Toshiba LCD PSU slow tick, not turn on...

[jesterace]

Hello,
I have two TV's i'm trying to get going again, and the funny thing was that both of them had the same type/model of parts shorted that i found initially... a single output diode.

Replaced ALL the output diodes in the one Bush TV and all is well. Done the same in the other one (Toshiba LCD) and it worked ok too... until the second time i turned it on then the power led blinked on/off red with a transformer ticking sound that seemed to get louder/screechier. I immediately turned it off, thinking maybe one or more diodes have blown again, but no. It turns out they are all fine.

This is a picture of the same board i'm dealing with:

https://flattvparts.co.uk/jmb-jt0126...11-v1-20558070



So far I've been randomly checking components:

Primary fuse obviously ok.
MOV ok (high resistance)
Bridge rectifier all tested normal in diode mode. No opens/shorts.
Switching mosfet, not shorted.
All other diodes on the board, normal.

I do have a blue ring tester which i have used on both the transformers, but i'm not convinced or sure of my results. The primary windings on both transformers give excellent readings, but the secondaries on both transformers give a shorted (no lights) result. What should i make of that? Can i be unlucky that both are bad or maybe it cannot read the secondaries properly with this tester?? or better to take them out of circuit?

Finally, i should mention i have powered on the TV since and it is ticking faintly and very slowly. Like maybe once every couple of seconds.

I guess the next obvious step might be to test the caps, but this stupid board has thick glue like tar under each one and most of them i cannot even see the spec to compare.

Thanks, any help greatly appreciated!
James.

[jesterace]

Re: Toshiba LCD PSU slow tick, not turn on...

Tested the electro caps with esr meter this morning and none show any high readings, their capacitance is also consistent with specs.

Measured main filter cap steady 325v or so. Output voltages are trying to come up, as measured with multimeter but drop to zero every sec when the ticking occurs.

Output diodes i replaced still in good shape.

Unfortunately my knowledge is very basic, I've read these toshiba/vestel psu's are quite a nightmare. Any ideas what i can look at/measure next?

James.

[budm]

Re: Toshiba LCD PSU slow tick, not turn on...

We need good clear pictures of top and bottom side of YOUR board.

[jesterace]

Re: Toshiba LCD PSU slow tick, not turn on...

Hi, just attached some photos hope they come out good enough.

[budm]

Re: Toshiba LCD PSU slow tick, not turn on...

So the ticking sound is coming from the transformer TR300 (top left of the picture 1)? and you also replace those 4 rectifier diodes D318, 319, 321, XX by that transformer, correct?

[stj]

Re: Toshiba LCD PSU slow tick, not turn on...

are you testing this connected to the rest of the tv or on it's own?

btw, the ringer should really be used on stuff that is removed first, it uses voltages high enough to trigger diodes and transistors.

[jesterace]

Re: Toshiba LCD PSU slow tick, not turn on...

As much as I try listen hard it actually sounds like both transformers are ticking at the same time. Unless there’s some way i can listen/probe them to my ear with a stick, I believe they are both ticking.

The diodes I’ve replaced are the d319, d320 and d328. I.e the three that are parallel under TR300. Only one of them was shorted when i began this job, but i replaced all three anyway with matching sb5100's. The diode that is on a slant has not been replaced - but it tests ok in and out of circuit.

P.S. tested with and without screen/main board/dvd connected. Same problem.

Ah I did wonder whether to try the ring tester on the secondary windings out of circuit, but it seems both trans. give matching results on all windings when in circuit anyway so I’m guessing they can't both be bad?

[budm]

Re: Toshiba LCD PSU slow tick, not turn on...

So as sti indicated, does it tick when there are no other board connected to this power supply board? The other transformer should not be ticking unless that power supply gets the PS-ON to turn on that section.
All four diodes by that TR300 look to be connected all in parallel.

[jesterace]

Re: Toshiba LCD PSU slow tick, not turn on...

Yes it ticks with just the psu board alone, and same with everything else connected. Makes no difference.

If the TV has been off a while and voltage drained away, the next time you turn on you'll get a satisfying/normal sounding powerup buzz of the transformers for a few seconds, then some silence... and bit of false hope that it might be ok now, but then the dreaded... tick, tick, tick from what seems to be both trans. but i will try to figure out if it is just one. Somehow.

[budm]

Re: Toshiba LCD PSU slow tick, not turn on...

There are 6 filter caps on the left section of the board, check the resistance between the two legs of those caps.
You either still have shorted circuit in the secondary side that cause the power supply to stuck in start-up-shutdown cycle, or the bad start-up-running caps (there are two small electrolytic caps by T300) in the primary side for that standby power supply section.

[stj]

Re: Toshiba LCD PSU slow tick, not turn on...

if you had a shorted diode, any smoothing caps after it are suspect even if they look o.k. on a meter because they have been hit with AC power.

[jesterace]

Re: Toshiba LCD PSU slow tick, not turn on...

tests with esr meter and multimeter:

startup caps (two small electrolytes near tr300)

resistance on both @ 20k scale: charges up, and goes open circuit. normal. no short.

upper start cap:

capacity: 97uf
esr: 0.4ohm

lower start cap:

capacity: 43uf
esr: 0.42ohm




filter caps:

resistance @ 2k scale: all charge and go open circuit. normal. no shorts.

cap 1:
capacity: 3809uf
esr: 0.04ohm


cap 2:
capacity: 988uf
esr: 0.07ohm


cap 3: (c314)
capacity: in circuit/leaky error
esr: 0.04ohm


cap 4: (c316)
capacity: in circuit/leaky error
esr: 0.08ohm


cap 5:
capacity: 984uf
esr: 0.04ohm


cap 6:
capacity: 3809uf
esr: 0.06ohm

[stj]

Re: Toshiba LCD PSU slow tick, not turn on...

you cant use a tester on startup caps because the important aspect is the capacity in ma or the ripple handling.
your meter wont show that.
however i suspect something after the transformer.

[jesterace]

Re: Toshiba LCD PSU slow tick, not turn on...

learn something new every day

without going too much off topic, out of interest what would you do to test the startup caps ma/ripple etc then?

P.S. maybe another bit of helpful info. When i first replaced the diodes and turned the tv on, i initially thought that it was still dead because nothing happened. It took about 30 seconds for it to think about displaying the standby led! Come to think of it i think i actually turned it off and back on again before it eventually decided to fire up with a picture. All seemed fine until the next time i turned it on, then this ticking happened and back to square one...

[stj]

Re: Toshiba LCD PSU slow tick, not turn on...

learn something new every day

without going too much off topic, out of interest what would you do to test the startup caps ma/ripple etc then?

i would throw them in the bin, pull the bottom parts-tray that goes from 0.47 to 47uf and take a couple of new ones out.

seriously, you cant test current capacity without spending several hundred on test gear and the caps are worth maybe .10 each.

[budm]

Re: Toshiba LCD PSU slow tick, not turn on...

You are using ESR meter to test the resistance between the two legs of those filter caps? I need you to use Ohm meter to check DC resistance between the two legs of the caps.

[jesterace]

Re: Toshiba LCD PSU slow tick, not turn on...

Where ive wrote resistance i used a DMM ohm meter on the legs. The esr/capacity were checked using an esr meter.

All the caps when tested with ohm meter did the usual charge up/down resistance and then went open circuit. i.e. there is no resistance to speak of on any of the caps. and no shorts.

should i have done something different?

[budm]

Re: Toshiba LCD PSU slow tick, not turn on...

Where ive wrote resistance i used a DMM ohm meter on the legs. The esr/capacity were checked using an esr meter.

All the caps when tested with ohm meter did the usual charge up/down resistance and then went open circuit. i.e. there is no resistance to speak of on any of the caps. and no shorts.

should i have done something different?

OK, thank you for clarification.
Since there is no shorts on the output side per your measurement, the there may be something going on in the primary side circuit that drives the power transformer.
Can you use hair dryer to heat the are around the standby power supply especially around the start-up/running caps to see if it becomes stable?

BTW, those capacitance and ESR readings are done withe caps are still in the board?
Those caps that show 3900uF does not look right.

[stj]

Re: Toshiba LCD PSU slow tick, not turn on...

caps 3 & 4 are probably in parallel.
desolder and test them.
and meter the pads on the board when they are out.

[jesterace]

Re: Toshiba LCD PSU slow tick, not turn on...

Hi again, sorry I've been away. Haven't been able to look back at this TV until today. I'd like to give an update... still not working though.

When i got back to look at the TV today i thought I'd remove and check all the optocouplers. They all checked out ok. Now i'm no expert, but after fixing one or two ATX PSU's that had shorted toroid inductors i wondered if the doughnut inductor could be shorted here.... and since i'm now clutching at straws and my knowledge is getting thin i'd try anything. Well, bugger me! I noticed that the inductor had a small burn between two windings! Before trying the TV again i gently separated the two windings so that they no longer touched. Then i plugged the board back in.

Success, i thought! For the first time since replacing the output diodes, i saw a green led! I immediately turned off the TV and hunted for a similar inductor, with same number of windings etc. from some old PCB's. After replacing the inductor with a near identical one, i fired up the TV again but... not working! Same ticking, sound once again. Severely pissed off, I put the old inductor back, making sure the damaged windings were again, not touching... but it still didn't work. So is it that the TV decided to turn on for a fluke??

The old one was certainly burned, you could see the windings touching and the lacquer had peeled away. So what do you make of that? I'm almost beginning to wonder if leaving the TV and coming back to it days later has had some effect. Like maybe a dry joint closed up... but I'm struggling to find any. However it is clear that the PCB is warped a fair bit, and it was like that from the first moment i opened up the TV.

...

Getting back the last post, i have now removed and tested all the electro caps, ceramic and poly caps on the top side of the board. All readings match the specs and ESR is ok on the electro caps.

I've also tested the small transistor near the SMPS transformer. The larger transistor next to the bridge rectifier was tested a while back. Thats ok too.

So now i'm well and truly up the creek. I can't believe it decided to work after leaving it a few days. Does any of this info spark any bright ideas?

Cheers,
James.