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-   -   RCA CTC16 question (http://www.videokarma.org/showthread.php?t=269528)

DavGoodlin 09-11-2017 01:42 PM

RCA CTC16 question
 
I have a chassis to repair for V-Ker's CTC16AE combo, and the 6JE6 (HOT) pin 1-7 G2 voltage is only 60 volts, needs to be 145. Grid drive is -55, so that is ruled out as a cause. The tube tested OK but that was emission only.

The 405 volts to the 13K screen grid (G2) resistor is correct and when checked out of circuit, so was the resistor. That leaves only the .1 mf orange drop cap.

I suppose this cap and tube is easy enough to sub but I ran out of time this morning. Could shorted fly turns do this, without red-plating the HOT? Is there anything I am missing?

Electronic M 09-11-2017 01:55 PM

One thing I can think of is a weak/dead damper. IIRC when the damper dies the fly can no longer draw plate current from the output tube so the screen grid of the output becomes the 'surrogate plate' for the cathode emission to go to. This unnatural screen current drags down the screen voltage, and strains that resistor.

DavGoodlin 09-11-2017 02:20 PM

Quote:

Originally Posted by Electronic M (Post 3189381)
One thing I can think of is a weak/dead damper. IIRC when the damper dies the fly can no longer draw plate current from the output tube so the screen grid of the output becomes the 'surrogate plate' for the cathode emission to go to. This unnatural screen current drags down the screen voltage, and strains that resistor.

I really need a good tube tester but that sounds good. Cathode current is only 70Ma, and that resistor was hot. Ill post the results of some tube subs:)

DavGoodlin 09-11-2017 10:04 PM

Another 6DW4B damper did not change anything. I changed the .1 decoupling cap also.

I lifted the HV cage and still the same. I hesitate to think the worst, a bad fly:no:

Time to look for a 6JE6A in the barn, no set to rob a good one from. All four of my RCAs use a 6DQ5

old_coot88 09-11-2017 11:39 PM

Quote:

Originally Posted by DavGoodlin (Post 3189411)
...Time to look for a 6JE6A in the barn, no set to rob a good one from.

What voltage do you get on G2 with the tube removed?

DavGoodlin 09-12-2017 02:06 PM

Quote:

Originally Posted by old_coot88 (Post 3189415)
What voltage do you get on G2 with the tube removed?

I got the full 405 vdc on G2 when HOT is pulled. It turns out the HO tube was dead, when my tester merely showed it to be slightly weak:sigh: I need a good tester BUT I do have plenty of tubes.

I put a slightly-used GE 6LQ6 in and got no HV but the plate got full red as the cathode current climbed rapidly to 430 ma!! I quick measured the screen G2, and it was up to 175, normally 145.

Pulled plate cap and disconnected screen resistor again, measured the horizontal drive, -55 vdc and a 200 voly p-p sawtooth on the grid G1. No issue there.

I'm fixing to condemn the flyback*, seeing this dreadful symptom on old color RCA's before, and using a flyback ring-tester should confirm. The fly's DC resistance may also give a clue.
Ideas?

* Owner has a spare fly, no hunting needed:thmbsp:

Electronic M 09-12-2017 02:24 PM

One thing to be aware of with those CTC-16 flys is that sometimes the rubber tire becomes conductive and it will arc and carbon track from the socket down either side of the doughnut...That can mess up a fly, but can be repaired if not too severe.

Before condemning the fly I'd look for shorts and or excessive loads on the secondaries...It was not in an RCA, but I've seen things like shorted HV caps in the secondaries load a fly down till the output tube redplates.

old_coot88 09-12-2017 05:15 PM

Is there any scorched odor coming from the fly? If so, it's likely bad. the '16 did love to eat flybacks.

DavGoodlin 09-12-2017 08:16 PM

Quote:

Originally Posted by old_coot88 (Post 3189444)
Is there any scorched odor coming from the fly? If so, it's likely bad. the '16 did love to eat flybacks.

It looks like this set was high-hours because the 1400 ohm resistor (405-260 volt dropping) terminal block under the 6BK4 was gone and all the wires were crispy.:thumbsdn:PW700 chroma board looks its age as well.

The ohms measured almost right at 596, has no tracking but a ring test failed miserably. Compared to the older CTC12 flyback, the ring test on both my Heathkit and Sencore VA48 was a total fail. I then tested a NOS fly and for kicks put a scope across it, a picture-perfect damped ringing wave.

Tom noted other items on the fly's primary winding can load it, notably the .015 caps on the efficiency coil. That is noted in the Carl Babcoke Tab manual.

DaveWM 09-14-2017 12:26 PM

deep fry that bad boy. may still save it.

DavGoodlin 09-15-2017 01:29 PM

Ill give it back to the owner, he will change the fly. He has several 16's and needs extra parts. I think it would be a good lazarus experiment for him. Like Zenith roundie VOT's are for me.


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