Dynex dx-42e250a12 will not power on

[dustman1747]

Hi guys,
We got another donor tv to work on here. The tv's model number on the back of the case is a Dynex dx-42e250a12. When we originally got the tv it would not power on at all and the power supply board showed no signs of power. We were unable to locate the source of the problem and decided to save time by ordering a used replacement power supply board. The power supply board is marked as a Hisense RSAG7.820.4543/R0H.

After receiving the new board, we now have a repeating single red blink on the front control panel. I was unable to located anything that referenced blinking error code for the light. There is a faint ticking noise coming from what sounds like the left side of the board. The voltages measured on the XP901 connector appear to rise at the "tick" and then fall back to near 0 as if its trying to start and something is failing.

The listed voltages for connector XP901 are:
GND
STBY
5V_S
12V
12V
GND
GND
PWM
SW
GND
GND
18V
18V

Actual measured voltages with all boards connected:
-
.5-1V
.5-2V
1-3.5V
1-5.5V
-
-
0V
0.5V
-
-
3.9-14V
2.9-15V

The LED connector has only a couple of volts on it.

With all boards disconnected the standby voltage stays steady at 5V.

There is no change with the LEDs or the TCON disconnected.

I was not sure if this power supply could be jumped to allow it to power up without the mainboard so not much testing was done with everything disconnected.

Checked for leaky or bulged caps and found none. Fuse also tested good.

Does this seem like we got another bad power supply board or is it possible that something on the mainboard is causing problems?

[budm]

Re: Dynex dx-42e250a12 will not power on

So if you run the power supply board by itself, is the 5V_S steady or not?
That ticking sound you hear is the power supply is in start-up/shutdown loop due to too much load, bad caps, etc.
For us to help we will need good clear pictures of the whole backside of the TV showing all the boards, the straight shot whole pictures of the top and bottom side of the power supply board.
It is hard to do long distant troubleshooting without seeing what we are dealing with.

[dustman1747]

Re: Dynex dx-42e250a12 will not power on

Without the mainboard plugged in 5V_S measure a constant 4.9V.

Ive attached a bunch more pictures hopefully they're good enough to get what you guys need.

[budm]

Re: Dynex dx-42e250a12 will not power on

OK, so it is either the 5V_S power supply is weak or the main board is drawing to much current that causes the power supply to go into shutdown.
You can load the 5V_S power supply with 5 Ohms (or 4.7 Ohms) 10W resistor to simulate 1A of current draw from the power supply to see if it can handle 1A per spec of the power supply.

[Andrew F. Ali]

Re: Dynex dx-42e250a12 will not power on

OK, so it is either the 5V_S power supply is weak or the main board is drawing to much current that causes the power supply to go into shutdown.
You can load the 5V_S power supply with 5 Ohms (or 4.7 Ohms) 10W resistor to simulate 1A of current draw from the power supply to see if it can handle 1A per spec of the power supply.

+1...It would seem to me that the PSU is good and the Main Bd. may be drawing a large current and overloading the PSU.

[dustman1747]

Re: Dynex dx-42e250a12 will not power on

Got the 5ohm resistor and attempted the load test this evening. With the resistor from the 5V_S to a Gnd the voltage was cycling from 0V to about 3V but never steady.

Out of curiosity I tried the load test with a 10ohm resistor for a .5A draw and the 5V_S pin held steady at 4.9V.

If I'm understanding you correctly this would point to the weak 5V circuit on the power supply?

[budm]

Re: Dynex dx-42e250a12 will not power on

The power supply is rated as 5V 1A as printed on the board, so it looks like the power supply is weak and not capable of delivering the load current per spec.
That can be due to bad startup/running caps C907, C821, in the hot side side or bad caps in the cold side or the combination of both.
You can try replacing those two caps in the hot side first and then do the load test to see if it can handle 5 Ohms load or not.

[dustman1747]

Re: Dynex dx-42e250a12 will not power on

So the company replaced the power supply board and the new one has the same exact symptoms as the first replacement. They are both considered "refurbished" and I would assume taken from tvs with broken screens, it seems unlikely that both would have the same exact problems.

I removed and checked the C907/C821 capacitors, both tested good. It still fails a 1A load test.

I also cut the 5v_s line and attached an external power supply to the mainboard. According to the power supply the mainboard never drew over 1A and settled on about .4A after it started. With the 5V_s line and the mainboard on an external 5V, the power supply puts out a steady 5V at the mainboard connector.

What are the cold side capacitors you suggest checking? Or any other ideas?

[dustman1747]

Re: Dynex dx-42e250a12 will not power on

Checked all the regular style electrolytic capacitors and all were found to be good. The board will still only support .5A not a full 1A on the 5v_S line.

Any other ideas?