perfecting a replacement part for the HP 1727A/1715A oscilloscope

[Alex_Baker]

Hello everybody.

This is a continuation of the long saga that is my 1727A storage oscilloscope from HP. As it stands everything is in pretty good shape except for the vertical output IC, which is a proprietary IC from HP and is thus likely unobtainable, though if anybody has one they could sell, let me know. Long story short I am starting this topic to perfect the replacement circuit for the aforementioned IC, all relevant pictures will be linked at the end, including the circuit schematic. 

To start out with I just used a couple of IRFZ44N mosfets that I already had, and they yield great results up to 150kHz. Beyond that the amplitude of the trace on the screen starts to decrease(and distort slightly), and by the time I max out my signal generator at 5MHz, the trace is pretty small. So what would cause that to happen? Are the Z44N fets not able to turn on all the way at that speed? Any insight would be appreciated.

[bdunham7]

I'm going to guess that the circuit driving the original chip doesn't have a low enough source impedance to drive the gate capacitance of your MOSFETs at high frequencies.  You probably need another stage.

[Alex_Baker]

The IRFZ44N that I used is supposed to have a max gate charge of about 63nC, and I am aware that the Z44N is no dainty mosfet. The mosfets that the original designer of the circuit used was something close to the STD16NE, which is supposed to have a max gat charge of about 32nC.

how much difference do you think that would make? I will do some testing to see if I can come up with some numbers, I also attached the original HP schematic of the vertical output board, which I should have done on my last post.