Capacitor Question for my Predicta
 

I'm doing a little work on the main circuit board on my Predicta tabletop. One of the capacitors that I would like to change has a value of "27 pf N330" according to the SAMS. I'm going to guess that this is a critical value cap (+/- 5%) or does it do more? I have a +/-5% silver mica cap of this value-will that work okay here?

That sounds like a temperature compensated cap (capacitance decreases as the temperature goes up.)

--Dave

okay...that seems to make a bit of sense. I guess I'll leave it in for now. Is there still a source for these?

You might try Pittsfield Radio for that capacitor; I used to get all kinds of capacitors there back in the early '70s--including the one you mention, I'm sure--while I was building radio prototypes. They may have some left in inventory. Next time I'm in there I can check for you. 413-442-0901 and speak to Alex. Or John. Best to indicate straight off that David Locke sent you. They will be closed the first part of this week, but should be open Weds-on from 1 to 5PM.

That capacitor may well be OK left alone if it is a ceramic disk, domino or dog-bone type, if you are merely "shotgunning" the assembly. If you have diagnostic reason to believe there are issues then that's another story.

If the capacitor is in RF-related circuitry then that N330 temperature coefficient will be apt to be critical. But sometimes it is not, and the coefficient is present simply because that's what the maker had in stock for the board stuffers to grab.

The N330 designation means that the capacitor will have a variance of 330 parts per million over a standard range of temperature, with the "N" indicating that the capacitance will drop as temperature rises.

The site http://www.mitron.cn/product/23-Resi...mpCompCaps.pdf gives a graph that well illustrates this.

That inductors and other components in a critical RF circuit application can change their characteristics with temperature engineers relied on this selection of caps with different temp coefficients to help them compensate for these tendencies so that the circuit would remain stable over a reasonable defined temperature range.

Caps like that don't usually go bad. So unless you have specific reason to suspect that it is bad, I'd leave it alone.

Thanks everyone for the tips. I'll leave it on for now. I did find some caps with temp coefficients at Mouser.com, but nothing close. I'm just doing a general shotgun replacement of parts right now. I kinda like the idea of a board with all new parts on it.

I've left the ladyfinger modules and couplates on for right now. Should I breadboard some replacements for these as well or leave them in situ?

On removing the crt shroud, I found that the picture tube is a low miles rebuild. That coupled with the new flyback should hopefully give this set a nice long life!

If you're up to making couplate replacements, and you want to use the set over the long term, why not do the work now? This article shows one of my caveman couplate replacements.

http://www.antiqueradio.org/philc12.htm

Regards,

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

I totally redid one of my Predictas a while back. Everything was either in spec or it was replaced. Electronically it was like new. Everything except the couplates that is. Left those alone. Wouldn't you know it, after putting the set back together, it ran for about two minutes and then died. Guess what failed? Those assemblies seem to be problematic. Not sure why, since they don't contain the type of parts that tend to fail.

--Dave

Somebody told me that couplates may contain paper capacitors, just as failure-prone as other paper caps.

It's hard enough to keep a Predicta working under the best of circumstances. I thought I had done a thorough job on mine, and it worked beautifully, but eventually the horizontal went on the fritz -- Grrr!

Phil Nelson

Why should it be any easier to keep a Predicta up and running now than it was when they were new?

They apparently were quite problematic way back when, and they haven't gotten any better with age.

Their only saving grace is that incredible "Jetsons" looking 1950's Space Age design.

Those blasted couplates can be a royal PITA. And they do fail. And finding out what's inside one of them, or finding a replacement, can be a challenge at times.

Perhaps unrelated to television, but I was working on a Thomas organ in a music store where I worked 25 years back. There was a series of frequency divider couplate networks for each of the 12 chromatic notes to derive lower octaves. One couplate was dead... and all attempts at finding a replacement lead to dead ends--strictly unobtainium. I had no service manual to the organ, meaning no info on the wiring and component values inside these packs.

Two "warts" at the top edge of the package indicated a couple of bipolar junction transistors that completed the Eccles-Jordan multivibrator circuit commonly used in divide-by-two stages in organs.

After taking some measurements and some educated guesses I felt certain that one of the pair of transistors was bad. Rather than try to reconstruct the circuit, I unsoldered the couplate from its PC card, marched over to the bench grinder, and ground both transistors down to lead level, as revealed by six oval metal spots in the ceramic coating. I surface soldered a matched set of transistors to the spots, epoxied everything in place, and reinstalled the couplate on the card.

Worked. :thmbsp:

I have a nice '60 Motorola console that I went through a lot of trouble to recap, did away with the seleniums as I recall, got it working very nice and buttoned it back up. It is now a very large paperweight, waiting for the day that I pull it all back apart to replace a couplet in the video section. I have a bunch of them NOS but I guess in replacing one new, seperate components is the way to go.

Paper capacitors are the most failure-prone as far as I know...

On my schematic, the couplate components were shown as discrete items within a dotted-line box, although it was a little confusing since the components for a single couplate were shown in two separate boxes. I suppose some schematics may show the couplate as a "black box" with unknown components, since the assumption was that you would simply install a new couplate.

Phil

Hey Phil, thanks for posting so much Predicta info on your site. If you remember that one couplate that looked like it had a diode in it, that had me stuck too, until I remembered your page had a saved newsgroup discussion on that very subject. Sure enough, the answer to a very obscure question was there, and I can carry on making my 'caveman couplates'.

Question-did the Princess Predictas come from the factory in anything other than antique white? I ask as The cabinet to mine was so damaged I had to strip it (it had been painted once before anyway) and I wanna get some fresh paint on there. I'm gonna use some kind of heat resistant spraypaint, as I've heard of bad restorations where the body paint began to bubble after a few hours.

I see the Telstar folks that make the repro Predictas make them in a variety of colors, but I thought Philco also issued cabinets in maroon and baby blue as well. Any thoughts much appreciated...

The Princess came in all biege, vermillion (bright red), and mahogany, which is dark-reddish brown color. The perforated front was beige on all models regardless....

The Debutante officially came in just charcoal (almost black), but biege was used for the motel model Debutante which is very common. These sets used the gold cloth fronts.

The Siesta (Princess with a clock) came in all biege too, as well as gold, with the perforated front being beige as well.

I know of no other colors than these for the 17" metal Predicta sets. I attached a pic of my 3, a gold Siesta, a biege Princess and a charcoal Debutante.

I don't know about repainting them, but I would say using a good quaility enamel paint would be best. Maybe go to a local auto shop and have them do it? Anyone out there with advice on this, and how best to match the original colors too?

I'm pretty good at painting, and I know of a good high temp paint that should be a very 50's vermillion.

The plastic screen is oh so shockingly scratched, but only in a very minor way. Your predicta screens look quite ready for inspection. How did you go about getting them so flawless? I have all three grits of Novus polish, plus somewhere around here I have a couple of grits of buffing compound for auto finishes...

This is exciting...I just got it working! I had it all hooked up and with raster, but no signal, even hooked up to the swamping signal of a DVD input. Well, after smacking around the tuner for a bit, I got a picture! I made a few adjustments, and it looks great! I have the next two days off, so let's see if I can get it back in a new red cabinet by Wednesday! No pictures yet, but here is the thread where I brought it home which has a pic:
http://www.audiokarma.org/forums/sho...light=predicta

Oh yes, I recall your posting when you found it now! Glad to know its working for you and you are on the way to restoring it!

My 3 Predicta screens are in really good shape, I so lucked out, no cracks on any of them and only a few small scratches here and there. I used the Novus plastic polish that you have too, to brighten them up. I have read that the plastic headlamp restorer kits for cars do wonders for clear plastics, and I am tempted to try it on the few spots that need work. One kit I saw uses super fine wet sandpaper sheets (up to 2500 grit) with plastic polish and like the Novus stuff it will buff the spots out in steps.

These 17" Predicta screen covers seem to be a harder, more durable plastic than the 21" ones, which evidently grow mold on the inside (!) and fog up easily. I would still be very care as they are 50 years old after all and may be fragile and dry. Have you removed it and cleaned both sides, as well as the screen front? That alone brightens up the CRT nicely...

Good luck with the repaint! My favorite Princess is the mahogany color, with the contrast of the beige face going well with the reddish-brown sides and top. I would trade in my biege one for that combo if I came across one!

My Barberpole screen had the typical molded/crusted over appearence too. I removed it and cleaned it with warm soapy water and a soft cloth laying on a towel in the bathtub. The outside was the moldy side, inside just dirty.....
And it smells better too!:D

Electronically, it's getting there, I watch it occasionally but I would'nt think of making it a daily watcher but it's cool to show off!!

It liiiiiives....here are some pictures with a new coat of paint and a polished up screen:

http://s288.photobucket.com/albums/l...t=100_1442.jpg

http://s288.photobucket.com/albums/l...t=100_1444.jpg


and barring that the hypertext won't work, here are some direct links that do:

http://s288.photobucket.com/albums/l...t=100_1444.jpg

http://s288.photobucket.com/albums/l...t=100_1442.jpg

Wow that was quick! It looks good! :tresbon: Did you use a spray gun or just use a can of paint?

Thanks! I did use spraypaint. I started with lots of primer and wetsanding to fill in the scratches and smooth out the crummy old paint job to a glass like finish. Then I applied the gloss red in a similar manner, and it shines great. Once the paint cures, I'll put a coat of wax on to protect the finish. I used Novus polish on the crt shell, and that took out all but a couple of small but deeper scratches. I think I'll leave it that way.

I keep losing HV after warmup and I can see the damper tube shutting down. Fiddling with the socket has made things better, but I think anyone stomping across the room would be enough to cause intermittents in this set. I may just add jumpers to the filament pins on the damper socket. I tested and retested the board for bad solder joints, but looks like one got by me.

Well nice job! BTW which brand paint did you use? I happen to be helping someone with info who came across my Flickr pics (http://www.flickr.com/photos/9998191...7604694313416/) and they want to paint theirs red too, so good timing for him!

The primer I used was Rust-Oleum Painter's touch primer gray. This wet sanded very well with 400g one the base coats, with 800g on the final primer coat. With using primer, all of those little nicks and imperfections will come out, affording your the opportunity to deal with them. I had a deep scratch on the top, and after every coat, I would sand it back down to the old paint, but the scratch would be filled with primer, creating an even surface.

The top coat is Rust-Oleum Painter's Touch Apple Red-in Gloss finish. I wet sanded the first two coats, and used a cloth to buff the final coat.

I soldered jumper wire to the damper filaments and it works flawlessly now. I have an RCA that uses these PC boards, only they still used hand-wired stuff for the HV tubes. If only Philco had done that, and these sets would not have had such major issues.