Predicta Holiday - First power up attempt

[N2IXK]

NTC thermistors are still made. Often referred to as "inrush current limiters". Digikey and others sell them.

[madlabs]

N2IXK, I'm digging through Mouser and Digikey. Not quite sure, but I am looking for 10 ohms @ 25c? I'm assuming the 25C number has to be at operating temps. All that I see so far seem to have a very high B number, which I assume is the initial cold resistance? As long as the B is higher than 400 all that would mean is a longer start up time?

Thanks!

Jonathan

[N2IXK]

25C is roughly room temperature, or the "cold" resistance. The hot resistance is usually specified at a given current.

[madlabs]

Thanks! Duh, did the conversion wrong. I'll keep digging through data sheets.

Jonathan

[bandersen]

The highest I've seen is 220 ohms cold. I used two of them in series: http://www.mouser.com/search/Product...lkeySL12-22101

[madlabs]

Bob, thanks! The price is right too. Added 4 of them to the ever growing Mouser order. Also been watching your Holiday resto videos, very helpful and informative.

Dug up some power resistors out of the stash that will work for the multi-tap sand resistor. I'm kinda surprised that the value of that had drifted so high, I've never seen a power resistor drift so much. Are those wirewound?

At work for 48 hours but should have some time to replace the power resistor on Thursday. Didn't get the order in so no new parts bonanza arriving this week. The 21FAP4 will get here this week though. Fingers crossed it arrives intact! And that it will work on a Predicta. We shall see. I was thinking of cutting the CRT wires I need to be able to swap up near the socket and putting bullet connectors on them. That way I can test the 21FAP4 works but continue to use the one the set came with if it has any life.

Thanks all! Really appreciate all the help and advice.

Jonathan

EDIT: I didn't put a link for the bullet connectors, but there is an epay link to actual bullet connectors?

[bandersen]

Yes, the tapped resistor is wirewound and I've seen them with increased resistance too. I noticed green corrosion on the leads and think that is the cause.

[madlabs]

Interesting. No corrosion visible on mine. Might be internal though. I'll do an autopsy when I pull it out.

Jonathan

[holmesuser01]

Moisture in the air makes those sand resistors corrode. They were never designed to last all these years, even not being used.

[madlabs]

Removed the multi-tap power resistor and subbed in some replacements and bingo! We got a raster and a little sound. The picture was dim and the sounds was very low, mostly the occasional scratch. The picture got dimmer after a few minutes of operation. Still, progress! We now know we have something to work with. And it was cool to see the little dot in the center of the screen when we shut it off, haven't seen that in years!

The power resistor had no visible corrosion. I may smash it to see the insides, I am still surprised that it had drifted so far off.

Working on a parts list, digging through my stash to see what I have on hand. The 21FAP4 arrived intact and in the original box with the pin protector. It is supposed to be NOS and it looks like it. Once I get the set working properly we'll give that a test.

Bob, after watching more of your videos, I am leaning towards replacing all the resistors and couplates like you did. What do you thing of replacing the sockets out of hand? After watching the trouble they gave you I am quite tempted to do so.

As to the couplates, I was hoping I could ask you for part numbers for the components. In return, I would take detailed construction pictures and post them in a separate thread for easy location for others in the future.

Thanks for all the help folks!

Jonathan

[holmesuser01]

You may find that the 12FAP4 that you have is perfectly good, and I hope you don't need the spare.

The capacitor replacements should make a big improvement in the image you see on that screen.

[madlabs]

Bruce, I hope you are exactly right. While I haven't actually pulled the CRT housing off to verify what I have in there I think it is the original 2V filament, 21EAP4. So while there is still a chance it has life it is a little less likely.

Big ol' cap order on the way, along with some other parts, Novus polishes ect. Not everything I need by a long shot but enough to get to work while I figure out what else we need.

Jonathan

[madlabs]

Hi All,

Rio and I got to a little work on the Predicta last night. We got the surge suppressors installed, gutted and stuffed one of the electrolytics and replaced a couple of paper caps. I am starting on the chassis side and will then go to the PCB. That will allow me to get familiar with the set and round up all the parts I will need to do the PCB.

The electrolytic was a nasty mess to take apart! It was hilarious seeing Rio's reaction to it. It was loaded with something that looked like a cross between something you find in an infant's diaper and lithium grease. Came apart pretty easily and went back together and looks good. Rio did a great job of peeling the rim back so we could pull the insides out. Of course, we are checking the resistors as we go along. Of the three resistors I have lifted and tested all three are off, 1k = 1.5k, 220k = 700k!, and a 18k = 15.2k. So, looks like ordering a complete resistor set is called for.

Overall, not a bad start. Should get some more time in over the weekend and get another parts order in on Monday. Fun stuff and good times. Thanks for all the help so far!

Jonathan

[madlabs]

Hi All,

Busy week but still managed to get some work done on the Holiday. The bottom of the chassis is almost done. I re-stuffed the two electrolytics that mount on the bottom but used a terminal strip to mount the three to replace the top mount can. The can was too skinny and my caps too big, would have been a job to get it to fit. Since this picture I have done some more work, just on cap to replace and three resistors to check. I plugged it in and ramped it up with the variac and it still works but with the same dim, flashy raster and very little audio.

Next week I hope to pull the PCB and start on that, but it is going to be another busy week. Anyone who has made some couplates want to share their recipe? Bob's videos are very good but I am not quite sure what the best type of ceramic caps to order is.

So far so good! Thanks for all the help.

JOnathan

[bandersen]

Originally Posted by madlabs

Anyone who has made some couplates want to share their recipe? Bob's videos are very good but I am not quite sure what the best type of ceramic caps to order is.

Only use C0G or NP0 ceramic. Other types drift with temperature. Use mica or plastic film is you can't get C0G in the value you need. I used mostly 630 volt, but a few 500 and 250 where I could get away with it. You should use a 1500 volt or better for the 1500pF cap in the vertical feedback network.