Predicta Tandem restoration

[bandersen]

You're both correct - it is a 1.5 ohm resistor. I wonder how that got mixed in with my 1 Megs ? Installing a correct 1M took care of my hold stability issue, but the boost is still 100 volts low.

I left the original boost filter cap in because it tested OK, but it seems like a good place to start troubleshooting.

[bandersen]

Luckily, I was able to pinpoint and repair the replica network. A little paint will hide the repair.

Note to self - do some basic tests before installing these!


Replacing the boost filter cap got the voltage up to the correct level. I often read about others able to run sets on original electrolytic caps, but I never have any luck.

Picture and sound are pretty good now. This original very weak 21EAP4, has those telltale bright white areas on faces.

[bandersen]

I pulled out an unrestored, nearly 100% original tandem chassis and hooked it up to an unrestored head unit to check the current draw.

About 1.25 Amps or 145 watts. That's the same as my restored chassis.

So why Sams states 1.65 Amps and the back of the set has 180 watts stamped on it remains a mystery

[bandersen]

I just received this 21EVP4 and will attempt to replace the nearly dead 21EAP4.

It appears to be physically identical. As for the CRT data, all I have is what's at the radiomuseum.
http://www.radiomuseum.org/tubes/tube_21evp4.html

2.68 volts at 0.45 Amps. So I figure I can add a current shunt across the filament to handle the 600mA string.

Does anyone have data on the G1, G2, G4 and anode voltages ? Is it identical to the 21EAP4 ?

[bandersen]

It tests like new so I proceeded with the transplant.




Time for a little math to figure out the appropriate filament shunt resistor then I'll fire it up.

[bandersen]

18 ohms was the right value to shunt 150 mA across the 2.68 volt filament.

The picture is bright and crisp now. It just needs to little tweak to the centering.

[old_coot88]

What's the actual voltage you're getting across the heater?

[bigaudioal]

Wow, nice improvement!!!!!

[bandersen]

Originally Posted by bigaudioal

Wow, nice improvement!!!!!

Using a dark Night Gallery episode probably wasn't the best example though Now I can crank the brightness all the way up without losing focus or contrast.
Replacing a Predicta CRT with their goofy metal strap mounting system is not the easiest of tasks

Originally Posted by old_coot88

What's the actual voltage you're getting across the heater?

2.71 volts with 117 on the ac line. Ideally I want 0.15 amps flowing through that shunt. 2.68 volts / 0.15 amps = 17.87 ohms. I rounded it up to 18.

I plan on added a little more resistance in series with the filament string so the set can be run on a high line voltage of 125 without stressing the tubes.

I'm curious as to how they could make an electron gun that runs on only 1.2 W (2.68 volts * 0.45 amps) that works as well as one that runs on 3.8 W (6.3 volts * 0.6 amps) ?

Did they use improved cathode material or is there some downside like shortened life ?

[old_coot88]

Originally Posted by bandersen

..2.71 volts with 117 on the ac line. Ideally I want 0.15 amps flowing through that shunt.

I'm gonna stick my neck out a little bit here.
Ideally, and for longevity's sake, i would run the CRT heater voltage as low as possible consistent with a nice bright picture (irrespective of what current is flowing thru the shunt resistor.
The logic behind this is that the 2V Predicta guns were notorious for going flat prematurely.
Other discussions have mentioned voltages ranging from about 2.25V to 2.5V as being appropriate.
I would experiment with adding a little more shunt resistance, say a hundred ohms at a time, across the 18 ohms you've already got there. And see how low the heater can be run and still give a bright pic.
The name of the game is preserving emission in a CRT that's probably irreplaceable.

I'm curious as to how they could make an electron gun that runs on only 1.2 W (2.68 volts * 0.45 amps) that works as well as one that runs on 3.8 W (6.3 volts * 0.6 amps) ?

Did they use improved cathode material or is there some downside like shortened life ?

2V Predicta CRTs generally had drastically short life, even when bought new.

Does anybody know for sure the logic behind the 2V guns?

[bandersen]

Originally Posted by old_coot88

I'm gonna stick my neck out a little bit here.
Ideally, and for longevity's sake, i would run the CRT heater voltage as low as possible consistent with a nice bright picture (irrespective of what current is flowing thru the shunt resistor.
The logic behind this is that the 2V Predicta guns were notorious for going flat prematurely.
Other discussions have mentioned voltages ranging from about 2.25V to 2.5V as being appropriate.
I would experiment with adding a little more shunt resistance, say a hundred ohms at a time, across the 18 ohms you've already got there. And see how low the heater can be run and still give a bright pic.
The name of the game is preserving emission in a CRT that's probably irreplaceable.


2V Predicta CRTs generally had drastically short life, even when bought new.

Does anybody know for sure the logic behind the 2V guns?

I wouldn't exactly call them irreplaceable, since I just replaced one I'll be using a 21FAP4 in another Predicta that will also require a little modification to work properly.

Perhaps I should add a zener diode clamp across the filament like some guys do with 3KP4s for extra protection ?

As far as I know, only the only the original 21EAP4 CRTs had the short life. This is a rebuilt non-Philco 21EVP4. Other brands used the low voltage heaters not just Philco. I assume it has something to do with reducing the heat / power consumption.

[bandersen]

Ah, nuts. The transient suppression devices suggested for 3KP4s only go down to about 6.8 volts.

http://www.littelfuse.com/data/en/da...iode_1.5ke.pdf

[Electronic M]

Originally Posted by bandersen

Ah, nuts. The transient suppression devices suggested for 3KP4s only go down to about 6.8 volts.

http://www.littelfuse.com/data/en/da...iode_1.5ke.pdf

IIRC form a class I took a couple of years ago those AC transient suppressors are just two Zener diodes diodes in series with either the anodes or cathodes connected together....You could probably make your own with two Zeners each of the max voltage you want across the heater and a suitable current rating.

[bandersen]

There is some geometry distortion. Primarily the vertical lines bend on the right side.
I don't think there's much I can do about it. I suppose it won't be noticeable when viewing actual programming.

[Kevin Kuehn]

Originally Posted by bandersen

There is some geometry distortion. Primarily the vertical lines bend on the right side.
I don't think there's much I can do about it. I suppose it won't be noticeable when viewing actual programming.

That almost looks like stray magnetic field interference. Did you try moving it around in the room?