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  #31  
Old 06-09-2017, 06:06 PM
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VintagePC VintagePC is offline
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Quick update before dinner. Got my tubes today, popped in a new 1B3 and a real 6AX5.

WE HAVE RASTER!

Baby steps...

No video signal yet, but given some of the things I have had to do so far that's not surprising. Ion trap magnet probably needs adjusting, as do all the controls (I had to disassemble and work many of them to clean them, so they are likely all out of whack).

To be continued...
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  #32  
Old 06-09-2017, 08:32 PM
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Quote:
Originally Posted by VintagePC View Post

Electronic M - In retrospect, I'm intrigued by your comment... One would think sets would be designed so that typical failure of any arbitrary tube would result in minimal or no damage. What is it about the circuit design that makes the HO tube red-plate if there is no oscillator? Are there other parts of the circuit that are similarly affected if another single tube fails?
There were a couple of chassis I'm aware of (IIRC the CTC-2 was one) with fail safes, but most neglected them. In most output stage designs the output tube is biased by DC, but in the case of TV Horizontal outputs the grid is normally biased by grid-cathode rectification of the horizontal osc drive waveform....No waveform no bias...Or more accurately the grid in absence of the waveform pulling it negative instead drifts positive in lieu of it's bias source and ceases to limit the cathode current. The cathode current skyrockets well beyond it's design ratings, the emissive cathode material rapidly burns off, the plate and grids get more energy than they are designed to handle and sometimes melt, and the fly is subjected to DC current well in excess of it's ratings and begins to heat...Sometimes until burning open.

It ain't pretty seeing a Horizontal output destroy it's self...Well maybe it is, but it ceases to be once you know what your seeing.
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  #33  
Old 06-09-2017, 09:25 PM
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Fascinating, it's the pre-silicon equivalent of SCR latch-up *grin*

Good news, we have video! Problem was "dumb user". Sometimes my cable box gets in a state where it will output sound but no video (while off). So when I powered the set and got audio, I foolishly assumed the box was on. Actually turning on the cable box got me a picture. It's watchably bright with the controls low, so it looks like the picture tube is in pretty good shape.



Spent some time poking and prodding. Had to adjust the horizontal drive waveform, probably because there was a cap right across the relevant coil that got swapped (all the ones that came out of the set were well out of spec).

A couple of issues observed that I will need to dig into. Some input here would be helpful, but if not I will start from square one since I have three different service guides with all the voltages and waveforms.

1. Looks like bad HV regulation, or low HV. Turning up the contrast or brightness controls makes the picture bloom and lose focus. It fades if you go further, and comes back if you turn it down. The focus control is also at the extreme end of its rotation.
I surmise this may have something to do with the non-connected doorknob cap?

2. Possibly related, picture takes an appreciable amount of time to come up, and does so very slowly. (A good 1-2 minutes. My previous tube set would have a picture not long after you heard the HV come up)

3. Observed that the width coil does not seem to have an effect when adjusted. I'll check if it's gone open.

4. I was setting up the horizontal oscillator per P 18 of the SAMs. There were times where I got to a point that something would go funny when adjusting the front horizontal hold. I'd hear a 'thup-thup-thup' noise both from the speaker and somewhere in the set, and the horizontal drive waveform flickers on the scope screen.
Something arcing somewhere? Didn't see anything immediately obvious.

Last thing... remember that rectifier's melted cathode connection? I just had a closer look at it. Check this out - there isn't much left of the cathode either:


Dayum, looks like this set basically had a complete meltdown in its past, must take some serious business to get that kind of sustained arcing in a rectifier. But it's all right now, it's been brought back from the dead!
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  #34  
Old 06-09-2017, 09:38 PM
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I know, it's been awhile since this thread started, but, may I see the set as it's playing now? Seriously, those focus rectifiers look as if they need seriously being replaced..... Thanks. Interesting thread. Thanks.
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  #35  
Old 06-09-2017, 10:39 PM
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Just remembered that I once had a halicrafters model 822.
The set was beyond with a dud tube, and, an arcing flyback.
I gave up.
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  #36  
Old 06-10-2017, 08:42 AM
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Not sure I follow, Bill. The photo in my last post is of it currently playing after replacing the 6AX5 and the 1B3. Did you want a video, or a photo of some specific thing?
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  #37  
Old 06-10-2017, 12:47 PM
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I think I'm on the trail of the HV problem... Looks like the 1B3 socket may be dirty or bad; I noticed that turning up the brightness slowly the picture just fades out; turning it up quickly I heard a snap from the rectifier area. Didn't see anything visible, but will disassemble the area and look closer/clean it.

Edit: Further update - Just finished cleaning the socket, also found the 3.3 ohm resistor was nearly 7 ohms. After replacing it and reassembly, things are looking better. No more fade-out when the controls are turned up, but the picture is still very jittery and I still hear corona hiss coming from somewhere in the HV area.

Last edited by VintagePC; 06-10-2017 at 02:45 PM.
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  #38  
Old 06-10-2017, 06:48 PM
EdKozk2 EdKozk2 is offline
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Is that old doorknob still in the circuit? If it is, it may be intermittently shorting, causing all kinds of strange effects.
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  #39  
Old 06-10-2017, 08:20 PM
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No, it was actually out of circuit. It still measures the proper value, and had a decent charge on it when I went to discharge the CRT... got two separate snaps, one from the picture tube anode, and one from the corona ring attached to the 'knob.

I put it back in to see if it would help any. Picture is maybe a little stabler, but still jittery and shifts left and right a bit. There's also no more discharge from the HV lead to the corona ring on warm-up, since it's now at the same potential.
Also replaced the HV lead, the old one was pretty crusty and the rubber parts were cracked. I had a spare more modern one I scavenged from an old monitor some years ago. Glad I never threw it out.

The jitter seems to get worse/better as I adjust the contrast control. Control is good though, I verified that it changes smoothly on a meter as I operate it.

Width coil is not open, but I haven't tried it since fixing the first HV issue.

I've tried looking around the HV area in the dark and nothing obvious is visible, but I'm also not able to see some parts well with the set on its side. I'll try again tomorrow - I feel comfortable wiring in the NOS dual pot for focus and brightness now that I know the replacement focus control isn't going to instantly burn up. That will let me flip the set rightside up again and take a better look around. There's still some electrical tape handiwork around the flyback that I haven't replaced yet, also worth investigating.


Maybe relevant but I also hear three or four "thup" noises as the HV comes up. Identical to what I heard when I was messing with the hold adjustments. I wonder if it's just the vertical hold trying to lock in.
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  #40  
Old 06-11-2017, 02:08 PM
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Doesn't bode well... I still hear the hiss and subtle "warble" that accompanies the flicker in the HV whine if I run the set without the HV rectifier top cap connected... so no HV to the tube or filament winding.

Does that mean what I think it means... that the flyback probably needs replacing because it's got an internal problem?
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  #41  
Old 06-12-2017, 12:14 AM
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Your problem may also be failing hv wiring. A good thing to do...
First, with tv turned off, look at high voltage donut coil. Does it have any pin prick holes in it? Try running the set with hv cage open, in a dark room.. Do you see any corona areas?
Where?

The earlier comment I made was just a note on the one I had. It's interesting that yours is an 815, and, mine was an 822.
Mine was very ugly. It used a 16gp4. It had a big rectangular metal mask.
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  #42  
Old 06-12-2017, 07:02 AM
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I think there's some more at play. I did suspect the filament wire on the 1B3 as that has to carry the HV too. Didn't see any obvious damage but I relocated further from the chassis to see if it would help. I'll retry the dark test tonight now that I have a better view of the area (I completely removed the HV cage)

I also found some interesting behaviour with the horizontal area while swapping around some tubes. That led me to dig deeper and find some more resistors that are way out of tolerance. I'll be replacing those first since it's low-hanging fruit.
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  #43  
Old 06-13-2017, 03:09 AM
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Smile



The old interest is coming back........
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  #44  
Old 06-14-2017, 05:38 PM
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Of course it stopped doing it when I tried to continue locating the source of the HV leak. Nothing obviously visible in the dark, but I did keep thinking I'd occasionally see faint blue-purple auras here and there before they'd go away. The orange glow from adjacent tubes reflecting off surfaces doesn't help. Will continue trying.

Side question, anyone have a picture of a stock flyback for any of the Hallicrafters 800 series? Best I have is the B&W drawing in the Sams, and it's not very detailed. Google is no help.
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  #45  
Old 06-14-2017, 08:31 PM
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Progress! Was still doing it after I replaced all the wonky resistors. Much scrutiny later, I still didn't see anything, but could still hear the HV hiss. It stabilizes and gets better as the set warms up but still has occasional bouts.

Figured I'd try to make the problem a little more visible.
Cobbled together a chicken-stick with a grounded lead and a multi-megohm several-watt resistor (so I don't draw a giant arc, just enough to make leakage visible), passed it all around the filament wire. Nothing, that's all good.

Started nearing the outer edge of the flyback donut (well away from the lead coming out). A-HA! started getting some glow going, some spots I even get ticklers. Looks like the HV insulation on the flyback has broken down some and isn't quite up to the task anymore. Added a few turns of electrical tape to see if it helped my theory. This time the image came up and stabilized MUCH faster than before. Still funny, but better, so there's something to that line of thought. It probably stabilizes after a bit once the air around the flyback has built up a charge.


I'll probably need to recoat this flyback. I know there are tubes of silicone specifically for this purpose as I've seen Bob A use it in his videos, but I'm looking for something cheaper and more single-serve... Is Corona Dope the suitable thing for this?

I'm thinking I'll have to carefully remove the old wax and give it some hefty coats - if it's breaking down then just smearing a coating over top of it isn't going to help much.

Thoughts/comments?
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