ฝัง
- เผยแพร่เมื่อ 18 ก.ย. 2024
- In this video I repair the Silver Mica Disease (SMD) in the IF cans of a Zenith H615 Tube Radio. Silver Mica Disease manifests itself as popping and crackling, and eventually can lead to no audio output at all.
You'll see how I removed the old mica and trimmed the contacts. I start by showing the mistake I made when I first tried the process. This way you can see what to avoid yourself.
Here is a link to the schematic of the IF cans themselves: meanmutha.com/z...
Thanks for sharing! The wires soldered onto the lugs appear to be litz wire. It's a good thing that you ohmed out those coils before putting that IF can back into the radio. As you know, those wires are exceedingly fine and can easily be broken. It's fortunate that those schematics for the IF cans included the values for the old mica caps, which made your job *much* easier than it would otherwise have been!
Thanks for the comment. Yes, they are litz wire, and it sure is fragile. Without the schematic you can pretty easily figure out the required values for the capacitors by connecting up a variable capacitor in place of the old mica, and then adjusting that for a peak (ideally with a scope or VTVM, but by ear if the slug is still tuned would work). Then take that variable cap off and measure its value. Use that value as your replacement fixed capacitor. (You don't use the variable cap permanently because it will drift heavily with temperature.)
So glad I found your video. In all my years I've never run into SMD until I delved into my Zenith H723 this week. Watching your method saved me a lot of time and likely a lot of frustration as I would have had to learn by doing.
Thanks for the comment. Glad this helped you out. That's the spirit of amateur radio, sharing info
Thanks for the video. I just got a radio with SMD and when I opened the can, I had not seen the design before so I wanted to be aware of how the parts fit together before taking it apart. Lucky for me, you provided this video. My radio only has the problem in one of the transformers but I will be retrofitting both for long-term reliability.
Very timely video! I just ran into these circular wafers, and I'm happy to see you showing the way.
Thanks for sharing! I did have this problem wit a Zenith C274. I know it was a "capacitor" problem but thanks to your video I dis save a lot of time.
Sorry but I do this a little bit differently. I snap a few pics and de-solder the coil wires, I also draw and make notes of the wire placements so there is no doubt where they go. Then replace the mica with cardstock (thick paper), eliminates the need for clipping the contacts. If you don't want to mess around with de-soldering, you can still do what you did, but there is no need to clip the contacts, just use cardstock between the contacts.
Thanks for the reply. I did consider using card stock between the contacts. It will work well, until it eventually disintegrates into dust (in a decade or four) and the contacts short out. This was why I decided to clip the contacts. No chance of ever shorting again.
There are certainly many ways to do this. This was my first encounter with the issue. have since improved my techniques quite a bit, but the processes is basically the same.
Note that even paper is a dielectric, and there will still be some capacitance between the plates if you put cardstock in between them. Of course it likely won't matter as it should be very minimal and you should be able to tune it out.
...I wouldn't use CARDBOARD- I'd use plastic shim stock...
Thanks for taking the time and trouble to film this process
I have done these a couple of different ways over the years. These big cans will allow you to put the caps inside to maintain original appearance, especially if restuffing old caps in the chassis. When I couldn't reach the inside of the pins (on smaller cans) i actually desoldered the wires and had a much better time of it, especially with the smaller, later, cans. Better to desolder the wires and have extra length than break them off short or (horrors!) from the top where they enter the coils.
On some cans (the clear plastic ones) you may have to add some hot glue or cement to stabilize the pins. And those with the rivet holding things together (Automatic Radio Co.) are a special pain. I remove the coils entirely before battling it (draw a diagram of where each wire runs - top coil top, top coil bottom, etc.) You may also have to mark the base so everything goes back in the right directions when putting the can back in the chassis. Some are marked with a dab of paint from the factory and you can use that.
Putting the new caps in the cans is a personal choice although I had one case where putting them outside caused issues with them picking up interference from nearby components. Position was critical due to a crowded chassis. Your mileage will vary, as they used to say. The coils themselves don't seem to care.
Thank you so much for having a link to Zenith's IF transformer cap values, I have a Zenith H845 with the mica disease and can't find anyone else with this cap. info.
Thank you for sharing this. Working on a Zenith 7e20 chassis now and this is a great help.
Excellent video. Thank you.
Rather than cut the contact plates, why not slide a thin piece of plastic material between the two contacts?
I have in the past slid electrician tape between the contacts and have had no problem with the end result.
I also note that each time I attempt the process I change the technique a bit.
I was musing about this in the video, and I think it is a good solution as well. This is my first experience dealing with SMD; so I am not an expert by any means. My only thoughts on this are that it might leave a very low amount of capacitance depending on what material you put in between the plates. This would like be not problem, and it sounds like it has not been a problem for you. The other reason I think I would just go ahead and trim the contacts would be because I would be concerned about whatever you put in between the plates, sliding around or deteriorating to cause a short later. This is probably not much of an issue either. But if I am already in there I don't see the drawback to just trimming them ( once you know not to over trim them like I did the first time!). Thanks for your comment.
I would think the chance of adding enough capacitance putting plastic in there would be very high and not worth the trouble, you did it just right by cutting out the tabs.
That what I was
Looking for your video is very helpful info ,I am somewhat of a novice ,I been doing electronic kits for over 40 years,and a lot more ,tube radios is something I just started, I have one I think is mostly done ,as soon as I figure out were the speaker wires go they snap off I change the filter caps
I like your idea to use some type of insulation instead of trying to clip the flat pieces. What would you suggest?
Thanks for the post. I had some discussions about this at the time with the very knowledgeable folks on the Antique Radios forum. Most seemed to think that cutting the contacts off was the safest bet. If you insulated them you would definitely want to use something like a thicker piece of plastic. Cardboard or other materials would break down. You wouldn't want these things shorting together in a few years.
Good luck!
I'm just wondering if the mica surface can be cleaned of silver with hydrochloric acid and Q-tips. If so, it could make for a more elegant restoration -- more original, and less invasive to the cans.
Thanks for your comment. I honestly do not know. It does sound like you might be setting it up to fail again in a number of years (or decades). But you are correct in that it would be more "original."
RyuDarragh Thanks for the reply. Very interesting, but it sounds like using regular caps is the way to go,
Old CB/Ham rigs suffer as well, thanks for the "Learnin'".21
Thanks for the comment. Yes, anything with those mica capacitors can suffer.
what meter should i use to measure the value of the mica wafer in these. I have three multimeters and none of them go below 100pf. If i get one that is below 100pf I cant read it and it goes in 1nf increments. I would like to be more accurate than this
I use a DER DE-5000 LCR meter. They are not terribly expensive. Some of the little Chinese made "Transistor Testers" off eBay will measure capacitance fairly low as well, but I am not sure they are accurate (or read small enough capacitance) enough for this task.
The value *might* be listed on a schematic or parts reference, or you might find something by web searching for the specific radio or part number on the can.
You can also simply try some capacitors out and see if they work for the full range. More time consuming, but definitely gets the job done. I would start with 100pF or 150pF and go up or down depending on the results.
You could also try to calculate the required capacitance based on the desired frequency, but for that you would need to know the inductance, which may or may not be listed or easy to measure either.
@@PelDaddy ok thanks. Yes the cheap Chinese ones bother me as you usually get what you pay for. I'm sure some trial and error may be necessary, but the radio when it doesn't exhibit the silver mica arc over is extremely sensitive and clean. But when I get the arc over holy cow the continuous static is enough to blow you out the room and it can last for a while before it clears.
I will look into the meter you referenced as I don't mind spending money on good quality meters. I want to try to get as close as I can to what's in there.
I have also heard of the pencil eraser or razor blade method, but figured what you are doing is probably best long term as im sure the problem will eventually come back and then i have to disassemble again.
@@kwacz The little Chinese Transistor Testers are actually pretty amazing, and good for a lot of things. But they can only read capacitance down to a certain level, and I think that may have been around 100-500pF, if even that low.
There is usually no ignoring the silver mica disease; indeed it sounds like thunder!
There are definitely a number of methods to fix the silver mica issue. Like you said, I wanted one that I knew would last a long time. I also like the idea of being able to easily replace the capacitors now because they are on the underside of the cans.
At 11:48 you said somethng about putting an insulator between the plates... wouldn't that make capacitor?
Ha! You are darned right it would! Better to just trim them off.
Looks like you could have twisted those lugs out an perhaps removed them (wire attached) for greater access to clipping. I have a H615 that needs this process as well.
Dave WM That might be possible, but when you get these things open and start seeing and feeling the wires you realize just how fragile they are. You just have to be very careful no matter which approach you take. Good luck.
Hear ya, just looks like it may have been designed to twist lock, but then that phenolic is 50-60 yrs old and may crack under the strain, and prob never meant to be removed so one twist per customer. I will eventually get to it and take a closer look. those 615's sure work well and sound great.
Maybe a dumb question, but why cut the tabs if the mica is all removed? Wouldn’t it be enough to remove just the mica sheet?
Thanks for the comment. In this particular transformer if you do not cut the tabs (plates) they will touch each other short out. The mica was the dielectric in between the tabs. Once it is removed, the plates can touch. Some people put an insulating material in there instead, but of course you are they making a capacitor, possibly of the wrong value (but maybe perfect). Whatever you do don't use some insulating material that will deteriorate, like cardboard.
Never seen one with a nut on the bottom,where did you get your information from on how to do this ,I bought a old sivertone radio I think from 1953 ,I might have to this too,I am looking for all the info I can find for my silvertone ,so when I recap I will have as much info as I can find for IT
Ron Thompson Thanks for the comment. I learned most of this from the Antique Radios forum. Look it up! Great resource.
How did you determine what size silver mica replacement caps to use?
Thanks for the comment. To determine the cap size either look at the schematic, or do the alignment first and then measure the capacitance with an ESR meter after you remove the can, but before removing the old mica.
If that fails the just try some standard values that you find in similar radio schematics and see if you can get the circuit tuned. Adjust the fixed values until you are able to get it within range.
I would also suggest posting about whatever radio you are working on to the Antique Radio Forum. The guys on there have all sorts of knowledge of all kinds of radios.
What's wrong with using a grid dip oscillator to determine the size of the capacitor(s)?
That would work too, if it is calibrated for very small levels of capacitance. My Chinese LCR meter ( transistor tester) was not up to the task.
Did you do another video where you put new calculators on the bottom as your replacements for the silver ones?
Ron Thompson Thanks again for the comment. Sadly, I did not do a video of this. I ended up putting mica caps below the can.
@@PelDaddy where did you buy your mica caps at .nice job on your video hope you do more
I got my locally at a St. Louis store called Gateway Electronics. You can get them from someplace like Just Radios www.justradios.com/mica.html or Mouser or Digikey. Good luck!
if you read this comment,please let me know,i need some info on this zenith chassis.thanks carl
I do read all the comments. Thanks. I will help if I can. Otherwise, I would highly suggest the Antique Radio Forum ( antiqueradios.com/forums/index.php )
Millersound speaker repair on internet s dad was millersound he probably has theese coils nos.their in PA.
Just replace the mica with a plastic washer or fish paper. Both will last.
Thanks for the comment. I have not tried that.
I still think I prefer putting caps below the can because that way they can be changed easily in the future. Also then you don't have to worry about the dielectric breakdown of whatever material you put in there at the voltages involved (since most washers and paper don't come with datasheets specifying breakdown voltages). But I am sure it could work, and I am guessing you have used this method with good results.
Thanks again.
I think I would have disconnected the coil wires, and taken my chances...
You mean disconnect the fine wires, and then try to reconnect them afterwards? My understanding is that they are very difficult to solder, but I have no tried it myself.
PelDaddy yes - next to impossible to solder
Helicopter s 38 b i I need fixed