Running the XJ-A140 With Missing Diodes
I finally got a hold of a new projector, and was able to resume experimentation. Due to the large amounts of pollution in the existing threads, I have created this thread devoted specifically to running these projectors with 1 or more missing diodes, please keep any other discussion in other threads.
Current Progress:
I first removed the diode array, unsoldered 3 of the 4 diode banks, and attached wire to each of the 3 banks. Note, there is a small surface mount component on each of the outer banks, this is a 10k NTC thermistor and removing it will cause an overheat error on the projector. It is important to leave at least one of them in good contact with the diode array, so the the computer can detect possible overheat conditions.
After this was done, the connections were insulated, and the array installed back in the projector. Space is tight, so don't use excessive tape, and keep in mind that the heatsink gets HOT which will cause most adhesives will loose their stick, so plan accordingly.
Finally the wires were connected to power resistors (its messy/undersized, I used what was laying around), and the projector was powered up.
Findings:
*Removing diodes does not adversely affect the uniformity of the output, I removed 18 diodes from my projector, leaving a single string of 6 at one end of the diode array, and the uniformity of the output did not change appreciably. For reference, the diodes are arranged in 4 groups of 6 diodes, in groupings 3 diodes 'tall' and 2 diodes 'long' stacked next to each other to form a 3x8 array of diodes.
note - don't trust this picture too much, it is testing my camera more than it is the projector. The white balance looks a bit better in this picture than in real life, however the grays actually do look gray in real life, not purple/brown as in the image.
*Despite the fact that the 445nm diodes are the workhorse of the projector, it is possible to remove large numbers of them without throwing off the color balance. I removed 18 diodes from my projector, and the output is still acceptable (see image below). The output does look a little yellow, but I have seen properly tuned projectors that looked worse (this is due to the fact that the human eye's color response fits a power law, so the 75% decrease in power only looks ~20% dimmer)
*The projector's power supply actually has a large compliance voltage range, and works with voltages as low as 20V (as opposed to the 30v typically encountered in operation). This means that you can remove 2 diodes per string (that is a total of 8 diodes) without making any changes to the projector, simply remove up to 2 diodes per string and short across the pads.
*This also means that you can remove an entire string, and replace it with a simple resistor. Resistances as low as 14ohm work, but I recommend a 15ohm resistor in case different projectors have slightly different compliance voltages. This resistor must be capable of dissipating at least 20W of power, I recommend getting at least a 25W resistor to play it safe. Something like the TBH25P15R0JE ($5 a piece) would work well if it were mounted to the heatsink internally. This might require some creativity/modifications, and in any case I hope to eliminate the need for this resistor altogether (see future work) so I am not perusing this too much further.
*Due to the pulsed nature of the output, you must be careful to keep the inductance of the dummy resistor low. In my testing I had appropriately 10in^2 of loop area, and while the projector did power up I was able to hear the 75Hz buzz on my radio! This can be done by twisting any leads together, and using a low inductance (NOT wirewound, thick film or the likes) resistor.
Future Work
*It looks like it will be possible to disable the voltage sensing entirely and just short out unused channels, I hope to investigate this further. If anyone would like to donate a diode driver board I would be happy to try and modify it. I have a simple schematic drawn up if you would like to perform the mod yourself and report back as well, but it will require cutting traces and rerouting signals. Those who get squeamish at taking a razor to their $800 projector need not apply!
*I am also investigating how to dim the red led, the current plan is to add a simple resistor in parallel which will divert some current through the resistor, and cause the LED to dim. I hope to try this tomorrow, I suspect that a ~0.25ohm 25W resistor will do the job, although it will get hot! Again, it would be nice to trace down the current sense line inside the projector, I hope to investigate this route as well.
One Request
I encourage everyone to perform this type of mod on their projector, however I would like to politely ask that you do not offer this as a service commercially.
If someone would like me to perform the remove 2 diodes from your projector I would be happy to, simply send my your diode array (you pay shipping both ways) and an I will remove up to 8 diodes from your projector, I only ask that you let me keep one of the diodes I remove for my time. Alternatively, I will perform the mod for $40 and you can keep all of your diodes.
Obviously there is no way for me to enforce this request, but it is a way to help recoup the time I have invested in reverse engineering these projectors. Needless to say, if people do begin ripping off my findings for commercial gain I will stop publishing them.
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Originally Posted by decix
