troubleshooting TV SMPS sequence

[ilikesteel]

Re: troubleshooting TV SMPS sequence

update
guys,
just a little recap, I should have included this.

separate 5V stndby on a small board. that's working. remote turns on the relay on the stndby 5V supply and that allows the 120vac to be applied to the main SMPS.

main SMPS shows 20V on the +24 rail and 2V on the +12 rail. I removed the PS connection, Conn3 which has all the outputs going back to the scaler board. so there is no load on the main SMPS. I still get the same results.

when I first received the TV and knew the stndby 5V was working, I arbitrarily
swapped out the 6 capacitors in the two output sections, 3=1000uf 16V for the +12 and 3 =1000uf 35V in the +24 rail. they were Panasonic FR series caps.

That's where we are right now. I made the above tests as indicated. so far from the replies and my testing, doesn't seem to anything jumping out as a bad component. I would like to save this from an early grave. I've included a pic of the 1W 5V stndby board and SMPS. my original post was" RCA L37WD12 Won't Power Up". I got so far with that one and the replies stopped coming. then badcaps was out for a week changing to a new server.

I thought I'd re-post a little differently because I wanted to find the logical sequence in troubleshooting a SMPS. If I can learn the sequence I may be able to make more checks before posting a problem. I got as far as checking the main cap for voltage. I did have 360V at the main cap. I was clueless after that, then I posted this post about troubleshooting sequence for a smps.

Thanks for everyone's patience. I won't try and fire this one back up until I've had some more input from any of you. there's no other forum like this one, not one.

Thanks,
ilikesteel

[R_J]

Re: troubleshooting TV SMPS sequence

why put a 1k resistor to ground? I thought you had problems with the +12 volt supply not working and the +24 was working but only +20 volts?
Does the power supply still work?
It can't be any simpler. T1 has 2 secondaries, 12-14 is for the +24volts. 10-11 is for the +12 volts, These supply a/c (20-30 khz) to ds3 and ds5 which rectifies the a/c to dc and goes through ls1b and is filtered by cs32 and cs7. the dc also flows through ls6 and is further filtered by cs8 which gives you +12 volts.
Try and measure the ac coming from the transformer, 12-14 should be close to +20vac
10-11 should be around 10vac depending on the meter etc.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

My apologies. here's a close up of the underside of the PS for the parts we're talking about. sorry!

ilikesteel

[ilikesteel]

Re: troubleshooting TV SMPS sequence

update
CapLeaker, Barton, R J,

ran into a dilemma trying to put a 1K resistor from junction of RS22 and RS23 to GND.

first I re-soldered the DS3 and DS5 diodes back on the board. I re-soldered the transformer, caps, and coil connections on both supplies.

when I went to identify the resistors RS22 and RS23 on the underside of the board, RS22 appears not to be on the board. it looks like a blank space where it should be.

RS23 is a 6800 ohm resistor. the marking on it indicates that (682). it truly measures 6.8K across the resistor.

RS22 should be a 15,000 ohm resistor (153). there are no resistors in that area marked 153. Also, on the schematic, RS24 should be a 15K ohm resistor, but it's not. It's marked 18 followed by a 0 or a C.

there's a blank for RS22 and also for DS23 which the cathode should be connected to RS22 as shown in the schematic.

I haven't tried applying power to the supply. I thought I should check first with you guys to see what the next step should be.

Thanks,
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

Resolder the transformer T1 and the inductors, diodes, capacitors on the 12V and the 24V. When you get it back together, if still doesn't work, I'd be putting an oscilloscope to it.

[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

[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, 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

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

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

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
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

[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.

[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.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

"bump"

[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

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

ilikesteel

[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.

[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.

[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