[time-nuts] HP Z3801A project update

[ed breya]

Since back in October, I've been working on getting the Z3801A in a 
5065A carcass project closer to completion. I mostly finished the clock 
control board and other details, and moved on to the power system. I 
decided to completely redo a lot of the mechanical arrangement, wiring, 
and the line powered section with nicer layout and all the facilities 
for battery backup.

A couple weeks ago, I had a major setback. I was really on a roll, 
having wrapped up the external battery control section, and ready to 
start on the internal battery interface and charging system. I had 
already built in the facilities for external DC input years ago, so it 
was mostly ready to go, once the controls were set up. I was doing some 
checkout on external powering, when the DC-DC converter in the booster 
crapped out - specifically, during hand-off tests between AC powered and 
external DC powered modes. I had incorporated all sorts of protections 
against faults, but apparently not enough, or I missed something that 
could happen in certain conditions. After inspection and rethinking, I 
still can't determine for sure how the converter could have blown out, 
but I have some theories.

First, I should mention that this Z3801A is of the "BTS" type, which 
runs on 20-30 VDC. This is handy for battery backing operation. The 
internal battery system is two 12 V, 12 A-hr SLAs in series. When 
backing up, it is used directly, without any extra conversion loss, just 
the forward drop of the blocking diode.

The AC power system uses the original 5065A transformer and choke, plus 
a bunch of other stuff to make everything work. The main output is 
called "Vs," and is the central point for all the power, running from 
about 21-30 VDC, depending on line voltage and battery modes and 
conditions. Everything goes into Vs. Its blocking diode is the rectifier 
bridge back at the transformer output.

The external source was intended to be 12 V nominal (11-15 range), like 
a car battery. For this mode, I had stacked a DC-DC converter output on 
top of the supply voltage, with all sorts of diodes to prevent (so I 
thought) damage under all conditions. The regulated output of the 
converter was isolated 12 V, and the input could run on anything from 9 
to 36 V. It was really nice, simply adding 12 V to the external supply 
voltage, giving around 23-27 V, less the blocking diode drop.

Anyway, it ruined my day when it crapped out. I had a pretty good deal 
going, with exactly three of these identical DC-DC converters - two 
deployed, and one spare. The other converter runs from Vs, to make the 
regulated 12 V wrt chassis ground, that runs all the peripheral 
circuitry. The common line for all the power side circuitry is separate 
from the chassis (but clamped by diodes), to better control any ground 
loops.

So, pondering the situation, I could have replaced the converter, and 
scrutinized everything thoroughly and added better protection. Or, I 
could have just dropped the feature for now, to be figured out some time 
in the future (likely never). I chose a third option, to build a fresh 
boost converter into it, where I know and control exactly what's going 
on inside, rather than guessing the limits of the not all that well 
specified last DC-DC converter module.

I'm happy to report that I finally managed to squeeze everything into 
the very limited space, and it's up and running fine so far. The new 
deal also opens up other options for improving and simplifying operation.

The converter is based on the LT1070, made civilized and protected by 
various circuitry. I'll have more to say next time about what's in 
there, and details of how all the stuff plays together.

Ed