 vdc
instead of 10vdc.

However, I've come to the conclusion that both of these boards have a problem on
the logic side of the board not the power side. Is there anywhere on the web
where there is a systematic guide to troubleshoot this board?


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Eric A.
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You already got where I was going. Those voltages are good enough to
get most DMD's to light up. The most likely problem is the 6264 SRAM
chip. After that, look at the data latch chips the interface to the
system bus. I don't know of a repair guide on this level, I just know
digital logic and follow the data paths. I've only been able to get a
few boards repaired on a chip level if more than the SRAM was bad. The
traces are so delicate, you end up ohming out every pin to make sure
you got it right. With a replacement board being so cheap, I only
spend an hour or so before throwing it into the reject pile.

K2
 
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Yeah but I'm a cheap bastard! :) Plus I like the sense of
accomplishment when I repair stuff.

I've made some progress though on this board. I now have the board
working with some garbage and it is not the Ram chip.

I went through the display tests and now I'm getting various "page"
errors. Page 1 test 4,6,12,14 and the same thing with page 2 test
4,6,12,14. And then I get a Ram data error (assume this is caused from
the input to the ram.

I know about U35,33,31 the 74ls175 chips.

Any other suggestions?

TIA,

Eric A.