Painting a Bakelite radio cabinet

Multiple sources online show them as in stock.

A lot of things evade you.

From the 35Z5GT data sheet: "Glass octal type used in power supply of ac/dc receivers. The heater is provided with a tap for operation of a panel lamp. May be supplied with pin 1 omitted. Requires octal socket."

Hint: It's not "center" tapped.

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Jeff-1.0 
wa6fwi 
http://www.foxsmercantile.com

has a good discussion of this.

He doesn't mention any specific sort of paint being required, but does recommend spraying on a coat of primer after a thorough cleaning... probably a good idea to help ensure good adhesion.

I recall that Rustoleum makes some paint varieties which are specified as being suitable for use on plastics. This sort of multi-surface paint might give you some additional defense against peeling, if you decide to omit the primer step.

Dave Platt wrote on 9/26/2017 3:42 PM:

It all depends on the details. I used a paint for plastic to use on a plastic container and it did not work well at all. I cleaned the surface well, but did not rough it up and did not use a primer. It was not too long before the paint started to peel. I don't know the type of plastic. It was a food storage container with a seal ring commonly available in stores. Just a data point for what it is worth.

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Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

Nothing sticks to polythene.

NT

I've seen seasoned restorers use car spray paint, vauxhall brazil brown for bakelite.

When getting a colour match I always give the sample a good clean first, otherwise the wrong colour is inevitable.

NT

Are you in the UK? Is that what we call polyethylene? How does it happen that we end up with different names for things like common plastic?

--

Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

Yes, yes, it's easier to say. Polythene is the next best thing to ptfe when you want to keep friction low. Handy for moving heavy loads. Food storage containers are most commonly polythene.

NT

Tradition. It dates back at least as far as the day when we Americans decided we couldn't afford to spell the new metal "aluminium" due to a severe shortage of printers' slugs for the letter "i". :-)

Pedant mode on. Most plastics have a short common name for obvious reason. IIRC Mr. Webster's rewrite of American spelling occurred well before plastics in the 1800s.

NT

This looks to be the schematic for the set. It's an AA6, with an RF amplifier ahead of the converter.

We used to use schematics of sets like these back in the 1950's and '60's as interview questions for EE and technician canditates. One very good one was "talk to me about the rectifier circuit."

Understanding the circuit starts with understanding basic electricity,

are calling a "center tap" between pins 2-3 of a 35Z5 is at the 21.5% point of the heater, not the center. According to the RCA data sheets, with 35 volts between pins 2 and 7, you'll see 7.5 volts across pins

2-3. Now add a #47 (6.3v 150 ma.) bulb across pins 2 and 3, and feed the plate from pin 3. RCA claims that the voltage across pins 2-3 (and the bulb), with a 60 ma. draw on the cathode is now 5.5 volts and between pins 2-7, 32 volts.

That, of course, doesn't "add up." Assuming RMS values, a 150 ma. heater plus 60 ma. of DC is only 210 ma. RMS, which should explain the lower voltage between pins 2-3. However, also consider that current flow in the plate circuit, with a 40 mike cap connected to the cathode is a pulse, not a full half sine wave, and that RCA measurements were made with a VTVM, which isn't a "true RMS instrument." Those pulses, and the low thermal inertia of the pilot lamp filament(s) explains why the bulb is at full brilliance when the set is warmed up and playing.

Another "trick" to the series circuit is that during warmup, the series filaments are a voltage divider that is essentially constant for any current passing through the heater string. These heaters have a steep positive temperature (heat/ohms) coefficient, so the startup current is more like 1.5 amps. If you are going to fuse the radio, you need to measure the resistance of the filament string cold, and work from there.

That's for starters.

Now, as to the rest of the set, take a look at the B+ circuit and voltages. The tube data sheets give 100 volts on both plate and screen as "typical operation" points for AA5 tubes. However, this set says 76 volts. Also note that "typical operation" specs an initial bias (-1 volt for 12SK7), while this set has the cathodes of the RF tubes grounded.

Yes, that schematic is a very good interview subject.

Don't get foxed by "Mazda." That was a GE trademark from around 1910 to denote "tungsten filament," and was licensed by other bulb manufacturers. GE dropped using it in 1945. If you need bulbs, a 1490 is a 1490, readily available today.

If you are going to play the radio without recapping, the critical caps are the coupling caps between stages, particularly the cap feeding the audio power amp. Also, check the value of the grid leak resistor(s). You should see zero volts DC at the grid of the 35L6.

The AA5 (and AA6) are, with the DC-3 airplane, and the GG-1 locomotive, standout classics of American design. They look "simple" but there's a lot of thinking that made them robust and reliable without being complicated.

Hank

If you are talking about 'Tupperware' type containers, that plastic is the lowest grade available. It won't hold paint, and common adhesives won't stick to it.

--
Never piss off an Engineer! 

They don't get mad. 

They don't get even. 

They go for over unity! ;-)

They're food grade. Nothing sticks to polythene unless you torch it first or weld it.

NT

Thanks for the detailed explanation. I had a suspicion that the "center tap" of the rectifier tube was not actually CENTER. The more I work on these old sets, the more I notice that they are all pretty much the same, particularly if it's the same brand. One website I looked at, said that the chassis is a number -xxxx-. (I forget the number, but it was a

4 digit number 10__). I guess that means this same chassis was used in other RCA radios, not just these 66X_ (1 thru 4) models. But considering the years they were made, that makes sense. They produced the chassis and made numerous cabinets to fit around them.

I have to say that I am highly impressed that this 70 year old radio still works, with it's old caps and all. I have considered recapping it, or at least the critical caps and the power supply filter caps. But then again, "if it works, dont fix it". Having it power up with no hum, was what impressed me most, since the electrolytics are usually bad.

Thanks for explaining that "Mazda". That had me puzzled. In all the years I have worked on this stuff, that is the first time I saw that word used. I should have suspected it was a trademark....

That schematic is clearer and better than the one I had, so that will help too.

Are you stupid, or do you just try to appear to be stupid?

"It works." Yeah, right up until it doesn't and fries an IF transformer or something else like one of the vacuum tubes.

A couple of examples:

1. A late '40s vintage Artone AM/FM/Phono console my parents bought new. It was working when I left home in 1972. I pulled the chassis around

2001 and turned it on. I worked for about 10 minutes and paaaf! one of the paper caps self-destructed. I replaced that one and tried it again. This time it lasted 3 minutes and pafff! another paper cap went away. After I changed the 4th one, I just replaced the remaining ones.' After another 15 minutes the one of the filter caps self destructed. I replaced all of them. The radio sits in my living room and has been working fine for the past 16 years.

1. A Hallicrafters SX-110 general coverage receiver. It looked like new and had the matching speaker. I set it on the work bench at the shop in

2007. Turned it on, and it worked perfectly. After 20 minutes the audio slowly dropped to zero. I turned it off to checked what happened. The filter can was hot enough to fry eggs. I recapped the radio and gave it to a friend of mine. It's still working perfectly and is one of his favorite radios.

But, by all means, do whatever you want. Solid technical advice is wasted on you.

--
Jeff-1.0 
wa6fwi 
http://www.foxsmercantile.com

Jeff was lucky, his capacitors failed quickly while he still had the radio apart. My experience was different, each one lasted months, so I ended up taking the set apart numerous times to replace one capacitor each time. I learned from that, now I replace them all at one go. But if you like taking things apart repeatedly, that's up to you.

--
Jim Mueller wrongname@nospam.com 

To get my real email address, replace wrongname with eggmen. 
Then replace nospam with expressmail.  Lastly, replace com with dk.

The Artone was the third set I worked once I decide I was going to play with old radios. The first was a Signatone code practice oscillator and every cap and resistor was bad in it. The second was a Columbia phono, AC/DC/Battery portable with a wind up clockwork mechanism.

After about ten years of playing with vintage tube gear, I opened a shop to do this for profit. This means when you fix a radio, it has to stay fixed once it goes out the door.

"You can't fix stupid."

--
Jeff-1.0 
wa6fwi 
http://www.foxsmercantile.com

Considering my age, it probably dont matter, but I bet all the modern caps will fail in far less time than those old paper caps. Nothing these days is made as well as in the old days. Caps that are still working after 70 years were not poorly made. I think a lot of that was because in those days, people had respect for their customers, anad wanted the name of their company to stand out. That was before our "throw away society". That is no longer the case.

Add to that, the fact that most caps are now made in China, and I know they wont last. I try to get all NOS Orange Drop caps. They were made in the USA and were top of the line. They cost more, but are the only ones I consider worth buying.

Capacior reliability is another topic to learn about. The 3 unreliable type s are electrolytic, paper and multilayer ceramic. Other types are extremely reliable. Unfortunately most 1950s or earlier small caps are paper, and mo st are well & truly shot now. Replacing old papers with new plastic film is a big reliability & longevity upgrade.

NT

Of for f*ck's sake. You are one ignorant son of a bitch.

Caps made "back then" were made with non-archival, i.e. acid bearing paper. They were sealed with bee's way which was hygroscopic. Moisture reacted with the acid in the paper and that was the end of that.

Modern capacitors are made with a metalized mylar film and have a service life measured in hundreds if not thousands of years.

Modern electrolytic capacitors have improved 10 fold over the years.

"A fool and his money are soon parted."

While you're at it, why not buy NOS Black Beauty capacitors.

--
Jeff-1.0 
wa6fwi 
http://www.foxsmercantile.com