VCR / TV : yesterday was a good day, I managed to fix that 32" LCD TV I was given, so I could see if THAT one was working any better than the previous monitors I tried.... and it does, on this one SCART works, phew ! Not just that, but all my picture problems are gone now, picture is just fine on this TV ! No more black and white problem, no more fuzzy/"unfocused" image, no more unstable picture that keeps jumping up and down all the time... there is nothing to complain anymore, picture is as fine as it can be from a 20 year old VHS tape running on an unserviced 20 year old VCR !
That's cool, at last I have a TV I can use when I need to. Plus it's very "manageable" as it's not too big and not too small, is super slim and feather light (you can hold it in the air with just two fingers easily !
)
So that's now an extra piece of TE in the lab... can use it to view SCART or TV if need be, when fixing or playing with stuff.
Does not have a composite input though, which is a bummer. So might replace it with another TV if one day I am given one that has both SCART and video composite.
However SCART has both RGB and composite signals available, so maybe I can make a little adapter to feed a composite signal through SCART. However that's assuming the TV uses the composite signal of the SCART connector, rather than RGB. Have not tested for this. Anyway, for now I have a working TV and VCR so that's cool, it's handy.
So what was wrong with the TV ? Yes, power supply again ! But no, not an electrolytic cap ! Still a cap though !
First inspection, off-line, I saw black burn marks under the miniature / sealed PCB mount fuse. Checked it, blown indeed. No other visual sign of damage on the board though...
So checked the diode bridge, made of 4 discrete diodes. Two of them were shorted. Hmm... there must have been a catastrophic failure / short circuit on the primary.
However a resistance measurement showed zero sign of a short ! As far as individual components, big filter cap looked fine, but pulled one of its legs anyway to give it a good test. Not shorted, not low impedance, capacitance still spot on..it's just fine. MOSFET ? Again looked fine and no obvious sign of a short between any of the pins.
So I was a bit baffled. So grabbed a scrap board and salvaged an appropriate fuse and replacement diodes. Covered the board with a big transparent plastic box for security, and plugged it.... glad I put that box !
Instant explosion and big fireworks !
Again fuse and two rectifier diodes blew, but again no sign of damage to filter cap of MOSFET, was still baffled !! How can I get such a catastrophic high power failure UP stream, and have zero blatant damage DOWN stream ?! Doesn't make sense !
I pulled the MOSFET ( a tiny surface mount D-PAK package) and tested it on my 10 Euro Chinese component tester... best 10 Euros I spent on the lab for sure, this little thing is worth every cent. MOSFET was fine, detected properly.
I had no more appropriate fuse (unless I spent two days going through my 3 large boxes of scrap boards....) and since it looks mysterious and probably requiring a few more fireworks events until I can figure it out... I decided to go down town to visit my local electronics shop to quickly get a few fuses and diodes, enough to make several attempts at fixing the thing.
I brought the board with me, so I could show it to the guy and just tell him just give me some diodes of the same size, and fuses that can fit this footprint... but when he looked at the board he instantly noticed a damaged cap that I had not seen ! A tiny blue disc ceramic, that I paid no attention to because it looked like small and located physically more in the low power part of the primary, not the main power path that blew.
He noticed that cap had arced. If you looked closely you could see a tiny black dot on it, and by feel with your nails, you could sense it was the result of tiny bit of material that parted with the cap.
So I looked at the traces leaving this cap, and hey presto, it WAS indeed related to the power path : one of its legs was connected to the positive output of the bridge rectifier / filter cap !
That sure would explain how it would blow the bridge without affecting the cap and MOSFET ! Would also explain why the short would manifest itself only when powered up, subjected to a high voltage, but NOT when doing a low voltage resistance check with a DMM ! So that looked very promising. Bought a replacement cap from the guy... expensive. 4,7nF 1kV, almost one Euro... and package was tiny, much smaller than the original. So I bought from him a 3kV one, which had the same package size as the original. Over specced maybe, but not taking any chances. The original failed so clearly was not specced high enough. So am just adding an extra safety margin.... at twice the cost, 1.50 Euro the cap !
Fuse expensive too, 1,50 a pop !
So bought only two of them.
So once back home, fixed everything up. Before putting the small D-PAK MOSFET in place, I tested it one last time and nooooo.... it's dead now somehow ?!: Tester sees it as a pF cap, not as a tranny anymore !
Maybe I blew it while handling it, too bad. Obviously no way I was going to order a new MOSFET of same part number, and pay 10 Euros of shipping (local shop obviously is small so can't have it in stock, there are only so many parts they can stock). I was already 5 Euros worth into that TV, with the diodes caps and fuses, it was as far as I was willing to go.
So, I went caveman style for once, just to see what would happen... I pulled a random MOSFET from a scrap SMPS board. Bad luck, part tested bad. Next board please ! Ah, this one is good. It's an N-channel which is what we need so great. Let's try that one ! Package is completely different. A big plastic TO-220 jobbie ! But same pinout and pin pitch as the D-PACK, so was painless to fit on the surface mount pads. I only had to bend the middle / drain leg backwards so I could solder it to the thermal pad of the D-PAK footprint.
Of course no idea of specs of either D-PAK or replacement TO-220 ! All I know is it's " big enough ", it's a MOSFET and N-channel, and it checked OK on the tester....so the fundamentals are good
Worst case, turn-on voltage is too high and the PWM oscillator/gate driver on the board can't turn it on and board won't work, or it will turn it on badly/partially and it will overheat (and not work probably). Or, Rdson is too high and it overheats again, or ... it blows... but the board already blew twice so no big deal, it can't possibly go more wrong that it already has... nothing to lose !
It's the best I could do with a budget of zero Euro, so either it worked, or I scrapped the TV.
So soldered that TO-220 in place, put the plastic box back onto the board just in case... and powered it up ! .... NO explosion ! GOOD !
So away with the plastic box, and I reconnected all cables going to the board, secured it to the chassis with all 4 of its screws, especially since they are often mandatory as part of the ground plane : sometimes the ground plane is made of several distinct pieces and it relies on the mounting screws to join the ground plane into one piece.
Applied power and.... YEAH, it works !!!!
So plugged the VCR via SCART, worked too, yes the repair was worth the trouble then, goal achieved ! Then played the Star Wars VHS tape, and picture was all good, did not exhibit any of the problems it had before when I was using video-composite on my computer monitor.
Would my random TO-220 MOSFET frankensteined onto the board, blow ? Put it to the test : watched the movie from end to end, for the first time with a good picture, and colour all the way, was great... especially since I also found a way to silence the super noisy rattling head drum ! I just sticked my little spool of solder wick under the front left corner of the VCR, and hey presto, silence I got !
So I could enjoy the movie. Lasted two hours or so. Then as soon as that was finished, I watched another tape/movie, 1h40min long.
So all in all 3h40 minutes of testing.... and the TV held up just fine !
So that's good enough for me ! Repaired for 5 Euros (and have some spares now as well...), and a salvaged/free MOSFET.
If the power supply ever blows again I can always fix it again, if I want / care. For now I call it a win.
Have a good day !
Home
Help
Search
Login
Register
