[Heathkit] SB-200 issues.

[Kenneth G. Gordon]

My SB-200 has recently started acting somewhat strange in that 
after it has been on for a while, both the power output, and grid 
current drop.

PO drops to about 1/2 of what it is when the rig is cold, and the 
tuning also changes as it warms up. Grid current drops to about 
10 mA and sometimes much less...like the needle barely 
wiggling at the bottom.  

As I recall it, and from looking at the schematic, there are a couple 
of resistors in the grid circuit.

Is it possible that these have become very heat sensitive and are 
rapidly changing value as they heat up?

Also, although I haven't yet taken the opportunity to check the 
fan in the bottom of the case, I am just about it certain it is no 
longer rotating.  

In the meantime, I put a fairly large 115 VAC "muffin" fan on top 
of the final cage to suck the heat out, and have propped the lid 
of the outer case way open to facilitate heat flow up and out. 
Things are a lot cooler now.  

The speed of the power drop is now reduced, but still occurs, 
and effects the tuning and power output of the driver, an HW-
101. With the amp off, the driver is steady as a rock.  

Ideas?

BTW, this SB-200 is still completely stock in every way except that 
the input circuits have been "tweaked" to provide minimum SWR 
between the amp and driver.

I am suspecting that the excess heat from the fan being dead 
has caused the tubes to go flat, and that is what I am seeing. 
Of course, then again, it could be the resistors have gone 
south...

I dunno. Suggestions?

Ken Gordon W7EKB