Groovy CTC-15

[frenchy]

Maybe I missed it but I haven't seen any mention of what you are doing with the guns' screen or drive controls? What are they set at now? Are you having to crank those all the way up to get a horiz. line to show up too or (?)
Frenchy

[blue_lateral]

There is a 6.8k resistor (big, 2w or 3w I think) connected from a four lead video peaking coil to one end of the drive controls. There is a 39k 4w resistor connected from the same end of the drive controls to ground. According to an old book I have, etther one of these can cause no line with the switch in the service position.

John

[Charlie]

Originally Posted by frenchy

Maybe I missed it but I haven't seen any mention of what you are doing with the guns' screen or drive controls? What are they set at now? Are you having to crank those all the way up to get a horiz. line to show up too or (?)
Frenchy

Frenchy, I have been setting the crt screen controls using a volt meter. The controls are turned up about 2/3 to 3/4 of the way to give me the 800+ volts at the CRT cap. However, it won't produce a horz line at all.

John, you are correct... there are a few resistors there at the service switch. I've checked those... but with them still wired in... and they gave perfect readings. I'm probably going to cut one end of each and check again... just in case those readings weren't a result of the circuit looping around thru another resistor close by.

I also checked the coil you mentioned. One side is supposed to read 7 ohms and the other 12 ohms... both sides checked good.

I've been going back and re-checking readings hoping that maybe I missed something the first time around.

[Charlie]

Yesterday, I changed a lot of resistors on the color and deflection boards of this set. When taking ohm readings at the tube sockets, the path taken is from the tube socket to ground or to B+. Easy way to find resistors off spec. However, there are many in there that won't be in the path between the points mentioned above. After checking several, I found quite a few that were more than 20% off from the marked value. I found several that had doubled or trippled in value. Many of them looked toasted, and i'm sure many of those probably drift in value as the set warms up. Altogether, I changed out about 40 resistors. I felt pretty good thinking that things would work well once finished.

Not so.

I powered the set up and waited to see how much smoke would pour out. No smoke... sound came in and then screen lit up. Looking good so far. Naturally, after changing that many resistors, I figured I'd have to make adjustments. The picture was terribly out of sync. I couldn't get a lock on the vert or horz. Tried putting in another sync tube... still no good.

I powered it down and did a quick check of resistance reading at the sync socket. The readings were WAY OFF. One of the pins should read 4 meg to going to ground... I got a reading of 30K. The other pin readings weren't much better. Well crap... I instantly figured I put resistors in the wrong places.

Back to the bench.

First, I looked at each resistor and compared to Sams. Then I went back and checked again. Then I checked under the PCB to make sure I didn't short any of the printed lines together with solder. Everything looked fine. I then traced each printed circuit from the tube socket to see if I could find a place in which I wired something wrong. Everything still looked good. Then I unsoldered resistors and caps surrounding the sync tube socket and checked them individually. They all checked fine. Hmmm... I'm running out of things to check.

Lastly, I decided to check the tube socket itself. Come to find out, something was wrong here. I was getting resistance readings in places I should get no reading. I took a piece of wire and worked it under the socket... thinking I'd find something wedged between the socket and PCB... perhaps shorting socket pins togther. Nothing turned up.

Finally, I unsoldered the socket the removed it completely. It looked fine. Took readings across the pins... nothing. Okay, so this means the socket is fine.

Hell... there's nothing left to check! Or is there?

I'm sitting there staring at the PCB. Keep in mind that the caps and resistors surrounding the area where the socket goes have been removed. For the hell of it, I placed my probes between a couple of the connections... one of which only has a few solder connection points... a couple of resistors and a pin to connect a wire. I looked at the meter and found a reading of 200K ohms. Okay... that's not possible... this meter must be malfunctioning. Got another meter. Same reading. What the hell??? How is the PCB conducting through the board? I checked from that same point on the board to chassis ground... got 500K! This would explain my sync circuit not working right... but why?

I got some alcohol and a q-tip to make sure there wasn't any dirt... even though it looked clean. Hell, it should be clean... I sprayed that socket a couple of days ago. Wait a minute... I sprayed that socket! There was oil in that spray! So, I cleaned the board with the alcohol and noticed my resistance readings changed. Now they're reading about 1 meg. I can't get it any better than that as of yet.

Sooo... I'm guessing that the oil in the spray has soaked into the PCB where the socket goes... probably thru fine hairline cracks. That's the only thing I can think of that would allow conductance thru the board.

And all this time I was expecting to find I wired a resistor in the wrong spot!

So, when you guys spray tube sockets, make sure you're using an electrical spray that DOES NOT have oil in it. It could really screw up your PCB.

[blue_lateral]

GAH! Oil shouldn't be conductive! What kind of spray was that?

For what it's worth, there was a real problem in some older sets with carbonized (burned) circuit boards. I think it was the early rectangular Magnavoxes, you know the chassis that looks like RCA at first glance, yet is pretty different when you start looking close at the boards? Well anyway it happens. As I recall it is the grid pins that really matter. A little leakage will grossly mis-bias the tube.

If you look at the IF board in the CTC-15, the board is milled out at the grid pins, and there are flying leads to the tube socket. I have often wondered if this was to reduce capacitance, or if it was deliberately done to prevent bias problems from carbonized boards.

Does your board look carbonized? Do I understand you correctly that all connections are removed from that trace and you still have a meg to ground? How far is the trace physically from ground?

John

[Charlie]

The cleaner was MG Chemicals Super Contact Cleaner. At first, I couldn't imagine this stuff being conductive... but so far, this was the only theory I could come up with. That sync socket was the only one I sprayed... and it was the only one I had screwy readings at. Also, it did look like an oily residue was there around the socket area... which made me think this spray was the culprit. I couldn't come up with any other ideas of why I was getting conductance thru two points that shouldn't have any... although I'll be glad to entertain ideas or theories if anyone has any!

Yes, all connections were removed from the trace. If I remember right, the trace I was using was at pin 3... and there were only 3 connections... so it was pretty small. When I would check readings from that point to other places... including chassis ground... I was getting readings. This particular trace wasn't too far from a ground... perhaps a half inch or a little more... and the ground trace nearly goes around the entire board.

I've been using this same brand for a couple of years... never had a problem before. For applications such as this, I think I'll start using spray that does not have oil in it. I usually keep both types on hand, but havent been down to the parts house in a while. I'll make sure to go this week and get some.

No, didn't look like any carbon residue on the board there in the sync area.... although some other areas on another board look like they've been cooked thru the years.

Since that post, I've been applying alcohol to the area with a q-tip and letting it dry.... then go back and do it again. Now, a couple of hours later, I'm not getting any resistance readings like before... now I'm getting readings around 20 megs where I was earlier getting 200k. I'm gonna keep applying the alcohol since it seems to be helping.

[Charlie]

After my experience with conductivity between isolated PCB traces, I went to the parts house to get some new cleaner spray and discuss my issue with the guy that runs the place.

I described in detail what happened and the man was very surprised of the results I ended up with. The man pointed out that the spray can be used on IC's. Like myself, he figured if it can be used on IC's, it can be used on tube sockets.

I picked out a can of MG Chemicals Electrosolve Contact Cleaner- Zero Residue, Non-Conductive. This stuff says it cleans and degreases all the normal stuff... including micro-processors, tuners, and printed circuits.

Now, things seem to be getting better. A couple of days ago, I would turn on the set, and it would take about 5 or 6 minutes for the picture to straighten out. Everytime I've turned it on since then, it takes a little less time for it to make a good picture. This morning, I turned it on and it straightened out after only a minute.

The set is producing a pretty good picture, however there are a few flaws. Sometimes when I turn it on, the screen is black with no raster (but has HV) and I've found that changing the channel makes the picture come on. At first, I thought it was something loose, but if I tried hitting the set or tapping various locations, that doesn't help. Only changing the channel helps. I've also noted that when the raster is missing, the 3rd IF tube is glowing twice as bright as it should. As soon as I change the channel and the raster comes on, the IF tube glow goes back to normal. I'll try changing that tube and see if it helps.

I'm having difficulty getting the verticle height and linearity set correctly. The top of the picture is a little stretched out and makes a circle look like an egg.

In a black and white thread, I mentioned having a light verticle line in the left side of the screen on my b&w sets... and that I hadn't seen it on a color set. Seems I shouldn't have made that comment, because now I have the same line in this color set. My horz drive is set correctly. Last night, I read in a book that changing the damper tube will sometimes correct this, so I'll try that and see what I get.

I'm noticing that the bottom part of the picture does not focus as well as the top. Focus voltage is correct.

I'm thinking it's possible that the focus, light verticle line, and vert height/linearity problems are all related... and possibly the culprit could be the flyback. The flyback is running cool... no hot spots. I had changed this flyback in the beginning, however, it was pulled from another chassis. It's a replacement part... not an original RCA flyback.

On the positive side, the color and contrast is really good.

It appears that this set was REALLY worn out when I got it!

[andy]

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