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 Post subject: RCA Radio Identification
PostPosted: Jun Thu 24, 2021 8:32 am 
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Joined: Oct Fri 12, 2018 8:26 pm
Posts: 41
Hi, Can someone help me to identify this RCA radio?
The model number seems to be QU6XX
Here are few pictures:
https://i.ibb.co/5rV3gCt/204053055-1008 ... 8895-n.jpg
https://i.ibb.co/Sxsc0qk/203667548-9939 ... 4230-n.jpg
https://i.ibb.co/P9BrFbR/203310227-2284 ... 8669-n.jpg
https://i.ibb.co/FKYgphp/203448557-3300 ... 3313-n.jpg


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 Post subject: Re: RCA Radio Identification
PostPosted: Jun Thu 24, 2021 11:43 am 
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Joined: Jul Sun 26, 2015 11:34 pm
Posts: 1085
Location: Hurdle Mills, North Carolina
RCA QU62 here is the schematic
http://www.nostalgiaair.org/Resources/675/M0015675.htm

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 Post subject: Re: RCA Radio Identification
PostPosted: Jun Thu 24, 2021 2:01 pm 
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Joined: Oct Fri 12, 2018 8:26 pm
Posts: 41
LarryAD wrote:

Thank you very much Larry.
This is the first vintage radio I'm about to restore.
I have some experience with working on vintage guitar tube amps from the 1960's /70's but not with radios from late 40's.

Beside re-capping all the electrolitics and paper caps and replacing old resistors that are out of specs, there's anything else I should be aware to before starting to work on this project?


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 Post subject: Re: RCA Radio Identification
PostPosted: Jun Thu 24, 2021 9:25 pm 
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Joined: Jan Thu 28, 2021 3:34 am
Posts: 76
Smilan,
You picked a beast for your first Radio restore.
Make sure you check it with a Dim Bulb tester to check the power transformer.
Do you have a Signal Generator and VTVM, these maybe needed to align it correctly.
You can get lots of help on this forum. Many members are highly skilled people.
Good luck


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 Post subject: Re: RCA Radio Identification
PostPosted: Jun Thu 24, 2021 9:30 pm 
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Joined: Aug Tue 28, 2018 9:22 pm
Posts: 3113
Location: Sanford Fla 32771 (USA)
As a suggestion, add to your profile your general location like city/state. Some one near to you may offer some hands on help.
I would if close by.

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 Post subject: Re: RCA Radio Identification
PostPosted: Jun Thu 24, 2021 10:16 pm 
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Joined: Oct Fri 12, 2018 8:26 pm
Posts: 41
kevin.groves wrote:
Smilan,
You picked a beast for your first Radio restore.
Make sure you check it with a Dim Bulb tester to check the power transformer.
Do you have a Signal Generator and VTVM, these maybe needed to align it correctly.
You can get lots of help on this forum. Many members are highly skilled people.
Good luck

Thank you Kevin,
I have here a bulb current limiter, a variac, a signal generator, DMM, a Hickok 600A and a scope.
I'm sure I will ask here some questions along the way.

pauls.ironhorse wrote:
As a suggestion, add to your profile your general location like city/state. Some one near to you may offer some hands on help.
I would if close by.


Hi Paul,
Thanks for your kind offer
I'm living in Tel Aviv, Israel (6575 Miles from you...), there're no so many vacuum tube technicians around here, but I might be wrong...


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 Post subject: Re: RCA Radio Identification
PostPosted: Jun Fri 25, 2021 8:11 pm 
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Joined: Oct Fri 12, 2018 8:26 pm
Posts: 41
From looking the schematic I have this question: What is the theory behind this speakers connection arrangement? What's the benefit of using an electrodynamic speaker with a permanent magnet speaker?
Image


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 Post subject: Re: RCA Radio Identification
PostPosted: Jun Sat 26, 2021 3:53 am 
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Joined: Feb Tue 24, 2009 8:20 am
Posts: 10172
Location: Aurora Colorado
The field coil was used in early speakers to create the magnetic field, permanent magnets were very expensive,

Does your set have 2 speakers ? If so, the plug shows that it can be left in the cabinet, when you service the chassis.

If you do not have 2 speakers, there may be an external speaker available.

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 Post subject: Re: RCA Radio Identification
PostPosted: Jun Sat 26, 2021 8:07 am 
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Joined: Oct Fri 12, 2018 8:26 pm
Posts: 41
Quote:
Post subject: Re: RCA Radio Identification
The field coil was used in early speakers to create the magnetic field, permanent magnets were very expensive,

Does your set have 2 speakers ? If so, the plug shows that it can be left in the cabinet, when you service the chassis.

If you do not have 2 speakers, there may be an external speaker available.


This radio have 2 speakers.
I'm familiar with electrodynamic speakers, but I don't understand why to use one electrodynamic speaker and one permanent magnet speakers together?


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 Post subject: Re: RCA Radio Identification
PostPosted: Jul Thu 01, 2021 7:29 am 
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Joined: Oct Fri 12, 2018 8:26 pm
Posts: 41
I have another question about this project,
I need to replace a .003uF/1000V capacitor and I can only find a .003/600V cap.
So I'm thinking to wire a .001/600V and .002/600V caps to get a.003/1200V.
I know it's better to add a resistor in parallels with each capacitor so to equal the current that passes thru each cap.
My question is how to determine the ohm value and wattage of those resistors?


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 Post subject: Re: RCA Radio Identification
PostPosted: Jul Thu 01, 2021 7:54 am 
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Joined: Feb Tue 24, 2009 8:20 am
Posts: 10172
Location: Aurora Colorado
They mat have been using up old stock, and it is an easy way to lower voltages. Also, in these sets, there is no need to add resistors for balancing currents

the tolerances can be plus or minus 100 % , and still work fine.

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 Post subject: Re: RCA Radio Identification
PostPosted: Jul Thu 01, 2021 8:15 am 
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Joined: Oct Fri 12, 2018 8:26 pm
Posts: 41
OK, after looking again at the schematic I've noticed that the maximum B+ voltage is 330V.
Those caps going from the plates (330V) of the output tubes to ground.
If so, it will be safe to use a 600V cap instead of the 1000V cap?


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 Post subject: Re: RCA Radio Identification
PostPosted: Jul Thu 01, 2021 8:27 am 
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Joined: Feb Tue 24, 2009 8:20 am
Posts: 10172
Location: Aurora Colorado
I am not very good at the theory, but I am thinking that the voltage can spike, because of a higher than expected

input voltage, and this will help stop damage.

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 Post subject: Re: RCA Radio Identification
PostPosted: Jul Fri 30, 2021 9:39 am 
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Joined: Oct Fri 12, 2018 8:26 pm
Posts: 41
I received the new caps, I would like to start recapping but I have something that I don't understand:
According to the schematic the high voltage center tap connected to the + terminal of C44-C and to the - terminal of C44-A.
Since C44 is a multi can capacitor this connection doesn't make no sense, by connecting the + terminal of C44-C to the - terminal of C44-A I'll bypass C44-C.
Am I missing here something?
Image


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 Post subject: Re: RCA Radio Identification
PostPosted: Jul Fri 30, 2021 1:42 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 10752
Location: Beaver Falls, PA. USA
C44C is used to filter the negative bias supply; its positive terminal will go to chassis ground.

The high voltage rating of the output tube plate bypass capacitors is needed, because the inductance of the output transformer winding can cause a voltage spike under some conditions, such as static noise bursts.

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 Post subject: Re: RCA Radio Identification
PostPosted: Jul Fri 30, 2021 3:07 pm 
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Joined: Oct Fri 12, 2018 8:26 pm
Posts: 41
Tim Tress wrote:
C44C is used to filter the negative bias supply; its positive terminal will go to chassis ground.

So the drawing of C44-C is wrong and should be reversed?
The CT should be connected to the negative terminals of C44 A to C and the positive terminal of C44-C is chassis grounded?


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 Post subject: Re: RCA Radio Identification
PostPosted: Jul Fri 30, 2021 5:06 pm 
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Location: S. Dartmouth MA 02748-1225 USA
smilan wrote:
Tim Tress wrote:
C44C is used to filter the negative bias supply; its positive terminal will go to chassis ground.

So the drawing of C44-C is wrong and should be reversed?
The CT should be connected to the negative terminals of C44 A to C and the positive terminal of C44-C is chassis grounded?
C-44 A, B, C. all have a common negative. In can Twist-Lok caps the negative is usually the shell and appears so in the schematic. One of the caps inside the can (+) terminal is connected to the chassis, Cap. "C" that makes the metal can (shell) at -23 volts potential this is bias for the output tubes. The symbol for C-44 is upside down BUT the common negative polarity for all thee caps remains. If you are using separate caps NONE of those three caps has a negative connected to the chassis and all the negatives are connected together and to the center-tap of the power transformer as shown.

If a mistake is made in these cap connections the caps may explode, the rectifier tube could be ruined or the bias incorrect causing red plating of the output tube and could ruin them by poisoning the grids.

The 1000 volts caps across the output transformer were foil/wax paper. C50 & C51. If a metalized is used even if it is 2kv the metalizing will breakup in time and the cap will loose its value and there will be no cap to absorb a spike generated by the output transformer when noise is received. The result will be failure of the audio output transformer. Use a cap in this location for buffer service or for spike reduction such a cap will have a dv/dt rating.

FYI cap MF values add when they are in parallel but voltage is the same as the lowest voltage in the group. Cap voltage ratings add when in series for identical caps in series the value in MF is divided by the number of caps in series.

For examples: two .01mf in series will be .005mf. Two .001 in parallel will be .002...

Do not shot-gun caps in the RF section, do not poke or bend any of those RF caps or move around any wires connecting coils. Any failure in the RF area can be found once the radio is in operation.

If you do not have a modulated RF generator for alignment non-disturbance to the RF area is important.

This receiver "appears" to have an RF amp, it will be a sensitive receiver if properly restored...

GL

Chas

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 Post subject: Re: RCA Radio Identification
PostPosted: Jul Fri 30, 2021 10:46 pm 
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Joined: Oct Fri 12, 2018 8:26 pm
Posts: 41
Chas wrote:
smilan wrote:
Tim Tress wrote:
C44C is used to filter the negative bias supply; its positive terminal will go to chassis ground.

The 1000 volts caps across the output transformer were foil/wax paper. C50 & C51. If a metalized is used even if it is 2kv the metalizing will breakup in time and the cap will loose its value and there will be no cap to absorb a spike generated by the output transformer when noise is received. The result will be failure of the audio output transformer. Use a cap in this location for buffer service or for spike reduction such a cap will have a dv/dt rating.
Chas


Thank you Chas for get into details.
I have a 3000pF 1Kvolts ceramic disc capacitors and a polypropylene metallized film caps with the same values.
Should I use the ceramic caps for this application?


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 Post subject: Re: RCA Radio Identification
PostPosted: Jul Fri 30, 2021 11:41 pm 
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Location: S. Dartmouth MA 02748-1225 USA
smilan wrote:
Thank you Chas for get into details.
I have a 3000pF 1Kvolts ceramic disc capacitors and a polypropylene metallized film caps with the same values.
Should I use the ceramic caps for this application?
I would use the ceramic. My experiments studying the failures of common metalized caps found the the peak frequency that well exceeds and unrated metalized cap is around 10mhz. That is what gets generated when a static pulse cuts off the output tubes and the magnetic field in the output transformer collapses. That would not get generated in an audio only amp unless the instruments had poor noisy connections.
Though not mentioned is any or all AC line bypass caps. The common metalized is a poor choice there too. The current trend is to use "X" or "X/Y" combination rated caps. These a designed to fail open to avoid send AC to the chassis connection.

The decision is yours, so long as your aware the common yellow metallized is not an"all purpose" cap...

Since this is your "first" radio do avoid using compressed air to dust. The mica trimmers on the tuning condenser body will vibrate then shatter into a thousand sparkly flakes, not good...

This radio may also contain capacitors that look like a "lozenge", others call these "deer turds". These are a brown plastic sealed mica sandwich. They are in critical tuning locations. I would consider replacement only: If the plastic body is cracked, they leak electrically or have changed value. These "lozenge" caps are of critical values, silver mica is the replacement. Of that, the values can be paralleled if not close to the original... Note, there may not be any of that style of cap in this model RCA radio...

An old school capacitor bridge can measure these even if severely leaking, to a point. Often will give a value when a Multi-DVM will not.

Be aware that any voltage readings printed on the schematic will conform to a meter usually indicate on the same schematic with "ohms/volt". A meter with a much high resistance will indicate a higher reading. Such readings can be deceptive if trouble shooting is required...

GL Chas

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 Post subject: Re: RCA Radio Identification
PostPosted: Aug Sun 01, 2021 1:02 pm 
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Joined: Oct Fri 12, 2018 8:26 pm
Posts: 41
Thanks again Chas.

I finished to replace all the electrolytics and paper caps in the main chassis.
I replaced the .003 / 1KV caps with new ceramic caps at the same values.
For now I left the mica caps in place.

Now I need to replace the old and rotten mains AC cable with a new one. Since I can't see at the schematic a "death cap" that should be removed it means I can install a three prong power cable and ground the metal chassis to earth?

Also I only have here the main chassis (the rest of the console located in a different place and I'll bring it to there after I'll finish to restore the main chassis) , there's a way to test the radio and preamp sections without the field coil and the speakers?
I'm thinking of feeding a sine wave from a signal generator to the grid of V5 (6SQT) and trace it with an audio tracer / a scop and see if it reaches to the grids of the output tubes (with no output tubes installed).
I was thinking to do the same test with the radio.
For doing so I can jumper J2, so the primary side of the PT will get the mains AC supply.
So I'm wondering if there's a way to let the B+ reaches the circuit?
Can I use a 40uF/450V to ground to replace the field coil?
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