ARE LATE 40s/EARLY 50s RCAs PRONE TO LOW VOLUME ISSUES?

[Tubejunke]

I have two old RCA sets. One is an 8T-243 and the other is a TC-127. The 127 has been recapped and the 243 is so ugly that I can't decide what I want to do with it. However, both sets have extremely LOW sound. In that I mean that you must crank up the volume pot to hear the sets in a small room.

The chassis' of this generation of RCA sets are virtually identical and I know that there are several folks here who specialize in them. I'm just curious if this was some sort of common problem. It seems to me that someone here once explained to me that the sound circuit in these sets is very different from the average set. I can't recollect the details. Thanks!

[Cruiseomatic]

I know I may be stating the obvious with my limited experience here but, Try cleaning the pots with contact cleaner. When I work on radios with low to no volume its usually because the vol. pot is so corroded inside with carbon/dirt.

[Tubejunke]

Thanks, but that was covered when I did the re-cap job. There is a tuning coil on top of the chassis that I can turn and make the volume at least somewhat louder, but I am not sure if that would just be the way around the problem and could possibly mess up another component by driving it too hard via the mis-adjustment. On the other hand, that very coil may just BE out of adjustment by some other hand in the past 50 years. I kind of doubt it because when I found both sets there was really no evidence of prior work by another collector.

[GeorgeJetson]

What?.....WHAT??!....I CANT HEAR YOU!!...WILL YOU PLEASE TURN UP THE SOUND!

[Phil Nelson]

I don't know that particular model, but around that time RCA and some other manufacturers used a "split sound" audio system. The audio and video signals have separate chains of IF amplification. Unless both are correctly aligned, the tuning spot where you hear the best (loudest) sound may not be the same as the tuning spot for best picture.

In later designs, there is only chain of IF amplifiers. The audio signal is picked off following that stage, and thus the sound is more likely to coincide with best picture.

Your sets may have other issues, of course, but this is a common problem. Some folks try to improve the sound by tweaking things "by ear." This is how many TVs got messed up in the past -- by people poking their fingers in the back and turning whatever screws they could find. The best cure is to do a realignment with the proper equipment, following the procedure in the service manual.

Phil Nelson

[Tubejunke]

Originally Posted by GeorgeJetson

What?.....WHAT??!....I CANT HEAR YOU!!...WILL YOU PLEASE TURN UP THE SOUND!

Thanks for the help George!

[kx250rider]

I think I remember seeing bad Micamold caps in the audio IF section, in a KCS-27. Those are often overlooked in re-cap jobs since people think that mica caps don't fail, but those aren't micas... They're paper caps disguised as flat rectangular micas.

Charles

[bandersen]

Originally Posted by Tubejunke

...It seems to me that someone here once explained to me that the sound circuit in these sets is very different from the average set. I can't recollect the details. Thanks!

In addition to the split sound IF Phil mentioned, some early RCAs used a Foster-Seely discriminator rather than a ratio detector. Due to a lawsuit with Armstrong I believe.

[TV Engineer]

Originally Posted by bandersen

In addition to the split sound IF Phil mentioned, some early RCAs used a Foster-Seely discriminator rather than a ratio detector. Due to a lawsuit with Armstrong I believe.

RCA NEVER used a limiter-discriminator in ANY of it units. It designed and employed the ratio detector in an attempt to screw Armstrong out of patent royalties, which resulted in a lawsuit by Armstrong against RCA.

If you haven't seen Empire of the Air, I highly recommend it. It tells the entire story in great detail with great accuracy.

RCA, like most manufacturers, used the split sound system for many years, but RCA hung onto it a bit longer than most because intercarrier sound was a General Electric development, and RCA would have to pay patent royalties.

Split sound worked well until the mid 50s because until then, pix carrier power and sound carrier power were equal. In the mid 50s, most stations realized that not as much sound carrier power was needed for equivalent coverage, and went to sound at 50% of visual. Later, in the 60s, it was determined that sound carrier power could be reduced even further to 10-20% of picture carrier power BECAUSE most sets had intercarrier sound by this time. While it did work well with intercarrier sets, each reduction in power resulted in a loss of volume at the split sound receiver.

Most STBs will maintain their sound carrier at the RF output at 10% of the pix carrier level. So, the problem has crossed over in the digtal age.

I hope this helps.

[Username1]

Yup to all of the above. Don't forget the set was aligned with the factory caps, then you replaced them, and they all have a tolerance range, new caps respond to heat differently than the old caps that manufacturers knew was going into a tube tv. You replaced all the caps and the tolerance range will allow frequency centers to be off in all tuned circuits, so replacing all the caps most likely means several items need realignment. When I was a kid I repaired arcade games, and the guy I worked for came across a little thing he wanted to add to all his games. It was a rapid fire button. This little circuit was just a 555 ic timer that sent multiple switch closings to the "fire" circuit of the game. I mass produced a board with the 555IC and the caps, and resistors, made the thing and put them in the games. All the parts for 50 of them came from one source. When I was done, all 50 had different firing frequencies ! ! So know that your caps are most likely part of the problem. And not that you didn't buy the best, just that you changed them all, and in some set designs mentioned above, the tuned circuits will need a touch up.

[Tubejunke]

Thanks folks! I am familiar with sets having the symptoms of sound and video tuning in at two different locations of the fine tuning range. That's not what is going on here. The sound is just very weak. It is clear, and seems to tune in pretty close to the video; it is just very low. I guess when I get time I will have to take a look at replacing the micamold/paper imposters. I feel like that may be the best approach to start with.

[Phil Nelson]

Originally Posted by Tubejunke

The sound is just very weak. It is clear, and seems to tune in pretty close to the video; it is just very low.

I have exactly that symptom with my current project, a DuMont RA-113. I'm going to try realigning the audio IF & discriminator in hopes of increasing the volume.

To practice up, I just tried that on my RCA T-100, which also showed weak volume when I quit working on it a couple of years ago. Tweaking the audio xfmrs made a big difference in the T-100, so tomorrow I'll give that a try on the DuMont.

Before you go there, however, I'd certainly replace any molded paper caps that remain in the set. They're every bit as bad as paper. In fact, they are paper, only in a different shell.

Phil Nelson

[Penthode]

Hi,

I have been following this thread for the past few years after being directed to it from a fellow member. I have a particular interest in the earlier RCAs having worked on a number of split sound sets over the years. I have been a professional broadcast engineer for about the last 30 years and a faithful 8T243 has been my friend for 42 years.

I have no trouble with audio level on any of the split sound RCA sets. I have also never had to replace the mica capacitors in the RCAs as they have always appeared to have been of the best quality. (That has been my experience to date). And because the audio is FM, broadcasters reducing the carrier level slightly will not affect volume since it is the frequency deviation and not the amplitude which affects loudness.

What I have found more critical in these sets is IF alignment. The best test for audio alignment is to just tune in a TV signal. Fine tuning on these sets aim for best sound. As you tune for best sound, you must tune through a null. The process is this: tune towards best sound through a null, the sound is then loudest and clearest and as you continue tuning, you will reach another nulll and the sound will then be weaker and diminish in quality. You must tune between the two nulls and the level will be weaker either side of the nulls and best and loudest between the nulls.

If your split sound set does not tune in this way then I would suspect the "phantom tweaker" has been at your set in the past and has meddled with the alignment. You can still get a bit of audio when misaligned but it will always be weak and not clear.

The last thing which must be mentioned is that the Video IF and Sound IF alignment is linked together and generally a set should have video and audio alignment done together. This is because you have to ensure that the best picture cooincides with the best sound. On the RCAs, the better sets all have four IF stages and a 4 MHz bandwidh and so should provide an excellent picture and sound when properly aligned. It is essential that the video carrier is at the 50% point on the IF response when the the sound system is properly aligned and tuned.

Please be assured that my 8T243 is plenty loud when properly aligned!

I wish you the best success with your set.

Terry

[old_tv_nut]

TV Engineer, thanks for that explanation - I forgot about audio carrier levels changing over the years - probably explains why I have trouble with my CTC-5 sound.

By the way, another reason for set top boxes to have 10% audio is that they sum the video and 4.5 Mhz intercarrier audio and then modulate onto the RF carrier, and the low audio intercarrier level helps prevent 920 kHz beat with the chroma.