[Amps] Odd Problem with SB-201

[Jim W7RY]

Your new choke system is probably resonate on or near 40 meters. Did you 
measure it? Remove it and return the plate circuit to stock and repeat the 
test.

As to your soft key... What happens when you by-pass it? Does still stick in 
transmit? If so, the relay is either physically sticking or has a magnetized 
coil.
If it doesn't still stick, then the soft key is bad.

The SB-200 is a great amplifier. BUT, the girds need to be directly 
grounded. To do this, you need to completely re-wire the metering circuits. 
I've done that when I add my QSK board. With the grids
grounded, it's more stable, puts out more power with less drive.

There are 2 different schematics for the SB-200 on the net. The difference 
is the tuned input values are completely different! I think the inductors 
are the same, but the caps are totally different. I can
dig out the best one that I have found and let the list know if anyone wants 
to know.

73
Jim W7RY


-----Original Message----- 
From: Mike Baker
Sent: Friday, October 05, 2012 2:31 PM
To: amps at contesting.com
Subject: [Amps] Odd Problem with SB-201

I have an odd situation that I could use some input on.
I reworked an SB-201 with the various Harbaugh mods and boards. New PS, soft
start, soft keying, protection diodes across the meter, glitch resistor. All
looked good and all seemed to work fine save for two issues.
1. The soft keying now seems to have a delay in the relay dropping back into
receive of a few seconds.
2. The most problematic.  For some odd reason 40 meters is not receiving
drive like it should. Like the input tuning network is now out of band or
??? All other bands seem fine. The input switch seems to be fine and not
open.

The only other change I made was to replace the original plate choke with a
new RFC that looks like a generic design single layer wound on a ceramic
dowel form and added a choke decoupling/bypass cap that screws to the
chassis.
Changing the plate choke and bypass cap shouldn't have any effect on the
driving ability of the input network should it?
I am stumped and could use some advice.

Thanks in advance.

Mike Baker  K7DD




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