This is a rehash of a time where in the the remote isolation, you decide to have a go just to see.... Not what a sensible person may do.
Those of you who have a reasonable electronics background may not want to see this, or it may cause nightmares for the rest of your time.
Enjoy if you can....
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Sometimes people expect you to do what appears to be simple to them, but is a problematic to me.
If you can load batteries into a torch, you obviously know all about electronics and can fix anything with a switch on it.....
Where do they get these ideas.
I'm posting this, just to give some hope to older fogies like me who have done limited electronic trickery in their time as a hobby..... and get tired of all the new fangdangled electronic thinggies... and how they are built, and how are you gonna fix em when they fail....
So.....
A sorry looking local looked me up, and asked if I would look at his computer screen. I figured it was a CRT type, and accepted the challenge. Yes I said, figuring if it wasn't the EHT transformer, I had a better than fair chance of fixing it.
The owner and the screen dutifully turned up and it turned out to be a very new looking 20 odd inch LCD device. I went cold to the core... cursed things, I bet they use those hard to see and even harder to get hold of surface mount bits and pieces. I was about to tell him to take it to the tip as I didn't feel too confident about it at all, but before I could get it out, he couldn't help but tell me that.... so and so said I could do it no problems etc. etc.
I was cornered by local legend and vanity.... egad.
Well, I shuffled off to the shed with it, mumbling things about Gordon and his proclivity for surface mount things... and anyone else who encouraged them....
I figured that at least the power supply would be through hole, and probably the fault would be there, and barring exotic components I didn't have, we'd still carry the flag.... Most of the satellite decoders, dvd players etc so far have through hole power supplies, and surface mount after that....we live in hope.
Electrolytic capacitors deserve special mention. They are built to fail.... and do with monotonous regularity..... maybe thats the cause in this case......just them.... if so, an easy fix may be on the way.
It took ten minutes to find out how to open the damn thing. What ever happened to a few well placed screws. It was all very cunning... but with patience and good management (and a few swift thumps), it came apart.
I didn't take any photo's at this stage, as I didn't think it would ever go again. The blown fuse, and black smoke marks across the board almost encouraged me to put the cover back on and call the owner..... but I just couldn't help but look in wonder at all the little bits and pieces too small to see almost, and marvel at how they must do these things.
Well, we'd gone this far, so how does it work anyway.
Popping off the screen and wiring harness to it, we find this...... the screen carries a mountain of very intricate whoop arse looking gear, and I ain't ever tackling that. All made in China by robots... no person could hope to do it.
Actually, the bottom of one of those boards has been butchered, as this is after the ordeal, not before....
My heart sank, as I didn't recognise any of those little bits as normal stuff. I pulled off the top board, and cheered up some.... there was at least some through hole bits to look at:
You can see that there has been a fair amount of butchery there too... no before shots I'm afraid.
The black marks have been cleaned up some. The Fet was blown up, the fuse was blown, and ...... where was the PWM chip.....
The PWM chip was one of those cursed surface mount things...yes, on the hard to see side of the board and surprise surprise, 320v going through one of them just leaves a smudge and some leads and burnt plastic.
Things looked grim about now, and I vowed to look at it some time later, and tipped all the bits back into the rear frame and let it sit for a few days... and then some.
Some days later, I decided to trace out the tracks away from the 8 pin pwm site, to see if I could guess what the part may have been. The 320v from the rectifier went straight to the chip.. pin 8 in fact. This told me that the chip was one of those "greenie" chips, which used the HV to bootstrap direct, and then turn it off and run from the tertiary field, rectified and then used as vcc... pin 6?. Output was pin 5, ground pin4 and feedback pin2.... etc. This narrowed it down.
The second green pwm chip I researched was an ld7575... and it matched pin for pin. I was shattered, I couldn't track one down anywhere.... I needed a plan B.
Looking at the data sheet for the 7575, we find a typical simple flyback circuit looks like this :
3.jpg (14.93 kB. 500x261 - viewed 3457 times.)
I did another quick trace, and whilst it wasn't the same at all, it sported an opto isolator, transformer with tertiary winding... and well.....that was enough bits to make a supply with.
I grabbed the trusty 3842's (pwm chip) out of the box, and proceeded to graft it onto the board... this was the tricky bit. Their entire PWM was built on the surface mount side, and 20 odd components used less room than a postage stamp.... what to do.
The big smoothing capacitor took up a lot of real estate on the top side of the board, so I resolved to use this space, and put the capacitor elsewhere.... anywhere..and a few (what I felt) unnecessary components were turfed out all together.
It now looks like this
You can see the capacitor was prized off the board and repositioned (where some other stuff used to be), the bridge was moved back a bit, and the 3842 through holed into a socket... (not very confident are we??). A few more holes and a few bridging wires, and we have half of it built. On the other side it looks just as woeful:
I scratched all the surface mount stuff off the board with a razor blade, and scratched all the tracks off as well. With some filthy soldering and slapping the bits on, I was finished. I replaced the Fet from out of a computer monitor from the tip, and last but not least, I wired up a rudimentary bootstrap circuit (80k resistor and 18v zener)......The bootstrap is the two messy looking resistors between the transformer and the fet.
After this, the tertiary should take over..... and it didn't. The 3842 I had (gotten cheaply from Rockby) didn't turn on until 16v... and the tertiary winding was only 12v...dammit... 16v wins, and the tertiary doesn't get a look in.
I'll worry about that later, the bootstrap will carry it until we find out if the rest of the thing works.
It was modelled around this circuit here:
Obviously were running it at 320v not 17v... but close enough...
Well it was time for the quick check around the board to see if all was well..
3 electro's were down from 1000uf to about 50uf. This is what appears to have o/loaded the fet which then blew everything up.replaced these and moved on to the 2000v screen supply PWM.... surpirse suprise.
At least this although surface mount, used a surface mount tl494... home territory. I used a 14v supply to mimic it's input voltage. The oscilloscope showed the oscillator was functional, and by driving the dead time pin down, the 2000v hissed into life for a few seconds. I tracked down who was turning me off, and bypassed that (the computer on the other board controls this chip for brightness, sleep etc.)... satisfied with this.... it was time to switch on.... and I was greeted with this.....
Feeling pretty smart about life now aren't we.......... and then it went blank...and then the panic sets in, I go over it all , and all seems fine. Switch it on, and it works and stops again.
These smart damn machines, I had forgotten to give it some video drive, so it switched itself off after 20 seconds.... nothing wrong at all perhaps.
I had found an old laptop at the tip....
After fixing it up (idle Fet), and tossing aside the screen/lid, and cleaning it up, ( yes that is it in it's cleaned up state), loaded up some software (Linux Ultimate2) and tried again.... this time it was all different..... but we will have to start another section as we have used up 9 pics thus far