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 Post subject: oxidizing plastic and bumblebee caps.
PostPosted: Sep Fri 17, 2021 8:37 pm 
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Location: Regina Saskatchewan Canada
Hi all,
Started checking my Motorola 52R out and found several things I hadn't seen before. First, I decided to diagnose what went wrong with the radio instead of just throwing caps and resistors at it. Tubes didn't light up, so that was as good a place to start as any. Trace problem to the 35w4 which tested "short on my tester. Turn over the piece of galvanized tin (another first for me) that it's all mounted on and discover a Bumblebee cap (pic4+5) has split open . Wow, my first Bumblebee cap! how exciting! I have a lot of AA5 tubes, but am waiting for some 35w4's to arrive. After finding the problem, I started replacing anything else out of spec and getting the cabinet fixed up and repainted. Still have to buff it out a bit as it's too shiny, but the color match is spot on.
I will find out if I missed anything when the tube shows up. Last, the question: what's best to neutralize the oxidation on (pic 3) plastic knobs? just about any product will wipe it off, but is there a way to stop the reaction?


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 Post subject: Re: oxidizing plastic and bumblebee caps.
PostPosted: Sep Fri 17, 2021 10:49 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Detroit, MI USA
The old exploding capacitor trick........ fortunately that one didn't catch on fire or spread shredded foil and paper everywhere under the chassis like they often did.

You can try clear coating the knobs once they are cleaned, but unfortunately unless you do that the problem will return again and again because the plastic itself is at fault.

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 Post subject: Re: oxidizing plastic and bumblebee caps.
PostPosted: Sep Sat 18, 2021 1:18 pm 
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Location: Beaver Falls, PA. USA
There are usually only six or eight capacitors to replace in the average AA5 or AA6, and it's time well spent. That usually gets them going, unless there is a bad tube or IF transformer problems; the large transformers used in the 1930s and 1940s sets do not have the SMD issues.

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 Post subject: Re: oxidizing plastic and bumblebee caps.
PostPosted: Sep Sat 18, 2021 2:21 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island NY
Looks like the "bumblebee" is the 47,000 pF capacitor on the schematic from the plate of the 35W4 to B-. Chances are when the cap shorted out it blew the heater of the 35W4, since as you'll notice, the plate of the tube is fed from the heater tap. You will never find a spec sheet from a tube manufacturer that recommends using the heater tap as a fuse, but that is exactly what many radio manufacturers did after UL let it be known they would accept that in lieu of a separate fuse. And as you can see, it worked as planned!

Regarding the knobs, the plastic is cellulouse acetate butyrate. It smells similar to the "stinkum" they put in natural gas so you can tell if you have a leak. The white stuff on it is a form of mold that grows on that particular plastic. There is no stopping it; in fact, coating it with something could make matters worse. Armor-All, for example, is itself susceptible to certain molds. Bleach will remove the mold but it harms the plastic. Alcohol is a somewhat less effective remover but it doesn't hurt the plastic, so the best strategy is to wipe the knobs down with some ordinary rubbing alcohol from time to time. The mold didn't get a chance to grow when the knobs were in constant daily use.

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 Post subject: Re: oxidizing plastic and bumblebee caps.
PostPosted: Sep Mon 20, 2021 1:39 am 
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Joined: Aug Mon 17, 2020 3:15 pm
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Location: Regina Saskatchewan Canada
That's the bumblebee alright, C4 according to the color codes. When I saw it, I connected the dots to the blown up heater on the 35W4. Thanks for the advice with the knobs. I will clean them up and stay on top of it. So far, only affecting the backside of the knobs. I know what you mean about the smell
as while back I bought pounds and pounds of knobs and the whole box stank until I removed all the decomposing knobs. This is some cheaply made chassis, but I like the radio as it seems the 52R is among the last before Motorola went to PLAcir circuit boards for their mini radios which drove me crazy
looking for an intermittent power loss on a tube portable not too long ago :cry:

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 Post subject: Re: oxidizing plastic and bumblebee caps.
PostPosted: Sep Mon 20, 2021 5:18 am 
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Joined: Apr Sun 08, 2007 6:47 am
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Location: British Columbia
The shell of the Bumblebee is made out of a Phenolic plastic like Bakelite, so it will not mold or break down, it likely broke from high leakage, or a short causing the capacitor inside to heat up, melting the wax, and the expanding gasses and heat caused the shell to split open. As for the knobs, it may just be mold, and not a breakdown of the plastic, the last few Motorola sets I had used knobs made or of polyethylene or polypropylene, not Tenite II, or one of those organic based plastics that like to mold, and disintegrate, they also have a different feel, Tennite II feels like a clear screwdriver handle, and smells like a dirty diaper if exposed to too much humidity.
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 Post subject: Re: oxidizing plastic and bumblebee caps.
PostPosted: Sep Mon 20, 2021 6:09 am 
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Joined: Jun Sat 15, 2019 7:43 pm
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There is no wax inside a Bumblebee capacitor, they are oil filled. Very high quality in their day.

DM


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 Post subject: Re: oxidizing plastic and bumblebee caps.
PostPosted: Sep Mon 20, 2021 6:25 am 
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Location: Victoria, Australia
Conservator's are having a lot of issues with plastic in artwork's. Plastic was promoted as an everlasting wonder material, however time says otherwise. Loss of plasticiser & decomposition are definitely making folly of that claim. Many radio formers & cabinets are becoming brittle & failing. Spurring a whole new generation of plastic.

Aside from radio stuff & art work many other early plastics are failing. Even the browning of the clear nitrocellulose plastic in dial covers and the Nitrocellulose finish is decomposition.

In several plastics one of the decomposition products is Hydrochloric Acid, this being the catalyst for further decomposition. Items here, in particular, are an AVO meter, a Wheatstone bridge and a Circuit & Tube Tester. These contain a lot of Bakelite & the fumant from it is acidic & responsible for the formation of Verdigris as far as I am concerned. That has lead to the secretion of porous bags of Soda Ash (Sodium Carbonate) within the devices and that has tended to solve that problem.

There is not much you can do with the decomposing plastic other than replace it and like the conservators, that is not easy.

I have found many oil filled caps in Leader Signal Generators and other stuff like Philco, BC-221-N Frequency Meter to be as bad as the worst Waxed paper caps and several of both have PCB' in them. I would strongly recommend leakage testing at their working, or rated voltage.

Marcc


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