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Johnson Ranger modulator screen voltage adjust modification.




 
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Author Topic: Johnson Ranger modulator screen voltage adjust modification.  (Read 506 times)
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Knightt150
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« on: January 21, 2018, 09:06:26 PM »

Is there some kind of kit on the market that would get rid of that 20K chernoble resistor, one that puts out low heat?

John W9BFO
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DMOD
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« Reply #1 on: January 23, 2018, 12:38:36 AM »

I don't know of any kits but here is the way I modify the Ranger HV power supplies:


Phil - AC0OB

* Ranger HV PS.pdf (27.45 KB - downloaded 39 times.)
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MikeKE0ZUinkcmo
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« Reply #2 on: January 23, 2018, 01:48:24 AM »

What is commonly referred to the Chernobal(?) resistor is the 18K in the VFO, but R-35, the HV bleeder/modulator screen resistor is a 20K adjustable 50 Watt 20K unit mounted on the right side of the chassis, also dissipates about 20 Watts as well.   Not sure which you speaking of but here goes.
      
The VFO resistor can simply be remotely located somewhere outside the VFO housing, and wire run into the VR tube.

If you are talking about the HV bleeder....

The Ranger is apparently well known for its high levels of wasted power dissipation, and the authors below address that and other issues using various approaches to the problems.   There are no kits that I'm aware of for any modifications to the Ranger, however there are several modifications proposed by WA1HLR, W3AM, and AD5X.   I'm sure there are others, but these seem to be the most prolific.

To answer your question, the only way to reduce the power dissipation is to increase the value of the bleeder resistor, R35.   Doing so will result in increased filter voltage in the high voltage power supply, which in turn requires replacing the current C77 with a series string of 3 450V filter caps, with 330K resistors across each cap.   Additionally, since R35 was a tapped resistor establishing the modulator screen voltage, I simply moved the screens to the low voltage supply.

Depending on what version of Ranger you have, there can be quite a bit of work involved to make the Ranger a much more efficient, dependable, and better sounding transmitter.   The example I recently purchased was a very early version which has required quite a lot of work, and I'm nearing the end, hoping I'll get many hours of enjoyable use out of it.   I'm also going to include a Hi-Lo power option so I can use it with virtually any amplifier, as well as doing some low power operation.   Don't know if It'll be able to do real, less than 5 Watt QRP, but I'll be close.  
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Mike KE0ZU

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Knightt150
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« Reply #3 on: January 25, 2018, 09:04:54 PM »

Thanks guys for all you efforts and the comeback on this.


John W9BFO
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