3LCD projecto ballast trubbleshooting

[kokodin]

Hello
Thais topic is being created, because i am an idiot.

I bought 3 used , non tested lcd projectors. No bulb, no refuns, 10$ a piece. All of the projectors that used AC lamps, i was able to test and evaluate that they would work, and one of them, the worst looking one is now the only one i fully fixed.

I couldn't test the one  that useds DC lamp, no bulb of this kind to hack it. But projector had the best oprics, so i bought replacement lamp, cheap chinese one but still should be a dc one. the lamp arrived and still no sign of life. if the ballast was working it should strike if even only once but no.

there is no known place to find a schematic for the balast and i have no idea how to trubbleshoot it to be certain it isn't the bulb that would be a problem. no way of thesting the bulb in other projector either, all of the othe projectors use AC lamps

Fun part is, all the other sanyo projectors i have, use the same chinese model of ballast and they all work, and the only one that use rubycon branded ballast does not

is there a way to safely measure how does ballast work. Strike voltage and stuff. Stucking multimeter probes doesn't seem to be the best idea, and i don't have a high voltage probe for the scope (i could build resistor devider, this should work with dc output even if the bandwidth be a mess)

most of components are very small smd stuff, ballast comunicate with the projector via some kind of smart serial bus, so at least i know that the brain is somewhat responsive, without the balast communication cable projector auto shootdown with a red light, with it connected it do normal startup cycle with 3 ignition try and then shootdown
all the mosfets and big diodes are ok, at least component tester recognize them as correct kind of component

I could try to draw a schematic starting from datasheet for dc converter used in it, but i already tryed to read smd component values and most of them look unreadable.

Projector in question Sanyo plc-xw57
 ballast board
Rubycon prd 7491ea based on 2SK3869 and 2sk2699 mosfets and m51995ap driver chip
oryginal lamp should be a dc Shp117 but i have no more info about the bulb itself

Can something like that be even trubbleshooted without a schematic?

by the way, new ballast bard cost about 90$ on aliexpress but now i am afraid the something else might be broken, so i am not going to buy one

[kokodin]

ok a little update
I still don't know what went wrong, but with some work and free time i might get to the issue.

What i was able to test and figure out is that the ballast board is in fact not working properly.
I got another broken projector of that model, to swap-test compnents. This one was with a working bulb, so i was sure that this ballast is ok. I paid around 30$ with shipping and with some care i might even salvage the whole woring projector out of it, but to the topic.

Power suply boards are both ok
Both bulbs are also ok and seems to work on the new ballast in both projectors
Logic boards seems to pe both ok
All sensors that could cut the power to the bulb are ok in both projectors
One ballast is as mention ok one is ded in both projectors

The problem is i am still not sure if i have a brain issue or the working part of dc converter issue.
Replacing opto couplers seems to be locking the projector so i did put the oryginal ones back . i put them back in random order so i guess none of them is broken, but still that only highly educated guess
Did not probe anything under power yet, did not fully tested srmiconductors either, but i got myself a hot air station so i guess i can desolder every single part and test it one by one... seems like fun

stupid question
would the pcb on a simple dc-dc converter most likely be 2 or 4 layers ?

[kokodin]

well one thing i know for sure, i have no idea what kind of smd transistors there are, but i figured out by swaping ICs that the broken board driver is ded. Because it doesn't work on the other board, and that the broken board isn't fixed by simply swaping ded switching controller to a working one. ;(
oh well at least the working one wasn't broken by my swaping and still works after putting the oryginal chips back on it.

unfortunetly i still did not figured out a way to probe those boards onder power. they live in the projector in cuch place there is no way to get to them  if everything is assembled and , well i would rather like to avoid probing live board under 300V dc floating around on the cables.

[kokodin]

well one more thing

i was playing around with those drivers on batteries. 8 9v batteries to be exact, around 70V.
Both boards work and behave very similarly.

when i send the signal to ignite the lamp by shorting one of the opto couplers both boars send constant voltage to the output.
it is actually the battery voltage  so it might ot be  working as it should , but both boards behave the same way which make me think. Does the driver behave normaly? and if it is, how do dc lamp strike an arch? there is no spikes on my scope, but for one thing, input voltage and current are only good enough to enable low voltage part of the circuit, and there was no load on the output so not really the ideal situation.
i wish i know how to make a fake load to test this off the projector. remove current shunts to make this board think it have a load?

[kokodin]

and now surprise surprise it works, only it doeasn't
i have no idea what i did to it, but i have ignition, lamp is lit for few secunds and then it turns off like it wasn't iginited and projector is cycling like it did not noticed the lamp ignition. All i did was playing around the output and measuring the output voltage. i did short the wires few times on batterys so it would hold high voltage on for longer, but that was it. it is getting late i will go to bed and rest a bit

this is actually quite fun, i did not replase a single component, i deslodered them and put back on after figuring out what they were and if they seems ok, but yes, this board is a mistery :] everything seems ok but the whole thing act as it was broken
i hope it isn't the microcontroller that talk with the main board