Weird issue with an RCA 730TV2

[Penthode]

Don't be concerned about the primary and secondary winding resistance. The failure mode of the transformer is leakage between primary and secondary.

Try taking in the replacement transformer.

Other than that, something is causing the oscillator to run fast. I do not think it is a resistor as they tend to drift upward in value. Capacitor leakage can. Check to ensure all the paper dielectric capacitors at oscillator grid including the integrator are replaced.

[Kamakiri]

Originally Posted by Kamakiri

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.

Fired up the Eico 944 flyback and yoke tester. Confirmed bad yoke. Welp, at least I know what to do next!

[earlyfilm]

Originally Posted by Kamakiri

Fired up the Eico 944 flyback and yoke tester. Confirmed bad yoke. Welp, at least I know what to do next!

Tim, just double checking before you sink some cash on a new yoke . . .

I'm assuming by "confirmed bad" you mean that the vertical windings check shorted.

I'm not familiar with the Eico tester, but other yoke and flyback testers will read "bad" if one forgets to open the resistor paralleling the winding. The 730 uses a 560 ohms across each vertical winding.

[Kamakiri]

I had thought of that. I wasn't certain if there was or not. I'll pull the yoke and confirm, thanks for clarifying

[Username1]

You have vertical deflection......... Full screen even..... I can't see it being the yoke....

.

[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....

.

[Kamakiri]

Right, but then I ran into this:

Originally Posted by Kamakiri

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.

[Username1]

Well, Ok, you have more than one problem... You need to do some troubleshooting.
First turn it on and hit it a few times..... figure out if it's got an intermitant.... Bad solder joints...
If you get a nice stable set while hitting it, then it's a part.... Look at what you don't have
and look in the part of the set that most likely lost it.... Raster no other stuff.... ??
Get out the block diagram and figure out where the problem might be..... Or at least
a place to start...

Hitting it... Use a wood dowel, 12" long, 1/2" dia. Bounce it off the chassis in different
spots. Gently hit the tubes, hit the coils, etc. You're just looking for a bad connection.

.

[Kamakiri]

Well, before I do anything, I'm gonna set the chassis on the bench and do some voltage checks to see exactly what is amiss. I'll have to be a little creative in suspending the picture tube, but it'll be pretty easy.

[Kamakiri]

Got the chassis on the bench. Found at least part of the problem. V1, the 6J6 RF amp, has only 88.2 VDC on pin 1 and 84.8 on pin 2. Should be 145 VDC on both.

V2, the 6J6 mixer, voltages are right where they should be.

V3, the 6J6 oscillator, pins 1 and 2 are high. Should be 75, I'm getting 98.

Problem has to be in those small white mmf caps that I didn't change. Are they anything special, composition wise?

[Tom Albrecht]

Those are ceramic caps that seldom fail. Generally OK to replace with mica caps.

[Penthode]

If the rf amp plate voltages are low, try adjusting the contrast (picture) control to minimum. There is no agc on this set and it may be the grid bias. I do not think the caps are bad and they are best left alone because of alignment.

[Kamakiri]

I was just working on an Admiral 30A1 and was thinking of THOSE caps. This set doesn't have ceramic caps. Oye. Working on too many sets at once....

Anyway, I discharged the HV and flipped the chassis over. Removed the screws on the lower tuner cover, and of course I can't see anything under the 6J6 because the tuner's in the way.

Still poking around in here.....

[Kamakiri]

Just to recap (har har), I have no raster on this set at this point but audio is fine, adequate HV (neon bulb test). Had a nice bright picture briefly, but doesn't now as outlined in the previous posts.

Usually at this point I'd start by measuring voltages working from the RF amplifier forward or going to the first video IF and working backward. Since I've got weird voltages right at the RF amp I'm kinda puzzled as to where to go from here.

[Penthode]

Originally Posted by Kamakiri

Just to recap (har har), I have no raster on this set at this point but audio is fine, adequate HV (neon bulb test). Had a nice bright picture briefly, but doesn't now as outlined in the previous posts.

Usually at this point I'd start by measuring voltages working from the RF amplifier forward or going to the first video IF and working backward. Since I've got weird voltages right at the RF amp I'm kinda puzzled as to where to go from here.

Did you set the contrast control to minimum and then check the RF amplifier plate voltage??!! Check the grid bias: I suspect the 6J6 is conducting heavily and pulling down the plate voltage. RCA did not include agc until the '48 models hence the contrast control feeds the rf and if grid bias.