[NostalgiaRadioTime]

I have a 49-1401 that has been recapped, resistors checked good, and it plays fine throughout the dial. However, it has the tell-tale signs of dreaded silver mica disease. My question is, can anyone tell me what value mmfd caps to use in place of the mica plates in the two IF cans? The schematic lists the ohm values for the coils themselves, but not the mica capacitance. Here's a link to the schematic at Nostalgia Air:
http://www.nostalgiaair.org/pagesbymodel...013645.pdf

Many thanks.

[Radioroslyn]

Generally speaking about 100-120mmfds should do it. Sometime it's a bit of an experiment the objective is to get a peak when adjusting the slug WITH A GENERATOR.

Terry

[Arran]

  Before assuming that the mica caps have a problem, and that it is not a dirty tube socket, dirty volume control, or a bad solder joint, rule the rest of those out first before breaking into the IF cans, hair thing wire and old plastic can ruin your day. The best way to find out the exact sizes needed is by removing the mica caps inside the cans, bridging a trimmer cap across each winding, and then peaking the IF through the trimmer, do not touch the slugs. Then you can measure the capacity of the trimmer at it's setting and order the closest size of fixed precision cap, they do not have to be mica at 455 KC precision ceramic will work fine.
Regards
Arran

[kiwi_steve]

<snip> they do not have to be mica at 455 KC precision ceramic will work fine. </snip>

Arran - just to clarify (for my own knowledge), when you say precision ceramic I assume you mean NP0 or some other tight temperature spec ceramic?  Tube radios get warm... and 'warm' changes the capacitance, albeit to a different degree with different cap types... which is one of the reasons I believe silver mica's are used - because they don't drift much with temperature change... 

I'm inclined to replace silver mica with silver mica, but NP0 ceramics (or equivalent) I believe are ok?  Or the old tubular ceramics?  I did try to find some NP0 caps down here in Hobbit land but they were 500 minimum order quantity... which was about 498 more than I needed.

Stunning radio by the way - we (to the best of my knowledge) never got those down here in NZ... might be something to import one day.

Cheers

Steve

[Arran]

<snip> they do not have to be mica at 455 KC precision ceramic will work fine. </snip>

Arran - just to clarify (for my own knowledge), when you say precision ceramic I assume you mean NP0 or some other tight temperature spec ceramic?  Tube radios get warm... and 'warm' changes the capacitance, albeit to a different degree with different cap types... which is one of the reasons I believe silver mica's are used - because they don't drift much with temperature change... 

I'm inclined to replace silver mica with silver mica, but NP0 ceramics (or equivalent) I believe are ok?  Or the old tubular ceramics?  I did try to find some NP0 caps down here in Hobbit land but they were 500 minimum order quantity... which was about 498 more than I needed.

Stunning radio by the way - we (to the best of my knowledge) never got those down here in NZ... might be something to import one day.

Cheers

Steve

 Both the tubular ceramics (which were used in some slug tuned IF cans originally) and the NPO kind will be fine, but regardless of what you choose they have to be under a 5% tolerance and not drift with temperature changes. Often you can't get a fixed value of fixed cap that exactly equals that of whatever the trimmer cap is dialed to, so you will have to go slightly higher, or slightly lower, or combine two caps to make up the value if it's a really odd value. Outfits like Mouser or DigiKey have search engines that will allow one to find the right kind, and as far as I know you can order whatever quantity you want as long as it's a value they normally stock.
Regards
Arran
Regards
Arran

[Paul Philco322]

I was convinced a Z brand I had was sick with it. Cleaned tube sockets and it went away. You never know. Good luck.

Paul

[codefox1]

Agree with the Master Paul, a good scouring of pins and firming up contacts of sockets using Deoxit and an old toothbrush or more drastic approaches like brillo on the tube pins, (rinse and dry carefully for a day,) would rule much of this out. Not a bad thing to do no matter what.

"Thunder" in an otherwise properly working set after connections, faulty caps and tubes are ruled out, is almost always a bad mica cap, usually one in an IF can that is a real pain to et to and replace, and can not be measured by any method published so far as I know. Of course you could isolate the problem by stage (IF,RF,) you probably know the drill, but as said, best of luck.

[OZ4]

I found this video on caps and where to use them  interesting.


Which Capacitor Do I Use? Tech Tips Tuesday
https://www.youtube.com/watch?v=67M7fsbLUIU

[radiohenry]

What is the recommended voltage of the replacement cap?
                                  Thanks,
                                         Henry

[Radioroslyn]

Old standard rating would be 500vdc for small micas. You could get away with 200v unit if available.

Terry

[NostalgiaRadioTime]

Thanks for all of this great information, everyone. I will double check the caps and give the tube sockets and tube pins a good scouring. If that doesn't take care of the problem, then it's got to be the mica's. I've had radios with silver mica disease before, so I've heard the tell-tale signs of it. This one isn't as loud or overpowering as others I've experienced in the past, so perhaps it is one or more of the sockets causing the 'whooshing' & crackling noise after all. One thing is for sure, I've never had good luck with removing the mica wafers from those small IF cans.