Predicta Holiday - First power up attempt - Page 8 - Videokarma.org TV - Video

Electronic M · #**106** 10-24-2016, 07:17 PM

Quote:

Originally Posted by WISCOJIM

You still haven't, it is a color set. In the book he is explaining that you do the degaussing to eliminate a tinted picture to obtain the best greyscale image possible on a color TV set. Therefore the color control is turned down.

Page 68 in the book shows him doing the degaussing on the same set, and the set says "color" on the control door below the on/off/volume control.

.

That makes a lot more sense. (IIRC I don't have any literature by that author.) I can't recall seeing someone do a degauss without the screen being a primary color or in normal opperation. In my experience only the worst gaussed tubes show it on a monochrome picture, but if you have a primary color raster even the smallest impurities show up clearly....

madlabs · #**107** 10-24-2016, 07:51 PM

Jim,

Ordered a copy, $5 delivered. Has to worth at least that! ;-)

Thanks for the suggestion. Anyone else got a favorite site or reference for TV troubleshooting?

Jonathan

WISCOJIM · #**108** 10-24-2016, 08:53 PM

Quote:

Originally Posted by madlabs

Jim,

Ordered a copy, $5 delivered. Has to worth at least that! ;-)

Thanks for the suggestion. Anyone else got a favorite site or reference for TV troubleshooting?

Jonathan

Can't take credit for that, it was N2IXK's suggestion. I just happened to have a copy of the book handy.

I agree that it was a good suggestion.

.

madlabs · #**109** 10-25-2016, 09:42 AM

Oops! Thanks for the correction.

madlabs · #**110** 10-30-2016, 04:15 PM

Hi All,

Just not enough time in the day. Anyway, I have gotten a little time in on the set. Rio and I polished the shroud and it came out nicely. We plan to start on the CRT cover this evening.

Got a lick in looking for the nasty hum. I still can't properly adjust the audio alignment due to a busted slug in A10/11. I have bought a parts set and hope to have a replacement soonish, but it will be some weeks before I pick it up. I don't think it is the alignment, you can see the 'scope trace below. It's just a really cheap digital 'scope. Nonetheless, you can see a clear, loud 120Hz signal and a lot of other nastiness. Things I have done and observed:

Volume control varies hum loudness.
Touching the volume pot reduces hum quiet a bit, although it is still there.
Removed the 3BN6 (audio connector) tube and put a 5 ohm resistor across the socket filament. Hum still there.
Connected an extra 100uF cap across the 140V supply. No change.
It's a new 12CA5. Don't have another at the moment. Old one is gassy and dead.
Cried, whined and complained. Didn't help, no change.

I gotta figure this out. While the picture still needs quite a bit of work, the set is still fun to watch. But the hum SUCKS!

Thanks!

Jonathan

old_coot88 · #**111** 10-30-2016, 05:42 PM

Try turning the vert. hold control fully stop-to-stop and see if the hum (or buzz) changes pitch.

madlabs · #**112** 10-30-2016, 05:53 PM

OC, without the CRT connected it makes no difference. With the CRT connected, IIRC, there is a high pitch whine at either end but not sure what the hum does. I can reconnect the CRT and and see if needed.

old_coot88 · #**113** 10-30-2016, 08:29 PM

The idea was just to see if the hum might be vert sweep bleeding into the audio. But since the pitch doesn't change with varying the vert sweep rate, that's not the source of the hum.

Electronic M · #**114** 10-30-2016, 10:20 PM

I'd recommend getting 3 examples of each of the audio tubes to try substitutions. Scope around. If it is not the tube it's self making the hum it must be on a PS rail or one of the pins feeding the tube.

madlabs · #**115** 10-31-2016, 10:16 AM

Tom, the noise persisted when I pulled the audio detector tube and put a resistor in-line for the series filament. So I *think* that pretty much confines it to the audio amplification stage and power supply.

My Tek 'scope is failing and I am too chicken to put my digital 'scope on the power supply. My gut tells me that is where it is but the caps are all new, so what else could it be?

Electronic M · #**116** 10-31-2016, 11:30 AM

It could be the tube it's self...I've seen tubes that just liked to pickup and amplify hum even when all sources in circuit had been killed (heaters running on regulated DC, B+ clean and regulated, shielding added, etc, etc).

old_coot88 · #**117** 10-31-2016, 12:01 PM

I don't have the schematic, but suspect that the set may use a 'stacked' audio output stage (that's where the output tube does double duty, serving as a big fat dropping resistor supplying B+ to the IF strip etc. Maybe someone could post a pic of the audio section schematic. There may be something wonky with the voltages on the output stage.

madlabs · #**118** 10-31-2016, 12:19 PM

Just on the way out the door for my 48 hour workday. I couldn't quickly figure out how to get just the audio section of the set, so here is a link to the whole thing:

http://www.earlytelevision.org/pdf/P...Sams-439-1.pdf

I don't have a second 12CA5 ATM. If I had known the one I had was dead, I would have gotten a second. I'll place a tube order soon. But until then I'd like to keep running down other possibilities. I also tried moving wires around, especially the ones that go to the volume pot and back, no joy. So probably not noise being coupled in.

Thanks folks!

madlabs · #**119** 11-05-2016, 04:06 PM

Howdy Folks,

Been playing with the Predicta a bit and have discovered a few things. Firstly, a lot of the noise was because I have to drive the video modulation and signal pretty hard to get the set to show a good picture. I'm guessing that is because I haven't touched the IF and the tuner circuits yet. However, careful tweaking of everyting has yielded decent audio with totally acceptable hum. I also put a .1 cap from the volume pot body to ground, which helped quite a bit. The volume is a little low but OK. Remember that I can't adjust A11-12 because of a broken slug. I do have a parts chassis coming something soon. The volume pot is supposed to be 2.5 megs and is actually 3.5 megs. I wouldn't think that was a big deal, but is it?

So, on to today's questions: If I replace the resistors in the tuner section, am I likely to throw the alignment way off? I don't want to mess with IF alignment, even though I may have the tools to do it. So if replacing the parts will likely mess up the alignment I'll leave it as it is.

One thing I have noticed is that the scan lines are thicker at the bottom than at the top. What causes this?

Oh, and got the book. Looks good, should get a chance to really read it this week.

Thanks!

Electronic M · #**120** 11-05-2016, 05:18 PM

Unless resistors have drifted high AND out of tolerance it is best to leave them be.

Your discussion of audio and video levels being high on the modulator to be right on the set bothers me. If the audio and video LEDs on the modulator are lighting then you are running it at over modulation (which causes audio buzz/hum). What you should do is adjust the modulator to NOT over-modulate/look right on a modern analog set, then adjust the AGC on the Predicta for sufficient contrast or max contrast before over-drive/sync tearing/negative video occur (ie make it bad then back off a bit till it is good again).

Scan line size is a function of vertical linearity. Have you adjusted the vertical with a grid test pattern yet?