8116A Code 42

[MarkL]

On BNC tees, Amphenol makes good tees for a reasonable price.  I always like Pomona components, but their tees are about double and I'm not sure worth it.  I would buy new from Mouser or Digikey.

Looking at the end points of C528/C530 in series, you can see it is between the base of Q504 and the collector of Q506.  A quick determination of their effect can be done by imagining a small positive voltage change on the base of Q504.  This would move Q504 closer to turn-off, and reduce the emitter current.  The emitter would move positive, being pulled up by the resistor network R530/R532.

This would in-turn cause the base of Q506 to go more positive, which would then pull down the collector of Q506.  The negative change here would be coupled back to the base of Q504 through C528/C530 which we imagined was going positive, therefore limiting extent of the positive change.

In short, what all this is doing is reducing the slew rate of this stage of the amplifier until the feedback can take affect and keep the waveform true.

On the positive driver side, C529 is performing the exact same function, and contributes to the slew rate control along with the C528/C530.  C529 is adjustable in order to tweak the overall slew rate of this stage.

Note that if C529 has insufficient range, C525 and C530 can be changed as per Table 9-1 with the procedure described on page 9-7.  C525, C551, and C552 also limit the high frequency response of this stage by the shunting the bases of Q503 and Q504.

A 200V rating on C528/C530 is not necessary, plus you have them in series which approximately doubles the voltage rating (ignoring imbalances).  You could substitute 50V, but I would be surprised if the existing 1pF caps were bad and needed replacing.