Weird issue with an RCA 730TV2
 

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.

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.

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.

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

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..

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 :sigh: . But that's what keeps this hobby interesting :)

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.

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.

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

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...

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?

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.

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.

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.

Check for tin whiskers on the hold and size controls. Isolate the controls from the chassis and see if there is any improvement.

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.

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.

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

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

.

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

.

Right, but then I ran into this:

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.

.

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.

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?

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

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.

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.....

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.

Call the repair shop...... Is it still under warranty.....?

.

Back on this one. Found a problem! The 3.3KOhm 1 watt resistor off pin 1 of the 12AU7 video amp that I replaced overheated and was open! Heady in my discovery, I soldered a 2 watt in place, and put the set back together.

Powered it up, and the horizontal output tube lit up like a Christmas tree. And then the smoke started coming from underneath the HV cage. Powered it down as quick as I could.

Dammit :tears:

Bright red plate, bright purple/blue ion glow or sparks and fireworks flying around inside?

jr

Neon letters that spell the word 'BANG!' ? :D

Bright purple/blue ion glow AND sparks and fireworks flying around inside. Wasn't too confidence inspiring. I don't get it.

Time to pull the chassis and see if I can see what went wrong here, but damned if I know offhand what it could be.

Just pulled the chassis, and the damage doesn't look too severe. Appears that a couple resistors on the chassis smoked, as shown on the circled schematic below.

Not really sure where to go from here, hopefully someone has a suggestion....

You might want to check wiring. Could a part have been connected to the wrong terminal? Blue light dancing around inside of a tube is sometimes indicative of a short circuit down stream.

I remember in an earlier life, I plugged in a Knight-Kit amplifier. The 6CA4 had quite a light show inside of it. Yes, it was a shorted filter. Never made that mistake again.

"Just sayn'"

Oh believe me, I'm not above making a wiring mistake. What KILLS me is that I had the sucker working and was trying to work through a vertical issue when I encountered this chain reaction failure last night.

I didn't change anything under the chassis since then, and nothing that I can see got bumped into anything else.

My usual power-up procedure leaves the horizontal output tube out until I verify the following:

Negative grid bias
Grid drive waveform
Screen voltage
DC plate voltage

Making sure that all of these look reasonable without the tube installed greatly reduces the chance of fireworks when the tube goes in.

Right, but I didn't have any reason to suspect any other problem. This is what I was trying to fix when the raster dropped out. Found an open resistor on the 12AU7 and thought I nailed it.

http://www.videokarma.org/attachment...1&d=1415031943