Changhong E32M250FD no standby

[maxvalutech]

This set well looked after and "won't turn on". No standby light so took main cap voltage 365v but only getting around 2.9v at both diodes and standby pin with mainboard disconnected, that voltage drops way down when main is connected. PSU is Megmeet model and found a schematic for it, replaced ec12, 13, 14 but little difference, diodes seem OK. The other circuits don't start and so no voltages. So am I right thinking the problem must be the transformer or a hot side component ?

[maxvalutech]

Re: Changhong E32M250FD no standby

Schematic herewith standby 5v is pin 8 of the 14 pin connector (from top on rear board pic.)

[budm]

Re: Changhong E32M250FD no standby

This is an always on power supply, only some of the cold side outputs are switched on when PS-ON is present.
Can you please put down all the pin names and follow by the Voltage reading for each pin since the pictures are not clear enough for me to read the pin names as printed on the bottom side of the board?

[maxvalutech]

Re: Changhong E32M250FD no standby

Thanks budm I have been focussing on standby only hoping for a quick fix - hasn't happened! No other voltages are showing at all and labels are for the top 6 pin connector is G G G 24 24 24 and I did think that perhaps LEDs should start independent of main but didn't follow through on that. The other connector pins are 12 12 G G G 5 5 5SBY PSON 5 5 G G and were difficult to decipher even with board in front of me. Standby led never flickered. Did run heat gun over board - no change. Guidance appreciated.

[CapLeaker]

Re: Changhong E32M250FD no standby

On your last post, check the 5SBY for 5V DC if it is there or not. The STBY power circuit is always on and has it's own STBY section on the PSU. If there is nothing there, disconnect the main board from the PSU and and check that same pin again. If there is still nothing, there is a problem on the PSU.

[maxvalutech]

Re: Changhong E32M250FD no standby

Just to reiterate with all connected there isn't any standby voltage of note as it reads 0.2v but with the main board disconnected comes up to 2.4 to 2.9v fluctuating all the time and that seems same at main cap on PSU which when I left leads on for a time also fluctuated 324 - 347v mainly which might be a bit low ? No other voltages are found at any time i.e. no 12 or 24v nor the various 5v pins.
Now the schematic is for MP022 - JD board and though similar it is a slightly different board - mine is MP022 - N . I'll just have to check all components on the front side of the circuit as I need standby voltage before anything else.

[budm]

Re: Changhong E32M250FD no standby

Try heating that small orange Lytics start-up running capcap in the hot side with hair dryer then power up the board to see if the 5VSTBY will be steady at 5V or not.

[maxvalutech]

Re: Changhong E32M250FD no standby

No luck with the heating budm but I replaced that cap anyway and standby did come up to around 3.2 but still fluctuating up and down, also then replaced the 2 blue ceramics at start of circuit but no change of note. The main cap now reads 339-346v which still seems a bit low to me as I'd like to see what I thought I had at first (wrong) 365v +, also the standby voltage I do get collapses to around 0.6 - 1.0v when mainboard is connected so does that mean the PSU circuit is breaking down under load?

[CapLeaker]

Re: Changhong E32M250FD no standby

Just to reiterate with all connected there isn't any standby voltage of note as it reads 0.2v but with the main board disconnected comes up to 2.4 to 2.9v fluctuating all the time and that seems same at main cap on PSU which when I left leads on for a time also fluctuated 324 - 347v mainly which might be a bit low ? No other voltages are found at any time i.e. no 12 or 24v nor the various 5v pins.
Now the schematic is for MP022 - JD board and though similar it is a slightly different board - mine is MP022 - N . I'll just have to check all components on the front side of the circuit as I need standby voltage before anything else.

That's what I thought. Your voltage reading of 324 to 347VDC is o.k. Here is the math: 240VAC x 1.414 = 339.36VDC. So take account of the input AC voltage too. Other than that I guess you have to start measuring voltages around the IC U2. Maybe there is something wrong with R77 and R78. Also check R52, the HV pin (8) and the VCC pin (6) of the U2 (LD7750).

[budm]

Re: Changhong E32M250FD no standby

No luck with the heating budm but I replaced that cap anyway and standby did come up to around 3.2 but still fluctuating up and down, also then replaced the 2 blue ceramics at start of circuit but no change of note. The main cap now reads 339-346v which still seems a bit low to me as I'd like to see what I thought I had at first (wrong) 365v +, also the standby voltage I do get collapses to around 0.6 - 1.0v when mainboard is connected so does that mean the PSU circuit is breaking down under load?

Is that without the main board connected to it?

[maxvalutech]

Re: Changhong E32M250FD no standby

Yes budm testing without main board connected. What I find puzzling is the fluctuations , almost like pulsing up and down, when taking those voltages and why I changed capacitors.

[dick_barton]

Re: Changhong E32M250FD no standby

From looking at the schematic the 5V, 12V and 24V should always be available at capacitors EC3, EC9 & EC12 with the 12V and 24V being switched to the output via mosfets when PS_ON is applied. So you don't need the PS_ON applied to check at the voltages at those points.
If those voltages are not there then it looks like a fault on the hot side of the transformer T1 possibly around U1 and the Mosfet it drives

What is U1 part number?

[CapLeaker]

Re: Changhong E32M250FD no standby

... or maybe there is a short on the secondary on one of the other voltage rails.

[dick_barton]

Re: Changhong E32M250FD no standby

Yes, that's also a possibility.