Burnt resistor on a Viking II - What does it do?

[KC2TAU]

My friend and I have been working on getting a Viking II up and running again that has sat for quite some time at our university's radio club. I've powered it up through a variac and I can apply filament and HV no problem. I can also go through the tune up procedure though it only has a few ma's of plate current. When I had first inspected the set I notice that both 6146's seemed to have lost vacuum at one point or another. Another item I noticed was that there is a resistor at the bottom of L11/L12 that seems to have burnt quite badly as only half of it is still there. I believe this resistor to be R35 which is a 56 ohm 1 watt resistor. What is R35's job? Is it being burnt related to the 6146's having lost vacuum? I just want to make sure that there isn't a component somewhere else in the rig that has gone bad so that I don't lose another resistor or set of tubes when I power it back up with a new resistor in place.

[KX5JT]

Looking at the schematic, it looks like R35 is a screen voltage dropping resistor for the 6146's

http://bama.edebris.com/download/johnson/viking2(2)/Johnson%20Viking%20II%20Manual.pdf

[KC2TAU]

Thank you very much for the explanation. Looks like I'll have to check R28 as well to make sure it hasn't gone up or down in value and perhaps contributed to the other resistor's failure.

[WQ9E]

L11 and L12 are in the plate leads to the 6146 finals (parasitic suppression) so I am not sure what you mean by at the bottom of these two since there on top of the chassis while R-35 is mounted at the socket.

If you are referring to R35 then it is possible a tube failure took it out but if so check the .001 caps on either side of it.  I imagine it probably failed mechanically rather than from an electrical event.  The replacement resistor should be another carbon comp variety.

An unrelated issue but while you are in there check the 5R4 sockets (and tube base bottom) for any evidence of carbon tracks and clean those if found.

If both tubes lost vacuum, it was probably due to the previous owner seriously overheating the tubes to the point that the plate seal failed.

Good luck with the Viking II repair, once finished it is one of the most reliable transmitters Johnson made.

[WD8BIL]

If both tubes lost vacuum, it was probably due to the previous owner seriously overheating the tubes to the point that the plate seal failed.

This would also explain the fried screen dropping resistor.

[WQ9E]

I agree failure could have been related to abuse but this is only a 56 ohm resistor so there is very little drop across it, the screen dropping resistor is R-26 which is a 20K resistor.  So excessive screen current would cause a very large drop across R-26 with little potential left across R-35.

I assume the main purpose of R-35 is some sort of decoupling/suppression resistor.  No comparable unit was used in the original Viking 1 (4D32 final).

[WD8BIL]

I assume the main purpose of R-35 is some sort of decoupling/suppression resistor.  No comparable unit was used in the original Viking 1 (4D32 final).

That's exactly what it's for. It's to suppress hypersonic oscillation in the screen circuit. The screen dropping resistor (20K) places a modulated voltage on the screen necessary for 100% modulation of the 6146s.

C45/R35/C28 form an RC pi filter.

There's little doubt an oscillation of some kind (most likely ultrasonic) caused the failures. This is confirmed with the burnt resistors in the plate parasitic suppressors L11 and L12. Once the screen starts oscillating all hell breaks loose with VHF parasitics in the plate circuit.

Check for any mods done to the modulator section and check the mod transformer for opens/shorts!

If the plate caps on the 807s were reversed for any length of time by the previous operator/owner this could explain eveything.

From the manual:
Pg13 - e. Turn "plate" on. If squeal is heard, turn plate off immediately and reverse 807 plate caps.

[Gito]

Hi

A large screen current that causes the 56 ohm screen resistor burnt,can be caused of a loss voltage (B+) at the plate voltages of the Tube (6146).
Broken L7 (RFC) ,or L21 (RFC)
With no plate voltage ,with excitation applied ,the screen current will soar up to a high level,it also can be caused that the transmitter is loaded to lightly ,
less plate current, makes more screen current.

The 807 is the modulator tubes,sometimes ,some off(a small) part the RF voltage(output) is "coming back" in the audio parts off the modulator audio (rectivied audio)
 ,and if the phase is right  it becomes a positive feed back that makes the squeal heard.
By swapping the plate caps ,we turned the positive feed back into a negative feed back.
So the modulator can work .

I think it's not the transmitter that is oscllating, but the modulator becomes an "audio oscilator"

Gito