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Author Topic: Viking Ranger VFO Power Regulator  (Read 2378 times)
WA7KGX
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Posts: 104




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« on: August 06, 2011, 03:35:57 AM »

The Ranger regulates the VFO screen voltage with a dropping resistor from
low B+ and a 0A2 regulator tube.  Both the resistor and tube are in the
VFO box.  So is the heat they generate.

Were these parts put in the VFO box to make it run warmer, or warm up more quickly?
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AD5X
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Posts: 1426




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« Reply #1 on: August 06, 2011, 04:26:14 AM »

The dropping resistor not only generates a lot of heat, but it's power dissipation rating (2-watts) is really not adequate in this application.  A very popular mod is to move this resistor outside the VFO box, and to replace it with a 5-watt resistor.  I rebuilt my Ranger several years ago and documented the changes I made.  Info is in the "Articles" section of my website at www.ad5x.com

Phil - AD5X
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W8JI
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Posts: 9304


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« Reply #2 on: August 06, 2011, 06:23:56 AM »

NEVER use a carbon resistor at extended high temperatures unless there is just no other choice.
Johnson had little choice because there were no metal resistors available.

Carbon with heat ages down in resistance, and that increases heat. Eventually the resistor can nearly "short".

This is true in every application! It's what caused so many TV set fires.

Phil is right. Move the resistor OUTSIDE the VFO housing and use a metal type resistor. Either a 3 watt metal composition, oxide, or metal film type resistor or a wire wound. 3 watts is enough, 5 is better, but the critical thing is NOT to use carbon because carbon is a semiconductor and can fail shorted.


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N2EY
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Posts: 3837




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« Reply #3 on: August 06, 2011, 04:57:56 PM »

Even the best designers make mistakes. I can see no reason to have the voltage regulator and dropping resistor inside the VFO box.

AD5X's changes are a very good idea. He has a whole list of them.

The 18K dropping resistor is between the 300 volt B+ and the 150 volt 0A2. In theory, it dissipates 1.25 watts, which somebody at EFJ probably thought was OK. (Of course that assumes the 300 volt B+ is really 300 volts).

But, in my limited experience, resistor power ratings should be considered as inflated by at least a factor of 2. I would never ask a 2 watt resistor to dissipate more than 1 watt. In a tightly closed space such as a VFO cabinet, I'd expect even less.

Why EFJ didn't use a 5 watt wirewound (available at the time) outside the box is beyond me. While economy is always a consideration, the price difference back then was maybe 20 cents, tops.

73 de Jim, N2EY
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W5JO
Member

Posts: 59




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« Reply #4 on: August 06, 2011, 07:25:33 PM »

NEVER use a carbon resistor at extended high temperatures unless there is just no other choice.
Johnson had little choice because there were no metal resistors available.

Carbon with heat ages down in resistance, and that increases heat. Eventually the resistor can nearly "short".

This is true in every application! It's what caused so many TV set fires.

Phil is right. Move the resistor OUTSIDE the VFO housing and use a metal type resistor. Either a 3 watt metal composition, oxide, or metal film type resistor or a wire wound. 3 watts is enough, 5 is better, but the critical thing is NOT to use carbon because carbon is a semiconductor and can fail shorted.

I have seen the subject resistors go so low in resistance that it appeared as a short causing the regulator tube to overheat so much that it sucked the envelope in and burned the board.  Move that resistor outside.
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G3RZP
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Posts: 4368




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« Reply #5 on: August 07, 2011, 03:39:59 AM »

I'm currently restoring a piece of 1950's test gear. In the PSU, there's a radial lead (that gives the age away!) 10 Kohm 1 watt carbon. It's obviously got hot. because some of the wax has melted out. It is a 10% resistor, and it measures 3% high.....VERY much to my surprise.

Some of the other resistors have moved by more than 50%: some are still well within tolerance.
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WA7KGX
Member

Posts: 104




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« Reply #6 on: August 08, 2011, 02:29:10 AM »

I   opened up the VFO box and snipped the 18k resistor.
It measured 19+ k and showed no signs of distress.
I replaced it with a 25k 5 watt resistor from the junk box.

A word of advice --- do not remove the top cover of the VFO box
unless you really have to.  The fiber shafts loosen up with age
and the springs tend to fall out.  Getting the shafts in place correctly
is a character building experience.
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W8JI
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Posts: 9304


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« Reply #7 on: August 08, 2011, 03:03:56 AM »

I   opened up the VFO box and snipped the 18k resistor.
It measured 19+ k and showed no signs of distress.
I replaced it with a 25k 5 watt resistor from the junk box.

A word of advice --- do not remove the top cover of the VFO box
unless you really have to.  The fiber shafts loosen up with age
and the springs tend to fall out.  Getting the shafts in place correctly
is a character building experience.

......Why did you add 40% to the resistance of the resistor that sets quiscient current in the voltage regulator tube?Huh Are you sure the voltage regulator will stay in range under all conditions?
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N2EY
Member

Posts: 3837




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« Reply #8 on: August 08, 2011, 03:59:01 AM »

I   opened up the VFO box and snipped the 18k resistor.
It measured 19+ k and showed no signs of distress.
I replaced it with a 25k 5 watt resistor from the junk box.

Let's do the math!

The VR tube operates at 150 volts. The B+ line is a nominal 300 volts. Difference is 150 volts.

With an 18K resistor, there's 150/18,000 =  8.33 mA flowing

With a 19K resistor, there's 150/19,000 =  7.89 mA flowing

With a 25K resistor, there's 150/25,000 =  6.00 mA flowing

The VR tube needs at least 5 mA through it to work right, so there's only 1 mA left for the VFO tube.

73 de Jim, N2EY



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WA7KGX
Member

Posts: 104




Ignore
« Reply #9 on: August 08, 2011, 04:39:22 AM »

I   opened up the VFO box and snipped the 18k resistor.
It measured 19+ k and showed no signs of distress.
I replaced it with a 25k 5 watt resistor from the junk box.

A word of advice --- do not remove the top cover of the VFO box
unless you really have to.  The fiber shafts loosen up with age
and the springs tend to fall out.  Getting the shafts in place correctly
is a character building experience.

......Why did you add 40% to the resistance of the resistor that sets quiscient current in the voltage regulator tube?Huh Are you sure the voltage regulator will stay in range under all conditions?
It drops the current from about 11 to 8 mils.  My RCA tube manual indicates the 6AU6 screen only
draws a few mils, leaving 5 or more for the regulator tube.   Besides, 25k is what was in my junk box.
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