VHF Tuner Alignment: where is the overall oscillator adjustment?

[LukeSimon]

Originally Posted by old_tv_nut

It should be easy to check the coils for opens and also check that the connection to the sharpness control is not open.

BTW, I'm scratching my head about how that sharpness control is supposed to work. Where does the luminance signal go next (which tube and what electrode of that tube)?

The 6LM8 acts as a cathode follower for the luma signal. So luma is output from its cathode and that is input to another luma amplifier. Dialing up the sharpness pot decreases the load resistance of the cathode follower amplifier which reduces its gain but increases its bandwidth.

I will clean the pot. While I am at it, I will try a few alternative tubes. Even though my NOS 6LM8 tests good… in the chassis performance can still vary.

[old_tv_nut]

Turning the sharpness pot does not change the DC bias by any significant amount, because it is in parallel with the 0.5 ohms of L15.

[old_tv_nut]

The sharpness control being open and having no effect is probably what's causing the ringing (echoes) to the right of the sharp letters. L15 and C48 will resonate with a higher, narrower peak in frequency without the 100 ohms in parallel, becoming underdamped and ringing in the time domain.

[Yamamaya42]

According to the handy-dandy LC Tank Circuit Resonance Calculator, L15 & cap are tuned to 2.131MHz, (8.2 mh coil & 680pF cap).

[old_tv_nut]

I don't have the SAMS. Is that where the 8.2 mHy is stated?

Anyway, I assumed 2 Mhz and calculated 9.3 mHy. Using that and using the tube characteristics to calculate the cathode impedances, I calulate that the 39 ohm and the cathode resistances result in an equivalent resistance of 128 ohms in parallel with the sharpness control 100 ohms (total 56 ohms), resulting in a net damping factor of 1.04 (assuming a 9.3 mHY coil). For a 8.2 mHy coil, the damping factor is 0.98, so still essentially 1.0

For either inductance, the peak current gain at the resonant frequency is 0.4426 at resonance, compared to 0.1822 at DC, for a gain increase of 2.43 times or 7.7 dB at resonance compared to DC.

[old_tv_nut]

By the way, if the sharpness control is open, the damping factor is 0.45, meaning underdamped and ringing.

[Yamamaya42]

Originally Posted by old_tv_nut

I don't have the SAMS. Is that where the 8.2 mHy is stated?

Anyway, I assumed 2 Mhz and calculated 9.3 mHy. Using that and using the tube characteristics to calculate the cathode impedances, I calulate that the 39 ohm and the cathode resistances result in an equivalent resistance of 128 ohms in parallel with the sharpness control 100 ohms (total 56 ohms), resulting in a net damping factor of 1.04 (assuming a 9.3 mHY coil). For a 8.2 mHy coil, the damping factor is 0.98, so still essentially 1.0

For either inductance, the peak current gain at the resonant frequency is 0.4426 at resonance, compared to 0.1822 at DC, for a gain increase of 2.43 times or 7.7 dB at resonance compared to DC.

Yes, I looked up the replacement specs in the sams, and it said that L15 was 8.2, (rf choke) along with a list of subs.

[LukeSimon]

I have never seen a tube amp used in this way. It acts as both a common cathode amplifier for chroma output from the plate and a cathode follower amplifier for luma output from the cathode. Wouldn’t the different sharpness setting also change the amplifier characteristics for the chroma signal output from the plate?

[old_tv_nut]

Originally Posted by LukeSimon

I have never seen a tube amp used in this way. It acts as both a common cathode amplifier for chroma output from the plate and a cathode follower amplifier for luma output from the cathode. Wouldn’t the different sharpness setting also change the amplifier characteristics for the chroma signal output from the plate?

I have the same question in my mind. Maybe the peaking frequencies are mostly outside the chroma bandpass - need to see more of the service info and schematic to make a guess.

[old_tv_nut]

The equivalent cathode load of V5A including its internal impedance varies from 214 ohms far away from the resonance to 259 ohms at resonance, so 17% max, probably much less over the chroma band. I'd need to crank up a spreadsheet to plot some curves.

[old_tv_nut]

Result from a spreadsheet:



At minimum sharpness, the curve is flat at 1.0 for all frequencies.

[Yamamaya42]

Any updates/ progress with your maggie?
To quote Spock from Star Trek II : The Wrath of Khan.
https://getyarn.io/yarn-clip/79475c4...f-a2b3598a16ab

I did try to adjust the 4.5 MHz (A11) on mine again, to see if it had any impact on the slight color shadow, it did not, it DID however have a point at where it started to interfere with the color and vertical lock, but really no noticeable improvement in adjusting it other than very slight reduction of herringbone patterns when FT is off tuned.

I did however have one annoying embarrassing incident that made me had to remove the chassis yet again, after I had it all bolted back in , while tuning A11 from extreme to extreme, I went a wee bit too far on the lower, and the damn core fell out the bottom into IF cage, I had to open it to get it out, I did not realize it would do that, I thought it would hit bottom and stop, oh well, no harm done other than a hour or so lost!

[LukeSimon]

Originally Posted by Yamamaya42

Any updates/ progress with your maggie?
To quote Spock from Star Trek II : The Wrath of Khan.
https://getyarn.io/yarn-clip/79475c4...f-a2b3598a16ab

I did try to adjust the 4.5 MHz (A11) on mine again, to see if it had any impact on the slight color shadow, it did not, it DID however have a point at where it started to interfere with the color and vertical lock, but really no noticeable improvement in adjusting it other than very slight reduction of herringbone patterns when FT is off tuned.

I did however have one annoying embarrassing incident that made me had to remove the chassis yet again, after I had it all bolted back in , while tuning A11 from extreme to extreme, I went a wee bit too far on the lower, and the damn core fell out the bottom into IF cage, I had to open it to get it out, I did not realize it would do that, I thought it would hit bottom and stop, oh well, no harm done other than a hour or so lost!

I have been reading the manual for my B&K 415. It has a chapter at the end that explains RF, IF, and chroma alignment for a couple models of TVs. Luckily, the first example explains the steps for the Magnavox T933 by showing the alignment steps called for by Magnavox in their OEM service manual and comparing how it is done using the B&K 415.

Both OEM alignment steps and B&K alignment steps are equally thorough. The B&K steps just involve less test equipment. It involves tuning coils and pots on the RF tuner, the IF board, and the chroma board. There is no way this can be done without a sweep and mark generator. Also, the steps in the Sams Photofacts are not as thorough as the OEM and B&K steps. The quality of the alignment wouldn’t be nearly as good if the Sams guide was used.

For example, the sweep generator sends a 400hz waveform through the RF signal and minimizing the amplitude of that in various parts of the IF circuit is how the sound traps are tuned. Much more scientific than the Sams guide. The bode plots provided also differ a bit from those in the Sams.

I noticed the same thing for Sams dynamic convergence steps, which are also inferior compared to the OEM steps. Ditto for the chroma AFC alignment which is very bad in the Sams. The hand written notes in the scanned copy of the Sams are a big improvement. Definitely makes me lose respect for Sams. OEM and B&K guides are superior.

The thing that worries me is the fact that some of the IF coils use two cores: a top core and a bottom core. Each must be separately adjusted. I have many alignment tools, but I worry that I don’t know how to “feel” my way between the top and bottom coil.

[LukeSimon]

Let’s just say that the IF sweep response clearly shows why chroma was smearing. My oscilloscope has the bode plot mirrored horizontally. So the chroma marker is on the right hand side.



Below is the result of my first IF alignment. Note that the achieved gain is 10x more than the curve above. This took hours, and was very frustrating, just like my first time converging a delta gun.




I am going to take another pass at it on another day after I decompress from the stress. It is pretty close to matching the reference curve, but I am sure I can dial in the sound traps a bit better and improve the response shape. Then I will align the chroma bandpass, and finally I will align the RF end to end.

[Yamamaya42]

From what I have seen of the videos posted to u-tube about this, there is no such thing as a “perfect” IF or chroma bandpass alignment, just varying degrees of ALMOST there!