My new predicta

[Phil Nelson]

Whether to replace the Predicta couplates (networks) pre-emptively is a tossup, in my view. If you build all of the replacements correctly, it won't do any harm. But you might be wasting your time and somewhat decreasing access to the PC board.

PRO:

Occasionally, a couplate does need replacement. Building a substitute is not rocket science; you can see how to do it in my Predicta articles or Bob's Holiday restoration thread.

Replacing them might increase the TV's long-term reliability, or at least your confidence in it. Replacing them might eliminate the need to pull the PC board in the future, a process that involves risk (risk of damaging the board or miswiring something).

CON:

Not all couplates go bad. If that were true, there wouldn't be a working Predicta left on Planet Earth. I suspect the majority of working Predictas still use their original couplates, or nearly all of them.

Tubular ceramic caps are extremely reliable. The resistors inside couplates may drift off value, but the question is, how much is too much?

Your replacement couplates will be bigger than the originals. Each new one decreases access to other components around it. This can complicate getting to test points on the board or even make it harder to replace a tube.

Each couplate has multiple terminals, and if you're too rough unsoldering and removing an old one, you may pull loose the foil on the board or otherwise damage it.

Replacing the couplates degrades the TV's originality.

SUM:

The decision requires you to judge the tradeoffs among several factors: originality, reliability, risk, and efficient use of the restorer's time.

Although I have replaced couplates myself, I don't want people to get the idea that automatically "shotgunning" all of them is a necessary part of every Predicta restoration. A more conservative approach would be to replace the high-risk caps and badly out-of-tolerance resistors on the board and then see how well the TV works.

Regards,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[josephdaniel]

If the video output tube (6aw8a) is gassy or dead would that cause the screen to be completely dark I just looked at this tube with the set on and it is very purple inside when on so is the horizontal output tube.
I have determined there is HV but I don't have a probe to measure the exact voltage but I can get a 1/2 centimeter spark to the housing of the flyback transformer.

[StellarTV]

Sounds like your HV is ok... disconnected video line will definitely cause a dark raster. That line runs separately from the harness coming from the back of the CRT as a single wire that plugs into a post on the board next to the video output.

[Geist]

Hi All;
Joseph, HV may be Ok..
But, purple tubes are not, that needs to be addressed.. It could be a bad tube or too much of a load on the tube..
Check with Old Coot, He would direct you correctly on what to look for and what's wrong.. The cause of the Purple tubes..
THANK YOU Marty

[josephdaniel]

Thanks, the video line is connected. I will wait on replacing tubes untill I get the rest of the paper caps and out of tolorence resistors out of the set. Also I will message old coot tonight and see what he has to say.

Ordering parts tonight

[josephdaniel]

Parts ordered with just radios since Mouser seems to always short me on some parts.
I actually ended up having to order more resistors than I needed to reach the minimum amount of 20.00

[josephdaniel]

Ok do out of curiosity I turned the brightness up all of the way and turned out some of the lights in my room and I was able to see a faint raster that is off center. Could this be a 6 volt CRT being run off of 2 volts I didn't see very much filament glow when I looked at the neck of the picture tube. I am hoping that it is just a combination of a weak tubes/aged caps on the PCB

[old_coot88]

Originally Posted by josephdaniel

Ok do out of curiosity I turned the brightness up all of the way and turned out some of the lights in my room and I was able to see a faint raster that is off center. Could this be a 6 volt CRT being run off of 2 volts? I didn't see very much filament glow when I looked at the neck of the picture tube. I am hoping that it is just a combination of a weak tubes/aged caps on the PCB

If that was a 6V heater, it would light up like crazy (since your heater string is constant-current* at .6A, and a 6V heater is rated at much lower amperage, it'd get around 12-15V across it). Hopefully the dim raster is not caused by flat emission in the CRT. If it is, has anybody ever had any success rejuvving those 2.5V jugs?

*Series strings are constant current, while parallel circuits are constant voltage. That's why series strings have tubes with such odd heater voltages all running the same current.

[bandersen]

Originally Posted by old_coot88

If that was a 6V heater, it would light up like crazy (since your heater string is constant-current* at .6A, and a 6V heater is rated at much lower amperage, it'd get around 12-15V across it). Hopefully the dim raster is not caused by flat emission in the CRT. If it is, has anybody ever had any success rejuvving those 2.5V jugs?

*Series strings are constant current, while parallel circuits are constant voltage. That's why series strings have tubes with such odd heater voltages all running the same current.

Both the 2.35 and 6.3 volt CRTs use 600mA of current. When replacing a 2.35v CRT with a 6v one, it was commended to short out a section of the big filament dropping resistor to compensate for the four volt difference. However, since AC line voltages are typically four or more volts higher now, I wouldn't do it.

I've tried rejuvinating a couple original 2.35v 21EAP4s with no luck.

[old_coot88]

Originally Posted by bandersen

Both the 2.35 and 6.3 volt CRTs use 600mA of current...

Uh oh, my mistake. Sorry for giving bad info. I erroneously assumed 6V heaters ran lower current. Last thing i'd want to do is give bad info to a newbie.

But the constant current/ constant voltage distinction between series and parallel circuits is maybe worth noting.

[Tubejunke]

I wonder if anyone has tried to duplicate these evidently troublesome PC boards yet. A little film (for lack of remembering the correct name), some copper and a chemical or two, and you can make anything you want.

[josephdaniel]

Ok i guess I will wait rebuld all of the couplets and see how it works with the orgional ones. Would it be a bad idea to replace all of the resistors on the PCB since I ended up ordering them to meet the 20.00 minimum order at just radios?

[bandersen]

Yes, it's definitely worth noting. This is also an issue with the 17" parallel filament sets where the low voltage CRT ran off a separate tap on the power transformer. If it's replaced with a 6 volt CRT, the filament lead should be switched to the 6 volt tap.

[old_coot88]

Originally Posted by bandersen

Yes, it's definitely worth noting. This is also an issue with the 17" parallel filament sets where the low voltage CRT ran off a separate tap on the power transformer. If it's replaced with a 6 volt CRT, the filament lead should be switched to the 6 volt tap.

Whatever was the reason for Philco's having the 2V jugs in the first place? I had just assumed, without researching it, that it was for matching current in the heater string.

[bandersen]

I think it was a consequence of making a tiny electron gun for the very short neck.