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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Mar Thu 01, 2018 5:28 pm 
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And try more than one vertical output tube, these sets are sometimes fussy about that tube and one that causes rolling as it gets hotter and hotter won't ever show up on a tube tester.

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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Mar Thu 01, 2018 7:29 pm 
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Mr. Detrola wrote:
And try more than one vertical output tube, these sets are sometimes fussy about that tube and one that causes rolling as it gets hotter and hotter won't ever show up on a tube tester.


One of my Zenith Portholes had poor vertical and the vertical output tube tested good. I wasted a lot of time under the chassis. I finally replaced the vertical output tube with a NOS resulting in perfect vertical.

What's weird is the old vertical output tube worked well in my other sets.


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Mar Thu 01, 2018 11:44 pm 
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That sort of thing shows up from time to time in lots of electronic circuits. Usually it's because the designer depended on some characteristic of the tube that wasn't controlled or tested for, and so could vary from unit to unit or among various manufacturers.
I once had to troubleshoot a device that was misbehaving and puzzled over the circuit until I realized that it was depending on an op-amp's input offset current to bias it properly. NOT a good design, though it did work a fair percentage of the time, depending on the particular op-amp.

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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Mar Fri 02, 2018 5:21 am 
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As a design engineer, when designing analog circuits you always try to make the design not depend on characteristics of active devices (e.g. in this case tubes) as far as possible.

But to do this you generally need some gain you can throw away with negative feedback. When a designer was struggling to meet a price point, a lot of niceties would be sacrificed.

Some designers, even in the 50s, did it properly though. I have a chassis on the bench right now that gives identical performance with a range of old and new vertical output tubes and quite large variations in mains voltage. To do this it uses an extra transformer to provide current sensing negative feedback from the yoke. This has the additional benefit it doesn't need a vertical linearity control. But it does have additional complexity and cost. It looks like in this company the engineers won out over the bean counters, this product has design integrity.

By way of contrast, the usual US practice at the time was to vary the bias on an output triode and use the triode's curve to correct the linearity. Very simple, but it does mean that height and linearity are affected by tube age and varying mains voltage.


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Mar Fri 02, 2018 10:09 pm 
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I have had more than one Predicta where the vertical size or linearity issues or rolling were affected by either the horizontal output or damper tube, as those got hotter the width would change slightly (which you couldn't see due to overscan) and that affected the boost voltage, which feeds the vertical circuit. A vicious circle but lots of sets were designed that way. The Predicta seems to be the worst of the worst though.

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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Mar Sun 11, 2018 1:57 am 
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OK, I swapped a number of tubes and re-soldered all of the connections to the various control knobs. Its much metter and I now have a steady vertical hold.

One final problem. There is about 2 inches of black on the right hand side of the screen where the picture ends at that point. I know there are some adjustments on the rear of the CRT, but I'm hesitant to fiddle with these.

Any thoughts, and thanks for help so far.

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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Mar Sun 11, 2018 3:07 am 
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Scroll down in this article to "Final Picture Adjustments" and you'll see a photo and diagram of the adjusters on the rear of the CRT:

https://www.antiqueradio.org/philc12.htm

Depending on the condition of your set, adjustments may not be the total solution, but they're a good place to start.

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Mar Sun 11, 2018 3:14 am 
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This is general advice for tube TVs:

Where there are width and / or horizontal linearity coils, these should be adjusted for minimum current through the horizontal output tube. Doing this may entail temporarily disconnecting the cathode connection to ground. Or, if there is a resistor, measure voltage across that. This adjustment generally corresponds with best linearity and allows the tubes and particularly the flyback to run cooler and last much longer.

If you have to deviate from the minimum current settings to improve the linearity or reduce the width for example, keep this deviation as small as possible and in any case never exceed a 20% increase in tube current.

For designs using a stablilised output stage there will be a pot that affects the width and in particular the B+boost. Follow manufacturers instructions to set the B+boost voltage. In general, the B+boost should never exceed 610 volts with respect to ground.

Regards the centering tabs on the back of the yoke - you can generally feel free to move these as needed (Phil's already posted a link to this) but note that non-optimal adjustment will usually degrade focus and/or cause vignetting (shadowing) in the corners. Don't try to use the centering tabs to cover up other faults.


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Mar Mon 12, 2018 3:08 am 
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A small separate issue. I dropped a clip lead where it shouldn't have gone, and blew out the fusistor - it did its job.

The spec for the fusistor is 5.6 ohms. I have what I believe are fusistors - one 7.5 ohm, one 4.7 ohm:
Image

Are these fusistors, and can I use either of them even though the resistance is different?

I know the other option is to use a fuse in series with a power resistor. What amperage for the fuse? The set draws 1.56 amps. Slow-blo or regular?

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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Mar Mon 12, 2018 3:01 pm 
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irob2345 wrote:
This is general advice for tube TVs...


Yes, that is good general advice, but Predicta TVs are a little different. No width or linearity coils and no centering tabs.

There is a width potentiometer located on the back left side of the chassis. It sets the screen voltage on the horizontal output tube.

As shown on Phils website, linearity is crudely adjusted by a magnet attached to a flexible strip of metal inside the CRT housing. Centering is accomplished by a couple rotating magnets set into the yoke housing.


Stan Saeger wrote:
A small separate issue. I dropped a clip lead where it shouldn't have gone, and blew out the fusistor - it did its job.

Are these fusistors, and can I use either of them even though the resistance is different?

I know the other option is to use a fuse in series with a power resistor. What amperage for the fuse? The set draws 1.56 amps. Slow-blo or regular?


Yes they are and, yes, you can use them. The resistance is to reduce the surge current on power up to protect the rectifiers. Modern silicon diodes can handle a higher surge than the old ones, but it's still a good idea to use one.

They only fuse the B+ circuit, not the whole set. So you would want something like a 0.5A slo blow fuse.

Oh and they do still make make modern fusible resistors. Here's a 4.7 ohm, 5W from Mouser.

https://www.mouser.com/ProductDetail/We ... KxZDtd8%3d


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