My new console radio

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My new console radio

Post by Ragwire on Sun Oct 12, 2014 9:00 pm

I got a pretty good deal on this Coronado model 94RA1-43-6945B, AM/FM console radio today. It's from about 1948, and uses a mixture of 7 and 9 pin miniature tubes and a couple octal tubes. I don't have much experience with FM radio or especially FM alignment, but I do have the Rider's service literature/schematic for it.
Wish me luck! alien

Rob












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Re: My new console radio

Post by 75X11 on Sun Oct 12, 2014 9:36 pm

Good looking unit. I would like to get a console with FM. I'll bet it will be a nice performer.
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Re: My new console radio

Post by Ragwire on Sun Oct 12, 2014 10:00 pm

I thought it looked pretty good for a franchise store brand. It's all there. The cabinet needs some refinishing. Good winter project for the man cave.
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Re: My new console radio

Post by Wildcat445 on Sun Oct 12, 2014 10:55 pm

Nice score!

WC

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Re: My new console radio

Post by Conelrad on Mon Oct 13, 2014 2:04 am

Great looking set, I'll bet it rocks when you get done with it!

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Re: My new console radio

Post by Guest on Mon Oct 13, 2014 8:30 am

I like it! Smile

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Re: My new console radio

Post by Ragwire on Mon Oct 13, 2014 9:43 am

Thanks, all!
I'll probably do like I did for the Maggy upstairs and hide an iPod adapter in the back to the phono input.
Would it be a crime to put a tuning eye tube in the dial face? I like originality, but it would be kind of a cool mod.
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Re: My new console radio

Post by Dr. Radio on Mon Oct 13, 2014 8:21 pm

Nice Rob.

I like the clean/simple lines, and NO, I don't think you'll hurt anything by adding an eye tube.

I could just picture that being in a basement rec room when new.

I spy a a number on one of the back of one of the IF transformers. "540". Possibly 40th week of 1955.....New for 1956?
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Re: My new console radio

Post by 75X11 on Mon Oct 13, 2014 8:38 pm

Having seen your work, I would imagine you will have the experts scratching their heads looking up that model with the OEM tuning indicator!
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Re: My new console radio

Post by frank1956 on Tue Oct 14, 2014 2:21 pm

Very nice!!!
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Re: My new console radio

Post by Ragwire on Thu Oct 16, 2014 11:15 am

75X11 wrote:Having seen your work, I would imagine you will have the experts scratching their heads looking up that model with the OEM tuning indicator!
Laughing
Dr. Radio wrote:I spy a a number on one of the back of one of the IF transformers. "540".  Possibly 40th week of 1955.....New for 1956?
Beitman's has it listed as a 1948 model. Oddly, the B suffix came before the A suffix in the model number. The 1949 model was the "A" and the only difference was a 12BE6, 7 pin convertor tube instead of the 9 pin 12BA7 mine uses.

cheers
Rob
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Re: My new console radio

Post by Ragwire on Fri Oct 17, 2014 7:30 pm

I have never seen both halves of a big chassis mount power supply filter resistor open before, but this one is. I suppose the capacitor was shorted or else the audio ouput 6V6GT was. That tube showed a short lamp on my tube tester. The next morning I tested it again and there was no short indication. I thought maybe I had a setting off because I was tire the night I tested the tubes. Well, I don't trust that tube and will be replacing it and the rectifier that tested weak. I have all the paper-wax and electrolytic capacitors replaced, and 7 small resistors that were wildly out of spec. I had to order the chassis mount power resistors from Mouser.

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Re: My new console radio

Post by Dr. Radio on Fri Oct 17, 2014 8:52 pm

Rob,

I'm sorry, I totally missed where you stated the date of the unit in your original post!

Keep us posted on your project.

Be sure to carefully inspect that B+ line and related connections. Anything intermittently pulling it to ground would definitely somoke those resistors.
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Re: My new console radio

Post by Ragwire on Sat Oct 18, 2014 10:13 am

Thanks...I'll power it up with a variac on the first try.

I learned something today (yesterday, actually) about FM that I had forgotten. I expected to see a limiter stage beofr the FM detector to prevent noise from getting through to the audio amplifier. This unit uses a ratio detector and they do not need a limiter because, unlike the Foster-Seeley type discriminator, the ratio detector is inherently immune to amplitude variations. I thought that was interesting. I have downloaded pretty much all of the 1958 RCA home study radio course, free here: http://www.km5z.us/RCA_Radio_TV_Course_1958.php
and man it explains things more clearly than any text we used in school.
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Re: My new console radio

Post by Ragwire on Thu Oct 23, 2014 8:45 pm

The chassis is done and working strongly on AM, but I have not been able to receive FM signals yet. I tried a dipole at the FM terminals for a dipole antenna, and it seems to be trying tp pick up something very vaguely. My signal generator (AM only) goes through and I went through the FM alignment with an unmodulated signal and VTVM. It made it more sensitive. The specs say the FM sensitivity for 0.5 watt output is 100uV, but only 10uV for AM (same output). The Eico 147 signal tracer is showing good gain in the stages, and the oscillator is working on the FM band.
Any suggestions?

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Re: My new console radio

Post by 75X11 on Thu Oct 23, 2014 8:49 pm

Yow! it looks like 1948 in there!
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Re: My new console radio

Post by Ragwire on Thu Oct 23, 2014 9:49 pm

Thanks...I did a lot of cleanup and repainted the transformer. The tuning gang was removed and given an acid bath cleaning. (It's tricky and has to be quick.)

I would like it to work on FM...and maybe it does with the right antenna. I have read somewher the old sets without RF amps needed an outdoor aerial for good FM reception, but I don't know what is normal for this one.

I want to clarify my last post a little: My signal generator is AM modulated only, and this will obviously reduce speaker output for a given input level because the AM is being more or less forced through the demodulator. I have to turn the generator up quite a bit to get a loud tone compared to the AM band, and between the modulation and the fact that the FM is inherently 1/10th as sensitive, according to the service literature, I expect that.

I still don't understand why I can't pick up anything on FM with a 1.46 meter dipole with twisted lead in. Now this antenna is just nailed to the wall of my shop in the basement, but any random modern FM radio can pick up stations just fine down there with no external antenna at all.
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Re: My new console radio

Post by 75X11 on Thu Oct 23, 2014 9:58 pm

Just out of curiosity, I wonder how it would respond to a fm modulator with your choice of applied tone. You know, like a sound feeder. It could provide a bit stronger signal than nearby stations and allow you to see if any detection alignment is in order.
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Re: My new console radio

Post by Ragwire on Fri Oct 24, 2014 10:00 am

That's a good idea...but my only FM signal generator is a digital function generator that goes only to 10MHz. Maybe I can borrow a signal from a working FM radio and inject it into the Coronado?
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Re: My new console radio

Post by 75X11 on Fri Oct 24, 2014 10:34 am

Something along this line was what I was thinking about. I have one saved up that I will probably use for testing when I find an older FM unit one day.

http://www.amazon.com/Transmitter-Charger-Wireless-Hands-free-Adapter/dp/B00L2A70R8/ref=sr_1_187?s=automotive&ie=UTF8&qid=1414157446&sr=1-187&keywords=fm+transmitter
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Re: My new console radio

Post by Ragwire on Fri Oct 24, 2014 11:56 am

Genius! I have one of those for my iPod to car radio!

I did an rough test this morning by applying what should be about 100uV into the FM antenna input terminals, but still using my AM generator. I do not have the means to set and measure the generator to that low of voltage at that frequency (about 100MHz) directly, but I measured the output of the generator using a high frequency RF probe and made a comparison to compensate for the probe's non-linearity at low levels. Long story short, I applied about a 50mVRMS radio signal across a capacitive voltage divider (a 10pF in series with a .005uF across the generator, and the .005uF was tapped for the signal to the antenna input) to cut the input to a calculated 100uVRMS.

Keeping the leads very short and straight, I believe I was able to inject something in the ballpark of a 100uV RF signal into the FM receiver. Then switching on the internal modulation--still with only an AM tone--I got a perfectly audible tone from the Coronado's speaker.

At this point, it is looking like, by design, it really just needs a good outdoor antenna to get FM. scratch
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Re: My new console radio

Post by 75X11 on Fri Oct 24, 2014 12:33 pm

There is a amplified antenna available at big lots that I use with my RCA Victor 8R71 to get the fringe signals. It is for video, so it would need a 2 wire transformer from radio shack.
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Re: My new console radio

Post by Ragwire on Fri Oct 24, 2014 9:00 pm

I might need one. I hooked up the iPod to car radio FM transmitter and got nothing from the Coronado but plenty from a solid state battery radio sitting next to it.

Also, on FM, the Coronado would periodically go silent (no backround noise/static) and become highly microphonic. I could here my fingers on the tuning knob...then it would snap back into quiet radio static mode without the microphonics. The chassis was on it's side so Maybe I have a tube with loose elements. The 6BA7 pentagrid convertor seemed the most sensitive when it was acting up. I have another of these tubes on order.
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Re: My new console radio

Post by 75X11 on Fri Oct 24, 2014 9:13 pm

My RCA 8R71 would do the microphonic bit also and I found a small open coil wrapped around a lucite dowel and immersed on a glop of duco cement that had come slightly loose and would periodically let 'er rip. Sometimes i think they were making it up as they went along with these early FM sets.
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Re: My new console radio

Post by Ragwire on Sat Oct 25, 2014 10:07 am

I'll re-examine the coils then, too...there is one that is one wrap of wire around a 15 ohm, 1/2 watt resistor that they call a "parasitic inductor" or something. Looks homemade. Supposed to dampen out any high frequency oscillations, I suppose.

Well, when the second batch of tubes arrives, I am going to replace the pentagrid convertor and both IF amps. I don't trust that 6BA7, and both IF amps are supposed to be 6BA6, and both are 6AU6. That is supposed to be a good substitute number, but somethimes they are not ideal. AM works great though.
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Re: My new console radio

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