In article , Remove _ for valid address wrote:
A colleague has asked me to look at an intermittent fault on
his BenQ video projector. This is the first projector I've
worked on, I'm more used to TV and VCR repair (and digital
asic design, my "day job").

There seem to be two (possibly related) problems, which both
its owner and I have witnessed:

1) Sometimes it shuts down during use. Lamp goes out.
The red LAMP error led comes on (and stays on), the power
led blinks green for 2 mins (the normal power down delay,
with fans running) then the fans stop and the power led
goes yellow (normal for standby). TEMP led stays off
LAMP led stays on.

[... snipped...]

It uses a EUC 250 P/01 lamp ballast, with a small (5-pin?)
control cable. It would be helpful to know the pinout of
this, I imagine it has a lamp fault line on it. Anyone know?
It also says "9137 001 71805 MADE IN HOLLAND" on it, which
makes me wonder if Philips were involved in the ballast.

I've now traced out the ballast control interface. In case
someone finds it useful, here it is:

logic board cable ballast
............................. ............................
_____ . .
|4 W . . _______
U5 |----+-------270R----5---yellow---5---100R---|k c|-- "dim"
_____| | . . +--------|a_____e|--
3K Z . . | _______
| +--270R----4---orange---4---100R---|k c|-- "on"
gnd | . . | +-|a_____e|--
Y c . . | |
-------------b +5V---3----red-----3-+------+ opto
e . . couplers
+5V | . . _______
| gnd gnd---2---brown----2----------|e k|--
10K . . +----|c_____a|-- "good"
X | . . |
-------+----------------1---black----1-----+
. .
............................. ............................

The top opto-coupler (associated with cable pin 5) seems to be on when
"economy" mode is selected, which dims the lamp.

The middle opto-coupler (associated with cable pin 4) seems to be used
to power the lamp on.

The bottom opto-coupler (associated with cable pin 1) seems to be on
when the lamp is operating normally. I suspect it goes off to signal
lamp fault detected. It is driven from a Philips P87LPC769BD chip
on one of the ballast daughter boards, this appears to be an 8051
derivative microcontroller. Presumably it measures and controls
the UHP lamp's voltage/current/etc and uses some proprietary
algorithm to decide when it thinks the lamp is unhappy and report
this (as well as shutting the lamp down).

Hopefully my logic analyser will determine the sequence of events
when it next fails...

Mike.