[41] Odd display problem

[emece67]

Hi all,

I recently fixed (almost) a broken posts 41C (fullnut) I own since the 90s. But, after the repair, it shows an odd display problem I'm not being able to fix.

Problem is: the display is always blank, except when a program is running or a key is pressed. In other words, the display only shows info in RUN mode, but it remains blank in SLEEP mode (as expected) and also in STANDBY mode (this is not the expected behaviour). As a side effect, the auto turn-off after 10 minutes in STANDBY does not work, the machine does not auto turn-off.

I've found some info on this problem in this forum ( see message 5 here ), but this was a 13 years old thread.

In order to diagnose and fix this problem I've done, by now:

• tested the machine with 2 different CPU boards (a 2-transistor C and a last gen. CV), both showing the same behaviour.
• desoldered the display board to check all tracks in the keyboard PCB. I'm sure there are neither shorts nor open circuits on such PCB.
• soldered again the display PCB checking that the metallic strips really connect both PCBs. I've checked continuity from the zebra strips between the CPU & keyboard PCBs to the tracks in the display PCB.
• cleaned keyboard and CPU PCBs with isopropyl alcohol.
• inspected the display PCB under a loupe trying to see any short/open. I cannot see any problem.
  

I'm suspecting that the oscillator in the LCD driver is not starting when the machine enters STANDBY mode. This explains both the dead LCD in such mode and also the non working auto turn off, as its timeout is controlled by the display oscillator.

Has anybody see this problem previously? Any clue about how to solve it?

Thanks & regards.

edit: I've forgotten to mention that the current consumption of the machine is, more or less, normal. About 5 mA in RUN mode, 500 uA in STANDBY and 35 uA when SLEEP. Batteries are new.

edit 2: lines PWO and SYNC work as expected. PWO high, pulses in SYNC when in RUN mode. PWO low and SYNC high in STANDBY (SYNC does not go low after 10 min). Both PWO and SYNC low in SLEEP.