Resistors for Predicta
 

Hello All, I normally do radios, and not tv's. So I'm going to start on pulling this 10L43 mainboard and replace everything (well caps, resistors, and couplates) Anyway, are carbon comp resistors preferred over metal film? (if so,..why) or is it ok to use metal film throughout this set ? Does it have anything to do with frequency stability, drift ? Other factors, than just replacing all the caps and maybe just out of spec resistors. I really want to do this right. Being in Canada, we just have to jump through more hoops than you guys in the U.S. and we end up paying more than twice for everything ( parts, shipping and duty hassles). I normally deal with Dave at JustRadios, but he doesn't have all the different resistor values and the one supply place in town leaves a lot to be desired.
Thanks !
Tony

Metal film are superior to carbon comp. Virtually no temperature drift and widely available in all values and wattages. I've had no problems using them in Predictas or any other set. Metal oxide are pretty good to.

Thanks Bob, I appreciate your expertise. I have gone through the set and checked every resistor and matched it up with the ones on the schematic. Most match, but not all. There are maybe 6 or 8 that seem to be way off in value. I don't mean the resistor changing value over time, but the color code not even matching the schematic. I take it, these were probably production changes and I should be replacing these with the value that is in there and not with the one shown on the schematic ? There are quite a few that have drifted as well. One thing I did notice is a small crack and what looks like carbon tracking on the damper tube socket (6DA4) between pins 5 and 6. That is looking down at the tube socket (from the top of the board) and counting clockwise from the first pin right of the keyway.
As far as I can tell, everything is original except for R54 (390 ohm) and cap C20 (.022 @400). They have been tacked on above board.
I like your idea of putting the electrolytics on top of the board rather than restuffing the cans.
Tony

Trust what is in the set unless you see evidence that it was previously replaced or that the resistors got overheated (which can discolor the color code paint). Might not be bad to order both possible values for parts that don't match the print. If the set don't work right and troubleshooting don't help changing the set from one rev of prod changes to another sometimes helps.

Yes, they did make production changes. Here is a two page Philco service bulletin describing some of them.

https://drive.google.com/file/d/0B37...ew?usp=sharing

Better: :D:D:D:D

So is it recommended to make all of these changes? I'm not where I can look but I think my set has some but not all of these changes.

I wouldn't unless your set is experiencing the problem the change addresses.

Would you happen to know where this "DOT" is located that signifies the various production runs ? It says Run 2 is RED dot, Run 3 is ORANGE and Run 4 is YELLOW. I know this has the 3 watt R15A resistor. I will check for some of the other changes. I don't see any painted dots on any sheet metal. Thanks Bob,.. this is good info.
Tony

Well I don't know about all that - just passing along my experience as a EE and tinkerer ;)

Sorry, I don't know. I reviewed pictures of several VOS boards but didn't see any colored dots.

Only thing I have seen is a stripe on the main circuit boards. One of mine has a yellow stripe and one has a green stripe, IIRC. Don't know if that has anything to do with the "dots" however.

Personal observations on carbon comps vs metal film resistors.

In small signal circuits where current is not an issue I have never had any problems with metal film resistors.

HOWEVER: I can't say the same is true in B+ power supply circuits. In old sets I have seen situations where 2W and 1W carbon comps were being pushed to their wattage limits. I have had more than one metal film self destruct under circumstances where the original ckt design was probably using a resistor that was under powered. In my experience metal film resistors will not tolerate overheating as well as a carbon comp.

As a result I have been using high quality NOS carbon comps in everything I rebuild. You never know when the original design might have been using a resistor that was under rated, and my experience is that carbon comps seem to tolerate abuse better.

Most high quality carbon comps have pretty good value tolerance and in old tube ckts the added accuracy you gain with metal film doesn't affect circuit function to any appreciable degree.

And besides, carbon comps are period correct, if you are into that sort of thing.

I should have mentioned I generally double the wattage when I replace resistors. 1W for 1/2W, 2W for 1W etc.

I use the PR01. PR02 and PR03 series from Vishay. They are a reddish brown rather than bright blue and the color bands are bright and clear. The 2W are similar is size to the old 1/2 watters.

Also very reasonably priced. Often as little as $0.02 each on sale. Available at Allied Electronics, Mouser and Digi-Key, Newark etc.

http://www.alliedelec.com/search/results.aspx?term=pr01

http://www.sm-elektronik.pl/galerie/...baec412385.jpg

You forgot author / preservationist-custodian......:D:D

Hey! Where can I get a copy? Just looked and don't see one for sale.