Barco 909 smps dead 2SC3998

Good idea!

You can easy disconnect the subboard and try the other subboard so you know for sure. Check them to be certain but I found no different schematics for different smps models.

Is it a newer model with a different R number your 2005 smps?

Look at these Panasonic FM for the standby voltage

Untitled by Radio Head, on Flickr

Ok I need some help here. I tried everything I could think of and am almost stuck.

I replace the transistor Q101 in the SMPS that switches the HTHD, the voltage for the horizontal deflection. The new Q101 is a 2SC3998 just like the old one. I tested it as a npn transistor and it seems good. Behind the Q101 is a thin sheet of plastic I applied cooling pasta on both sides.

I also replaced the pulsating TDA4601. A quick check on the pins made me belive it measured th same as the old one so I think the TDA was not broken.

Now when I switch on the Barco with the big button it should go to standby. From what I understand 325V (I measured) is going through fuse F100. It is on the collector of Q101 but since the unit is standby the TDA is not powered and Q101 has zero volt on the base so It should not conduct. I checked that with the fuse F100 removed and indeed the TDA has no power and Q101 has zero volt on the Base.

Now when I insert a fuse a normal SMPS gets 0.8V on the pin 9 of the TDA supply and it is still off (it needs 20V). So on my faulthy SMPS inserting the fuse should not activate the TDA in any way.

But what I see is that with the fuse the HTHD led burns on the subboard (subboards is tested ok) even in standby and the fuse blows. Also a crack is heared. After that only the standby led burns on the subboard. The subboard regulates the 9V standby that part works good.

So I conclude that the 325V that should be blocked somehow conducts through the transformator of the HTHD to ground. But in standby the base of Q101 is at zero so is the emittor. Q101 is tested again perfect. I took it out again. The TDA should be not working in standby but difficult to check. The only thing I can think of is that C102 is shorted at high voltage but checking with the MM gives it is ok.

So how to check the TDA further? Inserting a fuse blows the fuse. Perhaps disconnecting Q101 to make sure there is no conduction there? Than inserting the fuse and measuring if somehow the TDA is switched on in standby?

Or testing the other option that C102 is conducting at 325V? Seems unlikely to me and would cost me my last fuse I am afraid.

Ok I did a first test removing Q101 and removing C102. Inserted a new fuse and the fuse does not blow in standby!

The voltage in standby on pin 9 of the TDA is 0,8V and that is the same as the good smps so there is not problem with the power to high activating the TDA in standby. Also there is 0V on the base island of Q101.

So why does the fuse blow in standby? Is it the only option left that I see that C102 is breaking under 325V? I checked there is 325V on the island that normally the Q101 collector connects to.

This test seems to exclude the other voltages blowing the fuse at least in standby?

I checked the diodes and resistors but will check again.

I measured the components mentioned just to be sure.

D114 ok
R101 ok
D101 ok
D111 ok
R103 ok
R109 ok

And I feel kind of stupid because in the picture it seems I made a spark gab? I think I have to find non conductive coolpasta first and be more careful with it?


Untitled by Radio Head, on Flickr

You will have to clean up this mess ASAP! This silver stuff is conductive and doesn't work for this application. When applying the new paste, only put a small blob onto the middle of each surface. It will be pressed into a (hopefully) thin layer what it is supposed to do. For this stuff the more isn't the better. Also make sure that You don't have any broken traces. The one around D111 looks quite damaged...