Capacitor Question for my Predicta

[leadlike]

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?

[Dave S]

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

--Dave

[leadlike]

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?

[Old1625]

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.

[wa2ise]

Originally Posted by leadlike

One of the capacitors that I would like to change has a value of "27 pf N330" according to the SAMS.

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.

[leadlike]

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!

[Phil Nelson]

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

[Dave S]

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

[Phil Nelson]

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

[bgadow]

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.

[leadlike]

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'.

[leadlike]

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...

[spudz64]

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?

[leadlike]

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

[spudz64]

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!