Hi Johnny, Can you please let me know where RUN/SS1 (pin 13)voltage is coming from ( to LT1940 )? It is about 11V and FB1 voltage is about 7V. I think both voltages should be lower( RUN/SS1 about 4V and FB1 about 1.3V) As a result there is no 5V coming from that LT1940. Thank you for you help.
And for my vdc measurements, see this info.. But without the diode the box seems to work, and I think it's related to the scart connector.
Question: Can you please let me know where RUN/SS1 (pin 13)voltage is coming from.. If it's to, or from is i'm not sure with, but SS1 ans 2 goes to the U1105. But when I remove the R1140 0 ohm resistor it comes from the LT1940 chip. When R1140 removed, U1108 side 2.6v, U1105 side 1.2v. And if I place the R1140 back, and remove the 0 ohm resistor between the SS1/SS2 U1108 than I still have on both the 2.6v. So, to me it seems to come from the LT1940 chip.
Johnny, Thank you for your reply. I also thought that RUN/ss1 is coming via R1140 but then I also found out that if I remove that R1140, there is still the same RUN/ss1 voltage as it was with R1140. Are you sure that it can come from LT1940? If I understood well LT1940 datasheet that RUN/SS1 should start up the function of LT1940, in other words LT1940 does not start until there is a proper value of RUN/ss1 voltage. And do you think that high FB1 voltage ( about 7V) is related to high RUN/SS1 voltage?Or not? Thank you
The FB1 goes to the 85C resistor 75 kΩ, after it to the L1118 coil and to the 104 100 kΩ resistor to the PG1 , the other side of the coil goes to the SW1 which is the 5v output. I think that if the LT1940 is working fine that maybe the L1118 coil is defective (overheated), or the FB1/PG1 resistor or interruption between them. It's nearly the same as in the LT1940 datasheet, the typical application example. Only they use the SW2 for the 5v. Question: RUN/SS1 Run ss1 and run ss2 are connected to each other with the zero ohm resistor. Same as you can see in the data sheet. However it is also connected to the U1105 Pwgd pin. I'm not sure why it is, and why it has the 2.4v because as the datasheet shows that it may set to 0 voltage. Nevertheless if you have on RUN/SS1 11V and FB1 7V then seems to me more a LT1940 ic issue.
Johnny, Thank you for your reply. I was rather busy last few days, so I found some spare time to continue with the repair as late as on Saturday. I replaced the LT1940, now FB voltage is about 1.2V which is correct . Yet, the output voltages are not correct. I guess some of SMD resistors or capacitors have a different values. Johnny, do you happen to know what values of resistors and capacitors ,around the LT1940, should be? Thanks a lot.
No, sorry I have no parts list from the dm500hd but as for the resistor values you can use a SMD resistor code calculator from the internet. But I also advice you to disable the 3.3v and 5v outputs from the circuit to see if it's really collapse due the LT1940 or from another circuit by removing the 5v R1125 (print side) 0 ohm resistor, and for the 3.3v the R1136 (parts side) 0 ohm resistor. However, do not keep the motherboard to long powered on when you measure the outputs, because other circuits not really like this if it's taken to long.
