Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently May Sun 28, 2017 11:14 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 38 posts ]  Go to page Previous  1, 2
Author Message
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Tue 03, 2017 7:15 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 25226
Location: Detroit, MI USA
They weren't reliable when they were new, so why should we expect them to be now? After restoration, it doesn't seem to matter if you use them every day, or let them sit. Eventually there will be problems, usually minor but always annoying

_________________
Dennis

Experience is what you gain when the results aren't what you were expecting.


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Tue 03, 2017 7:15 pm 
Member
User avatar

Joined: Nov Thu 08, 2007 2:44 am
Posts: 2716
Location: Martha's Vineyard, Mass.
M3-SRT8 wrote:
It's like caring for a spoiled brat. 8)


Actually, it's like caring for a very old friend. That's better. 8)

_________________
“You can always count on the Americans to do the right thing after they have tried everything else.”


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Tue 03, 2017 7:21 pm 
Member
User avatar

Joined: Nov Thu 08, 2007 2:44 am
Posts: 2716
Location: Martha's Vineyard, Mass.
Mr. Detrola wrote:
They weren't reliable when they were new, so why should we expect them to be now? After restoration, it doesn't seem to matter if you use them every day, or let them sit. Eventually there will be problems, usually minor but always annoying


True. They are a challenge.

I'm usually deluded into believing that if I carefully recap using picked/tested/high quality parts, replace all OOS resistors, clean, test/replace all tubes, etc etc, then I've created a "Mil Spec" chassis, and expect superior performance.

It just doesn't work out that way with these very early TVs.

Oh, well. I asked for this madness. My biggest problem is that after a thorough and careful restoration I take any shortcoming in performance as a personal failure.8)

_________________
“You can always count on the Americans to do the right thing after they have tried everything else.”


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Tue 03, 2017 7:34 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 25226
Location: Detroit, MI USA
Doing the restoration properly does get you superior performance, there's never been any doubt about that. It's just that the best technology of the day isn't as reliable as we would like it to be sometimes. Kind of like the modern TV's that people throw away every few years out of frustration........

_________________
Dennis

Experience is what you gain when the results aren't what you were expecting.


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Tue 03, 2017 11:57 pm 
Member

Joined: Sep Tue 15, 2015 9:25 pm
Posts: 208
M3-SRT8 wrote:
Mr. Detrola wrote:
They weren't reliable when they were new, so why should we expect them to be now? After restoration, it doesn't seem to matter if you use them every day, or let them sit. Eventually there will be problems, usually minor but always annoying


True. They are a challenge.

I'm usually deluded into believing that if I carefully recap using picked/tested/high quality parts, replace all OOS resistors, clean, test/replace all tubes, etc etc, then I've created a "Mil Spec" chassis, and expect superior performance.

It just doesn't work out that way with these very early TVs.

Oh, well. I asked for this madness. My biggest problem is that after a thorough and careful restoration I take any shortcoming in performance as a personal failure.8)



Oh well I do too. I think we all do. That's why there are forums like this in the world!

George


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Wed 04, 2017 1:23 am 
Member
User avatar

Joined: Nov Mon 02, 2009 7:01 am
Posts: 1979
Location: Lincoln City, OR
Greetings to jimmc and the Forum:

Jim writes:

Quote:
I Don't recall which FM discriminator the 721 uses but the 621TS and other early RCA sets used the Foster-Seely circuit which is a crappy discriminator circuit.


I must take issue with the above. A properly designed and implemented Foster-Seely discriminator circuit is the best FM detector there is, short of a crystal discriminator. The reason that they are not found on later TV sets and cheaper FM radios is that the transformers are more expensive to produce. The Ratio Detector is much cheaper to produce and, as in most other fields, you get what you pay for. A Ratio Detector performing at optimum is marginal, and when mis-tuned (as most of them are) the results grate on the ears at best. Ratio detectors are the reason that sync buzz is so prevalent in TV sets.... a proper Foster-Seely circuit with a good limiter ahead of it will not buzz unless the video is over-modulated.

Just my $.02 worth.

Regards,

_________________
Jim T.
KB6GM


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Wed 04, 2017 1:36 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1224
Location: Phoenix, Arizona
Could you describe the difficulty in constructing the F-S transformer Please.

I always like to learn more.

The two circuits look similar until you see them side by side.

My only expierance with the F-S is in my 621 and it is quirky to tune in.

I was led to believe the F-S has poor noise immunity and the 621 is low on IF stages.

That with the fact that TV stations used to run 20% aural power until the early 50's when the FCC allowed 10% aural power.

And the early sets have no AGC.

I am aware that early TV sets ran the filiment voltage on the discriminator low to get rid of noise.

My 621 has plenty of audio level when tuned in.

Also the coil on the tuner has to be properly aligned as it is the sound pickoff as well as the sound trap.
Jim


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Wed 04, 2017 3:13 am 
Member
User avatar

Joined: Nov Thu 08, 2007 2:44 am
Posts: 2716
Location: Martha's Vineyard, Mass.
Did a bit of troubleshooting of the Video stages. Replaced a couple of suspect 270pF micas and V103 and V104.

Not bad. Not quite 630TS territory, but quite watchable. Sound is good.

Image

Image

_________________
“You can always count on the Americans to do the right thing after they have tried everything else.”


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Wed 04, 2017 8:57 am 
Member
User avatar

Joined: Nov Mon 02, 2009 7:01 am
Posts: 1979
Location: Lincoln City, OR
Greetings to Jim and the Forum:

It has been awhile since I looked into the differences, so I had to do a bit of research. The added expense is incurred not in the discriminator itself, but in the fact that the Foster Seeley discriminator must be preceded by a limiter stage; the Ratio Detector is relatively insensitive to AM and does not require a limiter.

Here's what Wikipedia has to say about it:

Quote:
The ratio detector is a variant of the Foster-Seeley discriminator, but one diode conducts in an opposite direction, and using a tertiary winding in the preceding transformer. The output in this case is taken between the sum of the diode voltages and the center tap. The output across the diodes is connected to a large value capacitor, which eliminates AM noise in the ratio detector output. The ratio detector has the advantage over the Foster-Seeley discriminator that it will not respond to AM signals, thus potentially saving a limiter stage; however the output is only 50% of the output of a discriminator for the same input signal. The ratio detector has wider bandwidth but more distortion than the Foster-Seeley discriminator.


I was interested in your mention of being in TV broadcasting. I am a retired broadcast engineer. I worked for CBS Television for 32 years, mostly maintaining the transmitter plant for KCBS-TV on Mt. Wilson (above Los Angeles). I remember back in the NTSC days how careful we were to ensure that the luminance didn't go over 100 IRE and that the proc amp clippers were set for about 104 IRE.... all to make sure that we didn't have intercarrier induced sync buzz.... then of course, the cable TV people would mangle the signal with over white text insertion and the like.... and our signal sounded like it was being produced by a bunch of amateurs.

Ahh.... the good old days....

Regards,

_________________
Jim T.
KB6GM


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Wed 04, 2017 4:33 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1224
Location: Phoenix, Arizona
I'm going to consult my library and see if I can learn a little more about FM discriminators myself.

I started working in television broadcast in 1961 and I still work in TV Engineering Management for a large broadcast group which has a couple of stations on Mt. Wilson I have no plan to retire. .

Back in the GOOD NTSC days we tend to forget the visual portion of TV was actually AM modulation. The visual carrier was at 1.25 MHz from the lower edge of the 6MHz wide TV channel . The lower sideband was filtered off by the lower edge of the channel. Video was mostly filtered off by the 4 MHz point as the Aural signal was an FM carrier its center frequency to be exactly 4.5 MHZ above the visual carrier. Aural carrier was phase locked to the visual carrier. The visual and Aural were essentially two seperate transmitters.

Analog composite video was moved around a TV studio as 1 volt peak to peak with sync going negative. We used the IRE scale on our waveform monitors so the overall video of 140 IRE units had peak white at 100 IRE which equaled 0.714 volts. Sync went negative 40 IRE or -0.286 volts.

The NTSC transmitter is modulated such that the Tip of sync causes the transmitter to put out its full licensed power, and peak white video is 12% of full power.

In the early transmitter days we watched carefully that we didn't sit in black too long as that forced the transmitter to run hard. and the plates to start glowing red.

In the early receiver s the Visual and Aural IF’s were totally separated at the tuner using the magical “RCA coil” which mostly trapped out the aural and blocked out the visual such that the AM noise was not present at the audio discriminator.

I believe both the Foster-Seely and the Ratio Det. Utilized a limiter as the final aural IF, the problem being in the 621 and 721 there wasn’t sufficient aural to saturate the limiter.

Back to ATSC . . . . Jim


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Thu 05, 2017 1:17 am 
Member
User avatar

Joined: Nov Mon 02, 2009 7:01 am
Posts: 1979
Location: Lincoln City, OR
Greetings to Jim and the Forum:

Yes... I am very familiar with the modulation scheme, although I don't recall having the visual and aural carriers phase-locked together. As I recall, the FCC spec was +/- 1 KHz for either carrier, but the intercarrier difference had to be maintained within +/- 500 Hz. Our exciters were phase locked to an external reference. When I first got there, this was a Motorola high accuracy 1 MHz crystal oscillator, with a chart recorder to compare long-term trends to WWVB at 60 KHz. Later, we went to Rb atomic standards. When the NTSC plant got scrapped out, I inherited both atomic standards... they are still on a shelf in my garage.

I have attached a "Pulses" log sheet from KCBS for a journey down memory lane. I had to do one of these once a week.

Attachment:
KCBS Pulses jpg R2.jpg
KCBS Pulses jpg R2.jpg [ 114.35 KiB | Viewed 617 times ]


I am quite familiar with the inverted AM modulation scheme for NTSC. We did our power meter calibration using a calorimeter. I remember once having to calculate the average power for black plus sync to ensure that we were performing the meter cal correctly.

Regards,

_________________
Jim T.
KB6GM


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Thu 05, 2017 1:43 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1224
Location: Phoenix, Arizona
The calorimeter methode was a big PITA in my opinion, but we had to do it occasionally, usually at 02:00 in the morning. We had to sit in black forever and watch the temperature spread on a couple of precission thermometers.

I don't recall the math but once you had average power you multipliet that by 1.68 to get peak power.

Now I have a calibrated HP meter and it reads average power using a -50 dB coupler and calculates the offset and we can do it during the news.

The lock between aural and visual came about after TV stereo.

I have a exciter I salvaged from a discarded transmitter. nice 2 RU frame. I have a little Extron routing switcher and the plan was to feed composit video and audio from several converters. Drake made cheap converters. I had the thing connected to an AMX control systen so I could route whatever into my Tx which was feeding my house antenna system.

Jim


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Thu 05, 2017 3:25 am 
Member
User avatar

Joined: Nov Mon 02, 2009 7:01 am
Posts: 1979
Location: Lincoln City, OR
Greetings to Jim and the Forum:

We were running BTSC stereo; hence the ICPM measurement on the above sheet. Our exciters were not locked together, exactly, but they were both locked to the same external standard, so in essence they were phase-locked.

We did have to maintain tighter frequency control because we were channel 2 and were required to use PFC (Precise Frequency Control) to minimize beat type interference to other channel 2 stations during periods of sporadic E propagation. I can't recall the tolerance for this system, but it was much tighter than the +/- 1 KHz for other stations.

We used an HP uWave power meter and the calibrated couplers for measuring power once we converted to ATSC. We had a calorimetric system installed for our channel 60 transition plant, but were never able to make it work properly due to the design of the Harris Sigma CD ++ transmitter system. The external heat exchangers for the liquid cooling system had fans that were controlled by a thermostat attached to the cooling plumbing. Thus, the fans cycled on and off during normal operation and the coolant temperature cycled up and down accordingly. We were never able to stabilize the temperatures long enough to obtain an accurate reading, so we finally gave up on the system.

Regards,

_________________
Jim T.
KB6GM


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Thu 05, 2017 3:59 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3681
Location: WI 54812
jimmc wrote:
Could you describe the difficulty in constructing the F-S transformer Please.

I always like to learn more.

The two circuits look similar until you see them side by side.

My only expierance with the F-S is in my 621 and it is quirky to tune in.

I was led to believe the F-S has poor noise immunity and the 621 is low on IF stages.

That with the fact that TV stations used to run 20% aural power until the early 50's when the FCC allowed 10% aural power.

And the early sets have no AGC.

I am aware that early TV sets ran the filiment voltage on the discriminator low to get rid of noise.

My 621 has plenty of audio level when tuned in.

Also the coil on the tuner has to be properly aligned as it is the sound pickoff as well as the sound trap.
Jim


What happened in the early 50's that inspired the FCC to lower the aural power limit? I've been searching through servicing texts of the period hoping to find mention of that change and how it affected performance of early receivers with no success. I'd be very interested in any source materials that would explain that situation in greater detail.


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Thu 05, 2017 5:49 pm 
Member

Joined: May Fri 29, 2009 4:35 am
Posts: 1912
Location: Chicago, IL USA
I scanned the RCA 721TCS service data and it is available to download courtesy of the Early Television Foundation. Owners manual coming soon.

http://www.earlytelevision.org/pdf/RCA_ ... e_Data.pdf


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Thu 05, 2017 10:16 pm 
Member
User avatar

Joined: Nov Thu 08, 2007 2:44 am
Posts: 2716
Location: Martha's Vineyard, Mass.
bandersen wrote:
I scanned the RCA 721TCS service data and it is available to download courtesy of the Early Television Foundation. Owners manual coming soon.

http://www.earlytelevision.org/pdf/RCA_ ... e_Data.pdf


Ah. Thank you, Bob. 8)

_________________
“You can always count on the Americans to do the right thing after they have tried everything else.”


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Thu 05, 2017 10:47 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1224
Location: Phoenix, Arizona
I just downloaded the service data for the 721 and I notice a connection from one of the diode plates of the 6AT6 audio amp headded off to the left.

Ends at the contrast pot.

The schematic is cut off so I cant see exactly what is changed. Maybe an attempt at AGC?

I have the schematic for the 621 in front of me and there is no connection at all from that same spot.

Jim


Top
 Profile  
 
 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Fri 06, 2017 3:59 pm 
Member
User avatar

Joined: Nov Thu 08, 2007 2:44 am
Posts: 2716
Location: Martha's Vineyard, Mass.
They are different. Even the 721TCS differs from the TS.

_________________
“You can always count on the Americans to do the right thing after they have tried everything else.”


Top
 Profile  
 
Post New Topic Post Reply  [ 38 posts ]  Moderators: 7jp4-guy, Mr. Detrola Go to page Previous  1, 2

All times are UTC [ DST ]


Who is online

Users browsing this forum: Pearson, wiseguy and 8 guests



Search for:
Jump to:  




















Privacy Policy :: Powered by phpBB