...

Here's one thread about the Portacolors:
http://audiokarma.org/forums/showthread.php?s=&threadid=13448&highlight=portacolor

There are surely others. Try using the search feature on the site, it's wunnerful (said in a Dinah Shore voice)

Anthony

...

I just love this forum! It's like hot rodding a car what with you guys coming up with improvements like this!

You said "irreversible" which I appreciate, that's my philosophy when I modify a car (unless it's a full bore custom job). Like adding disc brakes or electronic ignition to a old heap, it's great to upgrade without it being readily noticeable (except from a performance point).

Anthony

...

Andy,
I see money possibilities here. Maybe you could get in touch with the real GE and see about bringing back the portacolor. Portacolor III or something like that. Price em at about $99 or so. Volume discounts available if you buy them in quantity. The advertising tag line could be something like "The picture that beat Zenith and RCA"! Imagine the possibilities- tee hee.
Jeff

...

Simply great, Andy! Thanks for sharing those tips!

Just a couple of weeks ago, I was actually wondering how easy it might be to add DC restoration (probably using a single diode) to a Portacolor set..! [To me, the lack of DC restoration just gets visually annoying after a while, plus I thought it might be amusing to built a retro-looking cabinet for a cheap DVD player specifically to attach to my PortaColors and other sets from that general era. :-) ]

Now you just need to add automatic chroma gain... :)


--Marty

(yes, I'm the "In Living PortaColor" website guy...)
http://www.rwhirled.com/portacolor
I'm running a bit behind in updating that page, but, eh.

My dad actually works for GE. I'll run it by him next time I talk to him, lol.

Hate to say it, but all they'd do is slap a "PortaColor" label on something made in China...... :puke:

Originally posted by wa2ise

Hate to say it, but all they'd do is slap a "PortaColor" label on something made in China...... :puke:

In that case they'll need to change the spelling to:

"PortaPotty"

Anthony

Andy,

Unfortunately, my Portacolor with the clock/sleep-timer is currently in storage, but here's about the best view I can come up with of the clock knobs with the pictures I already have of that set.

The clock knobs are made of thin, flimsy plastic-- it's no wonder that yours are missing! Both of the clock knobs on mine are cracked and have bits missing from them, but they seem to be pretty much just simple knurled plastic cylinder-shapes; nothing fancy about them.

If you really need a better image, let me know, and I'll dig it up from storage and take some new pictures for you.

--Marty

Sorry to dig up the old thread, but could you re-post the schematic for the DC restoration circuit you made, Andy?

Ditto! I ended up getting two Portacolors after this was originally posted and by the time i went looking for this thread the scans were gone.
-Tony

...

Thanks alot Andy. Eric is giving me a portacolor and I want to experiment with it a bit.

Thanks Andy!

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.

...

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.

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, 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 (http://home.comcast.net/~jlrmsousa/portacolor_bradford_wtg79814_1171d24.pdf).

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

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

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 (http://home.comcast.net/~jlrmsousa/portacolor_bradford_wtg79814_1171d24.pdf). 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

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

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

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!

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

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

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

Eckhard,

The last thing I expected was higher Tint drift with the PAL receiver. I thought that immunity from tint drift was the great advantage of PAL.

So what is the mechanism for Tint drift in PAL?

Regards,
-Joe

Joe:

the tint drift depends IMHO on temperature and changing drive voltages. The Porta-color is not a standard PAL, but a simple PAL receiver with very simple circuits and without the PAL delay line. I will check whether I find the tint drift with other simple PAL receivers.

- Eckhard

...

This thread is so old, the schematics do not load anymore, so I am attaching the schematic that I found in another thread. If there is a more recent schematic, please re-post it to this thread for others.

https://i.ibb.co/S5xB8Yw/GE-Porta-Color-DC-Restoration-Improvement-Andy-AK-01.gif (https://imgbb.com/)

The mod only does DC restoration for the Y luma signal, but the mod does not also add DC restoration to the Y-R, Y-B, Y-G chroma signals that are attached to the CRT's G1 grids. So the described mod would improve luminosity consistency, that is, the black levels and white levels would be consistent, but chrominance consistency will still vary depending on the amount of average red, green, and blue in the scene. Since Y-R, Y-B, Y-G are independent signals that are AC coupled to the CRT's G1 grids, the level of color saturation for each color will depend on the average amount of red, green, and blue in the scene.

Has anybody tried DC restoration for the Y-R, Y-G, Y-B chroma signals that the GE Porta Color sends to the CRT's G1 grids?

The grid signals have about 70% DC coupling already. I think this was done to reduce sensitivity to tube drift in the color difference amps.

I think you could get 100% by removing the .0068 cap across the 270K in the diff amp output circuit. This would reduce the AC gain to the same as the DC, and the blue and green brightness pots would still work. I'm guessing the color gain has enough range to make up for the 30% reduction in AC gain.

I have thought of doing this fix to my CTC-5, but never got around to trying it. The reduced DC coupling is not noticeable in most pics unless they are close to a solid single color.

The grid signals have about 70% DC coupling already. I think this was done to reduce sensitivity to tube drift in the color difference amps.

I think you could get 100% by removing the .0068 cap across the 270K in the diff amp output circuit. This would reduce the AC gain to the same as the DC, and the blue and green brightness pots would still work. I'm guessing the color gain has enough range to make up for the 30% reduction in AC gain.



Here is a screenshot of the HC chassis schematic to help other readers follow our discussion.

https://i.ibb.co/XC1gGHX/Screen-Shot-2021-07-21-at-11-45-24-AM.png (https://ibb.co/tJgt1RB)

In this screenshot, wouldn't removing C92 cause the G-Y color signal to be attenuated by R103, significantly reducing chroma gain? Also, the output impedance of the color difference amplifier would be very high, significantly reducing the bandwidth of the amplifier.

Instead, if C92 was shorted, the result would be DC coupling, so DC restoration is not necessary, but then the green brightness pot no longer works. Ignoring that pot for a moment, why not just use DC coupling from the plate of the amplifier to the CRT G1 grid? I still do not fully understand that.

Removing C92 would reduce the AC signal by the same ratio as the DC signal, that is to 75% of the signal at the amp plate. I don't know what the CRT grid capacitance is, but a high frequency rolloff could be countered by a small cap in place of C92.

Example if CRT grid capacitance is 10 pf (wild guess): it is across about 800k (R104 and R8). 30 pf or so across R103 should fix it, if I've got my head screwed on straight.

I have one and will get a you a Pic tomorrow the plastic is very dirty and hard to see clock and I'm afraid of cracking it if I try to remove. Mine works OK and the funny thing the inside looks quality compared to whats made today

Regarding the use of less than 100% DC coupling in the chroma channels:

As you stated, this does shift the overall color balance towards the opposite of whatever the dominant (average) color is. In TV sets, this reduces the effects of DC drift in the color difference amplifiers.

In color photography, this is a simple form of correction for overall color shift in the lighting or the processing of prints from negatives. Kodak's processing machines went through several versions of this. Early ones did the balance individually on each negative and could suffer "subject failure" where, for example, a red sweater covering a large portion of the image would shift everything in the print towards cyan. Later machines averaged all the shots on a roll, thus coming closer to taking out the variations in the particular batch of negative film. At this point, the snapshooter had to use blue flashbulbs to match daylight when taking indoor photos. Later machines measured all the negatives on a roll, but additionally classified them into orangish (taken with clear bulbs or incandescent light) or bluish (taken with daylight or blue bulbs).

These technique are called "gray world" color constancy algorithms.
https://www.codeproject.com/Articles/653355/Color-Constancy-Gray-World-Algorithm

Regarding the use of less than 100% DC coupling in the chroma channels:

As you stated, this does shift the overall color balance towards the opposite of whatever the dominant (average) color is. In TV sets, this reduces the effects of DC drift in the color difference amplifiers.


Can you explain what DC drift in the color difference amplifier is? A drift in the DC voltage provided to the amplifier's plate due to the unregulated B+ power supply? Would 100% DC restoration for the output of the color difference amplifier really make the color consistency worse, or would it just be a small improvement since there is already 70% DC restoration?

DC coupling (which exists in the circuit) couples any change in the plate voltages to the grids. Changes can happen due to changes in components with temperature and age, or just changing the amplifier tube. DC restoration at the grids would remove such changes due to the preceding stages and would give the most stable result.

Both full (100%) DC coupling and DC restoration give the same result in the picture except that the first one can still drift over time and require readjustment of the gray scale tracking. The reduction in DC coupling by 25% (to 75%) is not really a big improvement in stability, but the designers seem to have thought it worthwhile.

It would make sense to me to try 100% coupling first, and see how well it works and whether it drifts noticeably. If it's OK, you're done; if not, you can proceed with DC restoration.

Maybe the color difference amps cannot make the necessary 33% gain increase; in that case you will need to do the DC restoration; but it's simple enough to try first.

Below are details of my implementation of the DC Restoration mod. First, the parts list:

Keystone Electronics 834 lug terminal (https://www.mouser.com/ProductDetail/534-834TS)
Two Panasonic film Capacitors 450VDC 4.7uF ECW-FD2W475K (https://www.mouser.com/ProductDetail/667-ECW-FD2W475K)
Vishay Fast Recovery Rectifier RMPG06G-E3/73 (https://www.digikey.com/en/products/detail/vishay-general-semiconductor-diodes-division/RMPG06G-E3-73/9600317)
18 AWG 300V solid copper hookup wire (https://www.digikey.com/en/products/detail/remington-industries/18UL1007SLDORA/11613269)


https://i.ibb.co/vZ64TWy/B01267-B4-EFB1-42-BE-AA89-FD8162-AC9-B26.jpg (https://ibb.co/dGw4VvC)
Two 4.7uF capacitors in parallel to keep the voltage coming off of the brightness pot very stable. The rectifier clamps the luma signal's sync pulses to the voltage coming off of the brightness pot.

https://i.ibb.co/S30mrRD/09-BE1-C1-E-EC60-440-C-9179-0-EAD31-D12-AEC.jpg (https://ibb.co/TgR0LmD)
Lug terminal is attached via screw to full metal chassis. Two caps are soldered to lug terminal, and hookup wire is run from the lug terminal.

https://i.ibb.co/XJmcR8j/E866-F13-D-DC4-B-4329-9012-AA31-D849-B259.jpg (https://ibb.co/tCyVrXQ)
Hookup wire is run through the TV's crawl space.

https://i.ibb.co/2Fq9K0r/45-C24-CAC-F89-C-43-AA-ADA2-2-B7-D415-C3-B10.jpg (https://ibb.co/9pnDYm0)
Rectifier is soldered on the underside of the TV, between two test points as indicated in the schematic.

https://i.ibb.co/bdCtwpx/D202-B7-EF-6-E76-420-A-A9-FE-22-B470-EDCC59.jpg (https://ibb.co/kcwZbYp)
Oscope readings of the CRT's cathode voltage.
Top row is before the mod.
Bottom row is after the mod.
Left column is a solid white test pattern.
Right column is a solid black test pattern.

"Max" is peak voltage of the CRT's cathode. Before the mod, max cathode voltage (which is used for CRT electron gun cutoff) varies by at most 33.6 volts between bright scenes and dark scenes, and after the mod, max cathode voltage varies by at most 3.2 volts between bright scenes and dark scenes. That is more than a 10x improvement in brightness consistency between bright scenes and dark scenes.

That is a very nice improvement! Have you watched some video on the set, and confirmed that it works as well as the numbers indicate it should?

I took a bunch of before and after pictures to show the benefits of DC restoration. The video source is a 1992 SNES that outputs a 240p video signal.


http://www.videokarma.org/attachment.php?attachmentid=203372&stc=1&d=1631554959
Top is before, bottom is after. Picture has high average luminosity, so without DC restoration, the brightness is too low.

http://www.videokarma.org/attachment.php?attachmentid=203373&stc=1&d=1631554959
Top is before, bottom is after. Picture has low average luminosity, so without DC restoration, the brightness is too high.

http://www.videokarma.org/attachment.php?attachmentid=203374&stc=1&d=1631554959
Left is before, right is after. This is an extreme close up of a 240p test pattern of 240 horizontal alternating white and black lines. The ideal picture is for the black lines should to be thicker than the white lines, since this is 240p video where every other line of video is supposed to be blank, and so each black line is displayed next to a blank line in the 240p video that is displayed on a 480i TV. It is a great test pattern for measuring cathode ray spot size (beam diameter), which is useful for tuning the electron gun for a maximally sharp image.

DC restoration helps keep the peak cathode voltage very consistent, which is crucial for making the cathode ray as sharp a possible. Here I set peak CRT cathode voltage, which the DC restoration mod clamps to, to be 250 volts (near max). CRT G1 anodes are 175 volts (near min). CRT G2 anodes are 670 volts (max). Focus anode is 270 volts.

Not sure why this is not talked about more openly in vintage TV discussions, but cathode ray beam sharpness is highest when cathode to G1 voltage is as high as possible. This means lowering the TV's "brightness" as low as it can go (this increases cathode to G1 voltage), so that the picture is completely black, and then achieving correct black levels or cutoff by increasing the G2 screen anode voltage. Many TVs will hit their max screen voltage, and brightness will be too low. So the "brightness" setting, (which adjusts the cathode voltage to G1 voltage) needs to be slightly increased to achieve correct black levels. Maximizing cathode to G1 voltage squeezes the cathode ray to a sharp point in the beam forming region that is in between the G1 and G2 anodes, but this also means that G3 screen voltage must be maximized to prevent the picture from being too dark.

Since this technique relies on setting the peak cathode voltage to a very high value, it only works correctly if DC restoration is in place. Once the beam is sharpened this way, then the focus voltage should be adjusted to further sharpen the beam. The reason why you should only tune focus voltage AFTER tuning K, G1, and G2 voltage is because changing the focus voltage changes the focusing power of the lens... and changing the focusing power of a lens impacts two aspects of the electron gun "optics": (i) focal length, and (ii) magnitude of spherical aberration.

Spherical aberration is never a good thing because it causes the electron beam to scatter, even when the lens has been tuned to have the correct focal length to the phosphor CRT face. So when you are tuning the focus voltage, you have to balance focal distance with the amount of spherical aberration to get the sharpest image. The thinner the beam is, when it enters the lens, the less spherical aberration occurs. So sharpening the beam in the beam forming region by maximizing K to G1 voltage means that the focus lens can potentially achieve a better trade off between spherical aberration and focal length

http://www.videokarma.org/attachment.php?attachmentid=203375&stc=1&d=1631554959
Top is before, bottom is after. Here is a comparison of the improved sharpness. Keep in mind that this is a first generation Portacolor that uses a low voltage focus CRT with a gigantic dot pitch.

Very nice thorough demonstration!