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Author Topic: Viking Valiant problems  (Read 12493 times)
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KG6YDI
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« on: August 16, 2015, 09:58:06 PM »

I'm working on a new to me Valiant. I've recapped it, put a 3 wire power cord on it, cleaned the rotary switches and all tube sockets and set the bias voltage to the 6146 finals. I have 8 mils of grid drive on 80. I'm unable to dip the finals.  I've also changed the 6146 finals.  I'm leaning towards the loading caps as a potential cause. Any suggestions?
                                     Marv.KG6YDI in Beautiful Down Town Bonny Doon
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Pete, WA2CWA
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« Reply #1 on: August 16, 2015, 10:29:06 PM »

Have you tried other bands? Are you following the procedure in the manual for the initial settings of the AUX and FINE Coupling controls for the band you are on? Are you using a dummy load?
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KG6YDI
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« Reply #2 on: August 16, 2015, 10:44:46 PM »

Pete-I've tried 40 and the same thing. I've got it connected to a 80 meter dipole and am using settings for loading that work on my other Valiant
                                          Marv
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K1JJ
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« Reply #3 on: August 16, 2015, 11:30:56 PM »

Marv,

Do you have an MFJ-259 antenna analyzer?  

If so, turn off all the Valiant power. Then connect the analyzer to the 50 ohm output at the loading cap. Then attach a carbon resistor from the 6146 plates to ground to simulate the 6146's plate load. This may be  3K to 5K or whatever... not all that critical.

Now try various bands and adjust the plate tuning and loading controls until you see a 1:1 swr.    Evidently you will not see 1:1 now because of a problem, but this way you can work on the rig and play around with no high voltage.

Another idea -  if you preset the Valiant tune/ load controls using the other Valiant as a reference, you will be able to watch the analyzer and bang around with a plastic screw driver looking for intermittents if the analyzer suddenly drops down near 1:1..

Also, divide and conquer... disconnect the plate tuning cap and loading caps and be sure they are individually doing their required capacitance ranges.  Be sure the tank coil is working right with the band switch, etc. Check that the plate choke has the required minimum inductance and is not shorted somehow.

Is the antenna relay working right? Key it and see if there is continuity from the antenna SO-239 to the loading cap.

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

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KA2DZT
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« Reply #4 on: August 17, 2015, 01:21:31 AM »

Could be an open coupling cap in the final tank circuit.
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N8ETQ
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Mort


« Reply #5 on: August 17, 2015, 08:13:17 AM »

yo'

     See if you can find a dip on 15 or 10M.
Loading Padders/Switch wafers are a common prob.
with the valiant.

GL

/Dan


      Johnson Valiant Loading



      As measured at the output connector with a DMM in
the "C" setting. Fine loading set full mesh or "0".


   Course Loading                       Expected Capacitance to ground

   0               4420pF
   1               4110pF
   2               3820pF
   3               3560pF
   4               3260pF
   5               2960pF
   6               2670pF
   7               2400pF
   8               2100pF
   9               1800pF
   10               1500pF


      The big thing here is the MAX value of 4420pF and
the 300pF "Step" size.  The max value pretty much assures you the
caps are all good, the step size verifies the switch. Fried coarse
loading switch contacts can make the expected results vary wildly.


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KG6YDI
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« Reply #6 on: August 18, 2015, 08:19:21 PM »

Thanks  for all the suggestions. I'll try to get some time to work on it this coming weekend and see what happens.
                                                 Marv.
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w1vtp
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« Reply #7 on: August 19, 2015, 01:34:12 PM »

As Pete suggested, you really need to connect that thing into a 50 ohm termination (load).  If you don't have one, buy or make one.  There are just too many variables if you try an antenna right away

While I don't have first hand experience with a Ranger, generally the loading cap (s) need to be at their maximum to start with a good plate dip.  I see some good suggestions preceding my post.

Al
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KL7OF
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« Reply #8 on: August 19, 2015, 02:22:09 PM »

Marv,

Do you have an MFJ-259 antenna analyzer? 

If so, turn off all the Valiant power. Then connect the analyzer to the 50 ohm output at the loading cap. Then attach a carbon resistor from the 6146 plates to ground to simulate the 6146's plate load. This may be  3K to 5K or whatever... not all that critical.

Now try various bands and adjust the plate tuning and loading controls until you see a 1:1 swr.    Evidently you will not see 1:1 now because of a problem, but this way you can work on the rig and play around with no high voltage.

Another idea -  if you preset the Valiant tune/ load controls using the other Valiant as a reference, you will be able to watch the analyzer and bang around with a plastic screw driver looking for intermittents if the analyzer suddenly drops down near 1:1..

Also, divide and conquer... disconnect the plate tuning cap and loading caps and be sure they are individually doing their required capacitance ranges.  Be sure the tank coil is working right with the band switch, etc. Check that the plate choke has the required minimum inductance and is not shorted somehow.

Is the antenna relay working right? Key it and see if there is continuity from the antenna SO-239 to the loading cap.

T
I like this....Steve
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KG6YDI
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« Reply #9 on: September 20, 2015, 05:02:19 PM »

Hi all-I've got my valiant working. the problem was a defective loading switch. I finally looked at the switch with a magnifying glass and found the broken wiper. I have a parts rig and i did a wafer transplant and i now have a plate current dip and can load it to 330 mils.
I have one other question/problem. The screen voltage on the 6146 finals is 300 volts rather that the 150 the manual specifies.
 I found a reference that a person set the voltage with the clamp circuit. This does not square with the manuals way of setting the clamp circuit.  Any comments would be appreciated.
                                              Marv. KG6YDI in Beautiful Down Town Bonny Doon
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KD6VXI
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« Reply #10 on: September 22, 2015, 08:10:05 PM »

Can't help with the valiant,  but I sure miss BD.   I lived in Felton and BC for some time working for the AAA contractor.

Good luck with the valiant....   I'm psyching myself up to tackle a new to me viking II soon.

--Shane
KD6VXI
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K1JJ
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« Reply #11 on: September 22, 2015, 08:33:28 PM »

Glad you found it, Marv.  Yes, a bad loading capacitor wafer switch contact would do it.

Dan / ETQ hit it right on the nose in his post above. All ya had to do was listen to him... :-)

He said:
"The big thing here is the MAX value of 4420pF and
the 300pF "Step" size.  The max value pretty much assures you the
caps are all good, the step size verifies the switch. Fried coarse
loading switch contacts can make the expected results vary wildly."

Right on prediction, Dan.

T

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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
KG6YDI
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« Reply #12 on: September 22, 2015, 10:10:49 PM »

Dan's suggestions were right the only problem was finding the defective switch problem. It wasn't burnt but rather the spring outside contacts had failed. They looked good but under close inspection when touched they fell off and the problem was obvious.
                                  Thanks for all the suggestions.
                                                       Marv. In Beautiful Downtown Bonny Doon
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AB2EZ
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"Season's Greetings" looks okay to me...


« Reply #13 on: September 23, 2015, 11:16:05 AM »

With respect to the high screen voltage:

The most likely cause is a drop in the value of the resistor that goes between the modulated B+ and the screens.

This resistor dissipates a lot of power, and it's value may have gone down with age and use

Stu
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N8ETQ
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Mort


« Reply #14 on: September 23, 2015, 03:45:37 PM »


  Hey Marv,
   

            glad U got him going,  The screen voltage depends on the
Mode, In CW 300 sounds about right, But as Stu points out the series
a "R" in the screen line to help it modulate in Phone Mode... 30 K
comes to mind.

/Dan





Hi all-I've got my valiant working. the problem was a defective loading switch. I finally looked at the switch with a magnifying glass and found the broken wiper. I have a parts rig and i did a wafer transplant and i now have a plate current dip and can load it to 330 mils.
I have one other question/problem. The screen voltage on the 6146 finals is 300 volts rather that the 150 the manual specifies.
 I found a reference that a person set the voltage with the clamp circuit. This does not square with the manuals way of setting the clamp circuit.  Any comments would be appreciated.
                                              Marv. KG6YDI in Beautiful Down Town Bonny Doon
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AB2EZ
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"Season's Greetings" looks okay to me...


« Reply #15 on: September 23, 2015, 03:53:18 PM »

From the RCA 6146 specification... 150V on the screen (at carrier) is the recommended value for plate modulated telephony.

The Valiant schematic indicates a screen dropping resistor value of 12,000 ohms... which is what you would want for three tubes in parallel... drawing their screen currents through the same resistor.

[In the Ranger, with just one 6146, the schematic shows a resistor value of 30,000 ohms]
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AB2RA
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« Reply #16 on: September 29, 2015, 02:44:00 PM »

Failure of the coarse coupling switches in the Valiant, Viking 2, DX100, 32V and so on are related to operating practices and some minor design changes. Always tune up in CW mode. Let up on the key before rotation of the coarse coupling switch. Then try again in CW mode till you get the right Plate and Grid current. Adjusting the fine coupling and plate tuning and grid tuning with full carrier is OK to get the final values right. Also NEVER change the mode switch from CW to AM while the plate voltage is ON. NEVER tune up in AM mode because the load presented to the modulators is not correct, because the RF stage is not at rated impedance.

Also, no DC path is provided to ground on the output of the RF stage PI network. This can cause failure of the capacitors for a variety of reasons. You can use a 2.5 mH choke as later designs use, but the resonances of the coil can be an issue. I have found that a 33K 2W carbon is satisfactory for most of the rigs I have tried. Besides DC, audio modulation voltage can be coupled across the loading capacitors if the antenna is a dipole (open circuit) or the antenna tuner is not link coupled (Johnson Matchbox).

The 32V is a different matter. The 32V3 addressed some of the issue by using a series parallel configuration for fixed coarse loading capacitors to increase the current and voltage ratings of the components. The loading circuit is part of a PI-L network at higher impedance than normal transmitters of that era, and the voltages developed are more than you would expect. However, the caution I give above on tuning up in CW mode applies. When you turn the loading knob and more numbers appear in the window nearby, that actually turns the switch with a cam. NEVER turn the 32V knob in such a way that the numbers for the coarse loading switch change at full carrier. NEVER tune up in AM mode either.

Static can build up on large wire antennas simply from wind blowing during dry weather. The resistor can bleed off that voltage.

SEPARATE TOPIC: Wrong screen voltage on RF stage.

AB2EZ and KG6YDI is correct that the RF stage screen voltage is incorrect. This is an issue that the Timtron mods address by increasing the value of the screen resistor. He also inserts some resistance in the grid circuit to cause more grid leak bias. I found with these recommended mods that the AM modulation linearity of the RF stage is dramatically improved and a good RF/AF trapezoid results if you use the RF stage bias setting recommended in the Valiant 2 manual of 100 mA in SSB mode, not the 50 mA in the Valiant 1 manual.

NOTE: The resistor for the RF final is the large resistor on TOP of the chassis near the 866s. It needs to be more wattage because the clamp tube pulls the bottom all the way to ground. The AF stage resistor is UNDER the chassis and only has the B+ to the VR tube voltage and is therefore smaller.

Many mods work solely on the AF circuit and ignore the modulation characteristics of the RF stage. W8JI discusses this also on his website with much simpler mods for the audio. At this point, I would suggest that you follow the Timtron ideas and use a better driver transformer.

I worked on my Valiant for quite a while and drew ideas from both of them, and added some of my own. I apologize that it is so lengthy. It was a blog to clarify the design and experimentation process which brought me to the final product. You can read about it at:
http://wireless-girl.com/Projects/AMTransmitters/JohnsonValiantSchematics.html

Good luck with your Valiant.
73
Janis AB2RA
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AB2RA
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« Reply #17 on: September 29, 2015, 02:56:32 PM »

I forgot to mention one problem I encountered on a Valiant when unable to obtain resonance. It is a rare occurrence, but worth noting. The large plate tank coil is secured to other components by a screw that mounts to a ceramic insulator. DO NOT OVERTIGHTEN the screw to fix this. A loose connection can happen there. Clean up the components, use some Rosin Flux to tin them, and a large solder gun to solder them together to prevent ever having the problem again.

Also at this point, fix the infamous Chernobyl resistor in the VFO and do the timtron mod for the drive pot to prevent failure of that unobtainium part.

I provide detailed photos of the implementation of these fixes at the website below.

Failure of this plate coil connection can destroy a lot of stuff before you find it. More on repairs and mods at:
http://wireless-girl.com/Projects/AMTransmitters/JohnsonValiant1.html
http://wireless-girl.com/Projects/AMTransmitters/JohnsonValiant3.html
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WA4JK
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« Reply #18 on: September 30, 2015, 11:54:11 AM »

This statement interest me"Adjusting the fine coupling and plate tuning and grid tuning with full carrier is OK to get the final values right."
Do you advise to reset the grid current to 8MA. under loaded and keyed down conditions?
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AB2RA
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« Reply #19 on: September 30, 2015, 02:11:22 PM »

After the plate current is on target, I always check the grid current and touch it up with the drive control (not the grid tuning) if necessary. It takes a bit more time to tune up, but you can always check it while on the air but not talking.

The RCA 6146 specs are for tuned up operation. As previously noted, screen voltage can be off by quite a bit due to the incorrect Valiant screen resistor value, and the screen dissipation is close to max rating, as noted by Timtron. The change in value on W8JI website and Timtrons notes, as reflected in my schematics, is the correct value for phone operation. If you load the transmitter lightly (reduce plate current) in an attempt to reduce power so that the stock modulator can make 100%, the screen current increases as the plate current goes down. If you simply unplug one of the 6146s to operate it with two RF finals, you will ruin the tubes because the resistor has to be optimized for the correct number of tubes. This effect is explained at length in Eimac's book "The care and feeding of power grid tubes".
http://www.cpii.com/docs/related/22/C&F1Web.pdf

On bands 20 meters and up, the grid current can shift due to inexact neutralization and tuning of the plate circuit. Even on lower bands, grid current tends to drop slightly in transmit. Be aware that the grid current can rise when the plate voltage is off in spot mode to above 12 mA. NEVER run 6146s at 4 mA or above for even short periods of time; see the 6146 specs.

Also, with the plates ON in CW and the key up (no carrier), if the grid current goes below zero, this can be a symptom of 6146s that have been run above ratings or operated off resonance or with excessive control grid current. The 6146s can be used for quite a while in this condition (they are expensive) if it is not too severe, but they will likely demand a bit more from the 5763 driver to get to rated grid current. But it is definitely time to shop for some spares. Also, if replacing 6146s in a triple parallel circuit, tube matching is important or they will not share the load properly. In that case the good tube that takes most of the load now fails due to overwork.

It is important to check the calibration of Johnson transmitter meter shunts. They are notoriously in error due to the wire used, and some kit builders were not precise about length of wire during construction. I strongly recommend replacing the RF and MOD plate current shunts with modern resistors, which are cheap and commonly available now. I describe exactly how to calibrate the meters with a simple power supply, a current limiting resistor, and a modern digital test meter on my wireless-girl website referenced previously. There are simple rule of thumb resistance selection methods which are quick and require no calculation. If you buy a Radio Shack resistor assortment, you will have all the values you need to trim the accuracy to 1% or better.

I use an MFJ-212 noise bridge to get the antenna matcher exactly on setting in receive mode. This results in a less than 1.1 to 1 SWR. Then I load the transmitter into a dummy load. Then I switch the transmitter to the antenna matcher and antenna and make any fine adjustments as touch up. Use of the MFJ-212 results in minimum QRM and stress on the finals due to off resonance operation of the 6146s while adjusting the tuner. Here is my write up on this:
http://wireless-girl.com/Projects/Antennas/MFJ212.html
I also built a clone using a Ten Tec kit for $20:
http://wireless-girl.com/Projects/Transmatches/TenTec1051.html

I hope this helps.
73
Janis
AB2RA
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