RCA KCS-47A Riddle

[7"estatdef]

What works fine then goes Zorch!!! and smells like burning rubber? My faithful 6T71 was working fine last night till it made a buzzing sound and the screen went dark. It also emitted an odor of burning rubber. Sound went dead but came back after a few seconds

About three years ago I when though it and replaced a bunch of resistors, all paper & electrolytic caps, + some bad micas. Set has been working just about every day since.

I got a chance to pull the chaiss out tonight. I was expecting to find something all burned up but I'm not seeing anything that looks bad. I remember Bill C mentioning something about the damper tube filament/cathode shorting and taking out the power transformer. HV fuse is good, maybe overrated as I don't think I could find that low current one when I was working on the set years ago.

Flyback resistance is about right as per the Sam's. Haven't had a chance to hook up the jumpers so I can fire it up outside of the cabinet.

Any thoughts?
Terry

[7"estatdef]

Found the answer to the riddle. Filament winding for the 6W4 was arcing at chaiss under the TV/Phono switch. Will repair by adding a 6v3A transformer. Hope it didn't damage anything else.

[ha1156w]

I spy a black beauty capacitor in the bottom left corner next to that transformer. You might consider replacing it while it's up for service.

[7"estatdef]

Oh that's just the one that shocks you when you touch the chaiss and ground.
Terry

[marty59]

Originally Posted by 7"estatdef

Oh that's just the one that shocks you when you touch the chaiss and ground.
Terry

Sure leave a nasty mess behind when they short out and blow!!
Get that thing outta' there!!

[7"estatdef]

Well I disconnected the original filament wiring and hooked up a separate filament transformer for the damper tube. Also replaced the lead on the flyback to the 6BG6 plate cap. Put it all back together and it worker great!!The best picture I've seen on that set. For a hour. Came in here to the computer for a bit and smelled something bad in the other room. I think the replacement transformer didn't have enough insulation and it arced over internally.I could hear it arcing. I not quite sure what happen after that but it toasted the width coil, that's what was stinking up the joint. Lost hv to the crt.
Dug up a transformer with a 3Kv insulation and install that. Replaced the width coil. Wasn't making enough hv and had shrunken picture. Long story short replaced the 6BG6 & 6W4 and it seems to be working great again. I think my wife thinks it's going to set the house on fire!
Terry
ps Anybody got a spare width coil for a RCA KCS-47A?

[TV Engineer]

This was a common problem with any set that used a 6W4 damper tube. The breakdown voltage from filament to cathode is rather low for a tube subjected to such high pulse voltages.

Replace the 6W4 with a 6AX4, or better yet, a 6AU4. This will prevent this problem from reoccurring.

[7"estatdef]

So is it common to smoke the width coil when the 6W4 goes? Or should I take a look for other problems?

Terry

[mattdavala]

My 21inch 1955 majestic console has a 6W4 damper tube and the heater is on a seperate winding with a special B+ connection to the heater.

Was this a way of reducing the potential of it arcing over? I've had good luck and many hours of using this set with its 6W4.

Thanks!
Matt Davala

[TV Engineer]

Originally Posted by 7"estatdef

So is it common to smoke the width coil when the 6W4 goes? Or should I take a look for other problems?

Terry

It is common for the width coil to fail when the trouble you have described occurs. When the cathode of the tube is at ground potential (as would be the case with an arcing filament winding) very heavy current is drawn through the coil. It is this pulse voltage that causes the tube to break down from heater to cathode.

If you use a 6AX4 or 6AU4, you may find that you can actually go back to the original filament winding. Since the high pulse voltage won't be present on the later tube type's filaments, no arcing will occur.

To Matt...

You are correct about the B+ connection to the 6W4 filament in your set. It was to limit the pulse voltage. However, I would strongly recommend replacing your damper tube too with one of the later ones suggested. Those tubes were developed for exactly this reason, and will save lots of heartache down the road.

Others that will work and give you the same measure of protection:

6DA4
6DE4

Always do yourself a favor... tap the damper tube when the set is on. If it arcs, change it. It's gonna short.

BTW guys...

Those tubes are SOOOOOO common you should be able to find as many as you want for a buck apiece.

Good luck!

[jeyurkon]

Something I read in Wallace's Telaides for Sylvanias. They said that some 6W4s sing because the filament is loose inside of the cathode. In later versions they packed the filament tight. I wonder if the vibration also led to the breakdown by damaging the aluminum oxide over time.

John

[7"estatdef]

I guess I had a little concern about switching to the newer tubes because of the difference in filament current between the 6W4 and the later damper tube as they are slightly higher.
Tnx much for your insights, I'm mostly a radio guy. These waveforms, pulses, and such make my hair smoke.
Terry

[bgadow]

Have you located a coil? I researched it tonight-they don't actually use a width coil, but do list a linearity coil which is RCA #71449, Merit #MWC-12, Stancor WC-5, Thordarson WC-14, Triad WLC-3. None of them in my stash, though

[7"estatdef]

Hi Bryan
Actually on the diagram it called the width coil on the chaiss it marked horizontal linearity. Haven't dug up yet. Seems like there's a lot of old RCA's out there.
Terry

[7"estatdef]

I have a question, Where does the 6W4 cathode/filament find ground? From the diagram looks like the plate, cathode, and filament are isolated from ground and all of the connection are at a high potential.
Tnx
Terry