[41] Odd display problem

[emece67]

As U0 does not oscillate because of the damaged clamp diode, and it is not possible to remove it, the obvious workaround is to replace the oscillator by an external one.

An external oscillator (with an output impedance lower than than of the damaged diode) driving OS1 will do the trick. The low impedance of such oscillator will "overdrive" the damaged clamp diode. The pulses will reach U0 that, being enabled by U1 in STANDBY mode, will pass them to its output and thus to the internal counters and, thru OS2, to the 2nd chip.

In RUN mode, the external oscillator can be stopped, but it is not a must, as in RUN mode U0 is disabled (and thus the oscillations do not go beyond U0). If the current consumption of the external oscillator is much lower than the overall current consumption of the rest of the system, it is allowable to keep it working in RUN mode.

But in SLEEP mode it is mandatory to stop the external oscillator, otherwise the current consumption is such mode will be high, seriously reducing battery life. Evenmore, in SLEEP mode, OS1 must be maintained low, otherwise the DC current consumption on the damaged clamp diode will also affect battery life.

Signal DPWO can be used to switch the external oscillator on and off, as such signal is high in RUN and STANDBY modes and low in SLEEP mode.

Thus, a low impedance, low current consumption, switchable astable multivibrator is what is needed to solve this.

The power consumption of the system in SLEEP mode is very low, in my case it is around 38 uA. This prevents the use of TTL, HC, and many analog chips because of excesive power consumption. So, the external multivibrator must be built around discrete transistors or using 4000 series chips. The later seems way easier. Evenmore, the 4000 series chips date back to the late '60s, so they are appropriate for a vintage calculator, even TASP will find it right ;-)

So, the patch is this:



A single 4093 gives 4 Schmitt trigger NAND gates. One of them is used to build the switchable astable multivibrator with the same architecture to that inside the LCD driver chip. It is controlled by DPWO. A 2nd gate is used to invert the output of the multivibrator to ensure that OS1 is low when the system is in SLEEP mode. The other 2 remaining gates are unused.

The thresholds of the 4093 gates are not the same as those of the Schmitt trigger inside the LCD driver chips, thus, to maintain the same timeout period of 11 minutes, it is needed to modify the value of the resistor from 200k to 470k.

Capacitor C2 in the CPU board is no longer needed. Opening the connection between OS1 and such capacitor will lower the power consumption in STANDBY mode a bit.

I have also tested another version of the multivibrator (with another more resistor and a diode in the feedback net) that produces the same frequency but drives OS1 high a very little time and low a longer time. The expected goal was to minimize power consumption in the damaged clamp diode, but the measured power consumption was almost the same (only a 5% decrease in STANDBY mode). Perhaps other parts of the LCD driver chips "do not like" to be driven by an asymmetric clock. Thus I finally used the symmetric multivibrator.

Finally, the current consumption of the whole system in SLEEP mode is the very same with or without the patch (my multimeter sees down to 0.1 uA). In RUN and STANDBY modes it has risen about 150 uA, which is almost negligible in RUN mode (from 5 mA to 5.15 mA) and acceptable in STANDBY mode (from 550 uA to 700 uA). The battery life will be affected very slightly.

Hope this helps, regards.