21-CT-55 power supply failure - Page 3 - Videokarma.org TV - Video

miniman82 · #31 09-15-2011, 09:34 PM

Bought some IF tubes today, didn't make much difference. The noise got a little lighter, but it's still there. I'm not sure how to tell what frequency this is, I only know that there are bars floating up the screen. I'd shoot a video, but I'm not sure the camera would pick up on it.

The noise is only there when passing a signal through the tuner/IF, when I put composite on it the tuner/IF were not powered and there was no noise.

Electronic M · #32 09-15-2011, 10:19 PM

I also have an unrestored heathkit cap checker I'd be willing to sell that is burried in storage, but can probably be located next time I go there, if you want to own one.

miniman82 · #33 09-15-2011, 11:11 PM

As long as you take that Silvertone in trade, I'm game. lol

Penthode · #34 09-16-2011, 09:06 AM

120Hz is ripple from the B+ supply. With 120Hz you will see two distinct bars floating up the screen.

60Hz will originate from the mains directly and you will see a single bar.

How many bars are there? Can you post a photo of the screen with the bars present?

And have you substituted the tuner tubes?

Electronic M · #35 09-16-2011, 12:15 PM

If you are refering to the console that I did not take then the only way that will happen is if I sell some TVs or toss some out. My folks have ordered me to stop collecting anything bigger than large table radios for an indefinite period of time.

Tom C.

CT-100 · #36 09-16-2011, 02:49 PM

Yes I can make the measurement, but this again, is going to take a few days. My primary delay is that I have not powered my chassis up since I am working on the CT-100. I am aware of your request and will do what I can as quickly as I can. If you lived in my neighborhood, I'd give you my chassis and you could take it to your lab for precise comparisons.

I have a lot of background with this particular set as my dad brought 5 or 6 of them home as they became obsolete; one even had an IF-RF problem like you have described. We're talking 1968, so I have to review the schematic and try to reverse engineer the solution he discussed with me. (I couldn't work on sets then, he did and I only watched the process)

If the grid bias is too high, then the grid current must be too low [OR] the tube is being over-driven. I need to get my diagram; is there a cathode resistor in the deflection output; if so, is it by-passed? I will look this weekend for sure.

I'm max'ed at work and have not fairly studied the responses ahead of this, so forgive me if I am repeating suggestions. I admit, I have not done my homework yet so please take these comments with that in mind.

Regards.

miniman82 · #37 09-16-2011, 03:23 PM

Quote:

Originally Posted by Penthode

120Hz is ripple from the B+ supply. With 120Hz you will see two distinct bars floating up the screen.

60Hz will originate from the mains directly and you will see a single bar.

How many bars are there? Can you post a photo of the screen with the bars present?

And have you substituted the tuner tubes?

To me it looks like a pair of bars on screen at one time travelling up through the pisture, but the power supply has no ripple so I'm not sure what to think. I've already subbed every tube in the tuner/IF and even the first video (6CL6), nothing made an improvement. If I had to pick a word, I'm flummoxed.

Quote:

Originally Posted by Penthode

If the grid bias is too high, then the grid current must be too low [OR] the tube is being over-driven. I need to get my diagram; is there a cathode resistor in the deflection output; if so, is it by-passed?

I think the tube is being overdriven, because it's DC grid bias is not what it should be. As I said before, I'm going on the assumtipn that the cause of the flat topped waveform at the grid is being caused because the tube cannot be driven any harder than it already is. Result: power comes from the only place it can, in the form of grid current.

Penthode · #38 09-16-2011, 04:47 PM

It seems you explored the most obvious plus some of the most obscure prblems. maybe signal tracing with a scope is the best way to track this down.

Can to apply to the antenna a test signal of a flat field of grey. Ensure you can see the bar clearly on the CRT. Then 'scope back from the CRT stage-by-stage. I'd expect you should see the hum when scoped at the CRT and I would trust at some stage the hum will disappear from the scoped waveform.

At this stage I am as flummoxed as you...

Terry

old_tv_nut · #39 09-16-2011, 04:56 PM

I'd suggest trying to duplicate the SAMS resistance readings at the tube pins in the IF and tuner - might lead you to an anomaly.

CT-100 · #40 09-16-2011, 06:43 PM

You know what's interesting? My SAMS shows 60 Hz Hum even though this is, in fact, a full wave PS configuration.

My RCA data is quite explicit, the drive waveform geometry is critical to the proper operation of this circuit.......

miniman82 · #41 09-16-2011, 07:24 PM

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.

CT-100 · #42 09-16-2011, 08:48 PM

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?

Penthode · #43 09-16-2011, 10:44 PM

Quote:

Originally Posted by miniman82

To me it looks like a pair of bars on screen at one time travelling up through the pisture, but the power supply has no ripple so I'm not sure what to think. I've already subbed every tube in the tuner/IF and even the first video (6CL6), nothing made an improvement.

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?

miniman82 · #44 09-16-2011, 11:53 PM

Quote:

Originally Posted by Penthode

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.

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.

Penthode · #45 09-17-2011, 12:30 AM

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?