20Y1 Admiral horizontal frequency problem

[6GH8cowboy]

I continue my efforts on a 20Y1 chassis where the freq of the horizontal osc is way off. So far off there is little HV. At this point ALL osc compoments have been replaced and double checked and tested. the tubes subd out even the HV rect, the yoke subed out, the flyback rung out with a scope and a square wave generator (rings well). The wiring and components have been triple checked against the physical reality.

http://i375.photobucket.com/albums/o...s/DSCF0196.jpg

The scope shows the present condition (top) of the osc as compared to the sync/ video signal (bottom-good). I need fresh perspective. Is the feedback signal from the HOT xfmr via the 5pf cap that important to the freq of is it just for locking. Is this still possibly an output section problem. Is there a good writup on the SyncroGuide osc, this looks like the RCA type. Has someone run into this before. I'm out of rabbits to pull out of a hat on this one.
At one time the set was running and just quit. The reason for checking wiring was to make sure I didn't create a new situation as I rechecked components and such.

[John Folsom]

Even with the HOT removed, the H oscillator should run close to on-frequency. Assuming you have check B+ to the circuit, all the R's and C's (including the silver micas), and have adjusted the synchroguide transformer frequency slug and the 'ringing' slug per the alignment procedure, what is left is the synchroguide transforemr itself. I don't know of any easy way to test it except substitution. But you should have no problem minding a substitute, either NOS for a used one from some chassis.

[6GH8cowboy]

Thanks John,
The osc trans has been replaced. I too feel that it should be running close without feedback from the HOT or sync. Oddly, if I try to "jump in" some experimental values the frequency dosn't change much at all. Can,t even drag it closer to any great degre.
All r's and c's and micas have been replaced, tubes swaped and even original Rs and Cs checked to make sure the schematic dosn't show different values. B+ is very close. I don.t suspect socket leakage esepcialy at this frequency, it's not like it's RF and the socket looks OK. I can tease some voltage off the HV rect with an insulated driver but very low level. At one time I could see a line up the screen in a dark work area but with it soo off freq I'm sure the DY and HOT are not resonant, hence the line. I disconected the yoke wires and put a frequency generator across and got some raster so I believe the DY is OK. Really strange.

[Zenith6S321]

I had the same kind of frequency problem when I recapped my RCA 721TS. After attempts at alignment failed, I rechecked my work and I found I had made a wiring mistake during recapping and replacement of out-of-spec resistors. Even though you have rechecked the wiring, you might want to have a fourth look. Could the schematic you are using possibly have a mistake in it?

[John Folsom]

Hmmm. I concur with Zenith6S321. Check your wiring, component values and the schematic (Sams is notorious for errors, as you probably know). We all make mistakes, even when we are sure we are being careful (I know I do). The HOT flyback and DY should have little to do with it. The problem has to be oscillator tube and associated circuitry. Try lifting one end of the 470pf coupling cap to the HOT.... this will eliminate all the circuitry downstream from that as a possible cause. There just HAS to be some straightforward defect. Gook luck, let us know what you find.

[miniman82]

On the scope pic: your top trace is not correct. The peaks should both be at the same height, so tweak the wave shape coil till it is. It's possible your actual frequency is not off by much, but with the wave shape that far off the whole thing may go haywire when attempting to sync an incoming signal. There was a really excellent explaination of the synchroguide circuit somewhere on the net, but I can't find it now. Something about how that double humped shape increases horizontal oscillator stability or something.

[teevee]

You're dealing with 2 circuits here, the horizontal oscillator AND the horizontal AFC. Best thing to do is disable the horizontal AFC, and see if the osc runs on/near frequency. You should be able to adjust it for a semi-stable picture (no synch) If that works, re-enable the AFC, and if it pulls you way off freq, look in that circuit.. If you can post that portion of the sch, it would help..

[bandersen]

Yes, those peaks should be equal heights.
You can download the Riders Service info here courtesy of the ETF.
The horizontal oscillator and AFC operation are described on page 14. Horizontal oscillator adjustment is described on pages 20-21 including a little picture of what that waveform should look like.

You might gain some insight from this video where I troubleshoot the same oscillator circuit running too high: http://www.youtube.com/watch?v=YXWX1YasFk4

[6GH8cowboy]

After comming to the point that its not a component in the H osc section I turned my attentions to the outout stage. As it turnes out the .002 cap (Riders c419) from the cathode of the HOT to the signal line has more to do with signal conditioning than I thought. My replacement was OPEN! I now have raster and the frequency is only slightly off. I have alighned acording to the write-up and I suspect I may have to tweak a component value in the osc, being the original values were flat values for the caps and replacement industry standard values are slightly higher or lower as provided. Has anyone found this to be the case when working with time base or osc circuits with the older flat values? H-Range c412 adjustment trimmer hasn't enough range.

Also I do not have a sweep generator (last used one 35 years ago) Is there a basic bare bones suggestion for an IF align or should I put in AV jacks being there ios no over the air stations anyway, or put up with the buzz when using a converter?

[John Folsom]

I don't think the difference between say a .05 and a .047 or a .02 and a .022 is going to cause you not the be able at attain lock. There should be a pretty good frequency adjustment range on the frequency slug of the synchroguide transformer. There must be some other issue, maybe one of your other caps or resistors is way off value?

[6GH8cowboy]

Thanks John.

I moved the H-osc adjustment to the outer end its throw and horizontal is within its range. I'm strugling for width and will sub the HO tube and damper.
At present I have slight retrace lines but don't see where blanking comes from as I peruse the diagram, maybe I will just have to settle with reduced brightness to eliminate them.
Photo shot from a mirror, yoke is not reversed.

[kvflyer]

Originally Posted by 6GH8cowboy

Thanks John.
...




Photo shot from a mirror, yoke is not reversed.

Nice progress. Please keep plugging away.

I had to laugh because I reversed the wires to the winding as a teenager one time. It was not hard to figure out. I had fun with my friends who just couldn't believe what they were seeing.

[bandersen]

You can eliminate the retrace lines with a minor modification as described in the Admiral service hints scanned by mbear2k http://www.videokarma.org/showthread.php?t=253004
I've modified a couple sets so far and it works great

[wa2ise]

Originally Posted by bandersen

You can eliminate the retrace lines with a minor modification as described in the Admiral service hints ...

I saw that as well, though my Admiral is in storage right now. But it looks like you find the 8200 ohm resistor and 0.047uF cap connected in series, one end to ground and the cap to the vertical oscillator circuit. This node also has a cap feeding the vert output tube's grid. You connect a 0.1uF or so cap to the junction of the 8200 resistor and 0.047uF cap, and the other end to grid one of the CRT. Insert a 270K resistor in the line feeding that CRT grid, so the bypass cap on the brightness control doesn't shunt the signal from the vert osc to ground. You probably should use a 630VDC or higher cap here.

[6GH8cowboy]

Lots of good info on that thread Bandersen. Did the mod and lookin good boyz.

Now for the width. Ive swaped the HOT, damper, osc and HV rect to no avail. Goin to play with caps across the width coil and monitor the cathode current.