coby 4025 no stby light power supply checks good.

[dbraffo]

hey everyone. great to be here. I got a Coby 4025 from my a friend. pretty new but he said it just quite one day. he bought it from the hh store and they gave him a new set, he had the replacement plan.
symptoms no stby light nothing at all. Pulled the set apart and unloaded power supply checked had 5v on stby. jumped stby and ps on all voltages check good. Hooked it back up have no stby light still power on main board 5v and 3.3 on ps on for logic.
Regs are 17-33
pin1 0

pin2 1.53

pin3 5.03

top flat 1.53

and 1117

pin1 .59

pin2 1.84

pin3 5.03

top flat 1.84

2 diodes both check good @ .539 and .545 and 0 in reverse
not sure about pin1 on the 1117. any help would be great and all the caps look fine but I know, should replace them anyway. a direction from here would be great thanks AB

[selldoor]

Re: coby 4025 no stby light power supply checks good.

17-33 reg looks wrong - should be 3.3v

1117 is an adjustable one so pin 2 - pin1 = 1.25v which probably means the reg is ok

You should check the voltage across any associated capacitors.

[dbraffo]

Re: coby 4025 no stby light power supply checks good.

thanks for the replay. rechecked the voltages all the same and rock steady and no fluxs at all . I think theses are the right ones. I have a old crappie magnifying glass. daughter broke my good one no to long ago.
Cap 5 =1.51 Cap 29 = 1.45

[budm]

Re: coby 4025 no stby light power supply checks good.

I agree that the 3.3V reg is bad.

[dbraffo]

Re: coby 4025 no stby light power supply checks good.

thanks, I'm ordering one up and be back with results

[dbraffo]

Re: coby 4025 no stby light power supply checks good.

Parts are in will post results tomarrow

[dbraffo]

Re: coby 4025 no stby light power supply checks good.

hey guys, installed new 17-33 but only getting 2.25 volts on pin 2. its up a volt but still down. thanks

[dbraffo]

Re: coby 4025 no stby light power supply checks good.

new info. I was reading a post on here about coby's having crapy grounds tin foil and such. strange I flipped the power board around reconnected the ground from from the wall outlet to the hot side and the other gnd wire from cold side to the logic board ( where they were when I pulled it apart) and what do you know, a standby led light. it also had a high pitch faint noise from the power board like it was warming up. tried the power button but nothing. scrambled for my meter but led and noise went off before I could get it to the board. checked the voltages and all are the same as posted. strange. I might try to freeze next. anything else yall can think of would be a great help and thanks

[newtothis]

Re: coby 4025 no stby light power supply checks good.

So the Reg voltage has gone to same as before?

Check for cold solder joints as well as bad grounds

[dbraffo]

Re: coby 4025 no stby light power supply checks good.

Hey guys all fixed. I wasn't paying attention. Chased it down to the 17-33 being bad and the grounds on and between the boards bad. The ground to the lcd was just foil taped in place. One note, this is what had me puzzled and is still strange. When the tv is off there is still power on the 24v. The 17-33 is showing the 5 in and 3.2 out, tv on voltage drops to the 2.2 volts out and holds steady. Thanks for all the help.

[tocold4u2]

Re: coby 4025 no stby light power supply checks good.

Yea that foil tape 4 ground is something else...then find a place to ground it in a plastic shell. lmao...im going throu a colby with A ground issue 2

[mmartell]

Re: coby 4025 no stby light power supply checks good.

Seriously ? Take a picture of this I can't imagine how one would try to pull it off.

[dbraffo]

Re: coby 4025 no stby light power supply checks good.

Thanks for the reply. still have a issue, TV ran great for about 6.5 hrs. then went back to no stby light ect... pulled cover all voltage are the same as before the replacement of 17-33. not sure how long it was off but didn't find anything overly hot. just curious I let it sit unplugged overnight, plugged it up and bingo we have TV. I didn't change any caps on either board but I do have them. they are HScon and I goggled and looked on here but not to much good or bad on them. I also noticed the TV had a greenish tent to it, more like the shadows were greenish than black.
I guess next step change caps and check for cold joints. I did notice I only have one audio channel. I saw on a Vizio that the audio ic would shut down the TV in a under voltage. chasing down stuff w/o a schematic is a pain. If anyone can think of something else please let me know and thanks a million.

[steves7839]

Re: coby 4025 no stby light power supply checks good.

When the tv is off there is still power on the 24v. Thanks for all the help.

The largest capacitor on the PSB bleeds down slowly from 300VDC when power is removed. I clocked it taking awhile, about 1 volt per second. While it stores some voltage, it can still power DC supplies.

[steves7839]

Re: coby 4025 no stby light power supply checks good.

So, I'm troubleshooting this same model TV and symptom, and asking the forum's help.

Coby TFTV4025 S/N 1092127-0002461
No standby LED or power
Done so far:

Confirm 120VAC supplied to PSPCB CN1 and fuse is good. Diode Bridge reads 4 diodes with DMM. The MOV reads .6 MOhm. All components look OK. CE1 (200V cap) reads resistance, and reads >300VDC when set is plugged in.

PS DC TEST: All DCV measure good and stable. 5V, 5STBY, 24V, V+ 24 to Inverter, (when TV plugged in). PSON is 3.0VDC

MAIN BD DC TEST: Edge conn DC from PSB good. Pins:
1. Power On 3V
2,3. 5VSTB 5.0
4,5 GND 0
6,7 VCC5V 5.1
8 GND 0
9,10 12V 12.1
11,12 GND 0V

3. ADJUST: 0
4. ON/OFF 5V When I short to GND backlight comes on fine.
5,6 12V 12.1V

I've concluded the PSB is good and the Main Board is faulty. Heating sections of the Main PCB make no difference.

I plan to do a reflow of the Main Bd at work. The board capacitors have two brand labels; HSCON and SW. No visible sign of leakage, I have no CSR meter.
Should I replace them?

I also suspect a dead oscillator crystal (I think, Its a metal oval case with C25000 stamped on the top).

What else causes a Coby TV to be dead without Stby LED?

[budm]

Re: coby 4025 no stby light power supply checks good.

"What else causes a Coby TV to be dead without Stby LED?" May be due to processor is not active (dead Xtal?), or it is locked up some how, bad EEPROM. Hard to troubleshoot without the diagram.

[steves7839]

Re: coby 4025 no stby light power supply checks good.

More troubleshooting after a long hiatus. I bought a "for repair" main board and it's symptom is a bit different. The blue stby LED comes on when set is plugged in. When I turn the set on, blue STBY LED turns off. No other response from the set. Main Bd voltages at 5VCC and 12V read good. But there is no 3.3V at the U3 EEPROM. The EEPROM data sheet leads me to expect 3.3VDC at pins 8 &7, <3.3VDC at pins 6,5,2,&1 (clock, data, CS chip select) and 0VDC at the rest (WriteProt, GND).

Instead, I measured around U3 with the set on: 0.5V at 8&7, 0.2 at data and clocks and 0.6V at CS. The board has a 3.3V regulator with 3.3V at the outputs, but I cannot find a trace going from it to the EEPROM VCC pin.
VCC must go thru a resistor, inductor or similar noise filter on it's way to the eeprom. Suggestions where to look for it?