In the tinkering cave this week we have a Panasonic AE700 HD Projector. These units are pretty old now, but they come highly recommended in many online reviews and the HDMI support means they remain a useful home cinema option.
The unit was first presented to me by a work colleague who complained that the unit would not illuminate. It powers on just fine, but returns to standby after about 30 seconds without displaying any picture. My colleague had already completed some diagnosis of his own and had determined, correctly as it would turn out, that the power supply is not generating a 15V rail – which is required for the lamp circuitry to work.
With the cover removed, it can be seen that this unit is a bit of a beast! It’s packed with a mixture of high-tech electronics, complicated optics, and extensive cooling components. Dismantling is slightly tricky; you have to remove the top sensor board first (the one with the buttons on it) and then you need to very carefully remove the dark grey cowling. The power supply sits underneath; which is why the cowling must be removed for further diagnosis to take place. There is a wiring loom which connects the PSU to the main board, and this will need to be disconnected temporarily (at the main board end) because it feeds through an opening in the cowling. You can connect it back after the cowling has been removed.
With the cowling removed, you can now take a look at the PSU. “Let the dog see the rabbit”, as they say.
The first rule of electronics diagnosis is “Thou shall inspect!” In this case there were no signs of explosion, trauma, or electronic stress of any kind to be found on the board. So it’s time to get the schematic out. Fortunately, the service manual for this unit was ‘freely’ available on the interweb. You can download my local copy of the service manual here. You might see a warning about “untrusted connection” – that’s because I can’t justify the expense of an SSL certificate. But I assure you there are no harmful files in my repository!
The schematic for the power supply is a little bit limited; it shows some discrete components but other circuitry is presented in block diagram form. Still, there’s enough information to start a line of diagnosis. The circuit I am interested in is shown to the left; click on it to open up a larger copy.
The second rule of electronics diagnosis is: “Thou shall measure voltages!” The circuit diagram shows P3 as the main connector – this is the wiring loom that had to be disconnected to remove the cowling. With the wiring loom connected back in, I measured the following voltages on P3:
P3 Voltage Measurements
|12||2.7||"MAIN / STB"|
Nothing to be too concerned about there. Here’s the voltage measurements for P2:
P2 Voltage Measurements
As can be seen – no 15V! Looking at the circuit, we have an output from the photoisolator which drives transistor Q107. The transistor is switching a voltage (which I measured to be 18V) to IC102 which is a 15V regulator.
One thing that I found really suspicious about this, is that we have a tiny transistor (only a small signal device, as it turned out) driving a much larger linear regulator. Going by the circuit we are looking at, the small transistor has to pass the same current as the linear regulator. I don’t know what current the linear regulator is supposed to drive, but it definitely has a much larger current carrying capability that the transistor which supplies it!!! It stinks of poor design choices to me. And Q107 has now become prime suspect for this failure.
And, sure enough, a quick measure of transistor Q107 revealed that it had indeed succumbed to its (inevitable?) destruction. 18V in, 0V out.
To remedy this, I replaced the component with one I had in my junk bin. The choice of transistor is not too critical here; it’s just switching a voltage. So I chose a ‘beefier’ device, which will happily support the current carrying requirements of the linear regulator – hopefully for the rest of this projector’s life.
A brief triumph – and a gift!
The projector worked after this, and I handed it back to its owner. Unfortunately it was returned to me a few weeks later with the same complaint. I was told that I could have the projector to play with, and keep if I could get it working! 🙂
I re-visited the power supply expecting a repeat problem, but everything checked out. Since the 15V supplies a H.T. power supply circuit for the lamp, and the symptom was that the lamp doesn’t illuminate, I began to wonder if the H.T. power supply needed some attention. Unfortunately this supply is cocooned inside metal shielding which does not seem to want to come apart very easily. As I was poking around with my screwdriver, I noticed that the projector had suddenly begun to illuminate, but she only fired up for a few seconds before there was an arcing sound and the projector switched back off. At the time I had been poking around a flat-cable signal loom which comes from the H.T. power supply and connects to the main board. I noticed that this loom was routed inside the H.T. leads for the lamp! This is almost certainly where the arcing had occurred!
I re-routed the small signal loom so that it gives the H.T. leads a wide-berth, and tucked it into the scart PCB. I imagine that the cable had become disturbed through repeated dismantling of the unit and had found itself tangled in with the H.T. leads somehow. You know cables, right? They tangle themselves up when you’re not looking!
After this I switched on the projector and voila! We have illumination. I took this opportunity to play Karate Kid: Classic.
Result! One working projector, saved from the scrap-heap.