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 Post subject: old tv no audio
PostPosted: Nov Sat 16, 2019 2:56 pm 
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Joined: Nov Sat 16, 2019 2:14 am
Posts: 4
hi new member,i have a 1946 rca kcs 27-1 good pix no audio just noise can send 21.25 signal at grid of ist sound if very good aligned sound, also checked rf alinment ok.
tried 2 converter boxes question is are these sets not able to reproduce audio from converter box ? i am stumped.
forrest.


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 Post subject: Re: old tv no audio
PostPosted: Nov Sat 16, 2019 4:52 pm 
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Joined: Jun Thu 25, 2015 3:21 am
Posts: 1352
i dont have a schematic in front of me but going by the age of that set it is probably a split sound chassis. if it is the video and audio have seperate if tubes. this means the video can get through fine if the video if is ok and there could still be a problem with the audio if.

to the best of my knowledge a converter box will work with any ntsc television. i have no experience with a television this old however. tvs from the 1940s were retired a long time before i studied electronics repair.

what tube did you inject your if signal into and how did you inject it? try injecting a signal into the point where the tuner connects to the if in the set and see if you can hear it. if you can your signal must be being blocked in the tuner. if not move forward till you can hear the signal. this should point to the defective stage. then voltage resistance measurements will point to which part of that stage is not performing properly and finally individual component testing will find the bad part.

i seem to recall reading that a split sound set was hard to get it aligned properly to get the video and audio to work well together.


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 Post subject: Re: old tv no audio
PostPosted: Nov Sat 16, 2019 6:01 pm 
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Joined: Nov Sat 16, 2019 2:14 am
Posts: 4
thank you thomas for geting back to me. i can inject 21.25 mc at take off point at tuner and get strong strong signal out i have checked sound if alignment aligns very good, also checked tuner alignment seems to be good, has any one had this problem before?
forrest


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 Post subject: Re: old tv no audio
PostPosted: Nov Sat 16, 2019 6:26 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2006
Location: Lafayette, CO
You might adjust that transformer at the takeoff. Is this the tuner with channel one? Craig


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 Post subject: Re: old tv no audio
PostPosted: Nov Sat 16, 2019 6:46 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3999
Location: WI 54812
Take off area from Sams 70-7


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 Post subject: Re: old tv no audio
PostPosted: Nov Sat 16, 2019 7:23 pm 
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Joined: Mar Sun 01, 2009 10:27 pm
Posts: 5051
Location: Ann Arbor, MI
Split sound sets do work with converter boxes. You have to adjust the fine tuning so that the converted sound carrier ends up at 21.25 MHz. It will tune much like tuning an FM radio. If the fine tuning does not have enough range they you may have to adjust the main oscillator tuning. Hopefully the picture will look good with the tuning set for the sound. If not, some fudging of the sound alignment may be necessary.

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Tom


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 Post subject: Re: old tv no audio
PostPosted: Nov Sat 16, 2019 9:13 pm 
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Joined: Nov Sat 16, 2019 2:14 am
Posts: 4
thanks for the replys,
i think i mite try changing the 62 mmf on the take off coil and try aligning again.
forrest


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 Post subject: Re: old tv no audio
PostPosted: Nov Sun 24, 2019 8:34 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1572
Location: Phoenix, Arizona
I watch an RCA 621TS regularly using a converter box and have no problems with sound.

RCA's model number system is.... 621-TS means 1946, 21 tubes, considered the first post war #2 set.

630-TS means 1946, 30 tubers, I know there was a 721 but I'm not going to question your number.

The first 1940's sets use what is called split system audio. RCA used an interesting coil on top of the tuner that has the outer winding tuned for Picture IF frequency and down inside is another hidden coil tuned for audio IF frequency. This inner coil also serves as a sound trap as it drains off all the audio signal and sends it to the sound IF It has an interesting core which is I believe the first use of Ferrite in cores.

The next issue is these sets were designed for 20% Aural carrier from the TV Transmitter and I know the 621-TS only has a single stage of audio IF. The next problem is these sets use what is called a Foster/Seeley FM discriminator which does not work as well as the later ratio detector. The circuit looks almost the same.

The fine tuning has to be dead on to get audio but video looks fine if mis-tuned.

I have a couple of the $40 coupon set top converters and I find the sound to be crappy on these.

The alignment has to be right on starting with the tuner or sound goes to hell first. I believe the only audio IF also acts as a limiter and the FS discriminator is touchy about AM noise.


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 Post subject: Re: old tv no audio
PostPosted: Nov Sat 30, 2019 1:49 pm 
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Joined: Nov Sat 16, 2019 2:14 am
Posts: 4
Hi thanks for reply,
I changed 62 mmf on take off coil and realined take off coil and finally got audio you are rite it tunes like an fm radio
I started in in the business in 1960 never ran into this system before.
Thank you.


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