Dynex DX-L40-10A Won't turn on.

[R_J]

Re: Dynex DX-L40-10A Won't turn on.

My manual says a good diode will read between .400 to .600 and not make a beep when the probes check reverse polarity. Those 2 read out to .150ish and beep

The manual does not seem to know about schottky diodes, they usually check around 0.2xx
And I will need to say this again: The BEEP is useless for checking components. most meters will beep between 0Ω and 150Ω.

[Funda]

Re: Dynex DX-L40-10A Won't turn on.

The manual only references silicon diodes. I'm surprised the military didn't list for every kind out there at the time.

[R_J]

Re: Dynex DX-L40-10A Won't turn on.

I had a closer look at the circuit and it looks like the relay does disconnect the AC into the main bridge rectifier, so in standby the voltage on the main cap in standby would be next to zero.

[Funda]

Re: Dynex DX-L40-10A Won't turn on.

I'm reading a 9.

[R_J]

Re: Dynex DX-L40-10A Won't turn on.

I checked the resistors on the PS board and the big 1k resistor in the lower right corner is bad for sure.
Does anyone know the wattage on that one? 5w? 10w?

I just went back over the thread, and this resistor (R901) is 100K/2Watt
It is ok because you have 5VS, +5 volts standby
9. volts across the Main filter in standby is FN zero.

[R_J]

Re: Dynex DX-L40-10A Won't turn on.

I suspect the problem is with the VCC voltage, with the power supply ON, check the voltage between VD814 (anode) and VD813 (cathode)
IF I remember the faulty capacitors on this board are: C811, C812 & C814
If you replace these three caps I suspect the +12 & +24 volts will stay up

[Funda]

Re: Dynex DX-L40-10A Won't turn on.

I'm confused now R_J. I could have sworn you told me to read resistance across RT801.

The voltage across is 0 without the resistor in. With the resistor in it fluctuates wildly.

[Funda]

Re: Dynex DX-L40-10A Won't turn on.

Well I'm pretty sure I fried it now. I have the board sitting on a plastic lid and I was about to check vdc at the 3 caps you suggested and it must have slid off it enough for it to touch the metal rail of the tv. There was a pop when I plugged it in and there's a slight smell of something burnt.

[Funda]

Re: Dynex DX-L40-10A Won't turn on.

Time for work, I'll pick this back up in the morning.

[R_J]

Re: Dynex DX-L40-10A Won't turn on.

I'm confused now R_J. I could have sworn you told me to read resistance across RT801.

The voltage across is 0 without the resistor in. With the resistor in it fluctuates wildly.

That was before I looked closer at the board, the relay does not short out RT801 like it does on some power supplies. This part of the circuit is working like it should
What is not working properly is the VCC voltage that the PFC circuit provides. If the caps I mentioned are bad, they can not supply the proper voltage to maintain the VCC for the pfc ic and the main drive ic. So as soon as the main ic turns on, the voltage drops and the cycle repeats. This is what is causing the fluctuating voltage

[R_J]

Re: Dynex DX-L40-10A Won't turn on.

Well I'm pretty sure I fried it now. I have the board sitting on a plastic lid and I was about to check vdc at the 3 caps you suggested and it must have slid off it enough for it to touch the metal rail of the tv. There was a pop when I plugged it in and there's a slight smell of something burnt.

Well this is not good, You should have removed it from the tv to work on it. all that needed to be connected was the ac cord

[Funda]

Re: Dynex DX-L40-10A Won't turn on.

I looked at the area of the board that had made the spark, which happens to be the area with the caps that need checking, and I'm guessing one of the legs of the transformer touched.
I'm still going to try to troubleshoot this board but what can I check for verification? Would rechecking the main cap be enough? It still has the relay click with the resistor in.

[R_J]

Re: Dynex DX-L40-10A Won't turn on.

The relay is driven by the standby voltage which is a seperate circuit.
Check the voltage across the main filter cap. in on condition.
Can you be more exact about which pin of which transformer touched (has an arc mark on it) this could help in troubleshooting.

[Funda]

Re: Dynex DX-L40-10A Won't turn on.

I don't see any arc flash or markings that show the contact point. Maybe I got lucky?

I'm still getting 411vdc at the filter cap.

These readings are with the resistor in.
c811 reads 8.66vdc
c812 reads 3.46vdc
c814 reads 12.44vdc

[R_J]

Re: Dynex DX-L40-10A Won't turn on.

Those voltages are too low, replace the capacitors. The voltage between, C812 (-) and C811 (+) should be around 25~30v
The voltage across C814 (12.4) is enough to keep the pfc ic above its low voltage lockout so it is working

There is a small capacitor C838 near the main filter, you can check the voltage across it, this is the VCC for N831, the main drive ic. I suspect it will be low

[Funda]

Re: Dynex DX-L40-10A Won't turn on.

c838 reads 12vdc, more like 11.94

[Funda]

Re: Dynex DX-L40-10A Won't turn on.

Surprisingly I have a matching replacement for c814 however the covers are different color. The one on the board is green and the one I have is brown. I don't know if that makes a difference but they're both 35v 100uf. I also have a matching replacement for c838 but different color covers. I'll have to track down some for c811 & c812.

[R_J]

Re: Dynex DX-L40-10A Won't turn on.

Surprisingly I have a matching replacement for c814 however the covers are different color. The one on the board is green and the one I have is brown. I don't know if that makes a difference but they're both 35v 100uf. I also have a matching replacement for c838 but different color covers. I'll have to track down some for c811 & c812.

What? the color of the cap does'nt matter, as long as the capacity is the same and the voltage is the same or a bit higher is ok.
You don't need to replace C838, I was just looking for the voltage across it. The ic's minimum vcc is 12.3 volts, you have 11.94 volts.

Are you sure about the capacity of the caps? They should be:
C814 - 22µf/50v or 100µf/50v
C811 - 10µf/25v
C812 - 10µf/25v
100µf will be too lagre a capacity. and they should be 105° not 85°

[Funda]

Re: Dynex DX-L40-10A Won't turn on.

Here's what I see.

You are correct about c811 & c812

[R_J]

Re: Dynex DX-L40-10A Won't turn on.

Then C814 is 100µf, The important ones are the 10µf, they are where the voltage is first filtered/developed, C812 seems to be the worst, C811 & C812 form a voltage doubler.