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Author Topic: Viking II, no grid drive on 20, 15 or 10  (Read 15631 times)
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WA2ROC
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« on: September 02, 2014, 09:11:40 AM »

I have been using my VII on 160, 80 and 40 and it works well.  I'm driving it with a 122 VFO and can get 5 to 10 ma grid current.  Yes I set it for 4 ma when in use.

But on 20, 15 and 10 I get zero drive.  I can hear the signal in my receiver and the oscillator and buffer controls will control the strength of the signal, but no indicated grid drive on the meter.

I suspect either a dirty contact(s) on the band switch or weak oscillator and buffer tubes.  Something is not doubling or tripling.

Any other suggestions?
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« Reply #1 on: September 02, 2014, 09:31:04 AM »

I have been using my VII on 160, 80 and 40 and it works well.  I'm driving it with a 122 VFO and can get 5 to 10 ma grid current.  Yes I set it for 4 ma when in use.

But on 20, 15 and 10 I get zero drive.  I can hear the signal in my receiver and the oscillator and buffer controls will control the strength of the signal, but no indicated grid drive on the meter.

I suspect either a dirty contact(s) on the band switch or weak oscillator and buffer tubes.  Something is not doubling or tripling.

Any other suggestions?

Do you have any crystals that would cover the bands of interest?  That would eliminate the possibility of the 122 being the problem.  Having a crystal or two around the shack can be a handy troubleshoot tool.  Otherwise, dirty switch contacts would deserve a good first look

GL, Al
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« Reply #2 on: September 02, 2014, 03:04:31 PM »

It appears (based on a Viking II schematic that I downloaded from BAMA) that the Viking II final RF stage employs -75V (nominal) fixed grid bias, in combination with self biasing.

If the fixed grid bias is too negative (e.g. more negative than -75V) and/or the buffer/doubler stage is not putting out a big enough 2nd, 3rd, or 4th harmonic of its input signal... then the final tubes will not draw grid current.

As Al (W1VTP) suggests, it is possible that the VFO is not putting out a big enough signal at the fundamental frequency to properly/fully drive the 6AU6 VFO/Buffer stage... whose output drives the 6AQ5 buffer/doubler stage.  In that case, the drive to the buffer/doubler stage will be too low... and the 6AQ5 buffer/doubler stage will not put out a big enough signal at the desired harmonic frequency.

It is also possible that the buffer/doubler stage is not functioning as it is designed to function. With a proper input amplitude (from the VFO or the crystal oscillator) the buffer/doubler circuit should be biased well into class C operation when it is operating as a frequency multiplier. In the schematic, the buffer/doubler employs a combination of fixed bias and self biasing. The negative fixed bias on the buffer/doubler tube is adjustable, via R15. Check the manual to determine if R15 is adjusted properly.

If you have an oscilloscope, you can (very carefully) measure the RF waveform on the grids of the final tubes. It's peak amplitude has to be higher than: [minus the fixed bias voltage on the grid], which is nominally 75V. I.e. the total grid-to-cathode voltage has to be driven by the RF component of the grid voltage to just above +0V... in order to cause the finals to draw grid current.

Since this is happening on 3 bands... it seems unlikely (to me) to be a band switch problem.

Stu
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« Reply #3 on: September 02, 2014, 03:31:09 PM »

Dick,

On 40 meters do you have your VFO-122 set to the 80/160 meter position (output is on 160 meters) or is it set to 40 meters?  If you have normal grid drive on 40 with the 122 set for 40 meter output then at least the VFO-122 seems to be working.

With the meter set to the buffer position while tuning up on 20 meters can you see any peak while adjusting the oscillator tuning control?
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« Reply #4 on: September 04, 2014, 09:04:57 AM »

Yes, I am using the 40-20-15-10 meter position on the 122

No I see no peak on the oscillator current when tuning the capacitor.  It's not zero, but only 1 or 2 ticks above zero.  Just like when operating on other bands.

Yes, I have a 6AU6 and a 6AQ5 coming today
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« Reply #5 on: September 04, 2014, 09:26:44 AM »

AB2EZ:  R15 is a 1.5K resistor and not adjustable.  But I will check the voltage at that point in the schematic (should be -25V) and replace all 3 divider resistors.

Thanks
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« Reply #6 on: September 04, 2014, 10:12:28 AM »

IIRC There's a set screw that can come loose on the driver section of the bandswitch and leave the multiplier in the wrong place.

Not saying this is your problem just remember it being a problem on a Viker 2 I had.
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« Reply #7 on: September 04, 2014, 10:16:09 AM »

W2VW.  Thanks.  I'll take a look at that too.
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Dick Pettit WA2ROC 
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« Reply #8 on: September 04, 2014, 10:49:59 AM »

Dick

Since the Viking II applies -75V of fixed bias to the grids of the 6146's, you can operate (e.g. into a dummy load if you wish) the transmitter even if there is no grid current. [This would not be true in a transmitter that employs only self biasing of the output tube grids... like a Ranger]

Adjusting the drive to produce the specified grid current is, in this case, a surrogate for adjusting the drive to produce enough RF grid voltage to fully drive the output tubes.

If the RF grid voltage is present (on 20 meters, etc.), but not quite large enough to drive the 6146 grids positive with respect to the cathodes ... the transmitter should still produce significant output power (but less than if the RF grid voltage were high enough to drive the grids positive with respect to the cathodes)

If you can produce output (load and dip the plate current) on 20 meters... then you should be able to adjust the 6AQ5 tuning control to peak up the output power... even if there is no grid current in the 6146's

Stu
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« Reply #9 on: September 04, 2014, 12:44:57 PM »

Yes, I am using the 40-20-15-10 meter position on the 122

No I see no peak on the oscillator current when tuning the capacitor.  It's not zero, but only 1 or 2 ticks above zero.  Just like when operating on other bands.

Yes, I have a 6AU6 and a 6AQ5 coming today

But what is your OSC buffer readings on 80 meters (it's working there, right).  Does the OSC current drop off when you switch from 80 to 40 and up?  Have you tried my suggestion of using a XTAL in place of the 122?  I get a definite peak on my V1 when I go through the tune up.

Al
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« Reply #10 on: September 04, 2014, 12:53:25 PM »

Yes it is higher on 160-80-40 with the VFO, maybe as high as 5 on the scale. 

I didn't have a chance to try a crystal but I nave 3 to use tonight.  That'll be the first thing I try, not including prying the top off the V2.

TNX
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« Reply #11 on: September 04, 2014, 01:31:02 PM »

Yes it is higher on 160-80-40 with the VFO, maybe as high as 5 on the scale. 

I didn't have a chance to try a crystal but I nave 3 to use tonight.  That'll be the first thing I try, not including prying the top off the V2.

TNX


Only for the purpose of "half-splitting" the problem.  No sense troubleshooting the V2 if the problem's in the 122 and visa versa.  I just fired up my V1 and without plate (HV) on, in the CW mode you can do everything up to and including the grid current.  Once that is established then the rest should be familiar ground.

Here's my V1 - I have a pristine v2 in the back room waiting for a smoke test  Grin

Al


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« Reply #12 on: September 05, 2014, 06:41:31 AM »

Voila!!!  It's now working on 20, 15 and maybe 10 meters.

First of all I used crystals to see if that would make a difference on 20 meters.  Same results.  Next I installed 2 known good tubes, a 6AU6 oscillator and a 6AQ5 buffer, and found significantly more grid drive on 40 meters (the crystals were of the 40 meter species) but still nothing on 20.

Checked all the voltages listed in the manual and found they were close, not exact, but within 5 or 10 percent.

I used my scope to track the signal from the oscillator to the buffer.  It was on the plate of the oscillator but zero after the coupling cap that goes to the grid of the buffer tube.

Hmmmmm.

I found a suitable substitute and soldered it around the bad cap and there was a nice signal on the buffer grid.  Using a dummy load and watt meter, I started tuning up on 20 meters and found no grid current but a decent amount of RF coming out of the old beast, maybe 70 watts.  Tuning was sensitive but it was putting out RF. 

I checked the frequency with my freq counter and it was exactly double of the 40 meter crystal I was using.  After a few more iterations of tuning the final and oscillator, there was 80 watts of blazing power heating up my dummy load.

Finally I connected up the 122 VFO, set it for the middle of the 20 meter band and tried for RF.  Once again, 80 fabulous watts  were heading down the coax to the load.

Mission complete.  Thanks to all for the suggestions, especially AB2EZ and the suggestion of tuning up with no apparent grid current. That's what I have, or actually don't have.

I'll button things up, maybe order a new set of 6146's and try 20 meters on Monday evening on the AM net on 14.330 at 8PM EDT,  (Shameless advertising message)
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« Reply #13 on: September 13, 2014, 09:51:17 AM »

Dick,

   I wonder how things went? Without grid current, I'd be hesitant to try to modulate that carrier. If I read what you did right, you bridged the oscillator to buffer coupling capacitor with another. If the old cap is bad, then it might be leaky. I think I would remove it instead of just shunt another capacitor around it.

   The EFJ Viking I and Viking II RF driver is not very robust, but it does work. So far you have found one or both tubes weak, and a bad cap.

   Since you have a scope, look at the 6146 grids. Do you have a low capacitance input 10X probe with a attached ground clip? If so, with DC coupling look at a 6146 grid (G1) as you drive it. It should start out at about -75 volts DC. Set scope to 20V/Div. Does the upward sine wave peak drive swing go above zero volts (scope gnd reference)? If it does, then you have grid current. Could be something going on with the grid current metering.

   We had over 20 check ins on 20M Monday evenings gathering. It was chaos, and you got lost in the process. Sorry about that Dick.  Cry

Jim
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« Reply #14 on: September 13, 2014, 03:17:36 PM »

Jim

You said: "Without grid current, I'd be hesitant to try to modulate that carrier."

I'm wondering what effect you have in mind... with respect to modulation (v. CW)?

I'm thinking that, because of the -75V fixed bias, the driver's RF output (the 14.xxx MHz harmonic, selected by the driver's output tuned circuit) might not be large enough in amplitude to push the grid-to-cathode voltage up to zero volts (actually a bit over 0 volts if there is some average grid current flowing). Nevertheless, the grid-to-cathode voltage is moving up and down around the -75V fixed bias voltage.... and the tubes are being driven into class C operation (although not with as much peak plate current as there would be if the grids were being driven hard enough to draw grid current.

However, I can see how there might be a problem... if the grid-to-cathode voltage does not go positive for a small portion of each RF cycle... since the diode that exists between the grid and the cathode does have an important (non-linear) role to play in reducing the effect of the feedback associated with the plate-to-grid capacitance (but only if it is forward biased for a small portion of each cycle).  

Stu

Dick,

   I wonder how things went? Without grid current, I'd be hesitant to try to modulate that carrier. If I read what you did right, you bridged the oscillator to buffer coupling capacitor with another. If the old cap is bad, then it might be leaky. I think I would remove it instead of just shunt another capacitor around it.

   The EFJ Viking I and Viking II RF driver is not very robust, but it does work. So far you have found one or both tubes weak, and a bad cap.

   Since you have a scope, look at the 6146 grids. Do you have a low capacitance input 10X probe with a attached ground clip? If so, with DC coupling look at a 6146 grid (G1) as you drive it. It should start out at about -75 volts DC. Set scope to 20V/Div. Does the upward sine wave peak drive swing go above zero volts (scope gnd reference)? If it does, then you have grid current. Could be something going on with the grid current metering.

   We had over 20 check ins on 20M Monday evenings gathering. It was chaos, and you got lost in the process. Sorry about that Dick.  Cry

Jim
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« Reply #15 on: September 13, 2014, 04:29:51 PM »

Jim
You said: "Without grid current, I'd be hesitant to try to modulate that carrier."
I'm wondering what effect you have in mind... with respect to modulation (v. CW)?

Stu,

  I will try to explain what I mean, and then maybe you can help me understand what I'm trying to say!

  This seems to be a class C RF stage that is plate modulated, but has only sufficient grid drive to get about 80% of normal carrier output. At this level of drive the grid meter reads zero milliamps. This suggests to me that the RF tube is not being saturated during a portion of the the normal ~90-120 degree conduction angle that the grid swing would be drawing grid current. This causes two problems;

1.) Reduced plate efficiency since we spend more time in the linear region
2.) Upward modulation will flat top way before 100% upward

a consequence of #2 will be that the load on the modulator might not be correct.

I cannot give a good theoretical explanation for #2 other then with reduced grid drive (no grid current), at no time is the plate at the saturation level; instead it is at more of a CCS level where the plate voltage will rise, but not the plate current.

If I'm correct here, the modulator will see a rising load impedance when the RF tube output flat tops on the upward modulation peak.

One thing for sure, using my Gonset G-76 for example, the neutralization null is not constant band to band. My VFO has 80M (2X the Osc) output, and getting to 20M and 15M requires the 160M osc to be multiplied 8X (20m) and 12X (15M). Running AM I get grid current, but at the bottom of the desired range. So what I do is modulate at reduced grid drive, and while doing so I tune the driver tuning condenser. I can usually get the positive peaks to rise 20% or more this way once I restore full drive. If the neutralization was perfect, I would not have to do this.

Good thing I use a Cumbria X-Lock 3.0 Stabilizer on that VFO. I can hold < 100Hz stability at 12X multiplication with that old Knight V44 VFO.

Now I hope I made a little bit of sense...?? :-)

Jim
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« Reply #16 on: September 14, 2014, 11:13:39 AM »

Jim

I needed to get a good night's sleep before thinking further about this. I also wanted to do a quick review the curves in the specification sheet for a 6146. As you know, the Viking II uses a 20k ohm voltage dropping resistor, from modulated B+ to the screen, to modulate the screen voltage.

If the peak RF grid-to-cathode voltage is lower than the design value... then the plate current and the screen current will be lower than the design value for each combination of plate voltage and screen voltage. Therefore, the modulation resistance will be higher, and the optimal value of plate RF load resistance will be higher.

The higher modulation resistance will result in some amount of mismatch between the modulator and the RF output stage. The peak  audio frequency modulation voltage will be the same. However, the mismatch will result in a higher load on the modulator stage  (v. the design value)... and therefore, the voltage produced by the modulator (for any given audio input) will be larger. The modulation power required will also be less. Therefore, I don't think the lower peak amplitude RF grid drive will have any significant effect on the ability to fully modulate.

To achieve maximum efficiency in the RF output stage, one will need to adjust the loading capacitor as usual. At optimal loading, the RF load impedance, looking from the 6146's  into the output network, will be higher... because the modulated B+ is the same, but the modulated fundamental frequency component of the plate current is less. If the loading is done properly, the output stage efficiency should be the same (actually, a bit better, because the tubes are conducting for a smaller portion of each cycle).

With respect to the output saturating on modulation peaks (i.e. the tubes cannot deliver enough peak RF current)... when I look at the curves of the 6146's, and assuming that the loading has been done properly in accordance with the reduced RF output stage plate current... I don't think this will be a problem... particularly with the reduced RF grid voltage peaks still being high enough to make the transmitter put out 80 watts (at carrier).

Having said all of that... there may be some subtle effects that we have not yet mentioned.

For example... the power supply voltages (including the effect of the AC line voltage) may not be "stiff" enough to keep the peak RF output voltage of the driver constant, during modulation, and/or the -75V "fixed" bias constant during modulation.  If the output stage of the transmitter is drawing grid current, then the "self-biasing" effect will tend to regulate the peak 6146 grid-to-cathode voltage. If there is no 6146 grid current, there is only the "fixed" -75V bias on the 6146 grids

Stu
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« Reply #17 on: September 15, 2014, 07:12:26 AM »

Thanks go out to all that responded to my questions and concerns about no grid current on the V-II.  I am able to tune it up getting 80 +/- watts out and made several local contacts who said the signal sounded "OK".  That probably means not broadcast quality, but readable.  

I'll just leave well enough alone until my replacement 6146's show up since the ones in the V-II are "I don't know how old".

Yes, I did finally clip the defective capacitor out of the circuit, leaving the good one in place.

Jim/JKO:  No problems.  I enjoyed listening to the folks checking in and when Tim got to rambling, I just powered down the Apache and listened for a while, then went inside to wind down myself.

If the weather conditions are favorable, that is no t-storms, I'll be back on the net tonight.
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« Reply #18 on: September 15, 2014, 07:46:12 AM »

Thanks go out to all that responded to my questions and concerns about no grid current on the V-II.  I am able to tune it up getting 80 +/- watts out and made several local contacts who said the signal sounded "OK".  That probably means not broadcast quality, but readable.  

   OK Dick, very good. I hope to be on tonight too, but there are storms posible tonight, so I might be in the "toss the coax outside mode'.

   Thinking about modulating a class C RF stage with drive "just" to the threshold of grid current. As the Rf B+ varies at an audio rate, at some point going upward the output will not follow the formula P=E^2/R where at 100% modulation the plate voltage at the peak of the audio cycle doubles. Here the PEP power is 4X the carrier level. I contend that without grid current we hit a point where the tube emission reaches a limit based upon the peak grid voltage available. When this happens, the R value in the formula rises, and upward modulation flat tops. Every plate modulated rig I've had does this; reduce grid drive, and modulation peaks flatten prematurely.

  In a related way, the same thing happens when we try to plate modulate a high mu triode such as a 572B or 3-500Z. Even with grid current, the peak grid voltage has to be way high to get 4X PEP power, and the conduction angle is higher then a low mu triode since these hi mu tubes can often run at zero bias, or just slightly negative to achieve cutoff. With a 572B, -10V grid bias at a plate voltage of 1500 pretty much cuts them off. This means the grid resonant tank spends more time supplying RF power to the grid (the % time above ground is longer). Anybody run a plate modulated hi mu triode, and get 100% plus upward modulation? I'm not saying its impossible, but just a lot harder then with a low mu or medium mu triode.

Jim
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« Reply #19 on: September 28, 2014, 12:07:17 AM »

In addition to everything you did, I'd replace all the 6AU6 tubes with 6AH6s in your Viking and the VFO. You will have better drive up through 10 meters with drive control at about a '3' setting from either the VFO or crystals.

I don't apply more than 5 ma of grid drive in AM. The 6146s run quite nice with 2.5 ma per tube and I get 100 watts of 100% modulated AM from a stock design Viking II, but all caps and resistors have been replaced in the audio section.
 
Load to no more than 200-210 ma plate current. If you load higher than that I sometimes see some negative modulation taking place.  Cry

The Viking II is IMHO one of the best designed transmitters for it's day, but it requires a careful adjustment of audio control. Use a scope to ensure it is not over modulated or under modulated.
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« Reply #20 on: September 29, 2014, 08:04:54 AM »

Thanks for the updates.  I planned to replace the 6AU6's with 6AH6's last time I ordered tubes, but they're on the way right now.

Yes, I keep the grid drive below 5 ma and plate current to about 200 indicated ma at all times. 

I also replaced the 50pf coupling caps in the oscillator/buffer stages and the pair of 25pf caps buffer to 6146 grids too,  but probably need to replace the bypass caps also.  All the resistors are +/- 10% of listed values. 

Another question:  the buffer current reads between 10 ma and 40 ma, depending on the drive pot setting.  But it reads this current even when not transmitting.  And the oscillator current never gets above 4 or 5 ma.  Does this sound right to you folks?

Thanks again.
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« Reply #21 on: October 08, 2014, 01:56:20 PM »

The 6AH6's did a lot to give me more grid drive but still none on 20 meters.

Looking at the schematic and the buffer section of the band switch, there is a 10K ohm 2 watt resistor (R36) between the RFC and the plate of the buffer.

What does this do, swamp RF?  Can't be a plate load resistor because it is shorted by the buffer coil.

I clipped one end out and measured it, getting close to 10K ohms. 

It may be time to start replacing lots of bypass caps in the oscillator and buffer, and maybe the finals too.  Oh yes, new 6146's gave me a few more watts out.

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« Reply #22 on: October 08, 2014, 03:15:52 PM »

It looks like this resistor is intended to limit the Q of the driver tank circuit on lower frequency bands (below 20m), and possibly to damp out parasitic resonances when those bands are selected. There is an analogous 4700 ohm resistor across a portion of the tank coil in the buffer tank circuit.

I think you should measure the resistance between the band switch wafer solder terminal that is electrically connected to band switch wafer's wiper and one end the associated tank coil, in each band position. To do this, you will need to disconnect every wire that is connected to the wiper solder terminal... so that the only path to either end of the tank coil is via: wiper=> band tap contact=> band tap wire=> coil. You should do this for the driver switch wafer, and you should do this for the buffer switch wafer. It's a "long shot"... but verify that the resistance is near 0 ohms in every band position (and, in particular, in the 20m position).

By doing this, you will also verify that the connection of each band tap wire to the corresponding tank coil has essentially 0 ohms of resistance.

I've seen what appears to be a properly soldered connection develop a high resistance... due to the formation of an insulating layer on the surface of one or both of the wires.

Stu
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« Reply #23 on: October 10, 2014, 09:53:18 AM »

After reading the post about someone's BC-610 problem with the Viking 122 VFO, I noted that I may have a potential problem that might cause low VFO output.

The coax between the VFO and my V2 is about 14" long.    That's what was there when I first got the rig several years ago.   Maybe whoever built the V2 looked at the schematic and saw that the coax from the oscillator/buffer tube to the VFO input was 14" long and just used that length for the coax from the VFO output to the V2 input.

 In the manual it says something to the effect of "...do not shorten this cable...or you will get lower output..."

It's supposed to be 36" long!

BINGO!!!

Tomorrow a 36" length of RG-59 will replace the short coax and I'll see how things work on 20 meters then.

Also, Best of luck with the 122 and the BC-610.  
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« Reply #24 on: October 10, 2014, 01:10:44 PM »

The cable between my VFO122 and Viking II is about 15 inches and it works fine on all bands.
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