CurtPalme.com Home Theater Forum

[Boet]

Hi all,

Last month the power supply on my BV701s suddenly died at switch off / on. I found the BU2525A transistor to be shorted. Replaced it with a 2SC**** (similar specs), failed within a second as well. Left it for what it was, since I had a spare power supply.

So I took my spare parts BV701MS, and put its power supply in the BV701s (same R******* number). That worked, for a while.
I noticed the image started to subtly horizontally contract / shake sometimes, and after a while this got worse. As I ran towards the power plug, the image collapsed and returned (albeit out of focus). Didn't wait any longer and pulled the plug.

Checked the whole SMPS for bad solder joints, fixed them, reconnected the power supply. Worked fine for about half an hour, left my brother with the Barco to watch a movie. Shortly after I left, the same shaky image returned (according to his vague description). Now there's no image after turn-on, and the SF LED on the power supply is lit.

Swapped the deflection board, no change.

I vaguely remember the shaky image problem from the BV701MS (donor), which would point the problem to the power supply, but I'm not sure.

Anyone who has a clue, before I try to reinvent the wheel?


(Also, anyone who has a BV701 service manual? Currently using the BV708 one which seems similar enough.)

[Curt Palme]

Replace the HV quadrupler. Sounds like it's either shorted or arcing internally.
Also replace the 100uF35v capacitors that are driving the BU2525 transistors.

[Curt Palme]

Replace the HV quadrupler. Sounds like it's either shorted or arcing internally.
Also replace the 100uF35v capacitors that are driving the BU2525 transistors.

[Boet]

Thanks Curt!

I should add that the horizontal LED on the deflection board is also on.

So if I understand correctly: take the quadrupler from the 701s power supply (with the blown BU2525A), and connect it to the 701MS power supply (which now gives the SF error)?

I'll source some new 100 uF caps as well.

[Curt Palme]

are you sure it's the SF light and not the EHT holddown? I think they are right next to one another. BUt yes, try swapping the quads out, they are the same between the two sets.

[Curt Palme]

are you sure it's the SF light and not the EHT holddown? I think they are right next to one another. BUt yes, try swapping the quads out, they are the same between the two sets.

[Boet]

Yep, checked it multiple times. The other three red LEDs are off.

Replaced the quadrupler, still SF error.
Also, when I put the power supply back in the 701MS (donor machine), it gives me SF.

Unlike described here http://www.curtpalme.com/Barco701_Layout3.shtm , the HTHD remains faintly lit green (the HTHD line measures only 22V).

So I suspect something in the HTHD supply itself or the feedback loop around the HTHD supply is wrong?
Because the HTHD supply isn't completely shut down (22V).

HTHD Overvolt isn't falsely triggered, neither are Low FBHD or Low +17V.

Other voltages:
+30V = 33.2V.
+17V = 18.2V.


It seems that:
HTHD too low -> SF triggered via deflection board.
Judging from the capacitor voltage ratings, HTHD of 22V is way too low.

I yet have to replace the 100 uF cap in the driver.

If only Barco had provided measured voltages around components under standard operation, like Sony did.

[Curt Palme]

OK, you'll have to 'scope the waveforms to the vertical output and H output stages to troubleshoot. The H and V drive signals come from the RGB board, which then go to the deflection board.

The big issue is, swapping some boards will cause problems, even though you'd think they should work, so I'd go back to your original boards and start troubleshooting that way.

[Curt Palme]

OK, you'll have to 'scope the waveforms to the vertical output and H output stages to troubleshoot. The H and V drive signals come from the RGB board, which then go to the deflection board.

The big issue is, swapping some boards will cause problems, even though you'd think they should work, so I'd go back to your original boards and start troubleshooting that way.

[Boet]

Thanks again!
Both projectors are back to original, except for the power supply.
One PJ is up on the ceiling, the other comfortably on my workbench (tested it before I pulled the power supply out).

H drive is being generated, measured 475V across the deflection coils (across 950-975V measurement node R43-R44) at 22V HTHD.

The fault: Q27 (BC547) in the overvoltage protection circuit had failed, conducting intermittently without being driven and thus pulling HTHD lower.

Replaced the transistor, now I'm up to 36V HTHD and 780V at the 950-975V measurement point. SF error gone.
Does that sound OK?

Now, on to the other power supply which blew the BU2525A (and its replacement) twice in the main SMPS...

[Boet]