This is a common problem. The grub screws cut a trench into
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the shaft when the knob is forced to go where it shouldn't, and
the trench can't get past some bump on the knob... probably the
grub screw, or its hole.
Take a light and peek into the hole, you will see a pair of
parallel tracks, I think.
Remove the grub screws completely, and keep turning while lifting,
and it will come off.
Kyle Rhodes wrote:
I just picked up a 577, all seems to work well except the Max Peak Volts /
Series Resistor Knob needs adjustment. The outer portion of the knob with
the white arrow that points to the voltage is off by one increment. i.e.
It points to 25 instead of 6.5, 100 instead of 25, etc.
I did the obvious and loosened (then completely removed) the two visible
set screws on the inner knob. When loosened the knob will spin more or
less freely, but it does not come off. This knob is spring loaded upon
pull, and aside from the slippage on the shaft, it seems just as connected
when pulled. I can't get it to budge, no matter what I do.
I figure there's a burr on the shaft, and the knob bushing is grabbing onto
it. If this is the case, I'm not sure what else to do besides pull and
twist until it works its way off...
In lieu of accidentally breaking any difficult to replace parts, I thought
I'd reach out to the hive mind and see if anyone has any suggestions. I
already looked in the service manual, and it implies the knob will come off
with those two set screws loosened, allowing access to the outer knob set
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