convergence selenium rectifier to silicone

[timmy]

how would i find out what resistor to use when replacing the triple section selenium rectifier in a ctc15 convergence board with silicone rectifier. its kind of hard to check voltage when tv is running, i have the voltage specs but need to know the resistor to then check it once its in or should i just replace 3 diodes in place of the triple section selenium. or is there some kind of formula to do. i would be using 3- 1n4001 subs. the sams says to use a resistor in series to match the voltage when using silicone diodes, would each resistor go on the positive end or the negative end of each diode.

[ctc17]

no need, do it all the time with just straight up diodes. no resistors needed

[timmy]

would 100v 1a diodes be any different then using 50v 1a instead for the selenium rectifier?

[ctc17]

Nope, all good. Why do you want to change it? Are you having problems getting convergence to work?
I have to say that out of the huge collection of tube color sets I have I dont ever remember changing the diodes.

I know your working on a 15, do you have the PDF sams for that? Can you email it to me?

[timmy]

well im changing the rest of the caps and the set works great but the rolling problem,and when i set up the set it did not converge that well as there were a few of the pots that were turned all the way to get it close but was still off alittle so im changing the rectifier since im told they leak and could cause problems getting good convergence. also changing caps in and around the verticle section. yes it is a ctc15 and i dont have a scanner otherwise i would send the whole thing. the voltage dependant resistor next to the 6em7 vert tube i get no reading at all with it out of the circuit and the sams dont give a value of it i do see it in the schematic but no value what should be put in place of it if its bad, or does this resistor not show any reading?

[ctc17]

That resistor is miliamp tested at a specific voltage, like 3ma at 450 volts. You can not test it with a meter. If it was bad te picture size would bounce all over the place. In that circuit its used as a very special rectifier to regulate and shape the feedback. Somewhere I have an article that explains all about rcas use of that.

[timmy]

if the video peak pot adjustment is to far up would it cause the screen to go out of focus and blurry and come right back. some picture scenes go blurry and then clears right up again.

[ctc17]

Thats sounds like the high voltage is dropping on bright scenes. Does the picture bloom? like expand all the way around?

You really need to do check the high voltage. The other thing you can do is measure the voltage drop accross the 1k resistor in the cathode of the hv regulator tube. It should be around 1 volt on a dim scene and drop on bright. (900ua drop accross 1k = .9 volt) If its dropping to zero you dont have enough high voltage headroom and theres another problem.
This is kind of explained in the sams under horizontal sweep adjustments.
It could be something like a weak horizontal output tube, weak hv rectifier, low b+, horizontal blanking issue, something in general causing low output of the flyback.
That shut tube needs to be always be conducting some current to maintain a steady high voltage and focus voltage.