SB-101 Driver Stability / Bias Voltage Wisdom Solicited

[Randall C. Iliff]

Hi,

I'm chasing down some transmit problems in an SB-101, and I suspect that
bias voltage is a key contributor.  Since the initial warning sign was a
wisp of smoke rising from the plate choke, I've been approaching the fault
location very cautiously. (Choke checks out ok, but I have no desire to push
my luck!)

On the theory that perhaps the finals went into oscillation, I wanted to
check out the neutralization more fully than the simple plate dip method in
the manual.

I currently have the plate and screen voltage disconnected to neutralize the
finals per the Heath technical notes, and noticed that the Driver seems
unstable when shifting the pre-selector setting.  (Suggestions on a logical
approach to neutralizing the 6CL6 would be appreciated- Heath basically says
fiddle with the wire until it seems stable.)

Receiver operation seems normal, but in the course of checking various
voltages I came up with something odd:

The Receiver DC Voltage Chart shows -65 volts at the grids of the 6146A
finals, and -2 volts on lug 5 of terminal strip BR.  I measure -65 at both
locations, which makes some sense given that the connection between the
points is RF choke L903 and it goes from there to the bias voltage source.
The other direction couples drive from the 6CL6 which makes me wonder if the
driver instability is linked with this issue.

Am I missing something here?

Also, and more obviously abnormal, is that this bias voltage remains at -65
in the Transmit mode instead of shifting to +118 per the manual.

The benefit of your experience and suggestions would be most welcome!

Thanks, and 73,

Randy
KE0BN

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