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 Post subject: RCA 124 schematic - capacitor questions
PostPosted: Sep Thu 28, 2017 2:33 am 
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Joined: Dec Thu 20, 2012 2:36 am
Posts: 86
Location: Birmingham, Alabama
Hello,
I am starting a new project on a 1934 RCA 124 radio. I want to figure out how to replace the capacitor bank and some of the other capacitors before I get to far into it. The radio has a 4 cap bank consisting of (3) 4 mfd and (1) 10 mfd electrolytics (there is also a stand alone 10 mfd). Someone in the past has replaced 2 of the electrolytic caps, so I am trying to determine what exactly they replaced as the cap bank was left intact. The cap bank has 3 wires (yellow, red, blue) and 1 pin connection on the positive side and 1 pin on the negative side for all 4. I am including some pictures. It looks like they replaced one of the 4 mfds that connects to the pin on the positive side with an 8 mfd (see orange / red cap in picture going across the length of the capacitor bank). However, I am assuming the original cap is still connected inside the bank and they are bypassing it. If that sounds / looks right, then I have a couple other questions.
1 - There is a newer 8 mfd (silver one in the picture) that has been added that connects to pin 5 of the 2A5 tube, which appears to be the cathode and grid 3 (suppresor?) in the tube manual. None of the capacitor bank wires connect directly to this socket. So, I am not sure if it supposed to be replacing a cap from the bank or replacing something else. I had someone else take a look and they thought it may be replacing the 4 mfd that connects to the field coil. But, I have not figured out how the cap bank connects to the field coil.

2 - There is a dual capacitor that connects to the tone control which has 2 pins. The cap has .035 and a .005 value and the 2 cap wires connect to each of the 2 pins. There is only one wire connected to one of the pins that goes to the 2A5 tube. From the chassis diagram (I will include the link below), it looks like the .005 connects to the pin with the wire connected and the .035 connects to the pin with no connection. I would like to be sure as I can't tell from the cap itself which value connects to which pin. Is there a way to tell from the schematic or I am reading the chassis diagram correctly?

I will attach some pictures. Thanks for any help. Replacing the cap bank is a little daunting as it has probably 10 connections on the positive and 6 to the negative. I am not 100% sure what the previous repair replaced.

Here is the schematic link
http://www.nostalgiaair.org/Resources/733/M0014733.htm


Attachments:
File comment: Picture with cap bank and red/orange 8 mfd going the length of bank connecting to the pins on each side
Chassis 3.jpg
Chassis 3.jpg [ 99.65 KiB | Viewed 279 times ]
File comment: Picture with the silver 8 mfd connecting to the 2A5
Chassis 2.jpg
Chassis 2.jpg [ 126.39 KiB | Viewed 279 times ]
File comment: Picture of the dual cap connecting to the tone control
Chassis 1.jpg
Chassis 1.jpg [ 71.96 KiB | Viewed 279 times ]
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 Post subject: Re: RCA 124 schematic - capacitor questions
PostPosted: Sep Thu 28, 2017 3:44 am 
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Joined: Nov Mon 02, 2009 7:01 am
Posts: 2221
Location: Lincoln City, OR
Greetings to rstuddard and the Forum:

I would need more detail to help you with figuring out the capacitor pack replacement. You would have to draw in the connections on a copy of the schematic and scan it or draw your own schematic of the affected area. It is just a little too difficult (for me anyhow) to visualize from a verbal description.

One thing I can say is that the silver 8uF cap is a modification. It is an attempt to perhaps improve bass response? In any event, it defeats the feedback network consisting of the 50K resistor, the 250K resistor and the .1 cap from the cathode of the 2A5 (the component designators are illegible on the schematic). I would lose this and put the circuit back the way it was designed.

Regards,

_________________
Jim T.
KB6GM


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 Post subject: Re: RCA 124 schematic - capacitor questions
PostPosted: Sep Thu 28, 2017 6:44 pm 
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Joined: Dec Thu 20, 2012 2:36 am
Posts: 86
Location: Birmingham, Alabama
Thanks Jim. I think I can figure out the capacitor bank - I am marking the connections with tape and taking notes. I mainly wanted to check on the silver cap (which you gave an explanation for) and the dual cap connected to the tone control.


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 Post subject: Re: RCA 124 schematic - capacitor questions
PostPosted: Sep Thu 28, 2017 8:08 pm 
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Joined: Mar Sat 14, 2009 5:56 pm
Posts: 3826
Location: RI, 02885
If I am seeing it clearly, it appears someone has tacked in newer filters on the old filter connections, leaving the old filter in-circuit. You need to work out exactly what has been done to the circuit before you can address any problems.

_________________
I'm not a hoarder, I'm a caretaker of scarce commodities


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 Post subject: RCA 124 schematic - output transformer replacement
PostPosted: Oct Fri 13, 2017 1:56 am 
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Joined: Dec Thu 20, 2012 2:36 am
Posts: 86
Location: Birmingham, Alabama
Hello, i made progress replacing the capacitors and when I powered the radio up I got no output. When I checked voltages on the plate of the output tube, it was like 7 volts. After research, I checked the output transformer and the primary appears to be open. The schematic shows a 600 ohm primary and maybe .38 secondary (not sure what that means)? The speaker is 3.5. I was looking at some universal output transformers and most start at 1200 ohms and go up. Is 600 a typical impedance or is it on the lower side for the older field coil speakers? I found a 119da transformer that has 600 ohms. Do you think it would be a good replacement?
[url]
http://www.radiodaze.com/600-ohm-speaker-matching/[/url]

As FYI, The field coil measured 1070 which should mean it is ok.

Thanks, Ron


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 Post subject: Re: RCA 124 schematic - capacitor questions
PostPosted: Oct Fri 13, 2017 2:01 am 
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Joined: Feb Sat 06, 2016 1:47 am
Posts: 2128
Location: Santee Calif. 92071
The transformer selected seems like a good match. Use the 4 ohm tap.


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 Post subject: Re: RCA 124 schematic - capacitor questions
PostPosted: Oct Fri 13, 2017 2:26 am 
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Joined: Jan Tue 31, 2012 1:55 am
Posts: 2609
Location: Mountains of Mourne. Ireland.
Legible drawings can be found in...
RCA Victor Service Notes 1934 pdf page-153 25.5MB
http://www.americanradiohistory.com/RCA ... ervice.htm

+ a somewhat sketchy tone control wiring schematic can be found in...
RCA Service Data 1932 - 1938 Vol B page-292-B 61.2MB

Rider's RCA 5-54
http://www.americanradiohistory.com/hd2 ... e-0117.pdf

Greg.

Edit: The Complete index of models can be found at the beginning of...
RCA Service Data 1938 - 1942 C


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 Post subject: Re: RCA 124 schematic - capacitor questions
PostPosted: Oct Fri 13, 2017 7:15 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 2139
Location: Lexington, KY USA
After research, I checked the output transformer and the primary appears to be open.

It's a good idea to check all the transformer windings and coils in the set before starting repairs. You may find a show stopper, depending on your budget.

The schematic shows a 600 ohm primary and maybe .38 secondary (not sure what that means)?

That means when you use an ohmmeter to measure the DC resistance these are typical values for a good transformer. These resistance values are on the schematic to help the serviceman check the components.

Is 600 a typical impedance or is it on the lower side for the older field coil speakers?

The 600 ohm number is the DC resistance, not the AC impedance of the transformer winding. The AC impedance will be much higher. Around 10X higher, in most cases. ( Could be 5X or 20X. 10X is a "ballpark" ratio.)

I found a 119da transformer that has 600 ohms. Do you think it would be a good replacement?

No. 600 ohms is far too low a load impedance for the 2A5 output tube. The tube manual suggests 7,000 ohms. This is not critical, close will be good enough. ( Even sort-of close may be fine for a given application. If you have some output transformers already, you might try what you have before buying one.)

Note that for the speaker, the AC impedance will not be so much more than the DC resistance. The schematic mentions 3.3 or 3.5 ohms for the DC resistance. A 4 ohm winding would be OK to drive the original speaker.

Having a field coil has no bearing on the speaker's voice coil impedance. Field coil speakers were made with many different impedances. However, the 3 to 4 ohm range may have been the most popular for many years for household radio speakers.

Ted


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 Post subject: Re: RCA 124 schematic - output transformer
PostPosted: Oct Fri 13, 2017 3:29 pm 
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Joined: Dec Thu 20, 2012 2:36 am
Posts: 86
Location: Birmingham, Alabama
Thanks Greg - Those schematics from those links are way more readable.
Thanks Ted for the for the explanation. I looked up the 2A5 tube and I can see the 7000k Load Resistance. I did some research and found the Hammond universal single use transformers have 5k and 10k connections. I found a classic tone that has a 5k and 8k (it is actually cheaper too). http://www.classictone.net/40-18030.html. Anyone have thoughts on using the classic tone or any other recommendations? There does not appear to be a lot of options out there.
Thanks,
Ron


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