[time-nuts] HP 58503A Display Board (Option 001) Problem Update

[Stan]

Hi all,

 

My 58503A Option 001 GPSDO had been running 24/7 for the past several years.
A few weeks ago I noticed that the same segment (segment E) in all of the
digit positions was not working, although the rest of the segments were
working as they should.

 

I unsoldered and removed the Vacuum Fluorescent Display (VFD) module (which
is the same as in the 53131A, 53132A , and 53181A) and found that I could
not get that segment to light up with the appropriate inputs applied,
although I could get the other segments to light up. "Aha!" sez I, thinking
that a new VFD will do the trick. Unfortunately, it did not. The new VFD
checked out just fine on the bench, but when installed in the 58503A the
same segment failed to light up and all of the other segments worked
properly.

 

So then I figured that the failed display segment in the original VFD
somehow damaged the output from the driver chip (HV518PJ, a 44-pin PLCC) for
that segment. So I got out my SMD rework equipment and replaced the driver
chip. Problem solved? Unfortunately not.

 

The behavior is identical to the original behavior, and also to that of the
new VFD with the old driver chip. Everything works properly except segment
E.

 

I've installed the display board into another 58503A and the display is
still defective in the same way. So it's not a problem with the motherboard.
I don't have another Option 001 display to compare this one to, so this is
the point at which I'm stuck.

 

The only other point of failure that I can think of is this: there is an
instrument-specific chip on the display board, a 58503-80023, that may be
some sort of decoder that turns the status messages from the motherboard
into a stream of serial data to send to the driver chip (which has no
decoding logic at all) to create the necessary characters for the messages
on the display. 

 

Any thoughts on where to proceed from here? Any theories as to how the
original VFD had the failed segment, but changing the driver chip and
putting in a new display didn't fix it? It's literally a straight connection
between the various output pins of the driver chip and the corresponding
input pins of the VFD. I checked the traces and there is continuity between
each driver chip output pin and the input pin on the VFD.

 

If the problem is a failed 58503-80023 chip, then how could a failure in the
VFD cause the chip on the input side of the driver chip to fail? Or is it
just some strange coincidence?

 

I've been referring to the CLIP for the 53132A to troubleshoot the display
board, but that only goes so far. The PCB in the 58503A is p/n 58503-60010.
Is there any detailed information available on this board? Anyone with a
58503A Option 001 board they'd be willing to part with?

 

Thanks for any help or suggestions,

Stan