troubleshooting TV SMPS sequence

[ilikesteel]

I posted a problem with an RCA L37WD12 TV awhile ago. It's definitely has a PS problem. I've followed a logical sequence of troubleshooting and now I've run into a dilemma. I'd like to troubleshoot the problem further. here's where I'm at:

1. standby 5V good, standby LED on red.

2. PWR-ON works, PS gets AC applied to main PS, led flashes green continuously.

3. the output voltages should be +24V and +12V. the output is actually +20V and +2V respectively.

4. checked across main cap 180uf, 400V, I get 360 DC volts.

what is the next logical sequence to check voltages with DVM, I don't have access to a scope. I know there is the HOT side and COLD side with different ground references.

What would cause both DC supplies to be affected with low voltage output? that's where I need to get too.

Thanks again everyone. Budm was the last person I communicated with before the forum went off the air for about 10 days.

Thanks,

ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

Look at a different way: What could be dragging the 12V down so bad?

I would disconnect everything from the PSU and force the PSU on by itself, then check the secondary voltages again. Maybe resurrect the previous thread again.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

thanks Capleaker. I did remove the cables from conn3, no load from TV set. still had lower voltages at the pin outs on conn3 of the PS for +12 and +24. I just can't get to the next logical step in the flow of troubleshooting it further.

ilikesteel

[R_J]

Re: troubleshooting TV SMPS sequence

I would suspect a shorted diode in the 12 volt section, ds3 or ds5.
The primary circuit is working fine because the +24v is at +20v. its only a bit low because somthing in the 12 volt section is causing it to be droping a bit.
I guess you could have a shorted cap. in the +12 circuit but I doubt it.

The area in blue is the primary that drives both the +24 and the +12 and is working fine.
The area in green is working although a bit low. the area in red is where your problem is. (unless there is something connected to the +12 that is loading it down) but it does not look like it if you disconected con3 and it was still low.

[CapLeaker]

Re: troubleshooting TV SMPS sequence

ds3, ds5 and check those three 16V 1000uF caps. matter effect, unhook the 24V and the 12V, take your ohm meter and measure those two against GND. Probably the 12V almost measures a short.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

Thanks RJ and CapLeaker,
in the initial rca L37WD12 post I mentioned I had already replaced all 6 caps in both output sections as a first line action, thinking the problem may be there.

3 1000uf 16V and 3 1000uf 25V were replaced with Panasonic FR series caps. got the same output results.

can I check voltages around those diodes, would there be any thing meaningful in a voltage measurement? those are the shottkey rectifier barrier diodes. can I measure them in circuit and get anything meaningful or do I need to remove them from the PS?


Thanks a bunch. I hope to fix this sucker!

ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

before you take them off, what reading do you get with your DMM in Ohms on the 12V and on the 24V output (TV off of course)? Well voltage measurement? We know already that the 12V is putting out 2V. Sure you can measure the 2 diodes in circuit with your DMM in diode mode. Lets see what you get.

[R_J]

Re: troubleshooting TV SMPS sequence

You should be able to compare the resistance readings in the 12 volt circuit with the ones in the 24 volt circuit as they use the same diodes. both circuits are almost identical except for the cap. voltages, and resistor values.
with power off, check the resistance between ground and +12, it should be about 700 ohms give or take. if its low like 10 ohms, you have something shorted.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

OK R J and CapLeaker, I will make the tests tomorrow and report back. I appreciate the help and guidance.

ilikesteel

[ilikesteel]

Re: troubleshooting TV SMPS sequence

UPDATE.

R J, CapLeaker,

DS3 and DS5 both showed same results.
meter NEG probe on cathode, POS on Anode = .292 on 2K scale

meter NEG probe on Anode and POS on cathode = .702 on 2K scale

for the hell of it I also checked DS2 and DS4.

DS2 and DS4 showed .574 with NEG on Anode and POS on Cathode.
NEG on cathode and POS on Anode is the same as DS3 and DS5 = ,292.

the NEG on anode and POS on cathode were different between the sets of diodes.


the +24V to GND showed .292 on the 2K scale.

the +12V to GND showed no reading on 2K scale.
the +12V to GND showed 7.38 on the 20K scale.

any help with those readings.

thanks,
ilikesteel

[ilikesteel]

Re: troubleshooting TV SMPS sequence

"bump"

[CapLeaker]

Re: troubleshooting TV SMPS sequence

sorry... didn't see you updated the thread. Well you get readings both ways on the diode, so next step is to measure them both ways out of circuit.

What I find weird right now that you almost measure a short on the 24V line now? Can you verify this?

What are the numbers on the diodes on the 12V and the 24V secondaries? A 0.2V drop is low, but for a Schottky Diode it is normal.

[R_J]

Re: troubleshooting TV SMPS sequence

Have you checked the coils Ls1b, Ls4a, Ls6. If the diodes are ok you could have an open coil. also check the continuity of the transformer winding 10 - 11 , it should be low around a 1 ohm, compare to the winding 12 - 14 they should be close to the same resistance.
12v to ground around 7 k seems high it should be at least measuring the two parallel resistors (rs10,rs11), 1.5k each = 750 ohms aprox.
when checking the diodes you should be using the diode test on the meter (the diodes are dual diodes) so ds3 has 2 diodes ds5 also has 2 diodes.
The 24volts is working so a slight charge in the caps could throw off the 24volt readings.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

Update
CapLeaker-R J,

double checked 24V to GND, on 2k scale, .290.
double checked +12V to GND, open on 2K scale, 7.40 on 20K scale.

DS3 and DS5 are MBR 20100FCT's.

checked both out of circuit:
forward direction =.360 on both diodes in each unit.
reverse direction open circuit on both diodes in each unit.

LS1B =.2 on the 200 ohm scale
LS4 = .2 on the 200 ohm scale
LS6 = .2 on the 200 ohm scale

T1 transformer secondary checks.
12-14 = .2 on 200 ohm scale
10-11 = .2 on 200 ohm scale

with both diodes disconnected from the circuit, I still get the same readings on the +24V to GND (.290) and the +12 to GND. (7.40 on 20K scale)

not feeling too good about this.

thanks very much guys,

ilikesteel

[dick_barton]

Re: troubleshooting TV SMPS sequence

There are 2 off 1K5 resistors in parallel on the 12V line at its output point so I would expect you to read 0.75K (750 ohms). There are also 5 off 3K resistors in parallel with the 24V output so I would expect you to read 0.6K (600 ohms)
Regards

[ilikesteel]

Re: troubleshooting TV SMPS sequence

UPDATE
Dick Barton,

measured across one of the 1.5K resistors and I got a reading of .700 ohms on 2K diode scale.

measured across one of the 33K resistors and got a reading of .576 ohms ohm on the 2K diode scale.

Both diodes are out of the circuit with the above measurements.

Thanks so much, I hope we can isolate this bugger.

Thanks
ilikesteel

[dick_barton]

Re: troubleshooting TV SMPS sequence

Hello ilikesteel,
The parallel combination of 1.5K resistors on the 12V line would give you a reading of .75K so that reading is close to what you would expect.
(I assume you mean .700K ohms and not ohms.)

If so the 5 resistors in parallel which on the schematic are 3K would give you 0.600K ohms (600 ohms) so the readings you have given above would be correct.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

Dick Barton, R J, CapLeaker,

What's the next logical step? it seems like the readings I've made are within spec, but I'm still in the doghouse. the latest results were done at the diode/2K range.

Thanks,

ilikesteel

[dick_barton]

Re: troubleshooting TV SMPS sequence

Assuming the +5V standby is working and you have you unplugged the power supply from the rest of the circuits, put a link to ground from the "On/Off" pin (junction of RS22/RS23) via say a 1K ohm resistor to bring the other supplies up.
If they come up and the +24V is working but not the +12V then I would have a good look at what has already been mentioned and check DS3 & DS5 and associated caps CS32, CS7, CS8
Edit
Diodes are best checked out of circuit.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

Dick Barton,
will try that tomorrow. I need to reinstall DS3 and DS5 back on the board.

thanks,
ilikesteel