321A 2N2207 transistor replacement: Ge-Si replacement sensitivity in the timebase

Dave Wise
 

If your timebase fails due to 2N2207 whiskers and you don't
have spares, this will save you some head scratching.

I replaced Q161 with a 2N3906, and it didn't work.
Symptoms included no trace unless the CAL knob was
all the way CCW, and then trace starting mid-screen,
with unstable triggering.

But if you replace both Q153 and Q161, it will work
fine - provided you also swap D150 and D153.

It's all about the idle loop. If you start at, for
instance, Q153's collector and go through D150, D149,
Q153, D153, Q163, and Q161, the diode and Vbe drops
must add up to greater than 0, or Q161 will not get
enough base drive.

For some transistors, it doesn't matter: Q173, Q183,
and probably Q194 can be changed without considering
the altered Vbe.

By the way, note the seemingly-meaningless Q199.
It does play a role: it keeps the -45 supply stable
by replacing drain not taken by other transistors
when the sweep is idle.

I still have a whisker in the vertical amp. I cleared
it by tapping, but I'm keeping my eye on it.

In Haste,
Dave Wise

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