Groovy CTC-15

[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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