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PostPosted: Apr Tue 01, 2008 6:52 pm 
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As far as load on the set's power supply, only the filament would be of concern (consistent with your comments) -- the B+ load is negligible.

For a color set, it's not obvious to me that you need three circuits. Since there is a single common brightness control for all three guns, I would study the brightness control circuitry and see if you can introduce a single blanking circuit there to "add" to the voltage level established by the brightness control. If there is no filtering in the intervening circuitry that would prevent a fast blanking pulse from getting through, you should be in business.

Even if you do need to apply three separate blanking signals (signal to each gun well decoupled from the other two), I would first examine whether feeding via three resistors would be sufficient. If, in the end, you need three separate low impedance feeds, I would just build one blanking circuit, and use a triple triode (like 6T8) to separately buffer (using each triode as a cathode follower) the signal going to each gun. But I would be very surprised if you really need to go that route.

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PostPosted: Apr Tue 01, 2008 7:17 pm 
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I hadn't thought about the brightness control, I'll have to look at the Sam's when I get home. I worried about possible "bleeding" since this circuit is in the video path - hence why three separate circuits seemed logical. I also thought of using one 12AU7 and then a triple triode tube like the 6U10 (The 6T8 is a triple diode/one triode). But a 6U10 uses .6A of heater current, the same as two 12AU7's. And 12AU7's are easier to get, the 6U10 is somewhat harder to find.

-Ian


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PostPosted: Apr Tue 01, 2008 7:32 pm 
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Interesting topic and great little single tube solution. Most tube color sets only have a single luminance/video circuit that feeds all three guns. Only one blanking circuit should be needed and it should interface with the video output circuit. On your CTC-15, it is probably using a 12BY7A (or is it a 6AW8A?) as the video output (haven't seen one of these sets in years). You need to look at that circuit. If you pull that tube, the screen will go essentially black (all three guns) unless the color level is turned up real high. Chroma is fed to the three guns separately.


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PostPosted: Apr Thu 03, 2008 3:55 am 
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This is a very interesting and informative thread. Many thanks to all here for the excellent contributions.


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PostPosted: May Sun 04, 2008 5:26 am 
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I received an inquiry by email to provide a little more detail on exactly how the new blanking circuit is wired into the Predicta chassis. I'll show pictures here in case anyone else is interested in the same information.

The blanking circuit is built onto a single tube socket, which is mounted by a U-shaped metal standoff on one of the existing screws attaching the back end of the tuner to the main chassis. The left mounting screw for the new socket also serves as the grounding point for the circuit (notice the solder lug just under the socket on the left).

Image

This location is very close to the socket for the cable to the CRT, which is the small 5-pin chassis you see right in front of the added blanking circuit. The large yellow cap you see under the 6CG7 tube in the new circuit is the 0.2 uF output capacitor that feeds the new blanking signal to the grid of the CRT. One end connects to the 6CG7, and the other goes through a hole in the chassis to the bottom side, as do the two wires from the filament of the 6CG7.

Now here's the view from the bottom side:

Image

The filament of the 6CG7 is introduced into the series heater string right at the CRT cable socket. From the schematic posted earlier in this thread, you can see that pins 4 and 5 go to the CRT filament. I disconnected the brown wire that was connected to pin 5 of the CRT cable socket, and connected it to the new yellow wire going to the 6CG7 filament. The green wire is the other 6CG7 filament wire, which is now connected to pin 5 of the CRT cable socket.

The original blanking circuit wire has been disconnected from pin 3 of the CRT cable socket, and pin 3 is now connected to the 0.2 uF capacitor wire (feeding down from the top of the chassis through a hole), and also to a 100K resistor, which is grounded to a nearby terminal strip ground.

The two remaining connections for the new blanking circuit connect to the main printed circuit board, shown below:

Image

The red wire is the B+ line feeding the new blanking circuit, which is connected to an existing 150K ohm resistor on the board (see schematic posted earlier in this thread). The vertical sync signal is taken from one lead of an existing 0.1 uF capacitor as shown.

And that's all there is to it!

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PostPosted: Sep Tue 08, 2009 1:03 am 
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Picture links restored. This time I put the pictures in the "Philco" section of the gallery, so perhaps they will stay active longer.

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PostPosted: Jun Mon 21, 2010 6:02 pm 
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Thanks for this great information! I built Tom's circuit and fitted it to a Predicta Debutante. Works great...no more annoying macrovision retrace lines! If anyone is interested, I built a small printed circuit board and can send a visio file with the info.


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PostPosted: Jun Mon 21, 2010 8:03 pm 
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I want a few of these kits for my old TVs...how much$$ and where do you send for them,S&H$$


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PostPosted: Jun Wed 23, 2010 4:40 pm 
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How would i install this new circut in a 1958' RCA CTC 7AH Color set,any suggestions...


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PostPosted: Jun Wed 23, 2010 5:59 pm 
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Very clever. If your big concern is Macrovision lines, a plug 'n play alternative is to use an inexpensive video stabilizer. I use it to remove interference of the type seen in the photo.

http://checkhere22.com/stabilizer/

Phil

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PostPosted: Aug Sat 28, 2010 4:24 am 
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Retrohacker and 47'Plymouth864: both of your sets already have a blanking circuit, you do not need the one outlined here. Use the stabilizer Phil mentions instead.

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PostPosted: Aug Sat 28, 2010 2:55 pm 
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I Stand corrected miniman82 about already having it from the factory!
but i own 3 old B&W TV's...1951'Capehart,332RAMX...1954'Zenith
M2250 "Sutton"console, with a 19M22 chassis,..and a 1956' Zenith
Table 19M21 chassis..that this kit would work greatly!, if also were built with it all ready? :D


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PostPosted: Aug Sat 28, 2010 8:17 pm 
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Sets made since the middle to late 50's have vertical retrace blanking circuits, but they were not designed to handle Macrovision. Some handle it anyway and some do not. I don't see why you could not add this circuit to a set that already has a blanking circuit. At the most you would have to disable the manufactures circuit. Another option would be to modify the manufactures circuit to produce a longer pulse.

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PostPosted: Aug Sat 28, 2010 8:24 pm 
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i may have to add that circuit to my 1951 admiral


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PostPosted: Aug Sat 28, 2010 9:02 pm 
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As you can see from the Predicta example shown here, even a set made in 1958/59 still has problems with retrace blanking and can benefit from a circuit like this.

And as Tom Schulz pointed out, early sets don't often have a special blanking circuit, but simply relied on the sync pulse of the broadcast video signal itself to do the blanking.

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