Color Classic Analog Board

[techknight]

Hey guys, the zener diodes at DL21 and DL22 was overheated and fell off my board. i cannot find them, so does anyone know what the value of these zeners are? or the numbers off the casings of the diodes? Thanks. im screwed without these diodes, because they regulate the B+ supply on this line.

The LC550 dont have these diodes. they have a spot for one, but nothing there.

 

[tomlee59]

The ones on the board I happen to have out on the bench are also toasted. But from what I can read, they are of the Vishay BZX85C series. At least DL21 seems to be an 18V zener, from what I can make out. The other one could be also, but it's hard to read the second digit. It could be 10, 16 or 18.

 

[techknight]

most likely are in my humbile opinion, since they are in parallel.

 

[techknight]

another thing that invokes my curiosity, i wonder why the LC550/575, etc etc doesnt have these. maybe something to do with the higher resolution.

its kind of gutsy, but whta happens if you run the color classic without them? wont sync the res?

 

[tomlee59]

most likely are in my humbile opinion, since they are in parallel.

Then they would indeed be the same voltage, and the designer would need to be spanked for hooking them up that way. Equitable current sharing between two paralleled zeners doesn't occur naturally.

As to your other question about what would happen without the diodes, I have no idea. I have not traced the circuit enough to know where these diodes are in the circuit, so I have no idea what would happen without them.

 

[techknight]

well the LC550 board doesnt have them. so the first assumption that comes to mind is maybe they are pumping a higher voltage in the LC550 on that supply line becuase the LC runs at 640x480 and the color classic does not. and the transformer mods that are floating around to boost the color classic res, may only be as simple as removing these zeners. LOL. well, anyway, either that or fry. hehehehe. (my analog board is screwed anyway, might as well try. )

and as far as parallel zeners, yeap your right because even though on paper they might be ideally matched, but thats only under ideal conditions. there are losses that might be slightly different between the two and that didnt work so well. hehe.

if it were me, i would use a bigger dumping resistor so the zeners wouldnt heat up as much, and leave the longest leads as possible on the zener to help disipate the heat.

 

[techknight]

Hey, did you get to figuring out what the actual voltage value was? thanks.

well if the diodes are still in circuit, you could meter them out in operation and it would give a good indicator.

 

[armadillu]

Did you ever figure out a replacement? Mine look really toasty as well... And so do their surroundings. They even managed to detach the traces from the board. They seem to test ok, but I'd like to replace them anyway, together with that resistor north of DF2, which I cant even read its ID.