Newbie's First Questions RCA TS 721 Restoration

[Chris K]

...and here's the schematic...half of it anyway

[Yamamaya42]

Originally Posted by Chris K

So my RF generator is set at 21.25MHz. The grid on the 1st audio tube, 6AT6, is pin 1. Do I need to ground the black generator output lead or just touch pin 1 of the 6AT6 with the red lead from the Knight? Sorry for the very basic questions here but this is virgin soil for me!

this set is very close to my FADA 895, which is a RCA630TS clone.

the 6AT6 is the 1st audio level amp, it also serves as a bias clamper ( diode ) as this set is pre-AGC and has a " picture control" like my FADA,

like someone mentioned above to test the audio out section, you can simply clip on to the center wiper of the volume control, and feed in any sine wave in the audible range, to test it. this will also insure that you are on the isolated side of the grid via the .01 cap , no risk of loading loading thing down hooking up to that point.

[Yamamaya42]

I'm using this!
https://www.earlytelevision.org/pdf/..._rider_tv1.pdf

[Chris K]

Originally Posted by Yamamaya42

this set is very close to my FADA 895, which is a RCA630TS clone.

the 6AT6 is the 1st audio level amp, it also serves as a bias clamper ( diode ) as this set is pre-AGC and has a " picture control" like my FADA,

like someone mentioned above to test the audio out section, you can simply clip on to the center wiper of the volume control, and feed in any sine wave in the audible range, to test it. this will also insure that you are on the isolated side of the grid via the .01 cap , no risk of loading loading thing down hooking up to that point.

OK...more idiot me on display...my Knight's lowest output frequency is about 160 kilocycles...out of audible range. Is there something else I can do???

[Chris K]

I'm not getting anything...even poking a screwdriver around and across the volume control's 3 terminals there's not a sound...no snap, crackle or pop...nothing. There isn't even a hum.

[Yamamaya42]

Originally Posted by Chris K

OK...more idiot me on display...my Knight's lowest output frequency is about 160 kilocycles...out of audible range. Is there something else I can do???

as was mentioned before, you should be able to crank it up and touch the center terminal of the volume cont with a screwdriver while touching the metal shaft and hear the telltale 60 cycle hum, like touching the unconnected end of an audio cable.


https://www.youtube.com/watch?v=r-WCPDdn9mk
My FADA in action!

[Chris K]

Originally Posted by Yamamaya42

I'm using this!
https://www.earlytelevision.org/pdf/..._rider_tv1.pdf

That's the one. Sam's has one too that's a little different. It's this chassis and a radio in a combo cabinet unit. I've found the Sams helpful but it's too different to follow verbatim

[Chris K]

Originally Posted by Yamamaya42

as was mentioned before, you should be able to crank it up and touch the center terminal of the volume cont with a screwdriver while touching the metal shaft and hear the telltale 60 cycle hum, like touching the unconnected end of an audio cable.


https://www.youtube.com/watch?v=r-WCPDdn9mk
My FADA in action!

The FADA is so crisp and bright. Just beautiful! I left a comment on YT! My 721 is audio dead from the volume control to the speaker. Volume control ohms out at 1 meg and the audio output transformer ohms out at 0.5k.

[Electronic M]

Isn't there 2 outputs on that knight (one audio one radio frequency)? There should be a fixed frequency audio output (something like 400Hz) that you can use to inject audio.

If there isn't an AF output on the Knight, do you have a junky (something won't be sad if you accidentally break it) radio or audio player (battery powered is preferable) with a working headphone or line output? You can make a cable that plugs into the output of your audio test source and has alligator clips on the other end. Put a roughly .1uF cap in series with the audio hot lead and feed that to the grid, clip audio ground to the chassis.

Alternately if you want even simpler just use a roughly .1uF capacitor to connect the heater hot lead to the grid...All you'll get is 60Hz hum, but it's a simple method to use when you don't have test equipment or sacrificial audio gear on hand.

[Chris K]

Originally Posted by Electronic M

Isn't there 2 outputs on that knight (one audio one radio frequency)? There should be a fixed frequency audio output (something like 400Hz) that you can use to inject audio.

If there isn't an AF output on the Knight, do you have a junky (something won't be sad if you accidentally break it) radio or audio player (battery powered is preferable) with a working headphone or line output? You can make a cable that plugs into the output of your audio test source and has alligator clips on the other end. Put a roughly .1uF cap in series with the audio hot lead and feed that to the grid, clip audio ground to the chassis.

Alternately if you want even simpler just use a roughly .1uF capacitor to connect the heater hot lead to the grid...All you'll get is 60Hz hum, but it's a simple method to use when you don't have test equipment or sacrificial audio gear on hand.

Hi Tom...you're right about the Knight. There's an RF out and an audio in/out as well. Anyway, I ran the heater voltage to the grid through the capacitor and nothing. The only noise this speaker has made is when I connected the audio output transformer leads to a 9v and the speaker crackles and pops as I touch the battery poles. So the fault, or the first fault, is somewhere between the first audio and the output leads of the AOT I would think.

[Yamamaya42]

That should be a start point then in tracking down the lack of sound, even though the linked Rider's may or may not be exact to what you have, the voltages shown should be close enough to give you a hint of the problem.

be it a bad tube, or resistor or cap or what ever.
the voltages on the 6K6 and 6AT6 should be pretty close to matching what you see in the schematic.

[Chris K]

Originally Posted by Yamamaya42

That should be a start point then in tracking down the lack of sound, even though the linked Rider's may or may not be exact to what you have, the voltages shown should be close enough to give you a hint of the problem.

be it a bad tube, or resistor or cap or what ever.
the voltages on the 6K6 and 6AT6 should be pretty close to matching what you see in the schematic.

So looking at the Riders and the audio output 6K6, I'm looking at 6.3 AC on pins 8, 7 and 2, -14 volts DC on pin 5, 185 volts DC on pin 3 and 200 volts DC on pin 4? Do you concur?

[Yamamaya42]

Originally Posted by Chris K

So looking at the Riders and the audio output 6K6, I'm looking at 6.3 AC on pins 8, 7 and 2, -14 volts DC on pin 5, 185 volts DC on pin 3 and 200 volts DC on pin 4? Do you concur?

pins 8 & 7 are tied to gnd so they are 0v , pin 2 is heater at 6.3v AC, pin 4 (ctrol grid) is tied to bias & the red wire of the output via 1k resistor to +225, should be about 200v, pin 3 is hooked to pin 3, plate and will be lower because of the 500 ohms of the output @ 185v, or so they say.
pin 5 is pulled negative via 470k

[Chris K]

Originally Posted by Yamamaya42

pins 8 & 7 are tied to gnd so they are 0v , pin 2 is heater at 6.3v AC, pin 4 (ctrol grid) is tied to bias & the red wire of the output via 1k resistor to +225, should be about 200v, pin 3 is hooked to pin 3, plate and will be lower because of the 500 ohms of the output @ 185v, or so they say.
pin 5 is pulled negative via 470k

Learning so much here...thank you very much. I'll get to work!

[Yamamaya42]

that 1k feeding power to the plate should be a rather big one, 5w or larger, and they tend to fry easily.