Weird issue with an RCA 730TV2

[Kamakiri]

Finishing up the restoration of a 730TV2, and I've got a weird issue with the vertical.....

It locks in fine. The height control is properly adjusted for the screen. Linearity is good. But, the image is about twice the size of what it should be.

I'm confused because I've never had this happen when there wasn't something wrong with the linearity or height......

Ideas? Someone mentioned that I might have an open in the linearity pot, but that seems to be fine.

[Tom Albrecht]

Your vertical oscillator is running at twice its proper frequency, so you're only getting half the image in each raster scan. That's why things are stretched vertically, and you see two overlapping images. Either the vertical hold control is not properly adjusted (most likely you would have figured that one out, of course, so that's probably not it), or something in the timing circuitry has gone bad. Check the resistors and capacitors right around the vertical oscillator. A short (leakage) in a vertical blocking transformer (if this set has one) could also cause this kind of problem.

[Username1]

Originally Posted by Tom Albrecht

Your vertical oscillator is running at twice its proper frequency, so you're only getting half the image in each raster scan. That's why things are stretched vertically, and you see two overlapping images. Either the vertical hold control is not properly adjusted (most likely you would have figured that one out, of course, so that's probably not it), or something in the timing circuitry has gone bad. Check the resistors and capacitors right around the vertical oscillator. A short (leakage) in a vertical blocking transformer (if this set has one) could also cause this kind of problem.

I'm placing my bet right here...... Tom is right, your problem is Vert Osc, and
the picture is too big vert. too....

.

[Eric H]

Check your caps in the vertical oscillator, I'm betting you have something like a .01 where a .001 should be or something of that nature. At least that's what happens to me.

[Kamakiri]

Possible. My 11 year old son did part of the recap. Think I'll have to double check his work and report back

[Sandy G]

Don't fuss at him, the boy's learning...If that's what it turns out to be... My dad would always yell at me over the slightest goof, which would make me even that much more nervous & uptight, & I'd INVARIABLY phck-up again... And then he wondered why I didn't like to do stuff w/him..

[Kamakiri]

Nah. Just the fact that he wants to learn this stuff is great. God knows I've made my share of mistakes

We installed the VOT in his Admiral 30A1 yesterday, and now there's no HV . But that's what keeps this hobby interesting

[vts1134]

Originally Posted by Kamakiri

Someone mentioned that I might have an open in the linearity pot, but that seems to be fine.

Looking at your photo here I can say that wouldn't be from the linearity pot. I didn't know you also had a "double half" image in addition to your picture being vertically stretched.

[Kamakiri]

Yup. Checked all the cap values in the area, everything looks good. There's one mica cap in the area but that specs out dead on. Vertical pot is right on. But, the set DOES have a vertical blocking transformer.

I have a feeling that's it. It almost looks like there's black potting that came out from it at one point in its life. Just a little bit, but enough to make me suspicious. Gonna ohm that out next.

[DaveWM]

you should be able to hear it running slow. check resistors as well in the vert circuit.

[Electronic M]

Had that happen to me on the first set I restored...I was moving up north during the extended process. I could not read those stripe coded bumbelbee caps back then, and went by the sam's....BIG mistake! The parts list, and the schematic had the same cap rated a factor of 10 different depending on which you looked at. My set had a 'no video' issue that (in adition to making me walk away from the project for ~ 1 year) made what was likely a sam's typo induced error go undetected until I had lost track of the original caps to confirm... I did not have a good stock of caps when I got video back and was confronting the issue, and unsure of the sam's being the cause, I fixed it by playing with resistors in the vertical stage until it worked...

That was the first TV restoration I did, and that set made me WORK for it every step of the way. A challenge, knowledge building experience, and accomplishment if I ever had one in my high school years...

[Kamakiri]

Ohmed out the vertical blocking transformer. Should be 165 ohms primary, 1310 ohms secondary. Mine reads 211 ohms primary, 1645 ohms secondary. What do you think?

[Tom Albrecht]

I wouldn't really be able to draw any particular conclusion from these particular resistance readings. The thing that would cause a problem like what you're seeing would be if there is leakage between the primary and secondary windings, or from one of the windings to chassis. Try measuring that and see if you see any conductance, say, in the range of a few hundred K to 20 M ohms.

[Kamakiri]

About an hour before your post just for grins I searched out the part number and managed to find a NOS one on ebay with a 9.99 BIN. For that price since this one's out of spec anyway I decided to pull the trigger. Will update once it's installed.

[Kamakiri]

Well, got the replacement transformer in, and it ohmed out wildly out of spec, something like 50 ohms primary, 750 ohms secondary....so I didn't use it. I did, however, while I was in there, replace a few more out of tolerance resistors along with two other sections of the chassis mounted multi-value resistor (there were three, and only one tested bad....the one I just happened to have handy).

So, for the heck of it, powered the set up and this time only got retrace lines, and a very weak sync of some kind that I was able to lock in....no discernable video.

The weird thing is that when I was powering it up this time, I got what looked like an intermittent blue flashing short within the electron gun of the picture tube that came and went with the adjustment of the ion trap. Every time it shorted, I lost raster.

After the shorts stopped, I left it at a certain point where the picture was really bright and spent about 20 minutes fiddling with adjustments to try and get a picture back. No luck. During that 20 minutes though, the raster was nice and bright, sweep was fine, and the transformer ran cool.

Found a weak 12AX7 video amplifier tube, replaced that, powered the set up, and lost raster

My plan was to trace back the work I'd done on the chassis mounted resistor, but now I've lost raster, so I'm confused. Picture tube tests good still, and I tried a magnetic ion trap just for the heck of it to see if I'd get a picture. Nothing.

I do have good HV though. Hm. Kinda lost as to what might have happened.