Element ELEFW401A No Power - suspected bad powersupply

[Ifixstuff9]

I have an Element ELEFW401A that appears to be completely dead. No standby LED, no backlight, nothing.

The power supply is a CVB42001 that was apparently used in a lot of cheap TVs a few years ago. I think its the issue with this TV because the the 12v rail going to the main board has no voltage. On the hot side I tested both main caps, 400v 68uF, and got 163v. On the cold side I got 0v on both capacitors. Any Ideas?

[R_J]

Re: Element ELEFW401A No Power - suspected bad powersupply

Using the negative side of one of the main caps (hot ground) check the voltage on the fet Drain (160VDC), and also the source(1~2vdc). then check if there is any VCC voltage (see picture) There are two small electrolytic caps (circled) they might be bad.

[Ifixstuff9]

Re: Element ELEFW401A No Power - suspected bad powersupply

Checked the voltages, got the following:

D - 165v
G – 0v
S – 0v
VCC – 0v

All of these were measured using hot ground

[R_J]

Re: Element ELEFW401A No Power - suspected bad powersupply

Check the values of R5,R6 & R7, they feed a/c to ZD1 which is the startup voltage, check if ZD1 and R17, If ZD1 is shorted, R17 could be damaged. It looks like U1 is not providing the drive for the Q3 & Q4 which drive the mosfet

[petehall347]

Re: Element ELEFW401A No Power - suspected bad powersupply

zd1 dont look too good on the pic .also q3

[Ifixstuff9]

Re: Element ELEFW401A No Power - suspected bad powersupply

Thanks for the help everyone. I tested r5, r6, and r7 and they seemed to be all good at 357 KΩ. I also tested r17 which was good at 151 Ω.

Do I need to remove zd1 from the board to test it properly? Because I got the same voltage drop when I tested it forward biased and reverse biased.

I also checked C2 and C3 as suggested. I had to remove them from the board to get an accurate reading, but both caps tested good.

[R_J]

Re: Element ELEFW401A No Power - suspected bad powersupply

If r17 is 151Ω, you should get a fairly good reading on zd1, There should be some voltage on C2, but it could be that U1 is bad and killing it, Can you make out the number/letters on U1?

[Ifixstuff9]

Re: Element ELEFW401A No Power - suspected bad powersupply

All I can see on U1 is MUP CH ... not sure if that helps at all. Maybe tomorrow I can try again with a microscope but that chip is really small.

[Ifixstuff9]

Re: Element ELEFW401A No Power - suspected bad powersupply

Also I just measured C2 and C3 and got 0v on both.

[R_J]

Re: Element ELEFW401A No Power - suspected bad powersupply

A guess would be that it is a LD7535. Check the resistance across C2 (basically checking U1 VCC pin to hot ground) The ic likely has an internal zener that might be shorted.

[Ifixstuff9]

Re: Element ELEFW401A No Power - suspected bad powersupply

The resistance across C2 is 10.4Ω

[R_J]

Re: Element ELEFW401A No Power - suspected bad powersupply

The resistance across C2 is 10.4Ω

Check if D1 is shorted? Also there should be a diode connected across R19 and C12. It is likely mounted on the top side. It could be shorted. If the diodes are ok, it could be the ic bad

[R_J]

Re: Element ELEFW401A No Power - suspected bad powersupply

All I can see on U1 is MUP CH ... not sure if that helps at all. Maybe tomorrow I can try again with a microscope but that chip is really small.

Did the markings look similar to this ic?

[Ifixstuff9]

Re: Element ELEFW401A No Power - suspected bad powersupply

Did the markings look similar to this ic?

Yes the markings are similar. I tried to get a picture of the IC, you can just barely make out the text.

[Ifixstuff9]

Re: Element ELEFW401A No Power - suspected bad powersupply

Ok something is definitely going on with that part of the circuit board. You were right about there being a diode across r19 and c12, its D3. I tried to test both D1 and D3 in circuit and both were shorted. However I then removed D3 from the board, tested it again, and it worked perfectly. I didn't remove D1 but it might be fine as well. So it seems that something is shorting that area. Its the same reason I had to de solder C2 to test it: when I tried to test it in circuit I got a short, but after removing it it tested fine.

Also I partially removed the heat sink on the hot side and uploaded a picture

[R_J]

Re: Element ELEFW401A No Power - suspected bad powersupply

I suspect the ic is bad, the important number is the P and the number above the CH, you may need to clean it with some alcohol But it does look like a LD75xx series

[Ifixstuff9]

Re: Element ELEFW401A No Power - suspected bad powersupply

Alright just checked the chip again, it says 36R above the CH.

[R_J]

Re: Element ELEFW401A No Power - suspected bad powersupply

Alright just checked the chip again, it says 36R above the CH.

That would make it a LD7536R, I suspect it is bad, likely the internal vcc zener is bad. 10Ω is way too low for that pin.

[Ifixstuff9]

Re: Element ELEFW401A No Power - suspected bad powersupply

Well I thought I'd update this thread in case anyone else has a similar problem with this TV.

Anyway a new power supply fixed the TV. I was going to order a replacement LD7536R chip, but I couldn't find one in the US and I didn't want to wait a month for a part I wasn't 100% sure would fix the problem. I was able to buy a new power supply and main board for this TV on eBay for less than $20 shipped. I didn't need the mainboard, but it was cheaper to buy them together than to buy just a used power supply.

Now, it turns out there are a few different versions of the CVB42001. One has two connectors for the backlight, the other has four. Other than this, the boards are the same. Because my TV had the two connector version and the replacement had 4 connectors, all I had to do is swap the connectors from the two board to the four board. I don't remember the exact process for this, but it was super easy to figure out, and works like a charm.