[Heathkit] SB-200 issues.

[jeremy-ca]

Ken, if it happens on all bands it could be tubes, the output blocking cap 
or the grid caps if they are the rectangular style and not the later dipped 
mica. Ive had many SB-200's come thru here with both cap problems.

The grid resistors usually drift up over time but not short term as in a key 
down test. They are there to both act as an aide to parasitic supression as 
well as providing a bit of drive balance. If one tube has gone bad it may 
have opened its resistor and you have one tube taking the full load 
hastening its demise.

Try using one tube at a time after checking the resistors and caps. Drive 
with 40-50W on 40M. The input VSWR will change but ignore that.

Replacement fans are available from Harbach but you can probably fix yours 
by taking apart, cleaning the shaft and oiling the bushings. When they lose 
torque and slow down and then stop they dont burn out.

Carl
KM1H



----- 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
>