[Heathkit] SB-200 issues.

[James M. Walker]

Ken,
Ola, I know you are familiar with the unit, but I have to ask, is
the AC line to the amplifier stiff enough. Reason I ask is if it sags going 
in
that effects the filament voltage as well as the high voltage.

Also I may have a left-over fan motor with the blades that goes on that I
would have to check. It is the same fan used on the TX-1 Apache. which
is also in my Johnson thunderbolts.

First place I would look is the AC line, then check the resistors cold, and
again after warm-up.

Also is this the rig with the 572Bs in it?
Jim
WB2FCN


----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at verizon.net>
To: <Heathkit at mailman.qth.net>
Sent: Thursday, January 17, 2008 11:07 PM
Subject: [Heathkit] SB-200 issues.


> 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
>
>
>