[Drake] T-4XB gain drops as warms up Why?

[Dan W. Dooley]

I have the same radio combination and though I have never experienced that 
problem, the fact that you have tried other tubes, leaves only one thing 
likely.  Or most likely, I should say.  I can't say which one, but it sounds 
like a capacitor is leaky somewhere and with the heat of extended operation 
is either changing value or more likely, leaky causing perhaps more shunting 
to ground if it's a bypass cap.  Or opening up if it's a feedthrough. 
That's not going to make it any easier to find but at least is one component 
to start checking out.


Dan W. Dooley WB5TKA
  Dooley's Treasure Chest
  email: dan at dooleysreasurechest.com
  http://www.dooleystreasurechest.com


----- Original Message ----- 
From: "Garey Barrell" <k4oah at mindspring.com>
To: "Drake" <drake at mailman.qth.net>
Sent: Sunday, March 01, 2009 12:36 PM
Subject: Re: [Drake] T-4XB gain drops as warms up Why?


> Jim -
>
> I can't say I've ever run into that one before......
>
> Does the  _idle_  plate current change, or just loaded plate current and
> output power?
>
> Just a quick guess, check R26 for value and condition.
>
> 73, Garey - K4OAH
> Glen Allen, VA
>
> Drake 2-B, 4-B, C-Line & TR-4/C Service Supplement CDs
> <www.k4oah.com>
>
>
>
> k9jmlinux at gmail.com wrote:
>> I have an R-4B/T-4XB combo. The gain varies during the first half hour of
>> operation. For example, on CW, to make full power when it is cold, the 
>> gain
>> knob will need to be at about 10 o'clock. As it warms up, the power 
>> drops,
>> eventually to about half power. Re-peaking the RF TUNE or the final 
>> tuning
>> indicates that neither has changed. To get to full power again requires
>> increasing the "GAIN" to perhaps the one o'clock position.
>>
>> I have check and swapped out tubes. No luck. I haven't dung in to find 
>> out
>> what stage is changing. The time constant is long enough that it will not
>> be a quick debug. Anyone experienced this and have any suggestions.
>>
>> Appreciated
>> 73 de
>> Jim K9JM
>>
>>
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