Re: Tek 547 A Sweep display during retrace

Dave Wise

Let's do some engineering and find out how much voltage Q373 has to take.
We can ignore the tubes since negative grid conducts no current.

R390 and R376 make a voltage divider.
The output is 49.9/(49.9+75) * 100 or +40V.
The Thevenin resistance is 49.9|75 or 30K.
Adding R375 in series gives 56.7K to +40V.
That's the top leg.
The bottom leg is R377 and R374 or 47.4K to -150V.
190V/104K is 1.83mA.
1.83mA * 7.4K is 87V.
150-87 = 63V. That's the maximum the transistor sees between collector and emitter.

The second criterion is speed. Q373 has to switch a 63V swing as fast as possible.
The main limit on speed in this case is collector capacitance rather than transition frequency. Storage and fall time are not factors since D372 prevents Q373 from saturating.

So you want a 70V (or more) transistor with small Cob.
According to the Common Design Parts Catalog, Cob for the 2N2207 is 2.3pF . The replacement doesn't have to be Germanium, but you will have a hard time finding a Silicon part with a collector that nimble.

Dave Wise
From: TekScopes@... <TekScopes@...> on behalf of yoyowzrd@... [TekScopes] <TekScopes@...>
Sent: Wednesday, November 08, 2017 10:16 PM
To: TekScopes@...
Subject: Re: [TekScopes] Re: Tek 547 A Sweep display during retrace

Figured it out. Q373 was bad. Dang 2N2207s! What are the suggested replacements?

Posted by: yoyowzrd@...


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