Newbie's First Questions RCA TS 721 Restoration

[Yamamaya42]

as mentioned, make sure the voltages are OK, AND, if the paper caps in the area have not been replaced, DO IT...

Leaky caps are the fastest way to kill audio output, and they are hard to test w/o a leakage tester.

[Chris K]

Originally Posted by Yamamaya42

as mentioned, make sure the voltages are OK, AND, if the paper caps in the area have not been replaced, DO IT...

Leaky caps are the fastest way to kill audio output, and they are hard to test w/o a leakage tester.

All of the wax/paper caps in the set have been replaced with new film capacitors throughout the TV. The only place I changed out the micas is in the horizontal adjustment section so any micas in the audio circuits are still in place.

[Notimetolooz]

The voltages you measured around the 6K6 seem pretty good. Have you tested and replaced tubes in this set?
What about the voltages on the 6AT6 pins.

A bit of clarification on how the sound stages work. 6K6 is the audio output tube, it is responsible with developing the power to operate the speaker. Most of the 6AT6 is used as a audio amplifier, it increases the voltage size (amplitude) of the audio signal. The 6AL5 is the FM discriminator (detector or demodulator). Before the 6AL5 the signal is 21.25 MHz with FM audio modulation, after the discriminator the sound is in the audio range. That makes the volume control a good place to try injecting audio. The audio stages are easier to troubleshoot than the IF.
You need a bigger audio signal at the grid of the 6K6 than the 6AT6 grid to hear the same volume.
The general way to determine the wattage required for a resistor is to use the resistor value and the voltage difference across it. That 1K resistor that feeds the screen of the 6K6 and then feeds the plate through the output transformer has 25 volts across it. Voltage squared, divided by the resistance is the wattage given off (dissipated), that come out to 0.625W. You always want to use a resistor rated two or more times the dissipated wattage so it stays fairly cool. So a 2W should be OK.

[Chris K]

Originally Posted by Notimetolooz

The voltages you measured around the 6K6 seem pretty good. Have you tested and replaced tubes in this set?
What about the voltages on the 6AT6 pins.

A bit of clarification on how the sound stages work. 6K6 is the audio output tube, it is responsible with developing the power to operate the speaker. Most of the 6AT6 is used as a audio amplifier, it increases the voltage size (amplitude) of the audio signal. The 6AL5 is the FM discriminator (detector or demodulator). Before the 6AL5 the signal is 21.25 MHz with FM audio modulation, after the discriminator the sound is in the audio range. That makes the volume control a good place to try injecting audio. The audio stages are easier to troubleshoot than the IF.
You need a bigger audio signal at the grid of the 6K6 than the 6AT6 grid to hear the same volume.
The general way to determine the wattage required for a resistor is to use the resistor value and the voltage difference across it. That 1K resistor that feeds the screen of the 6K6 and then feeds the plate through the output transformer has 25 volts across it. Voltage squared, divided by the resistance is the wattage given off (dissipated), that come out to 0.625W. You always want to use a resistor rated two or more times the dissipated wattage so it stays fairly cool. So a 2W should be OK.

I'll start my response as I will probably start all of my responses for the foreseeable future...Thank you so much!

The description of the roles of both tubes makes sense to me now. I work on the east coast of the US and live just outside Philadelphia PA in the suburbs. I'm at my day job right now, cheating my employer out of some time and money monitoring VK!!! I will check and report on the voltages on all pins of both tubes tonight. Both of them are tested old stock replacements I purchased from Antique Electronic Supply so they tested as strong tubes worthy of resale by AES in fact, I replaced almost all of the tubes in the set as well as all the wax/foil/paper caps and the electrolytics...and, as I said previously, some of the mica caps with exact value and voltage replacements...not an inexpensive change.

The resistor we are discussing is an original as I have not replaced any resistors yet except for the large, ceramic, dual bleeder resistor(s) on the back of the chassis. The 1K measures within 10% and it is considerably larger than most of the other resistors. I believe the Riders has no 1K resistors, other than the bleeders, listed at more than 1 watt. I'll do the voltage drop measuring and the calculation you described and see what I get.

The ceramic bleeder resistor was fascinating and beautiful but, unfortunately, one side was open. It's actually 2 resistors and not a strip with terminals coming off at resistive values along its length. The values are 510 ohms and 700 ohms by the Sams and 1125 and 610 by Riders. I went with the Sams resistance specs because the working side of the original gave me a reading of 507 ohms, a lot closer to 510 than to 610. The Sams lists them at 10 watts and the Riders lists them at 20 watts. I mounted 10 watt resistors at the Sams resistance values and the 700 kept blowing out. I did not mount it to the resistor housing however so there was no heat sink. 20 watt wirerwounds are hard to come by in exact values so I purchased two 15 watt resistors with matching values (Sams 700 and 510) from Mouser and flush mounted them directly to the large resistor box with heat conducting paste. The box gets hot but it seems to be doing its job as a heat sink adequately and I've had no issues with the bleeders.

Sorry for the long post! I'm stealing money by the letter!

[Chris K]

Originally Posted by Notimetolooz

The voltages you measured around the 6K6 seem pretty good. Have you tested and replaced tubes in this set?
What about the voltages on the 6AT6 pins.

A bit of clarification on how the sound stages work. 6K6 is the audio output tube, it is responsible with developing the power to operate the speaker. Most of the 6AT6 is used as a audio amplifier, it increases the voltage size (amplitude) of the audio signal. The 6AL5 is the FM discriminator (detector or demodulator). Before the 6AL5 the signal is 21.25 MHz with FM audio modulation, after the discriminator the sound is in the audio range. That makes the volume control a good place to try injecting audio. The audio stages are easier to troubleshoot than the IF.
You need a bigger audio signal at the grid of the 6K6 than the 6AT6 grid to hear the same volume.
The general way to determine the wattage required for a resistor is to use the resistor value and the voltage difference across it. That 1K resistor that feeds the screen of the 6K6 and then feeds the plate through the output transformer has 25 volts across it. Voltage squared, divided by the resistance is the wattage given off (dissipated), that come out to 0.625W. You always want to use a resistor rated two or more times the dissipated wattage so it stays fairly cool. So a 2W should be OK.

I fed an audio signal through the grid of the 6AT6 and the sound system from there forward works...speaker, AO transformer and tube and the volume control. I fed a 21.5 MHz RF signal through the plate for both pin 2 and 7 of the 6AL5 and didn't get a signal through the speaker with either. Am I doing something wrong here? I tried about 4 different 6AL5s but nothing changed. I'm sure I'm doing something stupid!

[Kevin Kuehn]

Originally Posted by Chris K

I fed an audio signal through the grid of the 6AT6 and the sound system from there forward works...speaker, AO transformer and tube and the volume control. I fed a 21.5 MHz RF signal through the plate for both pin 2 and 7 of the 6AL5 and didn't get a signal through the speaker with either. Am I doing something wrong here? I tried about 4 different 6AL5s but nothing changed. I'm sure I'm doing something stupid!

The 6AL5 Sound Discriminator(detector) plate circuits are not tuned to the 21.25 mhz sound IF. To hear sound you would need to inject your 21.25 mhz carrier with FM audio modulation into the grid of the 1st or 2nd audio IF amplifiers.

[Kevin Kuehn]

If you have Sams 70 folder 7, on page 6 there are instructions for sound alignment using a AM signal generator and VTVM. You should be able to substitute a digital VOM or scope for the vacuum tube volt meter.

[Notimetolooz]

The sound IF is tuned to 21.25 MHz.
The bigger issue in tracing the signal back is that FM discriminators don't work like AM detectors. A discriminator derives the sound by a method involving things like phase comparison which is difficult to break down into smaller blocks of operation and get audio.