Capacitor Question for my Predicta - Videokarma.org TV - Video

leadlike · #1 02-28-2009, 02:42 AM

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 · #2 02-28-2009, 11:59 AM

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

--Dave

leadlike · #3 02-28-2009, 03:40 PM

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 · #4 02-28-2009, 04:30 PM

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 · #5 02-28-2009, 05:10 PM

Quote:

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 · #6 03-02-2009, 05:32 AM

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 · #7 03-02-2009, 12:40 PM

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 · #8 03-02-2009, 12:50 PM

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 · #9 03-02-2009, 02:25 PM

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

Ampico-kid · #10 03-02-2009, 02:59 PM

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.

Old1625 · #11 03-02-2009, 04:14 PM

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.

bgadow · #12 03-02-2009, 10:13 PM

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.

Tubejunke · #13 03-02-2009, 10:29 PM

Quote:

Originally Posted by Phil Nelson

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

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

Phil Nelson · #14 03-03-2009, 12:17 PM

Quote:

Originally Posted by Old1625

finding out what's inside one of them, or finding a replacement, can be a challenge at times.

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

leadlike · #15 03-05-2009, 05:56 AM

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'.