Where Is The Tint Control?
 

I enjoy looking at old TV schematics, in particular the very first color TV schematics. It's interesting to see how circuit designers accomplish processing the signals.

I particularly like to see how the designers adjust the tint. Sometimes it's a pot; sometimes it's a variable capacitor. Sometimes it even looks like an inductor and capacitor combined.

That said, I was looking at the schematic for the first Admiral color TV, model C1617, on the schematic in the Early Television Foundation's technical information and didn't see any control labeled tint or hue. Where is it? Is it C507 in the plate circuit of V502A?

Curious minds want to know.

This is the first time I've looked at this set's schematic.
It has a crystal ringing circuit rather than a phase-locked loop, hence the need for a 3.58 amplifier and limiter. There are many tuned circuits that could affect the tint. I'd guess these were aligned as close to peak response as possible and then C507 was the final factory phase adjustment. It appears to me that there was no customer tint control, unless there was a mechanical shaft that brought the C507 adjustment to the outside.

Has anyone seen one of these in person to say?

By the way, showing the crystal as a diode symbol is a mistake.

Take a closer look at the crystal symbol on higher magnification. It's three parallel lines with the center one being a little bit shorter than the outer two lines. No arrow symbol here. Look at CR301 and CR301, the video and sound detectors. They have the typical arrow pointing toward the cathode in additional to a + symbol.

I worked for Scientific-Atlanta in the 1970s and we used CR for all diodes and XTAL for crystal designations.

Thanks - had to really blow it up to see it on my laptop.

I see a resistor spec I'm not familiar with. The video ouptuts have plate resistors marked 10% NIT. What is NIT?

One thing you can say about this chassis is there was no attempt to reduce parts count.

NI might be non-inductive, but don't know what the T would stand for.

Technology perhaps? :O

no way to know after all these years what they meant. :/

My bet is that it stands for Non-Inductive Technology. Sprague marketed a series of resistors designated NIT. The exact series had the power dissipation before the NIT. I.e., a Sprague Kool Ohm 10 NIT resistor was a 10 watt non-inductive resistor.

In the circuit for the Admiral, these resistors are in series with inductors so it is likely that the circuit designer didn't want any additional unknown amount of inductance added by the wirewound resistor.

It would be great if we could identify someone who owns one of these TVs. Just open the door on the front of the TV and see if there is a control labeled tint or hue.

There definitely a tint control. I was looking at an article on ETF about the C-1617A and it showed the front panel controls. There are four controls between the volume/contrast control and the tuner. The one closest to the volume control is labeled "Color Fidelity". A picture of the front chassis shows this control is not a pot. It has a phenolic strip which supports an aluminum shaft which probably goes to a variable capacitor, perhaps C507. The "Color Intensity" control is located in the lower, lefthand corner, next to the volume control. This control is definitely a pot which agrees with the schematic (R409).

Perhaps that TV is from...

https://www.youtube.com/watch?v=8CtjhWhw2I8

:D

It's also interesting to note that two of the first color TVs, the GE 15CL100 and the Westinghouse HK840CK15, do not have front panel mounted tint controls. The GE had a screwdriver adjusted variable capacitor on the back chassis and the Westinghouse had a flexible shaft mounted on the rear of the set adjusting a transformer core. I think all future sets had a consumer adjustable tint control on the front panel.

Aha! "Sprague" is what was needed for the search. There are some salvaged Sprague NIT resistors in a current listing on ebay marked as Sprague KOOLOHM 10 NIT.

I do think its interesting to note that my CT-100 never, ever, needs a tint control
adjustment. Sometimes it does need a minor color level adjustment. This is especially true if one uses different channels on it. But set even that and "forget" is just OK if the fine tuning is right. But I aligned the tuner very very carefully on every channel, 2-13 and 39.

The 15" Westinghouse color circuits are also pretty stable, and that one doesn't use a crystal oscillator, it's closer to a horizontal AFC circuit. I still believe the tint problems were more on the transmitter side, especially when network feeds or videotape was involved.

I've often wondered why color TV manufacturers didn't use the system similar to Westinghouse. Was it to eliminate yet another control (color hold) for the customer to mis-adjust?

I wonder how the PAL and SECAM systems handled the color subcarrier oscillator? Crystal controlled, I would think.

I always considered GEs system of using the gated burst to ring a tank circuit (that would resonate for longer than a horizontal line) to be the best. No expensive special components (crystals cough) and I've never seen it need adjusting to achieve color sync unlike RCA and Westinghouse sets.

Not to pick nits, but it's actually the reference oscillator, supplying the signal against which the (demodulated) subcarrier (i.e., chroma signal) is referenced.

All the while, the burst signal, coming as as quick "burst" during retrace, locks the reference osc. to a steady 3.58 Mhz. The burst is only a small number of cycles in duration. Then the osc. simply "coasts" on thru the scan period.

The tint (or hue or whatever) generally works by varying the phase of the burst signal.

(Edit.) GE's "ringing tank" is a shortcut method of achieving the same result minus the dedicated reference oscillator.

Ok, let's get clearer on this:

1) Color demodulation for NTSC or PAL needs a reference subcarrier to do the demodulation.
2) The carrier can be generated by an oscillator or a ringing circuit.
3) In the ringing circuit, there is no feedback to make a continuous CW. Therefore, the ringing resonator has to be very high-Q to keep ringing through at least one line time. Thus, a crystal is needed. The ringing decreases in amplitude between burst injections and therefore needs to be followed by a limiter to provide constant amplitude output to the demodulators. If the circuit is high enough Q to ring for several lines, the amplitude of burst and accompanying noise that is applied can be reduced. The circuit then takes more than one line to reach full amplitude when first turned on, but the effects of noisy signals are reduced.
4) An oscillator can be synchronized with the burst by use of a phase detector in a phase-locked loop. This is the most commonly used carrier recovery circuit. The loop filtering has low bandwidth (in the hundreds of hertz) and therefore responds less to noise impulses than if it were wider band.
5)An oscillator can be synchronized by injection of the burst, without use of a phase lock loop. In this case, it is like a ringing circuit with an infinite ring duration. Motorola used this in its single-tube "SODPIL" (Self Oscillating Detecting Phase Injection Lock) color circuit. Performance can be very similar to a PLL.
6)An oscillator can be based on a crystal resonator or an LC resonator. The crystal oscillator can be designed so it is nearly impossible to make it run more than 1 kHz or so off frequency, so that side lock is impossible. (Side lock is when the oscilator is off frequency by an integer multiple of horizontal rate, so that the output is back in phase with the burst when it occurs.)
7)An LC oscillator has to be designed with as high a Q as possible and very low temperature coefficient to prevent it drifting out of lock or even into side lock. An accessible frequency adjustment is a good idea in case it does drift too far.
8) SECAM uses FM chroma carriers instead of quadrature amplitude modulation, so the detection of the chroma is entirely different from NTSC or PAL. For example, SECAM can be subject to high frequency luma components exceeding the FM noise reduction threshold, resulting in bursts of color on strong details.