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The diode is shown in the 564 schematic, being added at 564 S/N 2980. From the serial numbers of my 564s I conclude that this must have been somewhere in 1964.
You undoubtedly were the first person with such bad luck ...
On Wed, Feb 10, 2021 at 02:35 AM, Charles wrote:
I had an "interesting" afternoon. First I made the extender (including running
coax for the other pins even though it looks like you're right and the 3S2
doesn't use them).
Then I slipped while measuring the voltages at the plug-in and took out the
regulator in the 561A. Actually the +300 was wide-open, almost to the +420
unreg. That took a while to find - because the added diode (to prevent the
grid on the 6AS7 from going positive) that shorted was 1) not in the manual OR
the change info in my copy and 2) well-hidden under the resistors on the
ceramic strips! Fixed that.
Moved the extender and another "zap". One tiny strand of braid had touched the
120 VAC next to it. Replaced the scope fuse and cleaned THAT up.
Plugged the extender cable back in - nothing working on the plug-in... then
red glow and bubbling from the 561A!! Fried half of R667. Then I realized I'd
plugged the cable into the wrong edge connector on the back of the 3T77A. Why
did Tek make them identical.... sigh.
Time for a break.