Viking 2 low plate current

[AB2EZ]

I hope someone will post after me, because I don't like to have the last word in these types of threads.

With respect to choice of the rated wattage of a replacement for R28:

You can assume the the screen current, at carrier, is approximately 22.5mA. That is, 22.5mA x 20,000 ohms = 450V. So if the 6146 plate voltage, at carrier is 650V, then the screen voltage would be 650V - 450V = 200V... which is about right.

If the screen current, at carrier, is 22.5mA, then the power dissipated in the resistor, at carrier, will be 22.5mA x 22.5mA x 20,000 ohms = 10.125 watts.

To take into account modulation, a reasonable worst case dissipation would occur with 100% modulation with a sine wave. That would produce more average dissipation in R28 than voice modulation.

With 100% modulation, with a sine wave, the dissipation in R28 would be 50% higher than it is at carrier. I.e. 15.2 watts.

Actually, the largest dissipation for R28 occurs when the operating mode is CW (HV B+ on), and the key is up (open). In that case, the clamp circuit is on, and the voltage across R28 will be almost the full HV B+. If the voltage across R28 is 600V, the power dissipated in R28 will be 18 watts.

Therefore, for a one-off replacement (as opposed to the production of many transmitters), I think the dissipation rating of the replacement resistor should be 25 watts or greater.

Stu

[KA2DZT]

If that resistor is in an area with little air movement (like under the chassis) you should further derate the power rating and use a larger power resistor.  Maybe go to a 50 watter.

Fred

[W3GMS]

I hope someone will post after me, because I don't like to have the last word in these types of threads.

With respect to the rated wattage of R28:

You can assume the the screen current, at carrier, is approximately 20mA. That is, 20mA x 20,000 ohms = 400v. So if the 6146 plate voltage, at carrier is 650V, then the screen voltage would be 650V - 400V = 250V... which is about right.

If the screen current, at carrier, is 20mA, then the power dissipated in the resistor, at carrier, will be 20mA x 20mA x 20,000 ohms = 8 watts.

To take into account modulation, a reasonable worst case dissipation would occur with 100% modulation with a sine wave. That would produce more average dissipation in R28 than voice modulation.

With 100% modulation, with a sine wave, the dissipation in R28 would be 50% higher than it is at carrier. I.e. 12 watts.

Therefore, for a 1-off replacement (as opposed to production of many transmitters), I think the dissipation rating of the replacement resistor should be 20 watts or greater.

Stu

Right on Stu!   So now you have two comments after your "closing comments"! 

My suggestion below is not meant to be at all critical, but just a suggestion as to my favorite way to tackle problems with these old rigs.  This can avoid what I call try this try that in order to bringing and old rig back on its feet!   

When folks work on rigs to look for problems, I strongly believe the first step should be taking voltage measurements or scope measurements or what I call the data gathering stage.  Collect the data to find out what is wrong and when you find out what is missing that will guide you to the failure.  A lot of trying this and trying that can be avoiding with some simple measurements at the beginning of the process.   This will generally lead to finding the problem quicker and disturbing less things.  All the concepts are the same so once that mindset is developed it can lead to a faster and more disciplined approach.

Joe, W3GMS           

[WQ9E]

When folks work on rigs to look for problems, I strongly believe the first step should be taking voltage measurements or scope measurements or what I call the data gathering stage.  Collect the data to find out what is wrong and when you find out what is missing that will guide you to the failure.  A lot of trying this and trying that can be avoiding with some simple measurements at the beginning of the process.   This will generally lead to finding the problem quicker and disturbing less things.  All the concepts are the same so once that mindset is developed it can lead to a faster and more disciplined approach.

Joe, W3GMS           

Exactly right Joe!  The data collection stage is very important because you are filling in the border and many pieces of a jigsaw puzzle and we all know that the more pieces that are in place then the easier the rest of the puzzle becomes. When you have enough data and apply a little logical analysis the underlying problem will generally become clear.

I became addicted to using a whiteboard in my academic life and I have one on the wall near the bench to fill in clues, hypotheses, and findings for difficult cases.  It forces you to maintain a logical approach and isn't subject to the whims of your memory and/or distracting events.

[W3GMS]

The whiteboard is my friend as well Rodger! 

I guess the process could be called a structural approach and the gratification from doing it that way is off the charts.  When that discipline is followed and learned, it really becomes a major attribute not only on the simpler tasks, but really shines when your working on a very difficult problem that has many dimensions.

Joe, GMS

[w5gw]

AES has a reasonable 25 Watt 20K Ohm resistor that should do fine for R28 - https://www.tubesandmore.com/products/R-L25J20K

[w1vtp]

My white board and the EICO 720 Heising hookup. Looks more grey than white but was handy for the project.  The black was post editing

Al

[N8ZBA]

W5gw thanks foe the info. I'm going to order a couple for the parts drawer.
Dan