Thread: Ct-100
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Old 11-21-2013, 03:12 AM
Tom Albrecht's Avatar
Tom Albrecht Tom Albrecht is offline
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Location: San Jose, CA
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Since the set is reasonably functional as a black and white TV at the moment, I'm looking into why there is no color. It looks like this will be a fairly labor-intensive bring-up.

The first problem I noticed is that the chroma reference oscillator does not run properly. It fires up for a few milliseconds, and then goes quiet for about 1/2 second. This cycle repeats continuously. I found that this behavior is occurring because as soon as the reference oscillator produces a signal, it causes the chroma sync phase detector to output a strongly negative signal, which has the effect of shutting down the oscillator (I'm not sure a negative signal to the reactance tube should cause the oscillator to shut down, but it does). Adjusting the tuning of the phase detector transformer has no effect. If I short the output of the phase detector to ground, the reference oscillator runs continuously. I haven't found any bad components under the chassis in the surrounding circuitry (except the 100 meg resistor in the color killer circuit, which I replaced), so I will check the coils and transformers next. Everything is recapped, including most micas at this point.

[new info] - When I short the phase detector output to ground so the reference oscillator runs freely, it turns out that its frequency is 2.47 MHz, not 3.58 MHz. Furthermore, it keeps right on running at 2.47 MHz if you pull out the crystal, and when the crystal is replaced with another 3.58 MHz crystal. So perhaps the tuned circuits do indeed need some attention. Will check that this evening. Maybe the phase detector is fine, simply doing what it does when the reference oscillator frequency is way below target.

Another problem I notice is that if I look at the output of the first video amp on the scope, the chroma sync burst on the back porch of the horizontal sync pulse is not visible. I gather this is probably a video IF alignment issue, causing insufficient bandwidth to pass 3.58 MHz? I suspect that's a big problem, so will do an IF alignment soon. Adjusting the fine tuning doesn't seem to provide any setting where the chroma sync burst comes through, at least not without a general massive increase in noise as I tune beyond the point where the black and white video looks reasonable. I imagine if I can't see the chroma sync burst, there won't be any chroma signal getting through either, so color is not going to happen even if the reference oscillator is brought to life.

Last edited by Tom Albrecht; 11-21-2013 at 10:30 AM.
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