A little 0000 steel wool and 600 grit sandpaper took care of the paint splatters easily :) After a little more sanding, I'll fill in the dings and clearcoat it with lacquer.

While working on it, I convinced myself that this is actually a photofinish. Odd considering the other Tandem cabinet is really mahogany veneer.
http://farm9.staticflickr.com/8122/8...599fbc5f_c.jpg

With another Tandem fresh on my mind, I decided to dive right into finishing this one off.

Rather than continuing with the original corroded, fire damaged chassis, I'll be using the "new" chassis.

The previous owner said it had been stored in a dank, damp basement for many years, but it looks pretty good to me. Minimal corrosion and no repairs. Just a few tubes have been replaced.

First, I tackled the main board.
http://farm4.staticflickr.com/3731/1...b83bee09_c.jpg

Here are my replica networks. I used 630v C0G ceramic for most. A couple micas only because Mouser didn't have those ceramic values in stock.

Also, one HV disc cap in the vertical feedback network. It's the yellow cap in the middle.
http://farm3.staticflickr.com/2863/1...db55e247_c.jpg

Here they are after painting, ready to be installed.
http://farm8.staticflickr.com/7390/1...d2f03157_c.jpg

Great job on those K-nets! (and everything else as well)

Thanks! Before I reinstalled the main board, I removed a bit of corrosion.
http://farm8.staticflickr.com/7318/1...e300d71d_c.jpg

Here's a closer look at that flyback. Some material has melted out the bottom but otherwise it's in OK condition.
http://farm6.staticflickr.com/5529/1...2b0db8a3_c.jpg

Now, I've moved on to working underneath the chassis. It appears to be 100% original. Even the filament dropper and thermistor test OK.
That sand coated dropper is showing green corrosion though so I'm inclined to replace it.
http://farm6.staticflickr.com/5521/1...1df9351d_c.jpg


This big multi-section cap is crusty and bulging :eek:
http://farm4.staticflickr.com/3833/1...6fc419b4_c.jpg

Very nice work on those couplates!

Thanks! Finally time for the first power up.
http://farm8.staticflickr.com/7334/1...da2acf30_c.jpg

First a test with the fusistor pulled so no B+. After a little cable wiggling, they lit up.
http://farm3.staticflickr.com/2808/1...dfe869eb_c.jpg

Now with B+ and a raster appeared :)
http://farm6.staticflickr.com/5523/1...57ef57ba_c.jpg

Next with a signal applied. Horizontal sync is impossible to achieve.
http://farm4.staticflickr.com/3705/1...7349bc58_c.jpg

Rotating the coarse hold control to one extreme, tweaking the hor. osc. and twiddling the hor. sync finally resulted in a somewhat stable image.
http://farm3.staticflickr.com/2815/1...d972b4f2_c.jpg

Swapping out a few IF tubes helped, but it still doesn't feel right. The sound is clean but not loud enough, the contrast is poor and the sync touchy. I suspect there's not enough gain somewhere.
http://farm4.staticflickr.com/3823/1...d904769b_c.jpg

I switched to a different head unit with a good 21FDP4 in it and was surprised that it didn't look any better. The set takes about 5 minutes to fully warm up and stabilize and the picture isn't very sharp.
B+ is about 10% low which might account for these issues. First I think I'll try swapping out the ancient silicon rectifiers with some 1N4007s.
http://farm6.staticflickr.com/5548/1...b26d937a_c.jpg

Just tested the original diodes and they're OK, but I'll try replacing them anyways. Something else odd. The total current draw is only 1.15A. It should be around 1.6A :scratch2:

Hi Bob and all,

I find these sets very appealing, mostly because of my engineering nature. But I wonder how these sets were used and how it was marketed.

What would the consumer do other than just put the CRT on top of the cabinet? One could have kept the cabinet near a viewing chair and used the cabinet as a remote control - and not have to get up to change channels. One could have kept the cabinet inside the house and brought the CRT outside on the patio for watching TV outside. Is the speaker inside the cabinet of the CRT using? Any ideas why these sets were/are populate other than the novelty of it?

-Steve

The speaker is inside the cabinet but there is an audio jack in the CRT base. I've never seen an actual external Philco speaker. They're much less common that than Holiday model that came out at the same time so I don't think they sold all that well.
http://farm9.staticflickr.com/8333/8...098e2804_z.jpg

I believe this is what the speaker looks. I suppose that could just be any old generic speaker plugged into the base though.
http://www.antiqueradios.com/forums/...e.php?id=32595

I bit the bullet tonight and dissasembled the CRT housing so I can service the 2nd video amp. It's been partially recapped, but I can see an original electolytic in there.

http://farm3.staticflickr.com/2874/1...c67e9561_c.jpg

It actually was a lot easier because of a great tip posted recently (sorry can't remember the author). Instead of fighting to undo the stiff spring on the decorative gold band, leave it attached and slide the band over the plastic housing.

http://farm4.staticflickr.com/3746/1...51343035_c.jpg

That needs a good cleaning :yuck:
http://farm8.staticflickr.com/7289/1...11d566c8_c.jpg

So does the inside of the rear cover.
http://farm8.staticflickr.com/7311/1...80762c23_c.jpg

There's an odd, faint "starburst" pattern on the CRT face :scratch2:
http://farm4.staticflickr.com/3733/1...049020b4_c.jpg

Finally, I can get at that little circuit.
http://farm3.staticflickr.com/2828/1...6a9e4ff2_c.jpg

http://farm6.staticflickr.com/5531/1...b7e23538_c.jpg

Looks like the CRT was replaced in 1965 in LA.
http://farm8.staticflickr.com/7346/1...d7332dd3_c.jpg

The small 'lytic, should have been changed, as well.
I like that shielded H.V. anode lead and the line interlock.

I tested that 4uF 'lytic and it's not leaky, but the power factor is around 20% so I'll replace it.

The plastic CRT cover is polishing up nicely with Novus #2. Here you can really see how dark the tint is.
http://farm6.staticflickr.com/5492/1...1b4b154f_c.jpg
001 by bandersen, on Flickr

Here's a side view showing how short the CRT is.
http://farm8.staticflickr.com/7349/1...65362ec9_c.jpg

I noticed some corrosion on the plug and the cable is loose.
http://farm4.staticflickr.com/3736/1...363b4afc_c.jpg

So I opened it up and found it's much worse than I realized. The pins are corroded, screws are very rusty and it's been poorly rewired.

Here's a nice, neat factory original for comparison. Now I can the see the rewired one is so loose because it should have a section of the vinyl outer jacket inside the plug.
http://farm4.staticflickr.com/3741/1...461f0404_z.jpg http://farm4.staticflickr.com/3790/1...55edbf6e_z.jpg

After more examination, i decided to rewire it myself.
http://farm6.staticflickr.com/5477/1...245a86c4_c.jpg

The point of no return :eek:
http://farm6.staticflickr.com/5496/1...446eb2ae_c.jpg

Here are the wires for AC line interlock, tube filament, audio, horizontal & vertical yoke, HV and video.
http://farm4.staticflickr.com/3682/1...f52c2ca7_c.jpg

That must have taken a few minutes to wire each one of those plugs at the factory.

Here it is cleaned up and ready for rewiring.
http://farm4.staticflickr.com/3812/1...47471c62_c.jpg

Unfortunately, I discovered one of the pins had broken off and been replaced with a bit of bent wire. Not sure what to do about it.
http://farm3.staticflickr.com/2861/1...92d05992_c.jpg


Also spent some time troubleshooting the chassis. It takes several minutes for the horizontal to stabilize and even then it's very touchy.

Also the 410 boost voltage is less than 300.
http://farm6.staticflickr.com/5500/1...d2e553b2_c.jpg

The horizontal is so touchy just bringing my hand near the tube has an effect.
http://farm3.staticflickr.com/2824/1...c60ca087_c.jpg

That got me thinking one of the pins might be floating. Sure enough - pin 2 is open.

There should be a 1Meg resistor between it and pin 7 of the AFC tube. It's buried inside one of the networks so I must have made a mistake somewhere :(

I tried tacking an external resistor between the tube sockets. It had a definite effect, but now I can't get hold lock at all.
So much as I hate to do it, I'll have to pull the board again.

That looks like a 1.5 ohm resistor...

The last band should be green for a 1 meg, looks gold here...

Brown, Black, Green is 1 meg but my eyes are quite bad...

Have a good one...

rrrhre2s

Ditto here, it sure looks gold to me too.

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.
http://farm6.staticflickr.com/5508/1...7507e2df_c.jpg

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!
http://farm3.staticflickr.com/2811/1...f24d4ea3_c.jpg

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.
http://farm4.staticflickr.com/3667/1...0930a85c_c.jpg

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 :scratch2:
http://farm8.staticflickr.com/7381/1...9503d878_z.jpg https://ycpi-farm8.staticflickr.com/...6296049b_z.jpg

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 ?

http://farm8.staticflickr.com/7430/1...a643b0aa_c.jpg

It tests like new so I proceeded with the transplant.
http://farm6.staticflickr.com/5472/1...dd697aa3_c.jpg

http://farm8.staticflickr.com/7331/1...16fb32c1_c.jpg

Time for a little math to figure out the appropriate filament shunt resistor then I'll fire it up.
https://ycpi-farm4.staticflickr.com/...0ba44431_c.jpg

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.

https://farm6.staticflickr.com/5483/...f2f02dde_c.jpg

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

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 :no:

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 ?

I'm gonna stick my neck out a little bit here. :nerd:
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?:screwy::saywhat::saywhat:

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.

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

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.
https://farm8.staticflickr.com/7409/...ac961fae_z.jpg

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.

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

What about the adjustments on the yoke cover? I don't remember if they're just for centering. The "T" shaped magnet, on the side of the yoke should have an effect on the pincushioning.

Yeah, that has no effect. This is a rebuilt CRT and the neck is a little crooked which may be throwing things off.

A little. Mostly it effects the horizontal linearity.

OK, I didn't realize you could not only bent the "T" but also twist it.

It's still a little warped in the upper-right, but not too bad I think.
http://farm4.staticflickr.com/3712/1...ca49b51dc2.jpg https://v4s.yimg.com/so/7370/12658691365_627214820b.jpg

That looks pretty darn good to me. Seems to have improved the focus, too.