Panasonic PT-AE700 LCD projector Power Supply Issue

[allspark]

Hi there,
This post is intended for those that are not afraid to apply a soldering iron to their beloved projector and have sound electronics knowledge.

Note: Be careful and do it safe. Lethal voltages are present. The responsibility is all yours...

I bought a defective Panasonic ae700 projector on ebay. The described fault, no standby light no sign of life...

I confirmed that it is indeed the power supply module that is faulty.
As this was supposed to be a cheap buy for the occasional use I certainly did not want to spend AU$200++ for a new module.

Unfortunately, the circuit contains 2 special modules and off course it had to be one of them

The module MR20T is a feedback generator for the SMPS. It serves 2 functions.
- regulate the output based on the 14.6V
- selectively rectify the 6V transformer winding to produce 3V3.

You will find that the 14.6V regulation is still being "attempted" but is somewhat screwed up.
The other voltages also "hover" lower than usual. The 3V3 can be as low as 1V or not present at all.
Buzzing can be heard as well.
Before you waste your time, this is what you need to confirm. Just measure what is supplied from the ribbon cable to the main board.
Normally you get:
14.6V
17V
6V
3.3V
The rest are control and GND wires.

So you have spotted some life in the power supply dungeon?
Time to unplug the projector and wait 10min to let all the HV discharge.

Then start peeling off the layers of ducting and screening. A magnetic screwdriver is recommended.
You need to undo the screw holding down the lamp supply on the left to be able to pull out the front plastic ducting of the main power supply.

The idea of this mod is to disable the internal 3V3 regulator/rectifier and just let the module do the 14.6V regulation job.
The 3V3 regulation will be done externally by a low drop linear regulator.
The tracks to the power input and output 3V3 of the MR20T have to be cut.
The output is then shorted to GND to tell the regulator to 'shut up'.
The little leaded filter inductor has to be removed, this will provide the tapping points for the linear regulator.
Now:
- install a 3A Schottky diode from the now isolated transformer output to the 1000uF 10V capacitor (the isolated 3V3 track). I used a HER303.

- Solder the Linear regulator to the screen for heat sinking. I applied a tiny bit of heat transfer compound and pushed it down flat while soldering the tab to the screen with a BIG soldering tip.

- Connect a 47uF cap or higher to the output of the regulator and at least a
100nF ceramic cap to the input. You must have at least 47uF otherwise
the regulator can not work properly!!!!
Regulator MIC29300 in TO220 package. Farnell# 121-3487

- The last modification is the primary current sense.
Because of the linear regulator the PSU has to produce a few more Watts.
The limits are adjusted very tightly so we must relax this setting by soldering a 330R 0805 resistor across the filter cap.

It is best to check for shorts, especially where the tracks were cut. Just for sanity!!

- Finally connect 3 short wires as per photo to the linear regulator.

Note: I have kept every part away from the isolation barrier so no safety is compromised. That includes the wires!!!

Install the board with the screws and connect the input power plug to the board only. ( The big one with 2 wires on the right).

Connect a little load of no less than 10mA ( a LED and resistor) to the 3V3.
You can stick the wires in the connector.
Power up, and measure the rails.

!!!!!DO NOT TOUCH THE HEAT SINK ON THE PCB THEY ARE LIFE AT 400V!!!!!

You should now measure:

14.6V
17V
6V
3.3V

If so, disconnect the power and have another cup of coffee while the HV is discharging.

Reassemble the projector.

Enjoy it as I do...



Detailed power supply modifications:


Installed regulator: