CTC-17 Screen control issue

[old_coot88]

Originally Posted by ctc17

... Would 15ma really make that big of a deal?

Oh yes indeed. The 6JE6 is already being driven to the max with virtually no "headroom" for overcurrent. It may be happy at 210ma but 225ma can spell doom. If the flyback doesn't fry first, what usually happens is- a hot spot develops in the glass of the 6JE6 which softens and sucks in at that spot. The tube goes to air in an instant, shorts out and kicks the breaker. Hopefully someone has not replaced the breaker with one of higher rating. I've seen sets come in with 7 amp breakers in place of the original.

If the tube is running at 221ma, keep a close eye on it and watch for any cherry red areas developing on the plate. Above all, NEVER, ever leave a vintage color set running unattended (even one with no overcurrent issues).

[Charlie]

Also, if that set still has the original RCA flyback, it would be wise to watch it close. Seems the originals were crap to begin with. If you compare the original to a replacement, you can tell the replacements were built like brick shit-houses!

[old_coot88]

Originally Posted by ctc17

As the horizontal drive tube/oscillator ages the cathode current goes up as the output tube looses drive.

Not necessarily. The first sign of the H.osc tube going weak is that the osc. will be off-frequency when the set comes on from a cold start; it'll take a little time for the lines to straighten up and the pic to lock in. A quickie test of the H.osc tube is to turn the set off for several seconds, then back on. If it comes on with lines before locking in, the tube is getting a little weak. If it comes on fully locked, the tube is good. A weak osc tube will generally cause off-frequency problems before drive level to the HO tube is affected.

[old_coot88]

Originally Posted by freakaftr8

... For some reason, my other 17 has a short horiz lock.

By "short", do you mean weak/touchy? If so, have you replaced the phasing diode? 95% of the time, weak horiz. sync is caused by that dual diode going bad. IIRC, it is common-cathode (not series) type.

[freakaftr8]

Oh duh. Your spot on old coot88. I had his issue on a magnavox. I'm going ro replace it.

[old_coot88]

Originally Posted by freakaftr8

... could filter caps cause the cathode current to be high?

Don't see how. Bad power supply filters would show up as hum bars or "bend bars" slowly creeping up through the picture.

I use a DMM rated max 400ma. I get reading from 221 to 241ma.

I would get another meter, preferably analog, to double check that current. Digital meters can go nuts if there's any strange waveforms present in what they're trying to measure. There's a helluva lot of horizontal sawtooth and even RF in that current, and even if you have a cap across the meter, some of that stuff can still get past it.

If the current is really running that high, the tube is gonna be sizzling hot. Old techies could tell if the tube was running too hot just by holding the back of a hand next to it.

[ctc17]

Try adjusting the hv adjustment for a dip. I remember a specific point where the hot current dipped in the hv adjustment. It may have been 500volts off from 25k but I was looking for the lowest current.

I was thinking more of the Zenith setup with the tipple triode that gets weak and cause lack of drive.
Im interested in the cap/damper trick. I may try that on a few of my sets.

[freakaftr8]

Yrp youre right. The current does dip with hv adjustment. Im like 24000 at 221ma, at 25kv 240ma. Im going to give this theory that old coot88 had with the pf caps across the cathode-plate.

But first im going to try a different meter, probablyt an analog simpson from work first. Ill see what I can come up with and let you know.

[Bill R]

The only problem with adding caps is that the set was not designed that way and you could be just masking the real problem. I would exhaust all componant checks first. Then if all else fails and as a last resort I would add the caps. I doubt the meter would make much difference, and a good digital meter would be easier to read and interpret thus more accurate results. A good meter not the $1.99 special from the tool store. For the most part you get what you pay for.