Waveform on pin 1 of the processor chip. Note the lazy fall
of
the waveform.
For comparison, I
displayed the
waveform on pin 28 to see what a proper functioning circuit looks like:
Waveform on pin 28. Note the snappy transitions.
The waveforms showed what I was hoping for. Before I started,
I
surmised a bad pull-up or pull-down circuit inside the processor
chip. This kind of problem could easily be fixed by
using
an external pull-up resistor to the supply voltage. Using a
decade box, I connected a pull-up to pin 1, and BINGO! The
controller started working again. By increasing the value of
the
resistor, I found that I needed at least 470kOhms of pull-up.
I
decided to use a value near 100kOhms for current margin. Note
that since the Maxi Controller uses a negative supply voltage with
respect to neutral (there is no power isolation), that the term pull-up
is still correct, although the transition is from high to low.
Waveform on pin 1 after the pull-up was added. Note the fast
fall
time is restored.
Photo of the added pull-up. Pins 3&4 is a convenient
source
of the supply. Pin 1 is the left most
pin of the added resistor. Pin 28 is directly opposite to pin
1
(down direction in this picture).
I added two pull-ups to the other Maxi Controller in the same way (on
that
one both pin 1 and 28 were inoperative), and was able to completely
restore
it to operation.
If you attempt this repair. Be warned of
live voltages inside the Controller. There is no isolation
from
the power line, and if you reverse the polarity of the plug, you could
have the hot terminal of the power line on the entire circuit
board. I would recommend you use a test board like I have
here
with GFCI protection.
My X-10 test board. Note the GFCI outlet with breaker
protection. A mini-controller and lamp
load is supplied to test wall switches such as is shown here.
The
black barrier strip (middle)
allows me to quickly attach and detach units, or they can be plugged
into the outlet.
Links:
- Schematic of Maxi-Controller
(courtesy of
David Eaton - davidreaton99.at.gmail.com).