3M1/Repairs

In order to allow my 3M1 to work with 40 V pulses, I added a second resistor in parallel to R104, increasing the quiescent bias point of the 6DJ8. The resistor I added was a 620 kΩ, and the resulting parallel combination measures ~185 kΩ. This now works perfectly in my 564B. It should work equally well in any 560 series scope.

You can see the extra resistor piggy-backed on R104 in the last two photos on the 3M1 page.

It's worth noting that the input circuit in the 3M1 is different from the middle (067-0500-00) version. The middle version has a Zener diode in the center of that resistor chain. That version may require a different approach, but the approach above should be a good start.