The blowers went on, the console was beeping constantly
and the console screen displayed complete garbage - it
really looked quite like the screen shown in the film
Matrix. So - obviously - something was wrong with the
machine after a rather long time without being in
As a first attempt in debugging this problem I removed
all boards which were not really necessary for the
operation of the digital console subsystem, so I removed
all analog computing elements, the DVM (digital voltmeter)
board, the coefficient potentiometer boards, etc.
Unfortunately, this did not change anything - the console
still displayed complete garbage and did not respond to
This was the moment I decided to check all voltages (which
was not too simple since the wiring is very dense inside
the machine) and this showed a problem: The -5V for
some of the old logic circuitry of the console subsystem
was only at about -2.3V. In the next step I removed the
rest of the boards which still were installed and checked
the voltages again - -5V was still out of range with
-2.3V, so the problem was in the power supply.
Fortunately the power supply of the EAI 2000 is quite
straight forward in its construction. Each output voltage
is provided by a non-monolithic linear regulator circuit
which consists mainly of an operational amplifier, a
darlington stage and some protection circuitry which
includes a foldback subsystem which gets active in case
of an overload situation. In addition to this an
overvoltage protection system is also present which shorts
the output of a regulator which has been out of range.
My first attempts to locate the problem in the overvoltage
protection stage were unsuccessful, but after a little bit
of measuring on the board which contains the -5V
regulator, I realized that the precision potentiometer
used to set the desired output voltage was defective.
So the whole problem with the missing -5V was eventually
solved quite simply by changing the potentiometer and
readjusting the power supply.
After this all voltages were correct, I reinserted the
boards of the digital console subsystem and powered the
machine on again. My first impression was that this had
solved the problem since the console displayed what I
had expected from reading through the manuals - the three
section display appeared and showed some informational and
error messages due to the missing analog components. After
a few minutes, the console display started to flicker and
jumped between complete garbage as before and a correct
output, so there was another problem in the machine.
Since all voltages still were in their ranges and showed
no ripple when measured with an oscilloscope, I remembered
that I had quite similar problems with my TELEFUNKEN
analog computers from 1959 and 1961 which turned out to
be caused by dirty connectors in the system. So I pulled
all boards of the console subsystem and removed and
reinserted all socketed integrated circuits (fortunately
most of the ICs are soldered directly into the boards so
this task did not take too much time).
After this the console display was stable even during a
quite long test run (still without the analog computing
elements being installed). The next step was simply
reinserting these computing elements. Since then the
EAI 2000 is working again and ready to solve differential
equations. :-) The only task which has to be performed in
the near future is balancing the operational amplifiers
since the integrators run quite fast out of range due to
misbalanced amplifiers, but this is just an inconvenience
since the system as such is working like a charm.