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  #1  
Old 03-09-2003, 10:35 PM
michael
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CTC-4 Work Continued

Got frustrated a few months ago and set this aside for awhile. HO plates were going red while bench-testing the chassis even though drive voltage was within 5V of spec. Noted that a 440 ohm resistor was between pins 3 & 6 in some schematics, absent in others; I removed that and replaced with a jumper . That helped somewhat but the red, while reduced, was still noted.

Service notes call for the horiz freq lug on L117 to be adjusted until minimal current draw is obtained between the clips of the HV fuse. No more red although the lug is turned in almost all the way to the bottom of the coil. Seemed like a lot of turns were necessary to effect a small reduction. That normal? I opened the cathode of the HO tube and have a draw about 215 mA there. Running for about 10 minutes results in a slightly warm--but not hot--flyback with just a tad over 25KV measured. Sound like I can hook up the CRT now? All tubulars and electrolytics have been replaced.

One problems persists: when the cap is pulled from the HO tube (no HV), I can hear the program's audio. When it's replaced, I lose the audio although I still have the background hiss from the speaker. What's up with that?

Suggestions please!

Thanks,
Michael
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Old 03-10-2003, 01:22 AM
Rob Rob is offline
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Michael,

I haven't looked at the schematic so I'm just talking off the top of my head here.

You speak of turning the H frequency coil slug in most of the way to effect a reduction in HOT current. Me thinks you have probably got the H oscillator way off frequency now and under these circumstances are also creating HF 'birdies' or unwanted oscillations, harmonics of which are able to sneak into and saturate the sound IF. Just a guess. Since you have no CRT on it you won't be aware of what H frequency you have unless you have measured it some other way than by synchronising to a broadcast picture.

Is there not an adjustable H. Drive coil on this set? That is what you need to adjust to bring the HOT current down.

Rob
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Old 03-10-2003, 07:13 AM
michael
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CTC 4 horizontal

The drive control is set to the point where I'm getting just -68V to the HO tube, its maximum position, and the instructions noted to turn the slug on the tuning coil to the point where the (current) meter is at its lowest reading. All related resistors have been checked and all caps replaced. Is it possible that the circuit's running 'wide open' without having the CRT hooked up, and that's causing the red plate? I can restore the slug to its former position or approximate, but I'm concerned about danger to the flyback. Would monitoring the cathode current of the HO be about the only choice I'd have to prevent such a catastrophe?

Thanks,
Michael
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Old 03-10-2003, 01:44 PM
Rob Rob is offline
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Michael,

I think the fact that it appears you need to move the tuning coil slug to an extreme rather than where it was once working happily is a good indication that something else in the circuit that is way off spec is still eluding you. Have you tried a different H. output tube? A gassy tube can hot plate at the proper voltages.

Not having the CRT hooked up shouldn't change the load seen by the HV supply if the 6BK4 HV shunt regulator is in place and functioning properly. That will be the same condition seen when the CRT is installed and the brighness is cut off, except that there would be much more capacitance applied to the HV lead.

Yes, I think the cathode to chassis gnd link on the HOT is the best place to monitor HOT current.

Perhaps some of the others here who have much more repair experience than I with these old RCA color chassis have some ideas for you?

Rob
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Old 03-10-2003, 10:26 PM
michael
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CTC-4

I could use the help! Checking voltages on some of the various tubes show something else is amiss as many pin voltages are way too high--perhaps a bias supply problem?--and a few about 10-15% too low. Hmmm. I ran into this with my first 621-TS; a number of voltages were off but the set works fine. Odd. Must be a designed-in variance, huh?
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  #6  
Old 03-12-2003, 08:00 PM
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Kirk Kirk is offline
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CTC 4

I agree with Rob, your oscillator is most likely off Freq. When you replace the HOT cap, you are causing your tube to conduct HARD and kill the B+, thus loading down the B+ to the Audio CKTs.

Can you use a scope? If so, I have a detailed procedure for checking the Output and Osc circuits on your set. It comes from a bench-training course on the CTC4 chasis.

The neg. voltage to the grid of the HOT is an approx. DC value. You need to verify the waveforms at this point. The "duty cycle" (time on and off) must be right.

Also..anytime a tuned circuit needs to be tuned to the end of it's range to work, a close look a the components is needed.

E-Mail me if you like, I'll scan the pages and send, or copy and mail.

Thanks,
Kirk
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