[time-nuts] HP5065A digital clock update

[ed breya]

Just finished rebuilding the digital clock, and all is well. The 
original clock IC was still good, so the display board stayed with 
minimal rework - only new interface and power system needed. Made a 
number of improvements:

1. Eliminated original power/interface board and clock setting buttons.
2. Isolated clock power common from chassis so no current through front panel.
3. Added electromagnetic shield surrounding entire module. All 
connections via feed-through caps.
4. Added local bypassing of LED currents at drivers and on clock IC power.
5. New power system with shunt regulator for LED power, making it a 
constant current load, to isolate the large step changes in segment 
and scan currents from the main supply. Also includes more filtering 
and fault protection.
6. Clock setting to be handled via 1 PPS signal - sped up or stopped 
according to controls to be added inside center compartment, 
accessible from front panel.
7. Added "dim" mode - display still visible when operating on battery 
backup mode (takes 15 mA extra).

Operating characteristics:
Input power: 12 VDC (regulated), approx 320 mA constant on AC power, 
40 mA on battery backup (dim), 25 mA (blank), "battery read" button 
still functional for normal intensity.
1 PPS input: TTL level, high impedance, 0-100 kHz.
Mode control input: AC on = +12V, AC off/fail = 0V.

Ed