Hello everyone. Just started DM repairs and first thing i stuck on is short to ground on 1V line from TPS 65251 (LX3 leg 36, 37). Track is going into the PCB and my suspicious is faulty CPU. Can someone confirm that or push me further in this issue?
Like the older dreambox receivers, it is likely that this 1v voltage is used by the cpu, and can therefore be caused by it. Provided it is not caused by a short-circuited ceramic smd cap related to this voltage. Through this picture you can see the 1v part and what I would do in this case is to remove the 4R7 (I think the L1906) or remove the DB1904 inductor next to the 4R7 to see which side the problem is coming from.
Removing DB1904 short is on U1902 (TPS 65251) side so there is a slight possibility that CPU is alive. I traced this 1V line into U2001 (TPS 56628) where i found no problem (by removing L2001). All capacitors related to 1V line i found are ok (C1655, C1611, C1612, C1966, C1965, C1968, C2005, C2006, C2007). When measuring voltages, 3.3V line drops down to 2V, i assume, because of short ... 5V is ok. Also U1902 converter is getting warm when powered and board takes about 200mA current stable. Removing U1902 short is still there ... but where?
The U2001 is another DC converter that only provides the 0.9 vdc output, so lower than the u1902 1v output. The u1902 provides 3 outputs, in this case the 5/3.3/1v dc. For proper operation it needs (I think) the 12vdc to the Vin pins (see this pdf datasheet for more on this) and needs to be checked, when it's too low it can give a wrong output. I think it is better to measure the outputs with ohms as the 3.3v output should be 2542 ohms, the 5v output ends up being around 35k and 1v 804 ohms. All ohm values should be close to these values and measure them only when the system board is completely unloaded otherwise you may get wrong values, and measure these values between a ground and the measurement spot.
Output voltages are now less important while having a short on 1V line. I just post my measurements for additional information. Even taking the U1902 chip out the problem persist, where i can't find a fault. Best option is to inject voltage, having U1902 out, to find a hot spot but I don't own a thermal camera.
Point is, that I ask for help from your community to trace the problem. Maybe someone knows a solution. I cannot find any schematics for this board or any other Dreambox.
First, there are no schematics of the dream boxes, and any other brand of receiver and other devices. Simple, because it was/is never released, nor were they required to deliver after it was decided many years ago. Only from the older ones there are some (kind of) schematics that were mainly voluntarily made by me and others for the Dreambox China forum/company that no longer exists. Anyway, enough blabla... Regarding your problem, maybe another member can help you with it, apparently not me. Good luck with it, and thank you for your contributions to this forum regarding the investigation of your problem.
So in the end the CPU, too bad because it will be difficult to find a cheap BCM7252SZBKFSBB3G. It is for sale on aliexpress, but too expensive. Nice picture btw, and thanks for mentioning if it turned out to be the cpu after all.
... but that's not the end. I did some maesurments on CPU and I did not find any shorts to ground (?!). CPU was reflow/replaced in my opinion. Someone did a terrible job at it. I found an alien flux under the chip. Some PCB pads are broken too, happily the NC ones. Next step is to reball the CPU and put it back to check if it's fine. And before that, I will make a map of connections for future troubleshooting.
