1) It did strike me the AC coupling capacitor at the BNCs, as it has a big metallic body, the input is soldered right there, where I expect this point to be critical in parasitic capacitance, maybe it has to do with the fact that both input capacitors in CH1 and CH2 must be closely matched.
The 7A18 and 7A26 dual trace amplifiers use similar input coupling capacitors configured the same way. The shielded outside of the capacitor is driven to help prevent EMI pickup. In the case of the 7A13 and presumably 7A22, those capacitors are matched to 0.4% as shown in the parts list to preserve AC common mode rejection.
Note that those capacitors are oil filled plastic film units for outstanding performance. At least I think they are oil filled; I have never cut one apart.
The second question relates to the first amplifier stage where according to the manual NPN Q152 is used as a switch, in a saturated state. Could they have used a JFET instead and why they didn't?
It is kind of a weird design, isn't it?
The differential clamp circuit which is not shown in your excerpt allows Q152 to disconnect the signal if it becomes 1.2 volts positive than the other side and CR150 disconnects it if it becomes 1.2 volts (?) negative compared to the other side.
My guess is they could have used JFETs but FET Vgs varies over a much wider range than bipolar Vbe making matching more difficult although this would only have affected the clamp levels. JFET transconductance is also lower making for a softer clamp and lower frequency response. JFETs are also more expensive and noisier; the general rule was to use them only as required.
Note that the 7A13 vertical amplifier was specifically designed for fast overload recovery time because of its use as a differential comparator.
The guys over at TekScopes2@groups.io and TekScopes@groups.io may have more suggestions.
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