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 Post subject: Johnson Viking II power out issue
PostPosted: Nov Fri 13, 2020 2:57 pm 
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Location: Fenton, MI 48430
My Viking II was producing 100 watts carrier out at 220ma on 6146 tubes. Now, 40 watts out 220ma and will not make 100 watts unless I drive 6146s to 400ma. 400ma will ruin the 6146s. HV is correct. Both 6146 plates showing a little red color at 220ma, did not notice that before low power out issue. Antenna SWR about 1.6 to 1.

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 Post subject: Re: Johnson Viking II power out issue
PostPosted: Nov Fri 13, 2020 3:18 pm 
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Check final grid bias to make sure that the bias supply doesn't have an issue taking the final out of its desired hard class C operation. One of your 6146 tubes may have failed. Check for the presence of HV on the output connector, the Viking II doesn't have a safety choke to blow the fuse if the coupling/DC blocking cap fails and if you have an antenna with a floating "hot" (i.e. a dipole without a balun) then it will continue to work in some fashion with a leaky cap while putting plate voltage on the output network and antenna.

Rodger WQ9E


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 Post subject: Re: Johnson Viking II power out issue
PostPosted: Nov Fri 13, 2020 3:33 pm 
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BTW- makes full power out on 40 meters. Lost a output fixed capacitor on 75 meters? Antenna has balun, HV would short.

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 Post subject: Re: Johnson Viking II power out issue
PostPosted: Nov Fri 13, 2020 3:42 pm 
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With that important additional bit of information... YES, the problem is almost surely in the output network.

With an open padder capacitor in the loading network, you will end up with a shallow dip and very low efficiency from the excessively high loading value.

An MFJ antenna analyzer is a useful tool for troubleshooting these output networks when there is any doubt. With the power removed, a resistor with the shortest leads possible is connected from one of the tube plates to ground. The resistor is chosen to represent the operating load of the tube via the formula Rl = Plate voltage/(plate amps x K) where K is a constant based upon the operating class. For your Viking II, K=2 because it is in class C, for class AB K ranges from 1.5-1.7 with AB1 at the low end and AB2 at the high end. This isn't exact but close enough.

With the calculated resistor connected, connect the analyzer to the RF output and the pi net should present a 50 ohm load to the analyzer when properly tuned. Using the analyzer allows you to test contacts and mechanical parts without fear of HV or RF burns and allows temporary substitution of low power capacitors to replace suspected bad HV/high current RF parts.

NOTE: The power tubes stay in place for this test because their capacitance is part of the tuned circuit.

Do NOT forget to remove the resistor and MFJ analyzer before power up!!!

Rodger WQ9E


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 Post subject: Re: Johnson Viking II power out issue
PostPosted: Nov Fri 13, 2020 3:53 pm 
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Location: Liberty, Missouri
Jim said,
Quote:
...Lost a output fixed capacitor on 75 meters?...
Could be, but I'd tend to doubt it as there are several 300pF caps switched in one at a time. You don't switch'em in/out when you're transmitting/tuning, correct?

Good TS/verification technique is as Rodger describes above.

Do you have normal drive on 80?

160 coil is switched "OUT"?

Tubes really shouldn't show any color, but I bet they do at 400+ mA.

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 Post subject: Re: Johnson Viking II power out issue
PostPosted: Nov Fri 13, 2020 4:51 pm 
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Location: Fenton, MI 48430
Mikeinkcmo wrote:
Jim said,
Quote:
...Lost a output fixed capacitor on 75 meters?...
Could be, but I'd tend to doubt it as there are several 300pF caps switched in one at a time. You don't switch'em in/out when you're transmitting/tuning, correct?

Good TS/verification technique is as Rodger describes above.

Do you have normal drive on 80?

160 coil is switched "OUT"?

Tubes really shouldn't show any color, but I bet they do at 400+ mA.

Normal drive and used to be enough for 100 watts. As stated 6146 at 220ma, but only 40 watts out. Increasing drive will increase 6146 plate current, but 220ma is the limit.
160 meter is switched out, of course.
Added 500pF on output, made no difference.
Verified output tuning capacitor in circuit.

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 Post subject: Re: Johnson Viking II power out issue
PostPosted: Nov Fri 13, 2020 5:02 pm 
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Location: Fenton, MI 48430
I found it. Poor connection of fixed capacitor switch on output.

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 Post subject: Re: Johnson Viking II power out issue
PostPosted: Nov Sat 14, 2020 1:41 pm 
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Hi Jim, glad you got it up and running.

Thanks for the end of the story. It gets somewhat disappointing to see how so many threads just kind of die mid stream.

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