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Dead TV, Magnavox, vintage 1995

I work on test equipment and experimental equipment a lot, and am familiar with high-voltage precautions, but have not worked on a TV in 15 years. Accordingly, let me ask...

The patient is a Magnavox 25-inch color TV from 1995. It went dead while in use the other day (no one was watching at that exact moment) and now shows absolutely no sign of power.

Where the AC power comes in, there is a tiny fuse, which is not blown, but it's something like 1/10 amp and can't possibly be the main power inlet for the TV. There is also an Omron relay or circuit breaker (at a quick glance I couldn't tell which).

What should I be looking for/at?

Thanks...

Reply to
mc
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"What should I be looking for/at? "

Standard tv troubleshooting:

Standby power supplies, system control and clock and data lines, on/off control lines from system control, main power supply regualtor system, horizontal start up, horizontal drivers, horizontal output, flyback.

Reply to
dkuhajda

Check for B+ voltage at collector of the horizontal output transistor (mounted on large heat sink in the vacinity of flyback). If nothing present, trace power supply backwards from collector. May be an open pico fuse in this path. If you don't have voltage at secondary of B+ transformer, check the voltage regulator and standby supply as suggested. A model # would help identify a possible common problem.

Reply to
Golf

Reply to
topstablala1

Give us a model or chassis number. Fuse that small is probably standby power supply, necessary but not usually the problem.

Reply to
Ol' Duffer

I forgot, but check the main fuse (glass) for open. If open, perform a diode test on the horizontal output transistor.

Reply to
Golf

Check the capacitor in the main power rectification for undischarge current.If so,the power supply is not funtioning,which you can check by reading the 110..145v at the secondary supply....

Reply to
topstablala1

Magnavox 25CR12 C121. Thanks!

Reply to
mc

Check the electrolytics near the B+ regulator. There was a common chassis around that time that tended to lose a large cap there. I think it also resulted in a shorted HOT. Andy Cuffe

snipped-for-privacy@psu.edu

Reply to
Andy Cuffe

That model number does not compute for me, unless it is a euro/asia model and I do not have access to that information. Are you sure it isn't 25TR12?

If so, it could be an X6 chassis that may be subject to the main B+ capacitor failure syndrome. 100uF 200V capacitor goes open due to surge/thermal damage. Once open, the supply loses regulation and climbs from its nominal 129V to something more like 145V, which makes the horizontal output transistor dissipate more heat. The HOT eventually shorts out, which usually blows the 2.5A pico fuse, occaisionally damages other stuff. The horizontal drive supply resistors also get hot enough to burn off their printing and crystallize their solder joints. I see these often enough to have it down to about a half hour.

Other possibilities are B8 or P6 chassis which are switch mode, run-all-the-time supplies that are much kinder to capacitors.

Reply to
Ol' Duffer

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