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 Post subject: RCA 66x3 questions
PostPosted: Mar Fri 02, 2018 4:06 pm 
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Joined: Mar Fri 02, 2018 2:18 pm
Posts: 26
Location: Fargo, ND
Hello all
I recently purchased a RCA 66x3 radio and installed all new caps in it, its sounds fine with no crackling or popping but the reception is my issue. I have to hold my hand on the back of the radio on the antenna part and I get like two stations at once and when I go to the right side of the dial I pick up like different stations but they are all playing together there is no defined station. This is my first resurrection on a AM radio so I have been reading books and watching videos so my knowledge is limited as of now but willing to learn. My only real test equipment is my digital Fluke meter and I can borrow a tube tester from a guy I know and all the tubes seem fine. I need to test the resistors yet but have not yet. If any one can give me some advice on what to check and where I could start that would be great. Thanks Donny


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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Fri 02, 2018 5:50 pm 
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Location: San Jose, CA USA
Sounds like your oscillator is not running. Two ways to check:

Measure DC voltage between (-) end of one of the filter caps and pin 5 (oscillator grid) of the 12SA7. You should see pin 5 several volts negative if the oscillator is running.

Listen for the RCA's oscillator on a nearby working radio by tuning the neighboring radio about 455 kHz above the RCA's dial setting. For example, if the RCA dial is set to 600 kHz, listen for a "station" (with no audio, except maybe a little hum) on the neighboring radio at around 1055 kHz.

If you confirm the oscillator is not running, first try cleaning your bandswitch contacts and see if that helps. Then check coil continuity of broadcast band oscillator coil windings.

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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Fri 02, 2018 9:04 pm 
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Location: Mountains of Mourne. Ireland.
Four page service manual.
viewtopic.php?p=2725133#p2725133

Greg.


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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Sat 03, 2018 1:51 am 
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Joined: Mar Fri 02, 2018 2:18 pm
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Location: Fargo, ND
I have negative .46 dc volts on the negative filter cap and pin 5 on the 12sa7, I took contact cleaner and hosed down some parts, still seems like two stations on one station.


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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Sat 03, 2018 4:06 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 37068
Location: Livermore, CA
Minus .46 volts isn't enough. When the oscillator operates pin #5 will be negative several volts.

Check voltages and components to your 12SA7 tube. Pin #4, 5 and 6 make up the oscillator circuit.

When the osc doesn't operate sometimes you can pick up strong local stations but they won't be tunable. The tuner will act more like a volume control.

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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Mon 05, 2018 12:39 am 
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Joined: Mar Fri 02, 2018 2:18 pm
Posts: 26
Location: Fargo, ND
Once I find some time I'll do some more digging into it, thanks for the info fellas.


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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Wed 07, 2018 3:23 am 
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Joined: Mar Fri 02, 2018 2:18 pm
Posts: 26
Location: Fargo, ND
Been doing some more checking, off my pin 3 and 4 on the 12sa7 I only get about 46 volts dc, about half of what my diagram says. Supposed to be 79 volts dc on pin 3 and 4 and still get .46 negative volts on pin 5. Any suggestions?


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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Wed 07, 2018 3:54 am 
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Location: Livermore, CA
With only 46 rest of the radio is also low in voltage.

Start with pin #8 of 35Z5. From there measure voltage on both sides of the 1200 ohm resistor (35). Have both 7A (30 mf) and 7B (50 mf), electrolytic caps been replaced.

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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Wed 07, 2018 4:17 am 
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Joined: Mar Fri 02, 2018 2:18 pm
Posts: 26
Location: Fargo, ND
Yes, I replaced those two also, I noticed it being low on other pins also on other tubes. I got a little confused when i replaced those filter caps, I think I have them right but my confidence level isn't quite 100%. I 'll rest my brain tonight and try again Tommorow night. Thanks for the info.


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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Thu 08, 2018 4:01 am 
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Joined: Mar Fri 02, 2018 2:18 pm
Posts: 26
Location: Fargo, ND
Ok, pin 8 on the 35z5 shows 65 volts dc but on the other side of the 1200 ohm resistor it goes down to my 46 volts, the 46 side of the resistor looked crusty so I replaced the resistor for my own piece of mind. Pin 4 where the 1200 ohm was on seemed kind of crusty when desoldered it also. What should my next step be? Thanks Donny


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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Thu 08, 2018 5:47 am 
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Location: Livermore, CA
Schematic shows pin #8 of 35Z5 should be 118 volts. B+ voltage starts there. Either 35Z5 is bad or electrolytic filter caps have a problem? Drop across the 1.2K resistor is reasonable just there isn't enough voltage to start.

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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Thu 08, 2018 3:41 pm 
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Joined: Mar Fri 02, 2018 2:18 pm
Posts: 26
Location: Fargo, ND
I noticed when I replaced that 1200 ohm resistor that I acquired a hum that I didn't notice before I replaced the resistor.


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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Thu 08, 2018 4:05 pm 
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Joined: Mar Fri 02, 2018 2:18 pm
Posts: 26
Location: Fargo, ND
I installed a 1/2 watt resistor but should that be a 1 watt resistor?


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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Thu 08, 2018 4:38 pm 
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Norm Leal wrote:
Schematic shows pin #8 of 35Z5 should be 118 volts. B+ voltage starts there. Either 35Z5 is bad or electrolytic filter caps have a problem? Drop across the 1.2K resistor is reasonable just there isn't enough voltage to start.

Agreed, either 35Z5 is bad, filter caps were installed incorrectly, or something is mis wired relating to the 1200 ohm...

If the was a enough current draw through the 1200 ohm resistor to reduce voltages by almost half, there would be smoke... A half watt wouldn't have a chance, replace with 1 watt and recheck value... No being 1/2 watt won't cause any of your problems, it'll just have a short life...

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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Thu 08, 2018 11:16 pm 
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Joined: Mar Fri 02, 2018 2:18 pm
Posts: 26
Location: Fargo, ND
Well back to the waiting game, i will have to order a 1 watt resistor and I will have to dig around and see if one of my other radios has a 35z5 I can try for a test example. Thanks for all the info fellas. I will keep you up to date on my journey. Thanks Donny


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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Fri 09, 2018 5:19 am 
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Location: Livermore, CA
Chassis is not used for negative terminal of electrolytic caps. Negative should connect to what is shown as ground. Symbol for chassis is a rake.

http://imageview360.com/radio/RCA%2066X ... ematic.pdf

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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Tue 13, 2018 2:46 pm 
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Joined: Mar Fri 02, 2018 2:18 pm
Posts: 26
Location: Fargo, ND
I got my parts in yesterday and changing out the 35z5 I am back to normal DC voltage, think I was at 121 volts before the 1200 resistor and 64 volt after the resistor if I remember right but way better than before. Any suggestions on the alignment aspect of it, just pick up a signal generator and i noticed on the alignment directions to install a .001 cap on the B- side and set it at 455 khz and then positive on the pin 8 of 12sa7. That seem correct to everyone? Thanks Donny


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 Post subject: Re: RCA 66x3 questions
PostPosted: Mar Mon 26, 2018 3:17 am 
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Joined: Oct Tue 24, 2006 5:53 am
Posts: 51
Location: Manassas, Virginia, USA
Mustang,
Thanks for posting! I'm currently restoring a 66X3 myself. I'll be keeping an eye on your thread in case this issue pops up for me!

Bryan


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