Rca ctc-5

[Penthode]

Originally Posted by miniman82

As Wayne pointed out, I think you're misunderstanding how the 6BK4 does what it does.

Sorry guys. I think I my post was misinterpreted. I guess I wasn't clear.

Since the 6BK4 is a "shunt" regulator, I realize it shunts current to maintain a constant load current, hence supply voltage. Also to regulate, the 6BK4 does not sense current, it senses voltage. The boost B+ voltage is sensed which should be directly proportional to the HV.

What I was suggesting is that if unregulated HV voltage is at or below the point the 6BK4 begins to conduct, the 6BK4 will not shunt any current, hence will not regulate the voltage.

I measured about 23kV with the 6BK4 cap disconnected. (The cap removed effectively removes all HV regulation). When the cap is connected, the voltage remains about the same. This means the 6BK4 is not conducting, probably because the unregulated HV is too low.

The question I have is if the focus voltage remain constant at the point the HV begins to drop (HV out of regulation), this means the problem is the unregulated voltage at the output of the HV rectifier is too low. This in turn implies that the HV pulse arriving at the HV rectifier is of insufficient amplitude. Further, because the focus voltage remains the same (constant CRT load), the HV transformer core is not being over saturated with the HV supply current demand. In other words, the transformer is okay.

On the other hand, if the focus voltage drops with the HV, the transformer core is perhaps becoming over saturated and I would suspect a fault with the transformer.

It is interesting to note that the schematic suggests the HO Tube cathode voltage is 7.5 VDC. This means the cathode current should be 7.5/24 = 312mA. I do not think the current should be appreciably lower than this because I believe the flyback pulse would be reduced which in turn reduces the HV.

[DaveWM]

while checking some other stuff, mainly I wanted to see if the two 5600 ohm resistors in the chroma matrix used to drive the green were ok (they were I noticed the 5600 ohm resistor in the grid circuit of the green diff amp was about 50% out so I replace it. while check other stuff I found R702 (6800) was reading over 100k. The odd thing was it was not wired as in the schematic. In the schematic it show it across C and D of the chroma take off transformer T701. In mine its connected from pin 7 of V701a to D with no direct wire from pin 7 to D as shown (which is why it was easy to check in circuit).

I replaced it as well. Turn the set on and there is too much green in the highlights now, so a minor tweek and all good again.

for fun I tried putting a 22meg in P with the ground side of the voltage divider on the shunt, increased about 1kv to 21.5. The overall bias with this setup is about 12v but of course it would change with brightness settings. the schematic shows a 15v (375/390) so its all pretty good.

adding the 22meg in p is within the tolerance level of the resistor so I will just leave it at that.

I don't know what happened but the HV started going up to over 25kv and I could see the shunt was not glowing green suddenly. I tried wiggling the tube and even a new tube, still not regulating. finally I pulled the chassis and check the filaments for voltage, all the resistors, put it all back in the cabinet and now its working again, I have no idea what was going on with that.

I am going to try changing the voltage divider resistor to 120k vs 83k (as done in later production runs) just to see if there is any effect on the cathode current in HO tube (the voltage divider resistor is on the horz osc board).

[DaveWM]

the 82k measured at 94k I went with a 120k. No drive bars, horz lin looks good,
21kv (with a 22meg on the ground side of the shunt) and the cathode is less that 180, about 178, which is about as low as I have seen on ANY of my color sets. there is a 'tiny' hint of a drive line that I can see on a blank raster, so I may go back to the 100k but will leave it be for now, and will check the fly temps as well.

The color looks good, but I still need to go over the purity and convergence again.

duh after looking a bit closer I think I am at 22.5kv the divisions are not 1kv I should have looked closer.

[DaveWM]

some progress pics

[DaveWM]

couple more OOT resistors, a common cathode resistor on the 2nd bandpass amp and blanking amp 270 reading 750, and I doubt it will make any difference but will replace anyway.

[DaveWM]

normally when I am replacing parts on old wafer style tube sockets, I just clip the lead and pig tail, this is to prevent breaking a tube socket pin connector and ruining my day.

In this case it was just the 270 ohm resistor and I could see it was not really wrapped around the hole, so I got out the solder wick and safely removed the resistor. Replaced the resistor all looked good, the only other connection to the tube socket was the wire that went to the blanker tube socket, but I did not disturb that.

I always confirm my work so if its a pig tail I make sure I have good continuity across the pig tail connection (if the wire is not clean its easy to have a good looking connection but no actual contact INSIDE the pig tail.

in this case I did not have to worry about that as I could see I had good flow and had filled the hole in the tab with solder. I did check to make sure the wire that goes to the blanker tube cathode was still good and surprise, it was open. a closer look and I could see the wire had broken right at the pin that I had just soldered to. it was partially corroded and I guess the minor movement done in the install of the resistor must have broken it.

I had to fish the wire out and strip it to get a good clean piece, lucky there was enough slack (just). solder wick out the connection again and redo thru the hole and wrap, solder back. Needless to say I was very careful as the socket tabs break easy.

when done I confirmed that I had continuity back from the pin thru the wire to the other tube socket, AND the 270 ohm resistor was still 270 and connected.

Watch out for those very fragile hookup wires used, in this case I think it was 26 gauge solid so not much to break it.

[DaveWM]

after fighting poor greens I decided to try the vectorscope again. this time I clipped on the other side of the grid coupling caps for RGB on the CRT. Much better, did not effect the CRT display at all.

While there were color bars, the blue and reds were fine but the magenta was weak as was the green.

looking at the scope you could see the fingers but they were much more of an elipse that an a circle.

My interpretation of this is the phase of the CW from the osc was not correct to one of the demodulators, and the transformer that changes the phase needed to be adjusted. Since green is derived from the X and Y demodulators, then a defect there would really mess up green.

What I like about the vectorscope is the instant visual response (not just that its changing but that its changing in the right direction.

as it turned out about 1/4 turn of that phase adj transformer was able to open up the elipse and give a nice circle pattern, when I looked up at the CRT it now had a very nice color pattern.

This is the second time the vectorscope came to the rescue for me, I highly recommend one. Besides they also have nice dot and cross hatch settings.

funny thing is how good you can get flesh tones even with green so useless, I guess its just the flesh tone does not need much green unless your looking as Spock.

[old_tv_nut]

Originally Posted by Penthode

... Also to regulate, the 6BK4 does not sense current, it senses voltage. The boost B+ voltage is sensed which should be directly proportional to the HV.

You are correct - don't know what I was looking at before.

[DaveWM]

I also found the vectorscope useful for touching up the matrix balance pot on the back of the chassis as well. this pot varies the cathode bias resistance of demodulator tubes. the effect on the vector scope is to squash down the circle pattern to get it as round as possible. I found that by adjusting the top core of the CW driver trans and the matix pot at the same time was the best way to setup the matrix.