Here I go trying to fix things again...4900

jay

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I've had an issue with my 4900 for a while now where the screen will dim and I get some color bleeding. A quick smack of the cab would fix her for a while. Today I decided to take the time and find out what the issue was.

Found it pretty quickly, I had a bad pot on the neckboard. When I touched it, it would fix it. When I unsoldered it, it basically fell apart in my hand.

SO.....I have another 4900 neckboard and I examined the numbers on the back of the neckboard pots and "502" is what I saw on a few of the pots so I soldered a new pot on.

Turned it on and nothing. Dead. Pulled the neckboard and looked again. Oh shit!!

The pot I replaced said 501 not 502. After checking a schematic 501 is 500ohms. I f-ed up and put a 5kohm pot on there.

Found a 501 pot and slapped her in. Again, nothing dead. Fuse is good. I have no HV no nothing.

Before I pull the chassis and start shot-gunning this thing....Where should I start?
 
A neckboard pot isn't going to cause your chassis to be bad, just by putting the wrong value there.

Look over the main chassis for cold solder, especially in the back right corner, along the back edge, and in the PS section on the left. Might have had a cold solder joint pop up while you were messing around with it.

Have you measured your B+ yet?

And if you didn't pull the chassis, then double-check the neckboard for cold solder and that the wire to the socket is connected good...
 
Cracked solder on the flyback would cause this too...

Basically anything that keeps the horizontal/HV sections from running will keep the monitor dead since the lower voltages are created by taps off the flyback.

RJ
 
I figured a pot on the neckboard wasn't going to blow something on the main chassis. With my luck, I figured it could happen some way somehow.

Before I did any of this, it worked. The only thing I did was discharge and pull the neckboard. Didn't move the cab, didn't plug it in to a different outlet, and a mouse didn't fart in the woods.

I went over everything I touched on the neckboard. Plugged it back in and nothing. B+ is 1.5Vdc and the fuse is good.

AC is 124V AT the chassis. This thing is just...dead.

Guess I need to pull the whole damn thing and take a looky.

Just when I was getting cocky with my last few repairs, this happens...:blackeye:
 
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The fuse in a 4900 are usually soldered in...test it with one leg unsoldered, betcha its bad. It'll test good while its soldered in sometimes.

If its bad make sure you didnt drop any solder or screws or anything into the chassis.
 
The fuse in a 4900 are usually soldered in...test it with one leg unsoldered, betcha its bad. It'll test good while its soldered in sometimes.

If its bad make sure you didnt drop any solder or screws or anything into the chassis.

I started to think the same damn thing a few minutes ago. It has to be!

I knew I should have modded that damn fuse "holder" when I had the chance but didn't. I even put that opaque brownish cover back on it. I used my VOM's continuity test to check it.

Doesn't Rat Shack have a real fuse holder that people use for a quick fix?
 
Fuse.......Won't be using that dumbass fuse cover anymore. Can someone explain to me why it would test good in circuit?
 
It's possible the filament burned thru in a very small area, and then as the two ends flopped around they ended up touching enough for the low voltage of the meter, but not the higher voltage of the incoming current. the act of pulling it out made it separate more...
 
Thanks mod, I'm not use to fuses blowing. Normally with my skills, everything else blows and the fuse stays in tact. Guess I got use to it.

Now, to find a tube....
 
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