Author Topic: Help with rebuild  (Read 2009 times)

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Offline TorqueRanger

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Help with rebuild
« on: February 04, 2013, 08:13:22 pm »
I am fixing a T.V and some how last while I was testing it the DVD drive came loose and touched the inverter board .. Well the T.V shutdown  and nothing happened that I could see.... I remounted everything and tried turning it on and the LED turns from red to green  but then nothing happens and when I try turning it off it does nothing  but stay green... I did some searching and found nothing wrong on the board I could see..I did some testing and found that the main board was not fully turning on because I override the inverter board and that's works fine but I have nothing on the screen .. I checked out the power supply and found nothing wrong except the 9V rail which is coming up 12v with no load and there is only a 5W resistor which pulls power from the 12v rail .. The resistor measures 4.7 Ohm's (Part number (PH)5W5R0J ) and I can't find any info on it.. Is there suppose to be 12v on the 9v rail or am I doing something wrong here ??? Also do you guys have any other input or help????

The T.V Spec's
Power board -MLT169A

Offline Jonny

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Re: Help with rebuild
« Reply #1 on: February 14, 2013, 09:09:56 am »
OOOHH I know the feeling. I recently killed a rather nice 5 CD changer when my meter probe slipped. OK it wasn't working anyway but now I'll never know if I could have fixed the initial fault LOL  :palm:

Depends what touched where really, if it managed to put voltage down a line at a level that wasn't made for it, it may have damaged ICs, which would make it history. Turn it on then go looking for components getting ridiculously hot, dead giveaway it's not happy. If there's an EEPROM, put a scope on the RX and TX lines and look for data flow, nothing happening might mean dead uC. Likely a long road ahead without a schematic.


Offline lapm

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Re: Help with rebuild
« Reply #2 on: February 14, 2013, 01:13:38 pm »
I dont even dear to admit how many machnes i have managed to cause extensive extra damage when doinr debug/repair work...  :-BROKE
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Offline TerraHertz

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Re: Help with rebuild
« Reply #3 on: February 16, 2013, 09:10:57 am »
<cringe> My sympathy.
I've a similar story, that will one day perhaps turn into another 'old scope restoration' thread here.
When I can overcome the shame.

I'd bought something I'd dreamed of for literally decades - a Tektronix 7104 1GHz scope mainframe. Advertised as working. And when it arrived it did work. However it was very dirty, with some corrosion of the frame, due to being stored in a bad environment.
So of course I decided to do a full strip-down and restore on it.

Now I've worked on plenty of things with CRTs before, and know all about discharging the CRT anode to a safe frame ground point before doing anything else.
I really don't know how I forgot this time. Something about the excitement of finally having a 7104 - the pinnacle of analog scope development. It had also been a while since the scope was on, but I should still have known better.

There's a HV flying lead connector inside, in a hard to reach place. I pulled it apart while reaching in awkwardly, and then... accidentally dropped it . The bare plug end fell past one of the deflection boards, and there was that horrible 'snap' sound of a few pF discharging several KV. To... something. It wasn't visible.

Worst sinking feeling ever.
Sure enough, the scope was (still is) dead. Does not even power up.
I have a service manual, but it turns out the 7104 switch mode power supply was designed by paranoids, and has multiple obscure fault detection schemes. If there's anything wrong, it won't even try to start. And the fault detection stuff is all analog, with parts in proprietary ICs, and really hard to understand.
I tried making a dummy loads setup, to fool it into thinking there's a functional 7104 scope attached, and couldn't even get that to work, or figure out why it wasn't working.

I'd like to mention that I am actually pretty good at repair, normally. This thing just defeated me, and I gave up in self-loathing and shame.
Someday, when I have a week spare in which to remind myself I'm a clumsy, careless idiot, maybe I'll try again. Can't wait to find out which of the extremely rare Tek hybrids on the deflection board I blew. Not.

Collecting old scopes, logic analyzers, and unfinished projects.

Offline PA4TIM

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Re: Help with rebuild
« Reply #4 on: February 17, 2013, 09:31:28 pm »
Sometimes it is luck. I was repairing a Philips PM2811 labsupply, lots of digital stuff documented but none of the analog. Checked everything, there was HV that could be right and I could not find the problem. I slipped a probe and blew a fuse. No HV anymore. I came to some powerfets, looked like one was gone. Desoldered it, tested it but it was good. Soldred it back and I had HV again but AC this time and at a high frequency, looked good, did some tests, followed an artificial made sense and found a smd transistor that had a base that never had been soldered. After that the PSU was OK again. Without the short I made I would not have found it. The mosfet probably had a rotten gate soldering and did not switch the 400VDC. After resoldering it did.

To bad I can not tell you the magic solution, good luck. my collection measurement gear and experiments Also lots of info about network analyse  repair of test and calibration equipment my youtube channel

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