K4915 Med Res Auto Protect?

schwaggs

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Donor 2015
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I have a k4915 that works fine for about 15-30 minutes then goes dark. If I power cycle the chassis, it will start right back up again.

I have re-capped the chassis

The tube was the worst I have seen when I checked it with my tube rejuvinator. ZERO emissions before restoring. I was able to get all 3 back in the green and balanced pretty decently.

I did both the above before I noticed this issue but to be honest, I only had the monitor a few days before capping and rejuving it.

Questions:

1) How do I determine what is shutting down the monitor?
2) Could an incorrect Cap in the cap kit cause this?
3) Bad flyback (it is an original from a scrap 4900 chassis I had)
 
You need to measure the b+ when it shuts down. It's probably going over voltage.

Thanks, I'll have a look. Shouldn't be too tough to measure and monitor.

Do you know if any info on the Auto Protect adjustment? It isn't in the manual.
 
The regulator

gets the voltage from the filter cap

to the collector or case of the transistor.

to see which pins these are by looking at a datasheet

for the part. or manual.

The base controls the emitter voltage

the emitter is the output of the regulator.

the base and emitter can be adjusted.

without the proper electrical load

on the PS it will in somes cases be as high as the collector voltage.

I go by the measurement of all the test points around the regulator

not just the the B+ test point.

You will not have a shorted HOT you have the same dc

on the collector.



The Protect test is to confirm that the chassis will shutdown

if the dc is to high, for the sake of the B+ and the high voltage.

when the dc is turned up and the chassis is working it does increase

the drive pulse across the flyback and that increase the high voltage

the test shows that the shutdown circuit

is working..

when it works it kills the oscillator and in turn killing the drive

pulse from the ic and the collector of the horizontal drive trans and H.O.T both

will be almost as high in dc.

just means it doen's have the correct load. or the ic is bad, no voltage to it.

a secondary short from the flyback?

which could be a few different causes.
 
I finally figured it out! Put a new flyback in and it turns out, the monitor wasn't turning off, it was getting much dimmer. With the new flyback, I must have adjusted the screen control just a little higher. When the screen would "go blank" you could just see a faint image. It wasn't going blank, it was getting very dim.

Long story short, turned out to be a cold solder joint on the HOT! I could have swore I reflowed all the power transistors (and high wattage resistors, header pins, etc) when I recapped it but I must have missed the HOT way over on the side of the chassis.

Colors are more pure and the brightness is rock solid (no flicker, etc).
 
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