InFocus LP600 Power Board Questions

[mbates14]

no no no. dont scope a startup pulse.

there are 0.1uf 400v starter charge pump caps that need looked at, along with the coupling cap.

youll need an analog ESR meter to read these for opens. on a TENMA brand analog ESR meter, they will check somewhere around 20 ohms. if the needle barely moves, or doesnt move at all (assuming the meter is calibrated). then the cap is bad and should be replaced.

[mbates14]

Yes theres an inductor in series with the circuit, on most magnatek infocus supplies, you have a big black elongated coil with the lamp wires coming out.

[mfruech]

I also have some (4) LP600's that are dead. If there is someone on this thread willing to repair the power board, please let me know. I am willing to buy a new board or send in my old for repair.

Mike

[mmclendon]

SUCCESS! Replacing the output caps with high ESR allowed the supply to start and run just fine. They were difficult to find but the 105o parts were used to try to extend the power supply life.

ROOT CAUSE of FAILURE: There is really nothing wrong with the Magnetek power supply design. However, the InFocus system design has a serious flaw in the power supply cooling when the unit is in standby ( only whatever convection cooling is available). So the unit that sits in standby for a period of time bakes the caps and one day, the baked caps in the control loop are in such bad condition, the supply cannot start up if it ever loses power. If you have these projectors installed permanently, remove the power completely when not in use. The lightning bolts at InFocus should warn the user about the issue. Thanks again daGimp for the lead.

Originally posted by mmclendon
Today's report from the engineer:

A careful visual under magnification does not reveal any visible faults. Will start at the output of the supply as the mains capacitor is at 170 VDC (normal with a full-wave rectification and 118VAC input).

The output section of this off-line supply is electrically separated from the main section by 1 grounding capacitor, 5 optio isolators and a ~1/2" wide gap in the copper on the circuit board.

Pulled one of the 100uF caps out of the output circuit and checked the ESR (as suggested by member daGimp) which was high. So, this poor supply has been running for about 3 years now and we'll replace those 85oC caps with the 105oC devices for a few cents more each.

Next step - order and replace output caps based on the assumption that if they are dry and out of spec, the opto-isolated feedback to the PWM chip will inhibit start-up.

Will also order a TOPS 245 PWM through-hole device since they are inexpensive and might as well have one around. It is believed that this device is the lamp switcher and the one in the circuit now is probably OK but what's a few dollars with the economy in trouble? The MC34... PWM device will be assumed good for now since it is a SMT device and a bit of a hassle to replace.

Will report back next week when parts are received, installed and another attempt to start the supply on the bench is made.

NOTE: If you are doing this work also, don't forget the lamp cover interlock must be defeated (shorted) for bench testing. This is the 2 port connector on the bottom of the board.

[mbates14]

You are correct to some extent, but your analysis is wrong.

there IS a design flaw to the infocus magnatek power supply.

If you look, the only cap that actually goes bad is the 100uf cap right behind the chopper controller output IC.

Why? if you look, for one, this chip has no heatsink, and 2, the capacitor is right behind it directly against it. this allows the heat to transfer from the chip streight into the capacitor, almost like they did this directly on purpose so it will fail soon.

remedy for that is to put the new capacitor in, but only push it in a little bit, soldering it in. which will have super long leads. then bend the capacitor over, away from the heat of the IC.

problem solved.

[75Shovelhead]

MBATES14 I have a 340b and the power supply is blown can you help me with this problem? Thanks feel free to email me.

[mbates14]

I dont like working with the 340s too much anymore becuase of the silicone mess they put on the power supply. i go through 3 razor blades and a half hour just to get it off enough to work with.

[MobileDick]

Yes, it works for me too!
Replace four 100uF/16v/85'C and one 4.7uF/35V/85'C electrolytic capacitors to new 105'C caps.

[craigowens]

Why? if you look, for one, this chip has no heatsink, and 2, the capacitor is right behind it directly against it. this allows the heat to transfer from the chip streight into the capacitor, almost like they did this directly on purpose so it will fail soon.

Not sure if you can answer this mbates, but are you referring to the capacitor at location C3 on that board? Thanks!

[pappascw]

This projector is kicking my butt.. Same problem, worked fine, power went out no lights.. Managed to get it going a few months ago was fine until power outage then back to same problem.

Where do I get these caps? I have a kids party in a few weeks and would like to give it a shot before I pull the trigger on a new one..

Or of anyone out there does a repair at a reasonable rate??

Thanks,
Chuck