Predicta Holiday - First power up attempt

[WISCOJIM]

Originally Posted by madlabs

And a bonus non-electronic question: Ordered the Novus polish. Do y'all hand polish or use a small buffing wheel? I may have to break out some sandpaper for some scratches.

I hand polish. I've heard horror stories in the past of melting plastic using a buffing wheel.

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[bandersen]

I hand polish too. All those controls should definitely be having a noticeable effect.

Buzz control might not be working because the alignment is throw off by the damaged slug. I suppose the slug could be replaced but finding one with the correct characteristics could be tough.

The centering controls are actually magnets. Unless they lost their magnetism, they should be moving the image around when you rotate them.

[Electronic M]

If it uses a Zenith style sound demod with a 6BN6 you may want to double check that circuit if the control has absolutely no effect...If it does not vary the level of hum in some way there may be other issues at play.

[madlabs]

The buzz control does have some effect. It varies the volume and only in a small range is the audio loud enough to hear and it does eem to affect some buzz. It does use a 3BN6 for audio demod. But the problematic hum sounds like 60/120Hz hum, like it had bad electrolytics on the power side or something. Although I do have to slightly off-tune the fine tuning to get sound, so maybe if it was all aligned correctly the hum would disappear. Any other causes of hum to look for?

Think there is any chance of gluing the core of the broken slug in the audio to a diddle stick? I only need to adjust it one more time. Although the slug is probably stuck, which in turn is why it got broken. Sigh. Anybody got a replacement transformer?

Jonathan

[madlabs]

Hi All,

Got a chance to poke around a bit on the set. The vert/horz centering adjustments do indeed work, I was spinning the shaft of the magnet, not the magnet itself! Duh. I have the picture nicely centered now. The vertical linearity definetely doesn't work. Just to confirm, the vert. lin. is the inner of the top (brightness) pot. The pot only has one wire coming to it, to the center contact. The center contact has about .2 ohms resistance to chassis ground. One side of the pot adjusts fairly smoothly from 3 to 1200 ohms or so and then goes infinite in the last bit of the turn. The other side goes from about 1.8 ohms to to 300 or so, then jumps up into the kOhms and gets worse from there. Measuring the resistance at pin nine of V9 I get 296 ohms and it doesn't vary when the pot is turned. So a bad pot. What I don't understand is why it only has one wire connected to it. The schematic clearly shows one side goes to R58 and the wiper to ground. If I understood how it is connected I could sub in a 1k pot to test while I look for a pot. I didn't have the time to disconnect everything to the set so I could look underneath and follow the one wire that is connected to the pot.

Didn't dig into the hum issue but did notice that it does vary with the volume and is there with no signal applied. The electrolytics have been replaced, of course. Later I will ground the control grid of the 12CA5 and see if it persists. If I can get the vert. lin. and hum issues resolved I'm going to call this set done.

Thanks!

Jonathan

[bandersen]

I suspect one side of the linearity pot is grounded and R58 is going to the center contact. It makes no difference if the wiper is grounded and one leg goes to R58 or vice versa.

[madlabs]

Well, both sides of the pot show resistance to ground that varies with adjustment and the center tap shows very low resistance to ground. There is no connection on either side. Later today I'll disconnect everything so I can flip the set over and follow the center tap wire.

[Electronic M]

If you have a tube tester look for heater cathode shorts in the tubes and try substituting tubes in the audio and audio IF....A HK short anywhere could add hum, and some tubes without a HK short seem prone to picking up hum from nowhere (I have a 12AX7 like that).

[tom.j.fla]

I think that R-58(270 ohm 3 watt) cold end is connected to ground. The pot may have failed and that was a work around for as a replacement pot was not to be found. It is always great to have a factory part. If you can't find a factory part and are not too fussy about factory look put a different pot on back of chassis till you can locate the correct part. Over the years I have seen that done and have done that a few times myself. All the best,Tom.J

[madlabs]

Aha! Figgered it out. The pot must indeed crapped out back in the day. There was a replacement pot installed where the optional UHF tuner would have been if mine had it. It looked factory to my uninformed eye, as the holder was indeed factory.

So, it does affect the vertical linearity. However, as I get close to getting the test pattern round, it starts getting egg shaped. So how it was is about as good a compromise as it gets.

One thing tough about working on your first set: I don't know how good was OK back then, picture quality and sound. So no way of knowing if it is as good as it gets or could be improved.

In other news, I bought another Predicta holiday (blond) and a parts set (holiday mahagony). I swear, I'm not a collector! :-) With luck I should have all the parts I need to keep a set running for a long time.

Jonathan

[Electronic M]

Linearity, height and centering all interact. Getting vertical right involves having one hand on height one on lin, and occasionally tweaking centering in most sets.

If playing with both controls at once don't give you a good linear raster that fills the screen, try tube substitutions and confirm all resistors in the vertical are within tolerance.

Note: if the horizontal over scan is much greater than the vertical and you want to further increase the vertical to prevent the image from being horizontally stretched, there may not be enough vertical range to match....If that is the case you may want to try to reduce the horizontal size (if no such control exists try H drive, H out/damper tube substitutions for weaker working specimens, or sliding a thin sheet metal copper sleeve between the yoke and the CRT neck) or live with it as is.

[madlabs]

Tom,

Thanks, that helps. I am off for my weekly 48hr shift so I won't get a chance to play with it for a few days.

I've been looking for a basic, how to adjust a set kind of thing but haven't really found one. And any recommendations for a book on fixing sets geared towards the set owner instead of serviceman?

Jonathan

[N2IXK]

Originally Posted by madlabs

I've been looking for a basic, how to adjust a set kind of thing but haven't really found one. And any recommendations for a book on fixing sets geared towards the set owner instead of serviceman?

This is a really good one, IMHO:

https://www.amazon.com/Practical-Han...sap_bc?ie=UTF8

[Electronic M]

Originally Posted by N2IXK

This is a really good one, IMHO:

https://www.amazon.com/Practical-Han...sap_bc?ie=UTF8

I've never seen anyone degauss a monochrome set till seeing the cover of this book just now. I can't think of why to do that on anything other than a color set. Looks really bassackwards to me.

[WISCOJIM]

Originally Posted by Electronic M

I've never seen anyone degauss a monochrome set till seeing the cover of this book just now. I can't think of why to do that on anything other than a color set. Looks really bassackwards to me.

You still haven't, it is a color set. In the book he is explaining that you do the degaussing to eliminate a tinted picture to obtain the best greyscale image possible on a color TV set. Therefore the color control is turned down.

Page 68 in the book shows him doing the degaussing on the same set, and the set says "color" on the control door below the on/off/volume control.

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