[AMRadio] SB-200 on AM

John Coleman jc at pctechref.com
Wed Jun 11 12:43:32 EDT 2008

Hi John:
	I use a SB200 as a driver for my 250THs it doesn't take much but it
is a little tasking for the little Kenwood.  I've mentioned this before but
here goes again.  I discovered a weird little thing when using the SB200 on
75 MTRS only.  At low carrier level drive like you might be using for AM the
SB200 was showing a small amount (maybe five percent) of hum modulation.
This modulation would roll and shift phase and modulation level as the
carrier input was varied.  I looked for all sort of things like ground loops
in equipment and I even changed all the electrolytic capacitors in the HV
supply.  It ended up being that on 75 MTRS only the RF choke and bypass was
not enough to keep the RF out of the power supply circuitry and the
rectifier diodes were acting a modulators for the final.  I found a small
torroidal choke and installed in the power supply lead that came through the
chassis.  This corrected the trouble but after about 20 min the toroid choke
got hot and certainly not because of the wire because it was very large.
The powered iron core was heating due to the RF so I had to find a regular
RF choke to add under the chassis to fix it.  This only happened on 75 MTRS
because the RF choke and bypass capacitor that is stock is not large enough
to filter out all the RF to prevent it from getting back into the power
supply and ruining electrolytics and or diodes.  This only seems to appear
at low input RF levels.  To look for it, connect a dummy load and scope to
the SB200 output.  Then very slowly start bringing in RF drive.  As I recall
it was some where between 10 to 50 watts of output where it started showing
up.  If you go too high on the drive so that you have over 75 watts it would
disappear.  As you can imagine this could play havoc with your modulation if
some one were comparing.

The SB200 now works very well not only as a RF carrier exciter for the
250THs but as a stand alone linear for AM.  It is equivalent to a DX100 on
output and as clean as your driver.

John, WA5BXO     

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of John Lawson
Sent: Tuesday, June 10, 2008 1:27 PM
To: amradio at mailman.qth.net
Subject: [AMRadio] SB-200 on AM

   I have found (and hauled home) a slightly-ailing SB-200 amp that I would 
like to use on AM...  I'll be driving it with an Alinco DX-70 (40W AM/CW), 
into an ATR-15 tuner, thence by ladder-line out to my "Sort-Of-NVIS Loop", 
the which system right now tunes 75 Fone down to 1.1:1 very stably and 

   Aside from the many pages of mods and tweaks, does anyone have any 
comments/experiences - good-bad-ugly - ?

   My previous AM rig was a Valiant - I've not had a separate amplifier 
since I sold off all my S-Line gear back in the late 80s.

    Thanks in advance for any info or thoughts on the SB-200 in AM service.


Carson City, NV

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