Problem replacing screen taps with pots
 

Okay, I'm obviously doing something wrong, so here's the deal.....

I'm trying to replace the screen taps with pots so I can better control the color for grayscale, etc. I've looked over the schematics (see attached) and came up with the need for a minimum of a 4K pot. I purchased three 5K pots and tested each one. They all are just at the 5K value.

Today I decided to test the red, so I took off the lead from the red (it was on Med) and put a jumper on the lead. I then put a jumper on the HI output. From the schematics, the HI should be directly from the collector of the red video output. On the other end of the wire lead, there is a 1K resistor and then it goes to the CRT socket. When I put the pot between the HI and the lead, the color gun came back and was very red. I expected this as the pot was fully CCW. As I rotated the pot, the red barely changed. I went full 5K and the red was reduced so little I almost thought it wasn't working at all. I then put the jumpers on the two end connections so it would get the full 5K and it was the same.

So where am I going wrong? The HI tap is full, the Med gets a 1.2K resistor and the Low gets another 2.7K. If you add all those up, that's 3.9K. So why won't a 5K pot give me the same kind of reduction?

Thanks!

I don't know the anwser to your question, but man, the effort you are putting in this CCII is commendable!

I hope you get 100% out of the old girl :)

...

I'm not sure I understand how that works. The Hi tap is full w/no resistors. So why doesn't a pot between it and the wire create the resistance like the ones in series on the Med and Lo? If I cut out the resistors, then the exact amount on the Hi would be present on both the Med and Lo. Leaving the pot to do the same it's doing now.

Andy is telling you how to use it like a voltage divider, on side of the pot is low the other side is the high the wiper (lead to crt) moves from low to high.

I'm trying to be the least destructive to the original setup of the circuit as possible. If I start cutting out resistors it will be hard to put it back to original condition.

I was under the impression that if I used terminal 1 from the Hi tap and terminal 2 to the CRT lead, it would act as a variable resistor? I'm must be missing something about how this circuit works.

What would happen if I connect terminal one to the Hi tap, terminal 3 to ground and terminal 2 to the lead to the CRT? Would that produce the effect I'm looking for, or would I short something out?

As I understand it, you did not remove R243 and R239? This means that you have reduced the resistance in this part of the circuit, and it will no longer bias the transistor or the CRT correctly. You must disconnect R243 and R239 and replace them with the pot (from H to L). 5k may or may not be close enough in this circuit - it is certainly outside the 5% tolerance of the original resistors. To get back to 4k ohms, put a 20k (18k or 22k is close enough) resistor across the pot (from H to L).

It sounds to me like he just put the 5k pot in series with the H tap and the 1k resistor going to the crt. That is the wrong place, as it needs to be set up as a voltage divider between the H and L taps with the center leg of the pot going to the 1k resistor, as explained by Andy. I share the concern about upsetting the bias on the transistor and would also recommend placing a 22k fixed resistor across the pot.
jr

Okay, so I need to remove one leg of the R240 (1.2K) and one leg of the R238 (2.7K) from the circuit. Then I need to place a 22K resistor across the 1 and 2 terminals. Finally I need to connect terminal 1 to the Hi tap, terminal 3 to the low tap and the lead going to the CRT on terminal 2.

Is it really necessary to use the 22K to bring it back to a 4K? I purchased the 5K based on a posting in an early thread. If 5K is too much, then would not turning it fully CW take care of that issue? I'm sure I'll never turn it that far as it would remove too much anyway.

5K should work fine, its an easier value to find than a 4 K
unless I am wrong ! The adjustment just wont be quite as precise.
Looking at the schematic the wiper(center) goes to the "plug" wire (CTR).
One end goes to C of video out.
Other goes to JCN of R239 & R238.

If you dont take out R239 & R240 they are in parallel with
the pot cutting the ohms in half & reducing range.
1)Nip off R240 at the C of Q206
2) hook up only the wire to CRT to center of pot.

If you nip the R deep in the TS you can tack it back on
& nobody will ever know if you change it back.

73 Zeno:smoke:

Since the circuit is a voltage divider, can't I just connect T1 to the Hi tap, T3 directly to ground and T2 to the CRT lead? Wouldn't that give me a separate voltage divider while being able to leave the current one untouched/unused?

No! Do not connect the other end of the pot to ground! That would put way too much voltage across the pot. Do you have a fire extinguisher?
jr

Your posted schematic is missing a lot of info on where some of the resistors are connected (what voltage is at each point in the circuit). There is only one way to make sure the circuit operates the same without knowing all these things and that is to replace R243 and R239 with the same total resistance.

I repeat, you will surely avoid trouble and get the result you want (a continuously variable tap instead of only H, M, and L) if you remove R243 and R 239 and replace them with a total of 4k. The way to do this is to wire one end of the 5 k pot to H and the other end to L, and also put a 20k (18k or 22k is close enough) from H to L in parallel with the 5k pot. Connect R253 to the wiper (center terminal) of your 5k pot.

Simple, and guaranteed to work without further calculation.

I think you mean R240 and not R243. R243 is a 33ohm wire wound 3W located in another circuit.

Okay, I'll look under the chassis for those two resistors and the other four for the other two colors. I'll then wire as noted and put a 22k across T1 and T3.

A resistor on the terminal and then to ground would be the right way?

No! the correct way is to connect it as old_tv_nut described.

jr

[QUOTE=TinCanAlley;3082296]I think you mean R240 and not R243. R243 is a 33ohm wire wound 3W located in another circuit.

QUOTE]

I mean the resistor between H and M - the number is smudged, so if you say it's R240, that's the one.

Okay, I'll go the route of cutting the resistor leads and such. Just out of curiosity, what wattage should the pots be? I had assumed they would need to be equal to or higher than the resistors in the current voltage divider circuit, but am not positive. The ones I got are for testing, so I didn't get quality ones. If/when I do it for use, I'll buy better pots.

Okay, going to put this project on hold. The scope arrives today and I've got a bit of reading, setup and practice to do so I can fix the jail bar issue.

Thanks for all the input and being patient with me while I learn.

Okay, while I wait for the replacement HOT, I've gone over the screen taps issue and have noted what needs to be done based on all the information given here, the schematics and seeing it up close with the chassis on a bench.

I'm attaching a pic of the underside of the taps. In it you will see three colors used to mark the Hi, Lo and Med taps. The green is the Hi side and is fed from the transistors. The red is the Med tap and is fed via the resistor from Hi to Med. The Yellow is the Lo tap and is fed via a resistor from Med.

Since the Med post is where the two resistors that need removal meet, and they're the only things in the post, I'm going to heat up the post and remove and isolate the leads of the resistors. This will allow me to put the pot across the Hi and Lo and the CRT lead on the wiper. If the day comes that I wish to put it back to factory, I'll just heat up the post and reinsert the resistor leads.

Any thoughts? Anything I might be missing? Any input on the wattage needed? Just realized the pots I got from a friend are only 1/2w. Is that too low for being used as a voltage divider? The circuit under 180V.

Thanks!