troubleshooting TV SMPS sequence

[CapLeaker]

Re: troubleshooting TV SMPS sequence

If I work on a PSU, it will be by itself and I force it on. As for the light bulb load, you can do either or. Probably I'd be having no load on it first, because of all the wire jazz hanging around, but it may be harder to troubleshoot. Here is another piece of advice: When resistors go bad they go higher in resistance. You find one out of spec, replace it. Zener diodes can be hard to check, as you need to take them out and actually find the "knee" with a PSU or use the oscilloscope with an component test circuit.

A few days ago I had a dual voltage PSU here on the bench. It only had half of the rated voltage on both rails. Turned out to be a bad capacitor next to a heat sink that was in the feedback circuit. Exchanged that cap and the starter cap (for good measure) and PSU is ticking like on day 1.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

OK, I'll forge ahead! give me a day or two and then I'll update the post. I think I'll reconnect the cables and connections back to normal, this way I won't have all the extra wire dangling in the way and the TV will be in its natural state. Ill also check the main cap for voltage again under normal load/connections.

Have you heard of the eBook "troubleshooting and repairing SMPS power supplies" by Jestine Pong?

http://powersupplyrepairguide.com/

Thanks again for sticking with me, and also to the other forumites that have commented on the post.
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

I have heard of the ebook written by Jestine Yong, but I don't have it to read.

[vinceroger69]

Re: troubleshooting TV SMPS sequence

I have heard of the ebook written by Jestine Yong, but I don't have it to read.

Same as ive been looking for this book too there is a torrent of the lcd monitor book online but not the smps power supply one.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

OK, I made some measurements this morning. some made sense and some where a little off, but I did get readings on questionable measurements. however, I did find one major issue. connected to pin 18 of the TDA 16888 is a resistor R36, according to schematic it's suppose to be a 100K ohm resistor.

It's not a 100K resistor, it's marked as a 514 smd resistor (510K). it's TINY. this resistor measured OPEN in circuit at all settings of the meter. I removed it from the circuit and checked it again, and yes, it was open. I almost lost it, it was so small.

could this one little tiny resistor be the cause? pin 18 is the PFC VC, Voltage Loop Compensation.

I did not measure any voltages due to the fact I found this resistor to be bad and it was in the feed back section of TDA 16888 just as you suggested.

anxiously waiting for a reply.

Thanks,
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

Well... I guess you are getting somewhere. Change it and put the same value marking back in there what you took out. Sometimes manufactures do changes on the fly without changing the schematic. That wouldn't be the first time. Anyhow... keep on metering maybe you find something else. BTW see if C40 is shorted. If the capacitor is shorted, it is bad. Also check R3, R7, R12 and R15, then R38.
Basic idea is to find everything you can find with the power off, then replace it and turn the power on and see what happens. In your case you can do that, as there are no shorted transistors etc hiding.

[CapLeaker]

Re: troubleshooting TV SMPS sequence

Same as ive been looking for this book too there is a torrent of the lcd monitor book online but not the smps power supply one.

Same here, found the LCD monitor one and a few other little bits and pieces, but not that SMPS PSU issue. It supposed to be a good book to read.

[vinceroger69]

Re: troubleshooting TV SMPS sequence

Same here, found the LCD monitor one and a few other little bits and pieces, but not that SMPS PSU issue. It supposed to be a good book to read.

Ive had nothing to fix this afternoon so had a really good search of the internet but cant find the book anywhere.

[budm]

Re: troubleshooting TV SMPS sequence

R2, R6, R11, R14, R38 (on pin 17 FB) are for setting the output Voltage of the PFC so you should check them out

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker, BudM,

I did check those resistors, but did not include them in the last post since I found R36 OPEN. here are the values of all the resistors near TDA16888. I found 4 resistors with a different marked value than the value on the schematic.

here's a pic of R36 empty and R38.

# marked Schematic Measured
---------------------------------------


R2 = 1504 820K 101.M

R1 = 514 1M 503K

R38 = 750 59K 56K

R36 = 514 100K OPEN
_______________________________________
"F" TAG

R2 = 1504 820K 1.01M
R6 = 1004 1M 790K
R11 = 1004 1M 790K
R14= 1004 1M 790K
____________________________________

"J" TAG

R3 = 105 1M 770K
R7 = 105 1M 768K
R12 = 105 1M 816K
R15 = 105 1M 767K
_____________________________________

"G" TAG

R4 = 205 2M 1.25M
R8 = 105 1M 820K
R13 = 105 1M 771K
R16 = 105 1M 820K
________________________________________

"P" TAG

R1 = 514 1M 503K
R5 = 514 510K 499K
____________________________________________


all the caps near TDA 16888 on the bottom of the board including C40 did NOT have a short.

I'm getting anxious!

Thanks!
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

I am not sure, but your DMM measures low. Are the batteries dead? Can you take a 1MOhm resistor or something out of your junk bin and verify?

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

measured a few resistors I had on hand.

resistor measured tolerance
--------------------------------------
4.7K 4.86k 5%
22 ohms 21.6 ohms 5%
22K 20.6k 20%
1M .95M 20%
1K 1.05K 5%
1.8K 2.0k 10%
1.5k 1.56k 5%

I can arbitrarily replace the battery if you think I need too.

thanks,
likiksteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

Hmm... okay then... I guess you got no other choice then to take out or lift one end of: R1, R2, R38 (that one is totally read wrong should be a 75C (59K 1%) not 750), etc.
First I would take those resistors that read really wrong one by one out of circuit and check them. Those SMD resistors are within 5%. Those SMD resistors don't only have numbers, they also can have letters! Double check the double check, hi!
R38 clean that sucker with alcohol and a toothbrush... maybe it will clean the marking off better. Unfortunately the switching PSU doesn't like it when resistors change value. A slight change in value makes a difference in these things.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

you're absolutely correct. I did misread some of the markings on the resistors. I also saw a couple of 823's with an underline which means 1% tolerance.

I lifted 4 resistors. I'm so reluctant to try to remove them all, only because of the delicacy in the removal operation. I only have a soldering iron and at best it's iffy.

here's the 4.

R2 1504 (1.5M) reads 1.49M, schematic says 820K.

R11 1004 (1M) reads .99M schematic says 1M

R6 1004 (1M) reads .99M schematic says 1M

R3 105 (1M) reads 1.0M schematic says 1M

_______________

random in circuit measurements:

R1 514 (510,000) reads 504K schematic says 1M

R38 75C (59,000) 1% reads 58.9K schematic says 510,000

R34 913 read 89.4K schematic says 91K

R41 823 read 81.9K schematic says 82k

R32 514 (510k) read 500K schematic says 510K

R33 514 (510k) read 500K schematic says 510K

R40 392 (3.9k) read 3.8k schematic says 3.9k

R43 390 (39 ohms) read 38.4 ohms schematic says 39 ohms

R37 392 (3.9k) read 3.8k) schematic says 3.9k

some of the resistors are barely a fleck of dust! amazing.

is a hot air iron better for removing SMD's?

we know R36 is definitely OPEN. to remove additional resistors, I'll probably have to make a chart of their location. it would be easier to remove them if they were on an end of row, and not in the middle of the pack. so I would start at one end and do it that way.

do you think something is wrong with some of the resistors? OPEN R36 is a problem, do you think something caused R36 to go OPEN. this is uncharted territory for me.

Thanks a bunch

ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

An open R36 is a problem indeed. I don't know why it opened up your guess is as good as mine. The cap next to it isn't shorted.

I do all SMD work with my hot air soldering station and ESD proof tweezers. I put them back on with "chip quik" solder paste.

Well... I guess you checked some resistors out of circuit and they turned out all right, so replace R36 with a new resistor (either SMD or bend up a thru hole one) solder it and fire the PSU up. See what happens.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

I don't have two 1M ohm resistors to use in parallel to equal the 510,00k value I need for R36. in the mean time I'll look for a 2nd 1M resitor. If I parallel two 1M resistors, would the value be accurate enough for the smps?

I think I'll order some SMD's from digikey in the US, solder paste and a pair of static free tweezers! it will take a few days for the parts to get here.

what do you think?

thanks
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

Well you could do that for testing purposes, but you have to get as close as you get to 510k +- a little bit. It's definitely better than an open resistor.
i guess you should have the Digikey order in 2 or 3 days. Getting curious now myself!

[budwich]

Re: troubleshooting TV SMPS sequence

If "open" is giving you some low voltage values, even just using a 1M alone should give you something to compare things in terms of "are you heading / looking" in the right direction. IF things don't change, then you can continue your "quest" while you wait for your parts.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

Capleaker, BudWich,

unfortunately, I can only find one 1M resistor. the value I need is 510K. I will order the parts from digikey along with some other miscellaneous parts I could use. give me a few days to get the parts, restore the board, and fire up the TV.

thanks so much for everyone who has commented and advised. special thanks to CapLeaker who has stuck with me and has kept answering my post updates.

I need to measure the size of the SMD before ordering. It's teeny weeney. I almost lost it because it was not very big, just a speck of pepper.

ilikesteel

[budwich]

Re: troubleshooting TV SMPS sequence

but as I suggested... just use the 1m ohm resistor. Then check the resulting voltage. That's it. It doesn't have to be exact at this time, you just want to see if things change... and hopefully in the right direction... if "open" (which is infinity resistance) gives you a low voltage result, a resistor between "open" and the "right one" should give you something a "bit better". IF it doesn't, then you likely need to look at the board further as something isn't right even IF you get the right resistor value. That's all the test should show.... it gives you something to do while you wait... :-) and potentially help check out the understanding of what is happening with the power supply..... remember the title of your thread.