NOS DANA 1992 not starting up anymore

[kawal]

Bought a NOS never used DANA 1992.  based on the chips inside the unit was made in late 88 or 89.

the unit worked great for a few hours. Now it starts up 1% of the time . Most of the time it has no display.  I noticed the flip flop IC26A is stuck . keyboard presses cause the Clock pin to go high but the output of the flip flop will not change. The processor is sending a reset to the reset pin.
 I guess even NOS  fails these 74 series chips. maybe power supply has transients on these units?
Now I need to get the 74LS74 and also the 244 just in case that is loading the outputs but i don't think so since  sometimes it works.
While probing the chip at pin 12  with a 10x probe I can make it startup 75% of the time. I thought the probe load was helping so i added a load resistor to that pin . That did not help maybe capacitance - so added a small cap - that did not help. It might be a broken internal bond wire and somehow the pressure on the metal pin  is helping it. I hope I am on the right track with replacing the chip.
I did replace the 2x1uf caps in the reset circuit as they measured bad in circuit but out of circuit they where OK.  The new caps are PP.  Also replaced the 47uf startup delay cap.  This one also measured ok. The startup  reset is 550ms which is close to the 500ms in the manual.
The processor is working (lots of bus activity even when stuck with display off.
The 10Mhz oscillator also working fine.
Kawal

[kawal]

I replaced the 74LS74 which seemed to be having some kind of issues sometimes with a  NOS (1983) 74LS74 in ceramic package from the local electronics store.

Seems to boot every time now.  I will stress test it for 25 tries to make sure.

the bad chip was made by national Semi - i heard the cheaper plastic packages  a re more prone to failure vs the ceramic packages.
Still a little disappointed about a brand new 1998 product failing after son few hours of operation.
BTW the custom chip in the gold/ceramic package  runs very hot so i added a heat sink to it. 




Kawal

[edpalmer42]

There are a couple of well-known problems with the Racal-Dana 1991-1992 counters that you should watch out for.

1.  The switches under the buttons age and fail.  There aren't many good replacements.  Look around the forum for more info.

2.  IC39 is a semi-custom IC that is absolute unobtainium.  If it fails, and it seems like it tends to fail, your unit becomes a brick.  It runs hot so you might want to add a heatsink to prolong its life.  I found a clip-on heatsink for a 40-pin DIP.

Also, any NOS piece of equipment can have bad caps, flakey sockets, etc.  If you haven't already done so, remove and install all the socketed chips, cables, etc. to clean the connections.

Ed

[kawal]

There are a couple of well-known problems with the Racal-Dana 1991-1992 counters that you should watch out for.

1.  The switches under the buttons age and fail.  There aren't many good replacements.  Look around the forum for more info.

2.  IC39 is a semi-custom IC that is absolute unobtainium.  If it fails, and it seems like it tends to fail, your unit becomes a brick.  It runs hot so you might want to add a heatsink to prolong its life.  I found a clip-on heatsink for a 40-pin DIP.

Also, any NOS piece of equipment can have bad caps, flakey sockets, etc.  If you haven't already done so, remove and install all the socketed chips, cables, etc. to clean the connections.

Ed

Ed

Thanks for the suggestions. The switches seem fine so far.  The keyboard request only  comes in when a key is pressed. I did add a heat-sink to the unobtanium  chip.

I did remove the only digital cable in the unit that connects the GPIB board and the oscillator cable.   I don't think there are any other cables in the unit.

My unit was on for 4 hours to test  stability and it went through 25 reboot cycles with no issues. . At the end of the test i rebooted it  and  its back to the blank screen again.  Seems like a thermal cycling issue or something like that . The flip. flop was dead for sure as it did not follow the logic table in the spec sheet. Perhaps the other national branded chips are bad as well. - Hope not but i got all of the 74 chips from my local store today just in case. 

[coromonadalix]

I've seen regulator leads  cut themselves / mechanical stress

I have to find all the 32x switches for my 2x  1992  sitting in a shelf ....  they where functionals last time  i've started them ??