GE Portacolor and DC restoration

[Tony V]

Thanks Andy!

[Chad Hauris]

Thanks for bringing this up again, I was working on Portacolors recently and know I read something about dc restoration a while ago.

Andy do you have a web server/site? I can post your schematic diagram on a site I have if that's OK so there will be a backup copy of it available.

[andy]

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[ChrisW6ATV]

Thanks for posting that circuit, Andy. I have always disliked TVs without DC restoration (including B&W), and I considered copying something from an old TV to add it to others.

[HuggyBear]

I'm new here, and this is my first post.

I *just* started restoring a Porta Color someone gave me 10-15 years ago. I don't know what model it is (model tag is missing) but it has the 10HE chassis.

I *DO* remember from working on them years ago that they sorely needed DC restoration. I am really rusty as far as working on TV sets are concerned (although a few months ago I successfully repaired a 2005 RCA that had a bad power regulator chip). I have a 1950 Philco I plan to attack this summer.

But I digress...can you make a suggestion for a type number for the transistor? Lacking that, does it need to be high voltage as the connection to the 280 volt source might suggest? How much power does it need to handle? Is 1/2 watt adequate for the 47K resistor or does it need to be larger than that?

What kind of a diode is that? Obviously the 4.7mfd cap needs to be 350-450 volts.

Thanks in advance!

[andy]

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[Joe Sousa]

Andy, just letting you know that I tried your DC restoration trick with the diode and got very good results.

I used a couple of 1N34A diodes in series. I knew that they would have negligible capacitance of a few pF. I tested the reverse breakdown at just under 100V for both diodes. The video swing at the CRT cathode is about 75V p-p, so I figured two 1N34A in series would give adequate margin.

I also added your recommended bypass cap. The DC restoration as observed on the scope was quite obvious. As the scene content varied, there was nearly no movement in the average video voltage. The minor DC variation that could be seen, if observed carefully, was mostly due to the non-zero impedance of the brightness pot.

The picture was indeed greatly improved. Now the color saturation stays put on someone's face, even if the back ground brightness varies substantially.

My motivation for adding the DC restoration was that I was finding that color saturation was very dependent on overall scene brightness.

Surprising that something else in the set could not have been traded in the original design for such a cheap and very effective image quality improvement.

By the way, I did not see any retrace from lack of blanking. The CRT on my set is pretty strong. It could be that on a set with weak CRT emission, the brightness control will be turned way up, making it more difficult to blank.

I did not add the 100k resistor to the bottom of the pot to limit the max brightness. I suspect a 50V zener or NEON bulb may be better than the 100k to keep the pot impedance low for best DC restoration. I have not tried this.

My Portacolor set is a Bradford 1171D24 sold by W T Grant as WTG79814 with the 10HE Chassis made by GE in 1974. I have the Photofact folder 1394 in scan form available for anyone interested at Bradford Portacolor.

Thanks for the original post. Saved me a lot of head scratching to figure out a good DC restoration circuit.
-Joe

[old_tv_nut]

Nothing to add to this discussion except Thanks! - I found it very interesting!

[Joe Sousa]

I have finished restoring my Bradford Portacolor.

I attached the final DC restoration circuit. I ended up using a NE2 neon bulb instead of the 100k resistor that Andy suggested because of lower impedance of the NE2, which is beneficial to get a stiffer DC voltage.

I also added a couple of shunt resistors that further helped stiffen the DC voltage coming out of the brightness control. The 47k and 130k/2=65k were chosen to match the voltage at the pot in the optimum brightness position.

One of the attached photos shows that 3 130V wires were originally wired to the bottom of the brightness pot and had to be rerouted for the NE2 bulb conneciton.

The black level still varies about 20% from full black to full white scenes, but that is much better than without DC restoration. In normal brightness scenes the variation is quite negligible.

The PNP buffer that Andy had would have been useful to stiffen the drive and prevent sync pulse attenuation due to the diode clamp or the black level restoration. I did not want to add the PNP because it did not seem right in an all-tube set. But there is merit in the PNP.

I attempted returning the 4.7uF cap to the cathode of the video amp pentode via a 1mH choke to get the diode current load recirculated. The DC restoration improvement was negligible, so I left this out. By the way, the cap only needs about a 250V rating. I used a 450V cap because that is what I had on hand.

Next, I tweaked convergence. The only fine adjustments are for vertical and horizontal position of Red and Blue. The horizontal is done by sliding the coils and magnets or rotating the magnets, and the vertical only by rotating the magnets closer to the front of the CRT. The magnet glue needs to be loosened with acetone on a stiff brush. After alignment, some nail polish glues the magnets back in place.

I also did a bit of blue horizontal size convergence by paralleling a 0.5 ohm resistance with the blue horizontal convergence coil. This was a very small tweak of about one half dot out of the white dot test pattern on the the screen. Just a bit anal...

Another adjustment was for the tint control. I got it to be right, when centered, by tweaking the transformer driving the 3.579MHz crystal. This is tweak A14 in the Portacolor Bradford photofact. None of my alignment tools was long enough to do the tweak from the bottom of the PCB, so I made one from a long thin wooden coffee stirrer from Starbucks.

This particular portacolor uses a toggle switch for power, so it is hard to know if the set is on unless the image or sound are also on. I had the set open, so I added a Russian INS1 neon bulb as a pilot light above the controls.

This set also has a variable focus pot. It may need cleaning. I noticed Focus fluctuations after I closed up the set. Working the pot back and forth improved focus stability. It needs a squirt of Caig pot cleaner.

Two of the attached photos show the final results. One has text from my DTV converter box to show the acceptable convergence. The other is an SNL scene, showing reasonable color.

Comments invited, and thanks to Andy for the original DC restoration post.
-Joe

[yagosaga]

Hello,

as an alternative to the GE luminance circuits, I have cut the luminance circuits out of the schematics for the Kuba Porta-color, see photo. The video signal is given over a R/C to the cathodes of the picture tube with C = 0.47 uF and R = 220K. This might be an improvement also.

I am satisfied with the picture quality of the Kuba Porta-color, and it is an improvement in relation to my GE Portacolors.

Kind regards,
Eckhard

[Joe Sousa]

Eckhard, good to hear from you.

So Kuba improved on the GE Porta-color design by adding DC coupling at the video pentode output.

Was the drive to the Video Pentode input DC-restored or DC-coupled?

It would be great to see the rest of the Kuba schematic.

Regards,
-Joe

[MRX37]

Okay so i tried this little experiment on a TV and I turned it on and heards a loud POP! and the power went out! It took them three damn days to fix the power and now the Tv stays on and shoots sparks and shit everywhere and I cant sleep because it hums all night long!

Seriously: I'm impressed. A couple dollar's worth of components was all that was needed? There's penny pinching, but sheesh!

[yagosaga]

Originally Posted by Joe Sousa

So Kuba improved on the GE Porta-color design by adding DC coupling at the video pentode output.
Was the drive to the Video Pentode input DC-restored or DC-coupled?
It would be great to see the rest of the Kuba schematic.

Hi Joe,

here are the entire schematics:

http://fernsehmuseum.net/kuba/img/CK211Pschematics.jpg

The drive to the video pentode input is D.C.-coupled. But note that the drive to the picture tube is not fully D.C.-coupled. GE has absolutely no D.C.-coupling, the Kuba Porta-color has some D.C.-coupling with R = 220K.

Best,
Eckhard

[Joe Sousa]

Eckhard, thanks for the link to the Kuba schematic.

I was surprised to see how extensive the schematic differences are between the Kuba and the original GE version. The differences extend well beyond the color decoder.

For example, the UHF tuner in the Kuba includes a preamp stage, the audio section in the Kuba uses two transistors for the IF, with a ratio detector, instead of the discriminator tube in the GE, power supply filtering in the Kuba has clever three tap choke, and the Kuba uses a triode-pentode tube for vertical deflection instead of the dual Triode of the GE version.

Have you noticed a difference in performance between your Kuba and GE Portacolor sets? Is the UHF sensitivity better in the Kuba?

Regards,
-Joe

[yagosaga]

Joe,

the picture of the Kuba Porta-color is better due to the better D.C. restoration. On the other hand, with Simple PAL, I have a little color drift. This means, I have to readjust the tint control after half an hour. The GE NTSC portacolor has no problem with color drift.

These are the differences which I could recognize.

Kind regards,
Eckhard