Predicta Holiday - First power up attempt

[madlabs]

Tom, the only AGC adjustment on the Predicta is the local/distance switch on the back and I have played with it quite a bit. I can get a decent picture with either setting but with different settings. I am using the one that allows the lowest contrast and brightness. Is there anything else I can do? I understand not wanting to over modulate and have it set just so I see some occasional flashing of the LEDs. If I adjust it any lower the picture gets poor and the sound volume too low.

As to a modern set, my little RCA AC/DC doesn't seem to be doing all that well. I may have to dig into that or see if there are any newer sets at the thrift stores in town.

So to make sure I understand, should I go through the tuner and IF and check all the resistors, replacing only those out of spec? And if I do so, will I need to re-align? And are there places that can align if I send them the board? I do have a sweep/marker generator, untested, and a 'scope. But I have never aligned a set and am aware that it takes skill as well as tools.

Thanks!

[Electronic M]

You might potentially achieve auxilary AGC by varying the RF output of the modulator.

Replacing only the off tolerance resistors might restore proper alignment or ruin it...It depends if the last alignment was with those resistors in proper tolerance. If so (likely), then it should help bring it into alignment. I'd stick to only the off tolerance units to avoid unnecessary changes.

It is possible there might be a service to align it for you...I've never looked for or used one so I can't help you with that.

Also be aware that these sets were never meant to equal a modern SS CRT set in picture, so knowing where to stop fixing it is important.

[madlabs]

Hi All,

Been taking a break from the Predicta for a couple of weeks. I needed a break and I had a couple of ham radios on the bench for other folks that I needed to get to. In the meantime I managed to pick up the parts set and complete set that I bought a few weeks ago.

Luckily, the parts chassis has an A11/12 that appears good. Didn't ohm it yet but the slug is intact and moves. So, I plan to remove it and install it tomorrow. Since I have to take the PCB off again, I am wondering if I should remove and test the ceramic caps. I now have a Heathkit C3 capacitor checker, which I did not have the last time I had the PCB off. I would like to track down any problems that might be the cause of the vertical linearity and horizontal hold issues I am having. Any thoughts on that? Any particular culprits to look at?

[madlabs]

Got the A11-12 tuning can off the parts set. I see that there is a slug that is only accessible from the bottom. I am assuming that is adjusted at the factory and only the top slug needs to be adjusted on the set. Is this correct?

While I have the PCB off again, I want to look for anything I can do while it is off. I checked C43, as I have read it can be a problem and is the only mica cap on the board. It tests fine, in the ballpark for capacitance. I leak tested it to 450V, which is as high as I can go. Should that be enough of a test?

And again anything else I can look at? My remaining issues are poor vertical linearity and troble locking the horizontal hold. One thing I should note is that I didn't have the correct value cap for C42. It is supposed to be a .0039. What I had on hand was three .0012 that I put in parallel for .0036. Think that should be ok?

Thanks!

[Electronic M]

RE: the slugs are they Hex like allen keys or are they screw driver adjustments? If hex then you need the correct alignment tool to access both from the top....They made plastic hex diddle sticks that had a hex end with a shaft above it thinner than the hex so you could push the hex clean through the top slug then into the bottom one.

Without a schematic to see the use and spec of those caps I can't really advise you much. Sweep stage caps can see high voltage spikes (which tends to kill sweep micas) and may be hard to fully test. If your tester is only able to leakage test below the rated voltage of a cap, and that cap is a likely culprit, then just change it. I've seen caps that only leak above a certain voltage....If you can't test to rated voltage only assume good if the circuit works.

[madlabs]

My recollection is that the top slug and the bottom are the same diameter. I'm at work for 48 hours so I'll look when I get back. They might be different diameters. If they are the same, I think it must be set at the factory. In the Sam's it only mentions tuning one slug in it, although it is a pretty generic "tune for best audio" kind of thing.

I can order a new cap for the sweep mica. Or I may try ginning up a ~ 1kV test supply. I have some HV transformers I could run with a variac. What are the symptoms of a bad sweep cap?

Jonathan

[Electronic M]

Originally Posted by madlabs

My recollection is that the top slug and the bottom are the same diameter. I'm at work for 48 hours so I'll look when I get back. They might be different diameters. If they are the same, I think it must be set at the factory. In the Sam's it only mentions tuning one slug in it, although it is a pretty generic "tune for best audio" kind of thing.

I can order a new cap for the sweep mica. Or I may try ginning up a ~ 1kV test supply. I have some HV transformers I could run with a variac. What are the symptoms of a bad sweep cap?

Jonathan

I was not trying to say the slugs are different diameter I was describing the taper of the diddle stick on the diameter..... The slugs should either have a hex shaped doughnut hole all the way through in the middle (which would allow both to be adjusted from the top with the correct diddle stick) OR have a solid center core with a groove in the top for a flat head blade screw driver to adjust them (the blade style will NOT allow both slugs to be tweaked from the top).

Your Horizontal and Vertical sweep stages have many caps doing many different things (and interact with most other stages in some way), depending on which cap fails a VAST plethora of symptoms could result.

Show us which caps or which symptoms and probability/nature of cause and effect can be assessed.

[madlabs]

Hmm, the post I made yesterday seems to be gone.

Tom, I didn't get what you said and posted a question up on ARF. Sometimes I feel bad pestering everyone here and try to spread out the pain.

I get it now, a diddle stick that allows you to reach the bottom from the top, with a thinner section to go through the top slug.

All the paper caps have been replaced. I will replace the mica cap or kludge up a ~ 1kV power supply to test this one properly. Current symptoms are:

Poor vertical linearity. A test pattern is egg shaped at best. The scan lines are thicker at the bottom than at the top.

Horizontal hold has trouble locking. It will lock for a while then roll. Part of this may be that I have had to off-tune some to get audio, so hopefully once I get the audio aligned correctly that will be less of an issue.

Thanks!

[Electronic M]

Non-linearity could be caused by the mica caps...In sets like these and older ones with older parts changing the sweep micas is very advisable.

As for the horizontal hold focus on the Synch Separator circuit and the area around the Horizontal AFC diodes (if those diodes are not tube then they should be replaced).

[bandersen]

There is only one mica cap in the whole set and it's the 390pF mentioned above in the horizontal multivibrator oscillator. The AFC diodes are in the 9BR7 tube.

Make sure that replacement vertical linearity control is indeed 1K and check the 390 ohm 3W resistor in series with it. Also try swapping out the 10DE7 vertical output tube.

Also make sure the 200uF cathode bypass on the vertical output tube is good.

[madlabs]

Digging through my mica caps I don't have a 390pF, but I do have two 150pF and two 50pF that measure out at 396pF. I would have to wait a week to get anything ordered and I am dying to see if the new A11/12 audio adjustment works. Is it electronically foolish to use the four cap combo? I don't care about style points and I'm going to order a few anyway and I'm sure I'll have the PCB off again or can take it off.

The Sams for my set shows and has a 270 ohm, 3 watt resistor. It does go to the vert. lin. pot. As it happens, that is one of the few resistors that I left the old one in place as it tested good. I think I'll peel that one out and replace it.

The vert. lin. pot measure 900 ohms, works smoothly. I will swap the tubes again. I did before but so much has been done and I'm not sure when I tried it in the process.

I feel like I'm getting close. Thanks for all the help!

EDIT: I forgot to ask, if ordering a new mica cap, is it worth it to get a N0 cap?

[bandersen]

Sorry, I meant 270 ohm resistor, not 390

Four caps together should be OK but you may introduce some stray inductance and noise pickup if the leads are long.

I'm not sure what you mean by N0 cap ? A standard 5% 500v mica cap is fine.

[madlabs]

Well, woohoo! Put it back together with the mica cap combo. Aligned the sound and it sounds great! I had removed and cleaned the buzz pot and that seemed to help a lot. Horizontal hold locked in! Scan lines are much closer to the same side top and bottom. Pretty cool. The vertical linearity is the same though. If this is as good as it gets, I'm happy and I think I can squeeze out some more.

I'm using a 6.3V CRT and the yoke that came with it was damaged a little, the housing partially apart. I can see that a straight line across the screen horizontally dips down on the left and up on the right. I'll see if I can kid it into shape and if not I'll put on the one that came with the set. I haven't really tried to tweak it all in yet, but I will. I'm kinda scared to touch it, it's working so well.

Many thanks! Progress!

[bandersen]

Cool! I'm glad the new coil worked out

[madlabs]

OK, now I want to get after the vertical linearity. Swapping out the 10DE7 has no effect. The 200uF cap is new. What else can be affecting the vert.lin.?

I wonder if it is just that I don't have all the controls adjusted correctly. I'm sure there must be a proper order and method to adjust everything but I don't seem to be able to find much information. Anyone have any guidance on this?

Once I get the vert. lin. straightened out it's on to other restoration aspects. Naturally I have a bunch of questions! First is: The HV wire to the CRT. It's old and stiff and has a couple of cracks in the outer insulation. Should it be replaced? I do have some neon sign wire around here somewhere or could buy something appropriate. I do hear some sizzle and smell a little ozone up at the CRT, is it worth it to figure out some kind of cup as later sets have?