Are you talking about that lower rail, with the .6 volt p-p wave on it? I think that comes from the vertical centering circuit, but I could be wrong.

I should have said ripple, not hum. I am referring to the waveform shown on the SAMS schematic at pin 5 of the ballast tube. It is shown as 60 Hz ripple. That is, two cycles at the 30 Hz rate. This must be an error since a full wave doubler should exhibit 120 Hz ripple, correct?

I am still not clear about the hum frequency on the screen. The distiction is important because 60Hz points to a heater cathode problem and the 120Hz points to a power supply problem.

Is it possible for you to post a picture or two of the screen with the hum problem?

Not easy to get a picture of what I'm talking about, this is the best I can do.

http://videokarma.org/attachment.php...3&d=1316235015

It's almost like ghosting. I can barely see it, but I know it's there. Whatever this is, it's related to the ballast overheating because both issues showed up at the same exact time. I figure something is loading down the power supply, which throws everything out of whack.

Some more questions:

Your right: it is hard to see whether there are two distinct bars or one. But the bar seems to appear sharply.

1. Which +285V is being pulled down: the +285V IF or the other +285V supply feeding the vertical centering control?

2. Did you change the rectifiers to silicon?

3. Have you tried putting back the old electrolytics for a test?

4. What did you last do before the problem appeared? (Did you see the problem before changing the electrolytics?)

5. What basis do you think the ballast too hot? (Between the two resistors, the ballast is going to dissipate close to 50 watts which will make it pretty darn hot!)

I wonder if by either changing the electrolytic capacitors in the doubler and/or changing the rectifiers you may have inadvertently introduced rectifier switching transients which are effectively being coupled to the RF/IF stages? A simple test for this is to place 0.01ufd 600v ceramic capacitors across each of the rectifiers.

My long shot guess is that if the capacitor impedance has changed maybe you are seeing the rectifiers switch on the screen?

Here's a rundown of voltages:

Mains input set at 116 with RCA Isotap.

Main 400v: 380
IF (coming from pin 2 of the ballast): 300
285 supply going to vert centering control: 260

As read at the filter cap of each respective line.

Yes I swapped some in briefly for a test, made no difference so I changed it back. Stock seleniums are still connected.

You mean the chassis originals? They are totally shot (open circuit), so no point.

I didn't change or do anything really, just moved it from one room to another and it worked fine in the new room for a spell then this popped up for no reason.

Hot enough to produce smoke, I'm not willing to let it go any longer than that...

I don't think it works like that with seleniums, but I could be wrong. Something about this set changed in a bad way to make this start happening, it was working fine one minute, the next it's all jacked up. I just haven't been able to track down the source yet.

I'm beginning to notice there are some issues with the horizontal oscillator. Pin 5 is right where it should be at 270, but the grid is sitting at -65 when it should be around -85. This in theory would cause the tube to conduct harder, leading to an increase in output waveform, which could in turn overdrive the HOT. This makes sense, since I'm no longer able to get rid of the drive lines in the screen even with the drive control all the way at minimum. There is a cap (C236 in the RCA manual) connected between terminal F on the hor osc sweep trans and tube pin 4 (grid) that if leaky, could cause this rise in grid voltage. Tomorrow, I'll see if I can find me a sub. This is exactly why I never trust these old micas, nothing but trouble and headaches if you leave them in.

I have a gut feeling the horizontal oscillator fault is not related. The Horizontal and vertical sweep sections get their power directly from the 400v line.

The +285v line appears to feed only the luma, chroma, sync and RF unit (tuner). I assume you injected a baseband composite signal at TP101?

As a curious subnote, I noticed on my RCA Service bulletin for this set (First Edition-First Printing 11-24-54) that the connections to the RF Unit to the main chassis do not match. Connections 2 and 3 are reversed! Funny, it has to do with the +285v supply and the other is the 6.3vac filament connection. Do you see the same on your schematic?

I do not see a particular single point of failure on the 285v line which stands out. Can you reaffirm that you see a full color image when it is injected the baseband video?

I would guess that maybe another electrolytic has bitten the dust? Some more voltage measurements are in order. Good luck and we will look forward to the next installment.

I agree, if the HOT is pulling too much current, that could be pulling down the 400v line, but I don't see how a horizontal issue could be overheating the ballast. Is there any way the vertical centering could be pulling down the 285v, possibly a short in the yoke?

Here is what I'd try to localize the hum bars. Insert a signal generator into the antenna terminals, and set it to a VHF channel visual carrier frequency. Connect a bias box to the AGC line. Connect a scope to the video amp. Turn on the AM modulation on the signal generator and adjust the bias to produce a somewhat undistorted sine wave on the scope (I say somewhat because the AM modulation of most signal generators is not that linear). Turn off the modulation. Now connect a detector probe to the scope and work your way back, looking at the plate of each IF tube until you see hum on the scope.

A possible source of hum is the AGC line, by the way.

I would not describe this as hum from what I saw when I was at Nick's - more like noise bars, which could be rectifiers.

Edit: also, there's some noise that seems video related, like the video or IF is radiating back into the antenna. This all goes away when the input is baseband into the first video.

Also, the voltages Nick reads show a much greater drop across the ballast resistors than what SAMs says, if I recall correctly, which would explain the excess heat. That says there's some kind of short, but finding it has been difficult. If it's in the IF or tuner, it might be possible to isolate it by resistance readings on the tube pins (as compared to the SAMs resistance chart). It's too bad SAMs doesn't have resistance readings from the pins of the ballast socket, which would isolate it to one branch or the other of the B+.

I had a bad short in a B+ line in my 71' Zenith reciently, and I tracked it down by unhooking the different branches of the rail, and checking the individual resistances to ground and then going after the one with the lowest resistance till I found the exact spot that was shorted. Since you can still run it you can probably find fault by disconnecting everything from the overloaded rail, and slowly reconnecting things untill the supply voltage drops drastically.

Just a thought.

Tom C.

I took a bunch of readings tonight, all done with an live picture on the screen and all circuits functioning.

Pin 3 of the ballast is a 430 ohm resistor, which feeds all 285v circuits that are not IF/tuner related. The voltage drop accross it is 96 volts, which according to ohms law means it's dissipating 21 watts. Inserting the current meter indicated that the circuits are drawing about 88ma from that pin.

Pin 2 of the ballast is an 800 ohm resistor, feeding IF/tuner. Voltage accross it is 70 volts, indicating a 6 watt dissipation. The circuits of this leg only draw 34ma, but that makes sense to me since there are a lot more plates to feed on the other leg.

Horizontal is still goofy. Something has GOT to be causing the HOT to draw excessive current, but I cannot figure out what it is. The loaded down horizontal section has got to be what's causing all the other issues, it's the only thing in common with every part of the set. Whatever it is, it doesn't seem to be affecting HV though.

You could always try adding a 100 ohm 10 watt cathode resistor on the HOT, just to see how much the current drops, and to what effect it has on the other symtoms. Is there any possibility there is leakage between the HOT socket pins?

Excessive horizontal output tube current can be due to a number of things. Is the horizontal drive correct? Excessive drive will cause excessive current.

What are the voltages around the tube? What does the horontal linearity look like? Depending how the linearity looks, that will provide a clue(s). How about a picture of a crosshatch pattern on the screen?

Have you checked and substituted the H O Tube? (Sorry, HOT to me means horizontal output transformer not tube). It could be gassy.

I am curious you say the EHT (+25,000v) is okay. How much curent is the 6BK4 shunt regulator drawing? It could be the EHT is low or high depending on the shunt current. Without the shunt, the EHT voltage should jump to +35000 to +40000. (Reminds me of the time my Grandma was watching her CTC11 and the shunt tube was bad. Her cat would like to sleep on the top of the set and she wondered why the cats hairs stood out. The EHT with my divider probe and VTVM measured +40000V. I suppose the cat had an especially good dose of Xrays when passing in front of the screen!

Terry

Drive is excessive, but I can't figure out why. I can't even get the drive lines to go away with the control and the oscillator tube has incorrect grid voltage as well, not sure where to go with this.

Lin is fine, nothing amiss there.

2 different ones, it's not the tube.

Don't know, when Wayne was here we checked the HV and it was fine. If the shunt were set incorrectly, I would expect it to be much higher, or if it were redplating much lower.

Since we can assume the oscillator is on frequency, have you set up the synchroguide with the horizontal locking range as per the service info?

Is the horizontal waveform amplitude applied to the grid of the 6CB5 correct?
Have you replaced C232 (0.01ufd)?

Does the horizontal drive control work properly eg do you see a change in the picture?



Are you sure they are drive lines? (I am suspicious because drive lines = bad linearity and you said the linearity is okay). If you rotate the horz hold so the picture moves to the right, do the lines remain stationary (drive lines) or do they move opposite to the picture movement (retrace line image)?



I was wondering if the HV reserve is there. I guess if you are getting lots of drive, the HV is okay. But if the brightness is advanced, does the picture bloom or does the width increase? (This would indicate the regulator is dropping out of regulation and the HV being generated is insufficient).

Yep, everything is set up right and I have equal peaks on the wave coil.

Yes, but the waveform has a flat top.


First thing I did.

It works, but because the oscillator is making such a large waveform it doesn't get rid of the drive lines.


The line moves opposite of the picture. That means it's not a drive thing?


No blooming, seems to have plenty of brightness. BTW, Wayne is probably right about the picture. It seems to be noise, not hum or anything else.

Yes, that means that either the retrace is a bit slow or the CRT is not blanked off during retrace. It isn't a drive line that you see.

The one white line is probably a video overshoot meaning that the perhaps a sync pulse edge is causing the line on retrace. The multiple lines to the left of it are the individual cycles of burst. The overshoot will generally be due to IF alignment or if the fine tuning moves the video carrier too far down the Nyquist slope. Do you see the line with the video injected at baseband?

I have seen the line made more obvious if the CRT RGB screen adjustments are set too high: they have to be set to ensure the beam current extinguishes on retrace.


Then that sounds okay to me. If it is picked up at the receiver front end, could it be your workshop fluorescent light?

Since it's not a vertical blanking issue, how exactly does this chassis blank horizontally? I don't see a blanking tube.

There is not blanking circuit as such. Blanking is done by the video waveform. Hence if the if there is a bad VIF response or misadjusted fine tuning, high video frequencies will overshoot producing a white line.

Try adjusting the fine tuning and reduce the RGB CRT screen adjustments.

Here's a scope trace, maybe you can get something from it.

http://videokarma.org/attachment.php...6&d=1316388575

Bottom is the composite output from the generator, top is from the grid of the second video amp tube. It does look like there's some fuzz on the blanking pulse, like the whole signal slid over a little. If so, are we talking sync issues or IF issues?

Maybe IF and maybe sync. Maybe both. What I am trying to say is that if the video frequency response is not flat or the phase response linear, you can get an overshoot at the sync pulse rise and fall. Getting a good wide bandwith with the tube technology of the past was a bit of a challenge. The problem is exaserbated with the use of vestigial sideband trasmission: broadcast transmitter design went to great lengths to try to mitigate the problem.

Also the broadcast sync waveform had a carefully controlled rise and fall time to help reduce this effect. Your test generator might have too fast a rise and fall time which may cause problems. Your modulator, like most consumer modulators, is double sideband so that should not cause a problem.

If your IF response is not flat or if the video carrier is too low on the Nyquist slope of the VIF response, this could lead to the white line appearing. I have attached a jpg photo I found on the web to illustrate what I had expected the sync to look like as applied to the CRT. Your waveform seems to have very fast rise/fall time sync pulses. Try expanding the waveform around the sync fall as an overshoot there may be seen in the video as the white vertical line.

Are you injecting at baseband or RF input? If at RF, if you adjust the fine tuning control to raise the video carrier to the top of the Nyquist VIF slope (that is adjust until you begin to lose picture detail and color and picture starts to smear) see if the line diminishes.

Lastly, consumer DTV boxes have proper broadcast sync rise and fall times so see if another video source causes problems.

It's getting worse, now an RF signal with a staircase won't even get through. All I get on screen is a jumbled mess, horizontal tearing, ect. I think it's high time to pull this chassis for a good while and put the B&K 415 on it, real detective work is in order.

I'm on mobile and cannot easily follow all responses. With regard to your hum, did you bypass your Si diodes with .001's? That looks like switching noise to me.

I decided to bypass the IF for the time being, injecting video into the 1st vid amp and disconnecting power to the IF strip. Found something odd: the bandpass amp grid is registering 0 volts, when it should be -15 or so. I can't figure out why it's not biased right, all the resistors in the circuit measure correctly and the caps are new. With the grid at 0, that tube would draw more current that it's supposed to which might explain the power supply drain. It might also explain the dot crawl I'm getting now.

Cured!
 

For continuity, this problem was finally fixed here: http://videokarma.org/showthread.php?t=252849


The voltages on the bandpass tube ended up being a Sams typo, the CT-100 chassis had the same readings. After fixing the problems detailed in the thread above, dot crawl is much better as well. I can probably make it disappear by tweaking the 3.58 trap in the luma line. So glad this is finally over. :smoke: