Capehart 661-P

[enrico6v6]

Great job, fantastic set! BRAVO!!!

[old_coot88]

WooHoo Good goin', Phil.
Is there a chance you could post an enlargement of the H sweep and sync area? When i try to blow it up, it goes to an all black/blank screen and nuthin' else. Trying to decipher how the 6L6 makes such a nice clean sawtooth without a conventional damper in series with the flyback's B+ path.
It looks like one of the HV tubes is acting as a shunt damper on the 6L6. I can't quite make out the designations of the two HV tubes (1B3s maybe?). oc

[old_coot88]

P.S.
Since your vert.sweep looks a little lacking, maybe you could post a blow-up of the vert osc and output too. Just noticed the osc. runs off regular B+ since there is no boost source. Some resistors in that area might be drifted.

[earlyfilm]

Originally Posted by old_coot88

Is there a chance you could post an enlargement of the H sweep and sync area? When i try to blow it up, it goes to an all black/blank screen and nuthin' else.

There is a much clearer schematic available at:

http://www.earlytelevision.org/
> Site Index
> S button
> Click schematics and go to Capehart 661

Just realized there are multiple links to schematics in this thread.

The PDF on post 35 is very good, but Micro$oft's IE seems to first compress and then muddy up the JPGs on posts 54 and 76.

Jas.

[Phil Nelson]

Thanks. Maybe I should go back and re-post those schematic bits in PDF rather than JPG format.

The schematic is arranged differently than most, which makes the sweep sections harder for me to follow. Anyhow, the horizontal/AFC subchassis communicates through its adapter plug. That subchassis labels its tubes as follows:

6SN7GT: Sync Amp
6K6GT: Sync Osc
6AL5: Sync Disc
6AC7: Control

I have replaced all resistors around the vertical tubes except for numbers 53 and 15. Until I did that, the vertical had major foldover on the bottom (Ingrid Bergman image) and it was impossible to get correct linearity through the entire height.

For the last photo (Scarlett O'Hara), I was trying to set the image size to fit the TV's little mask. The height is about right, but I can't bring in the width any narrower, suggesting (along with other symptoms) that the HV is insufficient. The designers may have planned for some horizontal overscan, but this amount would cut off way too much behind the mask.

The vertical controls have lots of range and they are very interactive. I can make the picture much taller, which looks better on the round screen, but that wouldn't work when I put it back in the cabinet.

I'm a little surprised that the TV looks this good. I initially turned up my nose when I saw that it lacks an HV cage and has minimal shielding on the tuner, but it can make a razor-sharp image with good contrast. The DC restoration is not the greatest, but I haven't looked at that part yet.

Phil Nelson

[old_coot88]

Originally Posted by Phil Nelson

...The height is about right, but I can't bring in the width any narrower, suggesting (along with other symptoms) that the HV is insufficient. The designers may have planned for some horizontal overscan, but this amount would cut off way too much behind the mask.

In later years, mfrs sometimes used a "width sleeve" to reduce width without affecting the HV. It was a sheet of very thin brass rolled into a cylinder and slid partway into the yoke. The shielding effect reduced the H sweep much moreso than the vertical (if at all), probably due to hysteresis differences between the H and V frequencies.

I don't know if this'd be doable in your situation, or if there's enough clearance between the CRT neck and the yoke for the brass sleeve AND an insulating layer to prevent any arcing issues. Just a thought anyhow. oc

[Phil Nelson]

That's an interesting idea. I have some heavy brass foil that should work.

This set has a rubber insulating sleeve that goes inside the yoke. I have to pull it off over the tube base every time I withdraw the CRT. I don't know if I can fit a brass sleeve -- and another insulating layer -- in there, as well. No harm in experimenting, anyhow.

Phil Nelson

[old_coot88]

If you try it, start with it maybe 1/4 of the way into the yoke and work from there.
I almost always ended up yanking those width sleeves and tossing them if sweep was borderline. But don't remember ever needing to make one for too much width. Man, that 6L6 is kicking butt, huh?

[old_coot88]

P.S.
I was thinking that if you should end up using the sleeve, you could also use the rheostatic "width" control to juice up the 6L6 a little more to bump up the HV a bit. oc

[Eric H]

Originally Posted by Phil Nelson

I'm a little surprised that the TV looks this good. I initially turned up my nose when I saw that it lacks an HV cage and has minimal shielding on the tuner

Phil Nelson

Phil, just a note, never turn down a Farnsworth era 10" Capehart if the price is good, no matter what it needs.

They are almost always a desirable set from a collectible standpoint.

I had and sold a Capehart 10" Combo once, it actually had a Farnsworth sticker on the CRT bracket, I liked it but it needed some serious cabinet work that I couldn't do and it was missing the turntable and antenna, it also took up an enormous amount of room.

[Phil Nelson]

Don't worry, I'm not likely to pass on other Capehart-Farnsworths in the unlikely event I stumble across any more.

I like this TV a lot. One of the cool things about earlier sets is that they're not all the same. My first impression made me wonder how well this one would work, but I was pleasantly surprised. Definitely a keeper.

On the topic of width sleeves, I ran across this interesting description in one of my old TV books:

http://antiqueradio.org/art/temp/WidthSleeve.pdf

I haven't gotten around to trying a width sleeve yet. I remembered that my metal foil is copper, not brass. Not sure if that matters, but if it does, I could easily get brass at the same source.

On the topic of HV output, this brief video clip shows the image size growing and shrinking as brightness goes higher and lower (watch the edges of the image).

http://antiqueradio.org/art/temp/Cap...FirstMovie.mp4

Correct me if I'm wrong, but I assume this is a sign of plain old weak HV output -- which I haven't really addressed yet.

Phil Nelson

[old_coot88]

Phil,
Regarding the width sleeve, it shouldn't matter if it's copper/brass or even alumunum, long as it's non-ferrous metal. Also, i don't recall it ever being necessary for the ends to lap. Some sleeves were completely clad with insulation on both sides, so the ends never made electrical contact anyway.
On that slight bloomy-shrinky thing it's doing, one's first thought would be a HV problem and a weak HV rectifier. But it's not doing what it'd do if that were the case. The pic is expanding with increasing brightness and shrinking when brightness is lowered, the opposite of what you'd see with a weak rectifier.
It's almost as if there were a designed-in coupling between the brightness control and the 6L6, controlling its gain. But i can't see any evidence of that in what's decipherable from the schematic.
(I must be a dummy, but can't seem to find any 'schematics' link in that website posted by Earlyfilm.)
oc

[old_coot88]

P.S.
You have a HV probe, right? What's the HV actually doing from low-to-high brightness settings?

[Phil Nelson]

Here's a link to the schematics page at ETF:

http://www.earlytelevision.org/tv_sc..._diagrams.html

Scroll down to Capehart-Farnsworth, right-click on 661-P, and then choose Save Target As... to save a copy on your computer.

You can get the whole 32MB manual from Chuck's website. Right-click on the following link and then choose Save Target As...

http://www.myvintagetv.com/Apple%20P...e%20Manual.pdf

The HV sags somewhat when I turn the brightness control from minimum to maximum. The control has a big range, going from dark raster to a bloomed, unfocused image far beyond normal. If you are moving the brightness control within a normal viewing range, the HV changes little, if at all.

I'm measuring roughly 8KV. The 661-P manual calls for a 10FP4, but I'm using a 10BP4. The data sheets say that a 10FP4 would normally take 10KV, a 10BP4 would like 9KV.

Phil Nelson

[old_coot88]

Hey cool. I was finally able to blow it up enough to see it clearly. That is some kinda novel horizontal circuit(!). It looks like the 6L6 gets its sync via direct injection on the screen grid (G2). The HV area now looks at first glance more like a doubler. But not a conventional doubler, since a step-up winding on the flyback is used. It's more like a "stacker", taking the pulse voltage off the 6L6 plate, rectifying it, and then stacking a second rectified voltage on top of that, to get the summed voltage for the CRT anode.

Presumably you've verified the values of those three 680K 1W resistors coming off the plate of the second HV tube. And also those four 20meg 1 watters going from that tube's filament to ground.

Darned if i can see what purpose the three 680Ks serve except to reduce the peak voltage on the second tube. They actually form a voltage divider in concert with the capacitive reactance of that 500pf 10KV cap in series with 'em. Wunner what'd happen if you temporarily opened that resistor string. If the HV goes too high, then you could put a fourth resistor in and experiment with the value.
oc