Electronics > Repair

NOS Dana 1992 just went crazy - fixed - broken CPU

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kawal:
I bought a NOS dana 1992 with manual and power cord. Seems like a spare unit  kept for the military. It worked great  for about 10..15 hours and now will not boot.  Trying to boot multiple times would bring it back but now that also is not working.  I read the eprom multiple times out of socket and its always reading back the same, so I don't think it the eprom . Now even multiple reboots are not helping and the unit is stuck.  The screen comes up either blank or with random segments lit up.  First I thought it was the 244 chip that was bad and replaced it . The symptoms are getting progressively worse if that's even possible.

The standby button would work  but nothing expect a reboot would bring it back from standby.  Now its will not react to standby most of the time.  I did check and the gate that is connected to standby is reading the standby button.
I did pull the keyboard and verified each key is working. There are no stuck keys, None of the keys are broken yet.
The reset circuit is  held low at power up and goes high which seems normal.  When the Standby circuit is not reacting I noticed the IRQ pin is being pulled low. 
When IRQ is high the PB port activity seems not normal.  Sometimes there is just spikes on the data pins and other times there is ramping signals  and sometimes some pins show normal square wave activity . Even when activity  looks more normal there is no screen activity  - it remains  at the same condition as when booted up.
The IRQ circuit is pretty complicated  but the more I look at it the more I think the MC146805 is toast.

The  voltage rails all are in spec.  The logic chips get a nice clean 4.995V .

On occasion when the keys are pressed on the front there is a lot more port activity  but nothing is getting updated in terms of LED or displays.

I looked at other threads but not exactly the same issue.
https://www.eevblog.com/forum/repair/more-racal-dana-1992-repair-woes/

Help  appreciated.

Stray Electron:
  FIRST, don't start replacing parts willy-nilly like the OP in the other thread did! Not only is that expensive, but there is also a butt load of fake parts out there today and there is a very good chance that you're install at least one bad part and cause yourself even more problems. And it's not just the fake parts, but also many 30 year old NOS parts could be failing by now. And add in the risk of damaging circuit runs, solder bridges and the like.  Take your time and actually troubleshoot each suspected circuit and only replace what is necessary.

  I would start by pulling every connector and every socketed IC and make sure that the contacts and the IC pins are clean and aren't tarnished or corroded.  A lot of this old stuff used socketed ICs which is nice from a repair point of view but the problem is that the socket contacts were made of one material and the IC pins another and they slowly develop poor connections due to galvanic corrosion. Especially if keep in any kind of humid environment.  After making sure that they're clean, it wouldn't hurt to apply Deoxit to all of the switch contacts, plugs and sockets, etc.

TheDefpom:
The front panel buttons are likely the problem, they go bad from age (not from use) and they can then confuse the uP on boot up.

I did a couple of videos replacing them on different units, I had links in some cases too to a source of the buttons I used.

Definitely worth re-capping it too, old is old.

David Hess:
You measured the DC voltages but did you check for AC ripple?  It sure seems like a worn out aluminum electrolytic capacitor problem.

coromonadalix:
I have two of them  sleeping somewhere, i did remove all the tact switches on one of them, and never finished  my job  loll

even ordered some tcxo clocks and pcb's  for a reference clock upgrade in them

And one of them  had the mentioned problem,  some tact sw  where stuck "pushed"

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