Viking 2 CDC replacing selenium rectifier

[N9axl]

I am starting to recap this Viking 2 CDC.  I plan to replace the selenium rectifiers with 1N5408 diodes.

This radio uses one 300 ma and (2) 35 ma units.  By my calculations this would require a 5 watt 285 ohm series resistor for the 35 ma units and a 33 ohm for the 300 ma unit to create the desired 10V drop one would have originally seen with a selenium rectifier.  (r=10/.035 A= 285) (r=10/.3 A = 33)  This combination of diodes and resistors would, I think, be robust enough to handle our current 120 VAC line currents.

Am I thinking this through correctly or is there some other consideration I'm missing?

[w4bfs]

are you sure that those selenium rectifiers are stock ?  I remember 2 5R4 hv rect, 1 5U4 lv rect and a 6al5 bias rectifier in the stock lineup...

since you are going to recap consider going solid state as you have stated by first considering what your line voltage is there ... the V2's were designed in the 110 vac line voltage days ... if you don't reduce it you will place excessive filament voltage on your tubes ... unplug your rectifiers and measure the 6.3 vac ....if you find it high (6.6 Vac or more) you should remedy
... the easiest way is to use the 5U4 fil winding to buck the lv xfmr .... it runs cooler and generally drops the fil voltg back to abt 6.2 .... definitely add a separate fuse to protect this unobtanium unit ....

or am I barking up the wrong tree ?

[N9axl]

I'm actually planning to run this rig through a variac and keep the line voltage at 110. I realize that changing over to solid state is preferable, but I'd like to get everything working properly as a stock radio before I start doing modifications.  I've seen one example of someone who, basically, built an outboard power supply for the Viking 2 which sort of makes sense -- or at least it would save my aging back.

My immediate issue is that this calls for an 8  to 10 mfd 1000 V capacitor (C9) .  The only ones I've found that could work are very expensive.

[N8ETQ]

     My V2 only had 1 Selezeium Rect. in him.
On the Bias supply, My manual shows a 6AL5
for the bias rect.  I snipped it out and went
with a 5408 as well.

GL

/Dan

[KA0HCP]

Attached are some data sheets for various selenium rectifiers with internal resistance listed.

You can look up your specific S.R. and see what resistance you need to add.

In my experience somewhere around 5-20 [Ohms] is typical.

[Correction]

[N9axl]

There are two on the PTT relay circuit and the 300ma coming off of T4 into the  phone/cw switch.  This is the CDC model but it appears to be the same circuit.

[Pete, WA2CWA]

In all the years of replacing selenium rectifiers with sold-state diodes, I've never added any series resistance.

[KA2DZT]

In all the years of replacing selenium rectifiers with sold-state diodes, I've never added any series resistance.

Thanks Pete,

Glad someone mentioned it.  I never added any series resistance either, not needed.

Too much overthinking on a lot of these topics.

Fred

[Jim, W5JO]

In all the years of replacing selenium rectifiers with sold-state diodes, I've never added any series resistance.

Thanks Pete,

Glad someone mentioned it.  I never added any series resistance either, not needed.

Too much overthinking on a lot of these topics.

Fred

Another thing is the radio was designed around a 115 volt AC input +/- 10%.  Digital meters have caused a lot of the overthinking.  In the day of the Viking II we used a Simpson 260 or some such.  You can barely tell the difference in 6 volts and 6.3 volts.

[Pete, WA2CWA]

In all the years of replacing selenium rectifiers with sold-state diodes, I've never added any series resistance.

Thanks Pete,

Glad someone mentioned it.  I never added any series resistance either, not needed.

Too much overthinking on a lot of these topics.

Fred

Another thing is the radio was designed around a 115 volt AC input +/- 10%.  Digital meters have caused a lot of the overthinking.  In the day of the Viking II we used a Simpson 260 or some such.  You can barely tell the difference in 6 volts and 6.3 volts.

And, in the case of the Viking II CDC, Johnson states in the manual that voltage, current, component values, etc. could vary 10% to 20% from specified and still be acceptable. I can understand precaution but some can take it to the extreme. In my many years of playing radio, I have never even had the inclination to put a variac on an operating position boatanchor rig to maintain some specific line voltage input. I have yet to lose a transformer or any other component due to the current AC line voltages. I'm also not one that leaves all equipment powered up 24/7 to keep it warm and humming. When I'm not in the shack, all AC power is killed except for the clock.

[N8ETQ]

Pete,


    Those are readings based on the DC parameters
at the tube sockets. Where the tolerance of components
involved come into play as well as line voltage.

    Applying that logic to tube filaments

           6.3 * 1.20 = 7.56

    This is the main reason all those Johnson 4W
25K DRIVE pots have crapped out. They ran at 3.5W
under DESIGNED line voltage.

    Back in the day these things were selling, I
think Low Line was a bigger problem than High Line
like we see today.  Turning the equipment OFF when
not in use will definitely help delay the inevitable.

    Just my 2 cents worth.....

73

/Dan

[Jim, W5JO]

    Back in the day these things were selling, I
think Low Line was a bigger problem than High Line
like we see today.  Turning the equipment OFF when
not in use will definitely help delay the inevitable.

Most of my life has been spent in rural areas and when I first started in radio (gad, 1955) they ran the voltage high in order to reach the end of the line.  The line transformers in those days were adjustable, and even the ring voltage on the telephone line was the same way, it regularly was ~105 + volts. 

I recall measuring line voltage back then in the 120 + range.  As for Pete's example of the tolerance, the electric company had the same, so if you were supposed to get 115 then it could vary between ~ 100 and 126 depending on where the capacitor bank was located.

I don't run my equipment through bucking transformers or a variac either and have not had any major failures due to line voltage.  In fact, I just replaced some tubes in my GK 500 whose date codes were in the Sixties.

[N2DTS]

I have had my 32V3 for at least 35 years, run it off whatever AC comes out of the sockets and on the 700 volt tap and have not lost a tube in it yet.
I have not had a tube fail in use ever.
I always solid state the power supplies, bias as well.
If a rig is that fragile I want to know about it and fix it.

[N9axl]

I have a particular problem in that I live about one block from a university campus of 10000 students.Its not only the residence halls but they have some major industrial equipment they use for instruction. 

  It shouldn't cause issues, but it does - we have constant voltage issues. I keep losing appliances to power surges, we get blackouts, etc.  I'm told its a location issue and nothing to do with my house. So a variac seems sort of a necessity in my situation just to keep everything steady.

I'm planning on installing a 10 amp variac which is fed from an isolating transformer - everything set at 115 volts.  Complete overkill, but I think that should solve the issue.

[Pete, WA2CWA]

I have a particular problem in that I live about one block from a university campus of 10000 students.Its not only the residence halls but they have some major industrial equipment they use for instruction.  

  It shouldn't cause issues, but it does - we have constant voltage issues. I keep losing appliances to power surges, we get blackouts, etc.  I'm told its a location issue and nothing to do with my house. So a variac seems sort of a necessity in my situation just to keep everything steady.

I'm planning on installing a 10 amp variac which is fed from an isolating transformer - everything set at 115 volts.  Complete overkill, but I think that should solve the issue.

I don't see where an isolation transformer is going to help your situation. If the input varies, so will the output, so what's the point.

Years ago our local electrical company installed two plotters one on each side of my 220 line entering the house to monitor the line voltage for a week. We live in a fairly populated area with many old homes and many new homes. I had a concern about line voltage variations. After a week, both plots looked like a picket fence depending on the time of day. There was never any constant voltage reading or minimal swing in the line voltage no matter what time of day. Even today in the shack, I have a panel meter monitoring my line voltage. The meter pointer very rarely ever sits in one place.

Unless you're going to use a "smart variac", that is, one that has circuitry to monitor the input voltage and compensates the input voltage ratio into the variac, you might spend all day long diddling with the variac control just trying to keep a steady state of 115 volts to your equipment. I also don't think that a variac would help controlling brief power surges or spikes unless you knew it was coming and have your hand on the variac knob the instant it was happening.

[Jim, W5JO]

  It shouldn't cause issues, but it does - we have constant voltage issues. I keep losing appliances to power surges, we get blackouts, etc.  I'm told its a location issue and nothing to do with my house.

What does your electric company say about all this?  Have they not offered to fix the problem?

[N9axl]

hmm. The answer was somewhat along the lines of "we don't find any issues in your area" along with an attitude of "what do you expect given where you live?" 

[Jim, W5JO]

hmm. The answer was somewhat along the lines of "we don't find any issues in your area" along with an attitude of "what do you expect given where you live?" 

I would hope they are interested in fixing any problem you have that they cause.  Can you hook a chart recorder to the line and then you would have ammunition to show.  I would bet the regulatory agency in your state has some standards that would be in your favor.   Around here if I found they are the cause of appliance trouble, they can be made to replace them.

[Pete, WA2CWA]

With most electric companies you can request and/or insist  a chart recorder(s) to monitor your incoming voltage generally for a week or two especially if you're experiencing frequent bulb burnouts, appliance failures, brownouts, etc. If neighbors are also experiencing the same issues, a group push on the electric company is in order. An isolation transformer and variac are a weak band-aid which really doesn't solve the underlying problem. It's hard to believe that an electric company would feed a campus environment and a residential area from the same drop electrical feed.

[N9axl]

Good idea. Didn't know that they did that and wasn't offered that as an option.  I will look into that. 

[N8ETQ]

   N9AXL,

        I have a MG set for a TRC-75 that will seriously
stress test your Pole Pig and Drop..  I've had 2 new pole
pigs since I acquired it.  It's AWESOME!  It scares children.
110 DC in @ about 90 or so Amps to get him going..  Short
of a rifle, quickest way I know of to get the nasty things
replaced...


GL


/Dan

[N9axl]

I'm terrified myself just reading your post - much less seeing that thing in action.  90amps. Geez..connect that up right and you could be powering your whole house and then some when the Big One hits.


Actually the power company guys were just here and found nothing wrong. They told me the standard voltage is 120 plus or minus 5 percent which, from what people are saying, is within the original tolerances of the Ranger. I'll bring it up on a Variac just to be able to test the voltages more safely, but it would seem I'm good to go.

[N8ETQ]

  Her ya go...