X-10 Maxi Controller Repair

X-10 image of Maxi-Controller (direct link)
One of the commercially available X-10 controllers is the so-called "Maxi Controller".  It can address all 16 unit codes and has the X-10 model number SC503.  In February 2004, two of my units stopped functioning with similar symptoms, and since they are $25 a pop, I decided to attempt to repair them.  One of my SC503s had two columns that stopped working (column with unit 1 and unit 3) so I opened that one first as it would provide me with the most clues to pinpoint the differences between a good and bad circuit.

The processor chip inside the case scans the keys in a matrixed fashion, and it turns out that both inoperative columns are associated with pin 1 of the chip.  Pin 28 is used for scanning the other two columns of number keys.  Using my scope, I displayed the waveform on pin 1:

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.

test bench
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.

  • Schematic of Maxi-Controller (courtesy of David Eaton - davidreaton99.at.gmail.com).

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