Bad LED segments on HP 8903B

Howdy, I picked up a HP 8903B that has some bad led segments. Looking around the interweb, I saw it's a fairly common problem yet I couldn't find any forums that posted a repair solution. I am hoping it just needs a good cleaning, been surprised how many times that has fixed bugs in electronics before. Its in the mail, I haven't received it yet but I am trying to research it so I can dive in and get her fixed. Thanks in advance!
 

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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
That reminds me somewhat of the Philips CD players that suffer similarly looking faulty LED displays. The problem there is that the display PCB itself has a monolithic driver chip embedded onto the back of the board (looks like a big blob of dark epoxy glue). The actually LED's are OK, its a drive issue, and that seems to be the case here as a whole line is out.

You might get lucky and find it is fixable by resoldering... assuming it uses separate discrete parts.

Would have to see one and see what the LED board and drivers consisted of.
 
That reminds me somewhat of the Philips CD players that suffer similarly looking faulty LED displays. The problem there is that the display PCB itself has a monolithic driver chip embedded onto the back of the board (looks like a big blob of dark epoxy glue). The actually LED's are OK, its a drive issue, and that seems to be the case here as a whole line is out.

You might get lucky and find it is fixable by resoldering... assuming it uses separate discrete parts.

Would have to see one and see what the LED board and drivers consisted of.

From the manual, I have the display assembly schematic and the interface schematic that drives the led assuming i read the schematic right and 17 is the correct line in.
 

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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
'17' looks to be a line of 4 inputs to the display driver chips. If this is a common issue then I would have thought there would be talk of any fix (assuming there is one). Maybe a driver chip has failed and its pulling a common data line high or low. You would have to try isolating each line from each chip in turn to test that theory out.
 
UPS dropped it off today! I took the covers off and saw what looks like a burn spot on one of the solder terminal on a diode. I took it off, tested it, forward bias .700V and reverse bias was 1.324V when it should be OL? The board is not labeled, the print is not readable and the X-Y axis locator doesn't seem right so I don't know which diode it is on the list. I think its the DO-7. I need some help identifying this and a compatible replacement. One of the big caps also tested leaky so I will replace them. If anyone can lend a hand I would appreciate it. I marked the diode in red on the fourth pic.
 

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Well, that turned out to be the easiest fix in the history of mankind. The power source was set to 220volts! Cripes! I started checking the power board test point voltages and they all were low. I pulled that cap I initially tested on the board and it tested good. After some reading I came across the voltage selection and sure enough. I don't know if I should be relieved or kicking myself for not checking that first. Pretty obvious I am a novice but I learn something every day...
 

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