Viking 2 low plate current

[N8ZBA]

Bought a Viking 2 cd last year and have using it weekly with no issues on 75/160 AM.  When it was my turn again last nice in a Qso after the rig has been on for a few hours I had very low output power. Under 10 watts of carrier.  while noticing the low output I seen the plate current is almost at zero while in transmit. I swapped out the 6146s today to see if there was any change and nothing. Just wondering if someone could please guide me to the next step. I'm new to working on vintage gear, but gotta start somewhere. Any help would be great.
Thanks
Dan
N8ZBA

[KC4VWU]

What does the final grid current look like under this condition?

...Phil

[N8ZBA]

The meter reads 6 ma of grid current.

[W3GMS]

Check your screen voltage as well.  If you lost the screen voltage or if its very low you will have very low output.  I am assuming the plate current still dips and all of that. 

I don't have the schematic in front of me but if it has a clamper tube make sure its good.  The purpose of the clamper tube is to keep the plate current from soaring if you loose drive to the final.  It pulls the screen voltage down when the drive is missing.  Anyway measure the screen voltage while your in the TX mode and see what it is.  I am guessing but it should be somewhat greater than 200V. 

73,
Joe, GMS     

[N8ZBA]

Joe,

Thanks for the replay. Yes it still dips, just really low reading on the meter. I will check the clamper tube & screen voltage.
Thanks
Dan

[N8ETQ]

  What Joe said...

/Dan

[w5gw]

Sounds like clamp or screen voltage has shifted. Try turning R30 a bit CCW to see if current increases. Resistors in Vikings are always going up in resistance, so I'd check that as well as R13, the big wire wound resistor.

good luck

W5GW

[N8ZBA]

Ok great, thank you for the info. Looked for my meters over the weekend and noticed I have everything in my tool box at work.  so Monday I'll get my house meter back home like it should be to start my testing of components.

Dan

[AB2EZ]

I agree with Joe, et al, that the likely cause of the problem is low 6146 screen voltage... probably because the clamp tube is turned on.

The clamp circuit in the Viking II is turned on or turned off ... depending upon:

A. The 6146 average grid voltage
B. The modulator screen voltage (R13 tap "A")
C. The setting of the clamp circuit's adjustment pot

The 6146 average grid voltage is nominally: the -75V fixed grid bias - 1800 ohms (R24) x the average 6146 grid current. I.e. -75V - 10.8V = -85.8V when there is 6mA of average grid current.

The following could cause the clamp tube to turn on, even through the average 6146 grid current is 6mA:

1. The AC line voltage is higher or lower than it was when the clamp circuit was last adjusted. I.e. the modulator screen voltage is more or less positive, and the fixed 6146 grid bias supply voltage is more or less negative... with the net result being to make the voltage on the grid of the clamp tube more positive. If so, follow the manual to readjust the resting modulator cathode current, and to readjust the clamp circuit.

2. The -75V fixed bias supply voltage is less negative than nominal. If so, check the -75V bias supply to determine the cause.

3. The modulator screen voltage has become more positive because the 20k ohm wire wound, adjustable tap resistor (R13 / tap "A") that produces this voltage has changed value, and needs to be readjusted. Check the resting modulator cathode current, and readjust tap "A" of R13, if necessary, as per the manual.

4. Fixed 470k ohm resistor R29 and/or 1M ohm potentiometer R30 have changed value. If so, readjust R30 (clamp circuit) as per the manual.

Stu

[N8ZBA]

Stu,

Thanks for this replay, it's a big help. Watch for updates.
Dan

[AB2EZ]

Dan

A simple test that can be done without any separate test equiment is as follows:

Turn on the Viking II, in CW mode, and tune it up ... with the low power and the low plate current (as you have reported). Make sure that the grid current is around 6ma.

Turn the audio control all the way CCW (off), turn the meter switch to modulator position, and turn the mode switch to AM. Verify that the modulator resting cathode current is between 60 and 80 mA. This will verify that the bias supply is working, that the HV supply is working, and the the modulator screen voltage (tap "A" of R13) is okay.

Then, turn off the Viking II, wait until the power supply capacitors have discharged, and carefully remove the clamp tube (probably still hot).

Then, turn the Viking II back on... VERY BRIEFLY going into transmit mode, while watching the plate current meter. Make sure to "dip" the plate current.

If the Viking II tunes up normally, then you have confirmed that the problem is in the clamp circuit (adjustment or repair required).

If there is excessive plate current, even when you try to dip the plate current, the problem is probably in the path leading from the grids to the -75V bias supply.

Do not operate the transmitter, beyond this confirmation, until you have fixed the problem in the clamp circuit or the bias supply

Stu

[N8ZBA]

The resting cathode current on the mod meter is about 110 ma in tx on the AM mode. I have removed the clamp tube and applied hv with the meter still ready hardly any plate current.

Dan

[w1vtp]

The resting cathode current on the mod meter is about 110 ma in tx on the AM mode. I have removed the clamp tube and applied hv with the meter still ready hardly any plate current.

Dan

Screen voltage on the 6146's under these conditions?  110 ma for resting modulator current seems very high

[KC4VWU]

Maybe the slider tap on R 13 has become loose and/or mis-adjusted? Mod resting current should be around 65mA, I believe?

...Phil

[AB2EZ]

If there is little 6146 plate current with the clamp tube removed... and if the resting modulator cathode current is higher than it should be ... I need to think about what could cause this problem.

Just to be sure... you removed the 6AQ5 clamp tube... not the 6AQ5 driver tube... and you found no effect on the 6146 plate current. Is that correct?

Stu

[AB2EZ]

Okay...

I wouldn't worry about the higher than normal modulator cathode current at this time. Part of the cause of that may be increased screen voltage on the modulator tubes because the 6146 RF output tubes aren't drawing much current from the HV B+ power supply. After you fix whatever is causing the low 6146 plate current, you can readjust the resting modulator cathode current.

Returning to the low 6146 plate current problem:

Verify that the 6AQ5 tube tube that you removed was the clamp tube (not the driver tube), and that removing it had no effect on the low plate current. You can leave it out (removed) for now.

Turn off the Valiant, and leave it unplugged... to allow the power supply capacitors to discharge.

As Joe and others suggested in their earlier posts... something (other than the clamp tube) is probably causing the 6146 screen voltage to be too low.

A. It is possible that the 20k ohm wirewound screen voltage dropping resistor (R28) has opened up as a result of aging and the heat it produces. Measure the resistance of R28 with an ohm meter. It should be around 20k ohms (the value is not critical).

B. It is possible that there is a cold solder joint somewhere between where this 20k ohm resistor connects to the 6146 socket's screen terminal (via R35, a 56 ohm series resistor) and where the other side of this 20k ohm resistor connects to the HV B+ (at the bottom of plate choke L21). Measure the resistance between one of the 6146 socket's screen pin and the bottom of L21. It should be around 20k ohms (the same as the resistance you measured for the 20k ohm wirewound resistor R28).

C. It is possible that there is a solder bridge or a wire shorting one of the 6146 socket's screen pin to ground, or that one of the 0.001uF 6146 screen bypass capacitors (C28 and C45) is "leaky" or shorted. Measure the resistance between one of the 6146 socket's screen pin and ground. It should be around 40k ohms (R28 + R13).

Stu

[AB2EZ]

Also check to see if one of the two plate chokes (L7 or L21) has opened up. Check the resistance of each. Check the resistance from the plate cap of one of the 6146''s to the bottom of L21. All of these resistances should be much less than 100 ohms.

If one of these plate chokes has opened up, the "plate" current you are seeing could be just the 6146 screen current... although I don't see how you would see any RF power output or how you would be able to "dip" the plate current in this situation.

Stu

[W3GMS]

As Stu said, don't worry about the high resting current on the modulator right now. 

Just because you removed the clamp tube does not mean that you have screen voltage on the final.  Unless you are not comfortable measuring the screen voltage at the pin of the tube socket I would do that.  If you find out that is missing or extremely low, then you can chase down what in that path has gone astray.  Just put a clip  lead on the screen pin and clip that to your meter lead or some meter leads come with little clips for the probes.  Do all of this with the power off and make sure all the supplies are discharged and then you can connect to the right points and you will be able to see what your screen voltage reads. 

Work safe. 

Joe, GMS

[AB2EZ]

Dan

One thing that we haven't asked:

If you adjust the output tuning capacitor to be on either side of resonance (either side of the position that produces the plate current dip), does the plate current increase to hundreds of mA, or does it just increase by a small amount?

If the plate current increases a lot on either side of resonance, then this would imply that the low plate current (and low RF output), at resonance, is being caused by too low an impedance across the RF output of the transmitter.

[The way a PI network works... A lower impedance across the pi network's output causes the impedance looking into the pi network (at resonance) to increase.]

What could cause the impedance across the RF output of the transmitter to be too low:

A. The loading control and/or then auxiliary loading switch is set to a position that adds too much total capacitance across the pi network output of the transmitter. In the parlance of radio transmitters: the loading is too "light". If so, increase the loading by reducing the capacitance via the loading control and the auxiliary loading switch.

B. There is a short (or very high leakage) across one or more of the auxiliary loading capacitors.

C. There is a short somewhere in the cabling between the output connector and: the antenna tuner/power meter/antenna.

D. There is a short, to ground, somewhere on the loading switch or the capacitors connected to it.

Stu

[N8ZBA]

 Stu,

When I tune the final to either side of the dip the plate current does not rise into the hundreds of ma. It barley rises above the dip and is still showing a extremely low plate current reading.  I've brought my test equipment home today from work and hoping to be able to site down tonight and take some reading.

I want to thank all that have replied so far. This web site has a great wealth of knowledge for wonderful guys that truly seem to want to help. Thanks agin. Be back soon enough with some updates.

Dan

[WD5JKO]

Dan,

   I don't know what kind of meter you are going to use. I'd examine the meter specifications before trying to measure The Hi B+. Most meters are good for 1 KV DC, but make sure. Also the test leads need to be up to the task. If your not sure, stick with the Lo B+ and PA screen voltage. I've seen a lot of test leads in really bad shape. An arm to anything shock from 700 VDC can easily be fatal. No hanging jewelry either. 

  I look forward to your next post.

Jim
Wd5JKO

[W3GMS]

I like the Triplett 630 and the Simpson 260 VOM's for measuring around RF.  Some handheld multimeter's really go buzerk with a little RF is fed to them!   Regardless, use what you have and by all indications you will find very low screen voltage on the tubes.  Not saying it can't be something else but the screen is the first place to hit with the meter.   

Good luck and let us know the results of your data taking session. 

Joe, GMS       

[N8ZBA]

We'll guys she is alive! The first thing I put my meter on when I got home today was the 20k ohm wirewound screen voltage dropping resistor (R28) and sure enough it was open. I went ahead and replaced the resistor (thank god I had one) and sure enough plate current is back. Loads up just like it should.

So I guess my next question would be, is something else bad that took out this resistor? Or would it just be from age and heat like you mentioned Stu. I do know the shack was pretty hot that night, rig was one for hours, and I was making some old buzzard transmissions. 

Thank you too all that had input and help me get the rig back to life. The number one thing I learned from this and the wife agrees is that I'm in desperate need of a work bench and test gear. 

73 for now
Dan

[W3GMS]

Dan,
If you getting the right screen voltage now, you should be ok.  The dissipation of the resistor is the voltage drop across it times the current going through it.  So if the screen voltage is now correct that says the drop is right based on the voltage drop and current going through it. 

Great job and I am thrilled you got it back working! 

73,
Joe, GMS   

[w5gw]

I'm not sure of R28's wattage - it is not specified in the manual. From it's size and vintage I'd guess perhaps 5-10 Watts - I'd replace it with a 10 Watt one if I could find one. Mouser has some wire wound Ohmite resistors that look good (in fact they have one that is a perfect drop in for R13, the notorious center tap 20K 50 Watt resistor)

Stu is right - heat and age are culprits in most wire wound resistor failure modes - not necessary that something else being bad. If you have good plate current reading likely all is well - remember plate current metering is a combination of both screen and plate currents on the Viking, I wouldn't worry about it.

Why does a wire wound resistor fail? Heat causes expansion and cooling causes contraction. Thus the initial linear resistance becomes non linear across the windings with use and age. Eventually some minor imperfection or stress, or stretching in the wire's metal will result in a higher than normal resistance at a point on the resistor and that creates a hot spot. In turn, increased localized heating and cooling, stretching, etc. until finally it gets red hot for an instant and opens up. The resistor may be properly sized, but remember, this is a part that is 60 plus years old.

While on the subject of resistors, the carbon resistors in every Viking or piece of Johnson equipment from that era I've worked on are much more notorious for increased resistance with age than in other equipment of the same vintage. Sometimes as much as 200% or more higher than they initially were -- just saying.