Another KCS47 Restore...No High Voltage/B+ Down 25V

[Chris K]

If you look at the first page of the thread where I have the picture of my version of your CRT test jig maybe you can see if there’s an issue

[Kevin Kuehn]

Originally Posted by Chris K

If you look at the first page of the thread where I have the picture of my version of your CRT test jig maybe you can see if there’s an issue

I can't see the yoke in your picture. Looking at a similar RCA yoke I have here I don't see any way to rotate it unless you loosened the steel clamp around the yoke windings and forcefully rotated it. But why?

[Chris K]

I got it. There’s a band that holds the nut for the adjustment screw and there’s a screw that can be loosened separating the band so the yoke can be rotated. I adjusted it and now the picture is oriented correctly.

[Chris K]

Can’t tell you why I did that other than when I serviced it I took it completely apart.

[Chris K]

F#*K!!!! I turned it on and tried to decrease the width using the pot on the back of the chassis and when I turned it I heard a big arcing pop. Now there's less than an amp draw and no picture. Damn it.

[Kevin Kuehn]

Pot could have been dirty/intermittent or previously damaged. I think it would be a good idea to keep your scope on the grid of the 6BG6 when powering this up so you have some indication of what you're feeding the horizontal output with. You might want to ohm out your flyback windings to see if it went open, again. Hopefully not. Before this happened did you ever get back to looking at the waveform and bias on the 6BG6 grid?

[Kevin Kuehn]

The width control is a little unconventional on this KCS47. It varies B+ going to the plate of the first triode section of the horizontal oscillator. Check that the 1/4 amp fuse feeding the horizontal circuitry is still good.

[Electronic M]

I remember when I resurrected a KCS-47 (I wanted it gone and wanted to get some YouTube content out of it first) that the width pot would spark when blown on...I think they're prone to tin whiskers. You may want to confirm that it's not being shorted (terminal to terminal or terminal to case) by whiskers...And also that no fuses went when it sparked.

[Chris K]

Keep in mind I'm not at home so I can't do any troubleshooting, but the fuse occurred to me as well...but not for the same reason. First, I assume if the 375V circuit is blown, I would still get something close to the 0.75 amp draw the TV had after the pop correct? Second, the yoke is OK...I ohmed it out and it seems fine. I think I know how this happened. The jumper I put on the speaker plug to connect the 375V to the rest of the set was an alligator clip wire. Every time I worked on this, I had the speaker cable and jumper dangling off the workbench so it wouldn't contact anything accidently. Last night it was on my disorganized and tool filled bench and when I turned the chassis to the side to get to the rear controls, the jumper was close to the metal base of a gooseneck desk lamp I was using as a work light. I think it may have contacted the metal base and shorted the 375V line, taking out the fuse. The usual stupid, stupid, stupid on my part. We'll see. I'll check it tonight.

[Chris K]

Originally Posted by Electronic M

I remember when I resurrected a KCS-47 (I wanted it gone and wanted to get some YouTube content out of it first) that the width pot would spark when blown on...I think they're prone to tin whiskers. You may want to confirm that it's not being shorted (terminal to terminal or terminal to case) by whiskers...And also that no fuses went when it sparked.

Equal chance that happened Tom. Never dealt with tin whiskers before but I certainly know of them and what they can do. Would a big shot of contact cleaner (Deoxit Fader) into the width pot possibly clear them out?

[Chris K]

Originally Posted by Kevin Kuehn

Pot could have been dirty/intermittent or previously damaged. I think it would be a good idea to keep your scope on the grid of the 6BG6 when powering this up so you have some indication of what you're feeding the horizontal output with. You might want to ohm out your flyback windings to see if it went open, again. Hopefully not. Before this happened did you ever get back to looking at the waveform and bias on the 6BG6 grid?

No on looking at the bias or the waveform but if I get it up and running again tonight, I will. I bought a full set of Pomona (SP?) test sockets for octal and 7&9 pin vacuum tubes so testing is pretty easy.

[Chris K]

Originally Posted by Kevin Kuehn

The width control is a little unconventional on this KCS47. It varies B+ going to the plate of the first triode section of the horizontal oscillator. Check that the 1/4 amp fuse feeding the horizontal circuitry is still good.

Not sure I see how it does that Kevin?????

[Kevin Kuehn]

Follow from the bottom of the width pot up to the resistor in series to the right side of the horizontal hold pot. That's the B+ path to the plate of the horizontal AFC tube.

[Chris K]

Oh I see that. I was looking at just the oscillator section of the 6SN7

[Kevin Kuehn]

Yeah, both of those 2 tubes are labeled horz osc on the RCA service manual I'm looking at. RCA was into some unusual design practice at this period of time. A lot of it came and gone so fast it never got much coverage in TV servicing texts. I'm still trying to wrap my head around how the width pot in this circuit works. So far it's making my brain hurt.