Re: 577 D1 storage & brightness control


On Tue, Nov 12, 2019 at 9:11 AM Dennis Tillman W7PF <@Dennis_Tillman_W7PF>

Hi Siggi,
Congratulations! You have found an excellent curve tracer.
There is a circuit in all 577s to prevent the CRT from being burned when
you turn the collector voltage down so low that it essentially becomes a
dot on the lower left corner of the screen when you are testing NPN
transistors or on the upper right corner when testing PNP transistors. That
bright spot would eventually burn the phosphor at that spot.
Hey Denis thanks for the reply.
I hadn't realized that the collector supply would affect the intensity as
well, but I can see it now in the schematic. I'll have to look for that
when I dig in.

The brightness control should work regardless of how long the collector
voltage trace looks like when it is sweeping across the screen. You should
be able to adjust the brightness when the beam is anywhere.
The only time the trace is automatically dimmed is when the trace is
reduced to the size of a dot or when it is so very short that it might burn
the phosphor.
Right, thanks.
BTW: Nothing like reading the manual - on page 2-3 of the service manual
it says:

Provides continuously variable flood-gun current duty cycle from about 10%
to 100% (when the collector sweep is turned down or disabled), permitting
extended retention of displayed information. Also controls the degree of
spot dimming when collector sweep is turned down or disabled.

And on the schematics I see that the intensity modulation and the flood gun
current duty cycle are indeed tied together. It all makes sense now.

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