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JOHNSON VIKING II issue

C

Crambone

Well-Known Member
Jan 22, 2019
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Hoping someone can help me out with a NO MODULATION CURRENT issue. The radio keys over 100 watts and shows a good sine wave on the scope. Only issue is ZERO MOD CURRENT. I checked the large tap resistor and it’s showing as good 20K end to end and 9 something and 9 something ends to center.
Put in a new modulation XFMR also as I had a new one. All the tubes were checked and all showed like new. Checked all the caps and resistors in the V1 & V2 area also and changed any that were out of tolerance.
When I received the radio it wasn’t working as I had no oscillator Current and found they had the cap coming off the crystal board going to the wrong pin. Repaired that and everything else started coming into line. Now just the MOD CURRENT ISSUE.

Just not sure where to start looking now?
 

I'd start looking at the center tap of the transformer that drives the 807 modulator tubes. If the negative DC voltage is much more than 40 or 50 Volts this will cut off the tubes' anode current. R16 is the control that sets the bias current for those two tubes. If the voltage doesn't change when R16 is turned up and down, that's a problem.

And if the 807 tubes are just worn out and flat, that would do it.

73
 
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I thought I'd post the Block and Schematic for all to view. It seems to me it could be almost anything along the audio chain. By others looking at the schematic maybe they would be more knowledgeable about places to start. I helped operate one of those rigs as a novice years ago at a field day event. I remember the open wire feed line and being told not to touch any of the dials on the Johnson, just the receiver and transfer switch. For me it was strictly set to one frequency crystal operation while others took their break.
Viking II Block.jpg Viking II Schematic.jpg
 
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nomadradio said:
I'd start looking at the center tap of the transformer that drives the 807 modulator tubes. If the negative DC voltage is much more than 40 or 50 Volts this will cut off the tubes' anode current. R16 is the control that sets the bias current for those two tubes. If the voltage doesn't change when R16 is turned up and down, that's a problem.

And if the 807 tubes are just worn out and flat, that would do it.

73
Click to expand...
Im showing the following voltages at the 807"s
V3 RX TX
Pin1 0 0
Pin 2 -.08 .6
Pin 3 -41.6 -40.6
Pin 4 0 0
Pin 5 0 0

V4 RX TX
Pin1 0 0
Pin 2 -0.8 .5
Pin 3 -41.5 -40.6
Pin 4 0 0
Pin 5 0 0

Both Plates show 770VDC in TX


R16 is working and tubes are tested at 95%
 
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UPDATE, I rewired the mike jack to pin one with coax and correct resistors and now when I key and talk into mike I am seeing very small amount of movement on Oscilloscope. Audio turned up to 10 shows slight MOD Current?
 
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You essentially have the full plate voltage appearing at the modulator cathodes. That is an indication that one or both of the resistive meter shunts SH1 or SH2 is open. Check the meter for damage too.
 
Well I just went to check and needle pinned and poof went fuse. I checked the 20k tapped resistor and it shows 90ohms end to end but 9k end to center and 10k end to center so I pulled it and it checked good. I then found 2 purple wires and one yellow showing continuity to ground! I then checked T4 Modulation Xfmr as it’s been a suspect all along. It shows continuity to ground on almost all the leads attached to the 5 lug terminal strip. I’m going to guess that’s not a good thing?
 
Shockwave said:
You essentially have the full plate voltage appearing at the modulator cathodes. That is an indication that one or both of the resistive meter shunts SH1 or SH2 is open. Check the meter for damage too.
Click to expand...
Is the SH1 & W resistive wire? Do I measure or just check if one or both are open?
 
Crambone said:
Is the SH1 & W resistive wire? Do I measure or just check if one or both are open?
Click to expand...
Because the schematic does not give a value, it is likely they are resistive wire shunts. Normally, they will be below 1 ohm. Something between .1 and .47 ohms typically. Other than these shunts, or a simple open connection in the modulator cathode circuit, I don't see any other components that would cause plate voltage to appear at the cathodes.
 
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Shockwave said:
Because the schematic does not give a value, it is likely they are resistive wire shunts. Normally, they will be below 1 ohm. Something between .1 and .47 ohms typically. Other than these shunts, or a simple open connection in the modulator cathode circuit, I don't see any other components that would cause plate voltage to appear at the cathodes.
Click to expand...
Well don’t I feel like an idiot! I wrote Cathode and was refusing to Plate. Sorry for that i had a brain fart, comes with age!
I’m going to check them anyway!
 
Crambone said:
Well don’t I feel like an idiot! I wrote Cathode and was refusing to Plate. Sorry for that i had a brain fart, comes with age!
I’m going to check them anyway!
Click to expand...
While that does change the outcome, don't feel bad. I've had brain farts that took a lot longer than that for the smell to go away.

Now it's looking more like you don't have any AF drive at the grids of the modulator. Start by checking the voltages around V1 and V2, like you did with the modulators. Using a scope, to confirm signal presence, or a tone generator to inject a known good signal, is very helpful when the volt meter alone fails to pinpoint the failure.

Today, you can even use your smart phone as a simple tone generator. Use the earpiece 1/8 inch jack through a resistor divider (potentiometer) to adjust level, and a DC blocking cap to protect your phone from voltages in the radio. The volume on your phone may have enough range to do without the potentiometer but, never omit the DC blocking / coupling cap.
 
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I got things kind of going I found the ceramic cap going to the RF Choke L7 was shorted closed to ground. Replaced and seen modulation but not for long as I only had a 3amp fuse in it for extra protection and I am out of 5amp. Going to grab a few 5amps tomorrow and go back to it.
 
I made a video of my issue it might make it easier than me explaining
After I made this video I was checking voltages on the tubes and on V7 & 17 (6146's) when I went to connect my probe on top to the plate in TX it makes a zap not allowing me to get a voltage. The resting voltage in RX is 2VDC. With the caps off the top of the tubes I'm seeing 834Vdc on the connector and -10Vdc on the tube cap itself?
V7 & 17 6146 tubes.
On the underside Im seeing
Pin 2 6 Vac on RX & TX
Pin 3 0 on RX and 198Vdc in TX
Pin 5 -79Vdc RX & TX

 
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Ahh.. If you have a carrier, the 6146 tubes will not be the focus of an audio problem. Especially a "no audio" problem.

The modulator plate current is the key hint here. Until the negative grid-bias voltage on pin 3 of the two 807 tubes comes down you won't have modulator plate current, or audio.

Leads me also to wonder how strong the 807 modulator tubes are. But until you can get a plate-current reading with no mike audio there's no good way to tell without a tube tester old enough to test them.

Really makes it sound as if R15 has gone bad. Maybe?

Or maybe the two 807 tubes are just flat as a pancake.

73
 

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