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 Post subject: Need help with Hallicrafters 506 HV
PostPosted: Feb Fri 21, 2020 1:18 am 
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Joined: Jan Thu 14, 2016 1:18 pm
Posts: 18
Location: Niagara Falls, NY 14304
Hi everyone,
I'm doing my first restoration of one of these and I'm in need of some help. It has been recapped and most resisters replaced as well. Sound and video are good but HV seems low and crt is dim. Deflection and focus seem fine. I set up the 10 meg (5) in series and checked the micro amps per instructions expecting it to be below the 95 required to have 4700 volts. Instead it reads 380 micro amps and adjust up to about 450 and down to 200 or so before raster disappears. Same with brightness all the way down. Am I doing something wrong?


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 Post subject: Re: Need help with Hallicrafters 506 HV
PostPosted: Feb Fri 21, 2020 3:21 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 753
Location: Crystal Bay, NV
Your 5 resistors are in series, giving you 50 meg, Right? Just checking.
I ask because 300uA through 50 meg would take 15kv, and this is not possible.
=====
Ron


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 Post subject: Re: Need help with Hallicrafters 506 HV
PostPosted: Feb Fri 21, 2020 8:37 pm 
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Joined: Jan Thu 14, 2016 1:18 pm
Posts: 18
Location: Niagara Falls, NY 14304
Yes Ron, 5 10 meg resisters in series with meter across .001 mmf 6000v cap. I forgot to mention that the transformer windings resistance readings are right on except for the 450 ohm winding that comes in at 320 ohms. I'm thinking shorted turns on that winding. The windings look and smell good but that doesn't mean much. If the low voltage power supply wires were reversed, I'm sure it wouldn't work at all, but I'll check the voltage at the plate of the 6C4 osc to be sure . All caps except two small pf in HV enclosure have been replaced. All the resisters have been replaced.


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 Post subject: Re: Need help with Hallicrafters 506 HV
PostPosted: Feb Sat 22, 2020 2:48 am 
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Joined: Mar Sun 01, 2009 10:27 pm
Posts: 5263
Location: Ann Arbor, MI
A shorted turn would cause the high voltage to be low. But your measurement says that the high voltage is impossibly high. There is no fault in the transformer or anywhere else that could do that. I wonder if your 10 Meg resistors are not really 10 Meg. Do you have an ohmmeter that can measure 10 Meg ohms? Try checking the resistors and see if they are the right value.

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Tom


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 Post subject: Re: Need help with Hallicrafters 506 HV
PostPosted: Feb Sat 22, 2020 4:34 am 
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Joined: Jan Thu 14, 2016 1:18 pm
Posts: 18
Location: Niagara Falls, NY 14304
Thanks Ron and Tom for your help. The resisters were purchased specifically for this purpose and measure 10 megs on my ohm meter totaling 50 megs. One end of the series string is connected to the ground side of the cap. The other end is connected to the negative lead of my digital multimeter and the positive lead is connected to the other end of the cap. I've tried different ranges on the microamp scale but reading is the same. Maybe my meter is reading wrong. Would an analog meter be better for this test? Can I use a high voltage probe to measure the hv?


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 Post subject: Re: Need help with Hallicrafters 506 HV
PostPosted: Feb Sat 22, 2020 7:51 pm 
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Joined: Mar Sun 01, 2009 10:27 pm
Posts: 5263
Location: Ann Arbor, MI
I wonder if having the meter at the hot end of the resistors could have an effect. Try putting it at the ground end of the string of resistors. And if you have another meter, try that. A high voltage probe should work fine. The string of resistors is a homemade way of making a high voltage probe.

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Tom


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 Post subject: Re: Need help with Hallicrafters 506 HV
PostPosted: Feb Sun 23, 2020 12:45 am 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1457
Location: Champaign IL 61822
The meter itself should be at ground potential as should its negative lead.

The resistors go from HV supply to the positive meter lead. There should be
a capacitor across the meter leads, perhaps 0.05 uF, to filter out any
residual AC.

I've had fits measuring CRT HV with digital meters, and I have resistors
of 50, 100, 200, 750, and 2150 megohms.

Its also possible to check the HV using a resistor chain and a scope, just be sure
the voltage divider divides down into range of the scope input!


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 Post subject: Re: Need help with Hallicrafters 506 HV
PostPosted: Feb Sun 23, 2020 4:32 am 
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Joined: Jan Thu 14, 2016 1:18 pm
Posts: 18
Location: Niagara Falls, NY 14304
Thanks everyone for your advice. I don't have another meter at this time, that measures microamps but I will get one. Obviously the reading can not be correct and the HV is low. I will try the meter I have( digital multimeter) inserted at the ground or negative end of the series resisters to see if that changes anything. If the current is the same, I'll get a different meter. In the mean time I'm looking at the resistance of the windings. The 450 ohm winding reads 320 ohms. After reading other post on this forum discussing moisture in the transformer lowering the HV, I decided to test this by putting the transformer in the kitchen oven at 225 degrees with the ohm meter connected through 3 foot leads to the countertop where I could observe the winding resistance. In one hour the resistance had gone from 320 ohms to 440 ohms. That tells me that moisture is a probable cause for the HV being low. So I'll dry it out over several hours and seal it with spar varnish as another member has suggested on this forum and then see what I have. By then I should have a different meter to check the microamps.

Thanks for the help. I will let you know how things work out. Please post any other ideas that might help me solve this weird problem.

Larry


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 Post subject: Re: Need help with Hallicrafters 506 HV
PostPosted: Feb Sun 23, 2020 4:52 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 753
Location: Crystal Bay, NV
I just don't trust your meter. I use an old RCA VTVM that's from the same era as the TVs that I restore. Very trustworthy. I don't understand how the resistance would change that much just by heating a little and driving any moisture out. You can probably find an old analog meter for about $20, and it will be easier to notice changes as the meter swings around (as when aligning a sound discriminator).
=====
Ron


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 Post subject: Re: Need help with Hallicrafters 506 HV
PostPosted: Feb Thu 27, 2020 3:20 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 753
Location: Crystal Bay, NV
After thinking about my last post, I decided to check on the temperature coefficient of copper wire. I found it to be .393% per degree Celsius from room temperature. Using the 225 degree oven and a 75 degree room, gives an increase of 150 degrees or 83 degrees Celsius. Multiplying by .393 gives an increase of 32 percent. So, 320 ohms times 1.32 gives 422 ohms. So, I was wrong and your resistance reading looks correct. However, the change you saw was not because there was moisture; any good coil would show the same change.
====
Ron


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 Post subject: Re: Need help with Hallicrafters 506 HV
PostPosted: Mar Wed 04, 2020 5:25 pm 
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Joined: Jan Thu 14, 2016 1:18 pm
Posts: 18
Location: Niagara Falls, NY 14304
Hi everyone,
Ron, I now agree with you about the moisture in the windings. After cooking the windings and insulating them, I found that the resistance returned to about 340 ohms (it was about 320 ohms) instead of staying over 400 as I was hoping. What I did find though was that the connections in the HV circuit were not correct. This chassis had under gone work before I got it and I've found many mistakes. My guess is the wrong schematic was used to repair it. I found that Sam's 91-6 matches some of it (like IF's being 6au6) but Sam's 48-10 a better match for other circuits. Some mistakes just serviceman errors. Anyway, the current now reads 93 microamps with brightness turned down with my original meter. Picture brightness and contrast have improved greatly. I want to thank you and all who gave me guidance !


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