N Channel JFET in Micro Sample & Hold

[roglok]

Would one of the wizards on this board please be so kind to tell me if I can use a PN4391 instead of the 2N5457 in the latest version of Ray's S&H?

Most humble,
Roglok

[Uncle Krunkus]

It's listed on the schem as an option, so I would say yes you can.

[roglok]

oh, thx! i knew it would be stupid to ask. i have only really looked at the new schematics.

[roglok]

I've built the Micro S&H on stripboard and I'm having trouble getting it to work. The sample clock generator seems to work just fine. I was using a triangle wave as input signal. The input attenuation and level shifting works, but the waveform appears slurred at PIN 1 and correspondingly at the CV output. The wave at PIN1 is also shifted above ground.

The Glide part of the circuit was omitted and I'm using a 4.7nF styroflex. Other than that, everything should be as in the original schematics.

EDIT: I'm using a PN4391 instead of a 2N5457...

Here are some pictures of the scope:





Any ideas what could be the issue here?

Thanks for looking!

[roglok]

PS: The Rate knob doesn't affect the CV output. However, there are tiny spikes (barely visible on image 4), which look like bleed through from the clock triggers, since those change in frequency with the clock rate.

[jordroid]

Maybe triple check the orientation of D1, suspect a cooked C1 if you used a styrene type, if that doesn't do it suspect the FET, recheck pinout or try a different type if you have any around. From the scope shots at the output of U1-A it looks like the FET is not doing it's job correctly, i think?

good luck,

jordan

[roglok]

Thanks Jordan. I posted the issue over at Muff Wiggler's and it turned out that the PN4391 doesn't seem to work in this circuit.

I exchanged it for a BF245C, which does the job. Not sure if a 2N5457 will yield any different/better results, but I'll compare once I've got one.

I sent a note to Ray Wilson about this, but he has yet to respond...