[Heath] SB-200 problems...again...

[Glen Zook]

Not to ask a stupid question, but have you checked the value of the meter shunt resistor?  It is a 1.5 ohm resistor, R-15 on the schematic.  Also check C-19 which is a 2 mfd @ 150 volt electrolytic.  If the shunt resistor has gone low the meter reading will definitely be low.
 
Glen, K9STH


Website:  http://k9sth.com


________________________________
From: Kenneth G. Gordon <kgordon2006 at frontier.com>
To: Heath at puck.nether.net
Sent: Sunday, September 18, 2011 9:47 PM
Subject: [Heath] SB-200 problems...again...

OK. This time I'll try to get it finished before I hit "Send".

My SB-200 has been odd in that since I have had it, the plate idle 
current has NEVER been 90-100 mA as the manual states it should be.

And, for that matter, grid bias voltage has never been exactly -2.0 
VDC either...

I finally got fed up with this state of things, and decided to 
carefully examine every component in that grid-bias string, or in the 
plate metering circuitry, to see if I could find 
something....ANYTHING....that was out of spec.

I have been through practically every component, and have, so far, 
found nothing out of spec.

Yet, at best, the plate idle current never rises much about about 20 
mA (which is ONE "hash-mark" above the lower meter stop, BTW).

Checking the bias voltage at the RF-cold end of the grid RF choke, I 
find that when I use one of those Harbach soft-switch units (the one 
specially made for the SB-200), the bias voltage is -11.99VDC, which 
indicates to me excessive resistance in the switching device.

If I bypass that Harbach unit by jumpering the relay switching point 
to ground with a clip-lead, the bias voltage is then -1.8 VDC which 
at least is in the ballpark, and SHOULD make the plate current read 
90-100 mA....at least with the old Cetron 572Bs...but it still reads 
very low, 20 mA at the highest.

As far as I have been able to determine, everything is within spec, 
and wired correctly, yet I still have this problem.

Power output, with new tubes, into a dummy-load is 650 - 750 watts, 
depending on band.

So, I guess what I am concerned about is linearity.

Before I continue here, I must ask: has anyone else here experienced 
such ?

I think the next step is to carefully examine the signlal output with 
a scope.

The way things are, I may be trying to fix something that isn't 
really broken.

Ken W7EKB
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