[Collins] 30s-1

[Dr. Gerald N. Johnson]

What is happening to the idling plate current? The bias voltage is 
adjustable for SSB operation to set the PA tube idling current for 
minimum distortion. On CW its left at the maximum voltage from the bias 
supply to run the tube more towards class C and not so linear. The bias 
voltage dropping should be accompanied by the PA plate current rising 
significantly.

The drift in bias voltage can be from the top side series resistor or 
the bias adjust pot drifting higher in value. Most resistors used by 
Collins only drifted high. Could also be from the capacitors shunting 
the bias circuit going leaky with heat like C210 (50 uf electrolytic), 
C140 and C104. Or the grid current overload relay coil developing 
leakage to ground.

The metered bias voltage can also change if the PA tube is developing 
grid emission. That should should up as negative grid current without RF 
drive. That's inside the tube and comes from cathode coating migrating 
to the control grid and the only cure is a different tube.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.

On 11/25/2015 4:44 PM, Björn Magnusson wrote:
> Hi.
> I have a Collins 30s-1 and I have a problem with it.
> When I start it up and wait a few minutes to get it warm the bias is correct.
> But when I start to transmit the bias will slowly goes down.
> And after a half of a minute it will only come up to approx 0,1 on the multimeter.
> If I change to call mode the bias is correct.
>
> Do you have any idea of what the problem might be? I have checked the electrolyte capacitors that I can think Bill be wrong with but they seems tone ok.
>
> Thank you.
> Best regards Bjoern.
> 73 de sa6axu.
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