Help troubleshooting Dual Gate Slew

[dstrength]

I'm troubleshooting a Dual Gated Slew module, and both sections 1 and 2 are exhibiting the same behavior:

1) In LIN mode, the slew time is very short even at values of 1M on P1.

2) In LOG Mode, the slew time works normal, but the rise / fall direction switch doesn't seem to affect the slew. That is, I get slew in both directions all of the time. (The direction seems to work in LIN mode, but it's hard to tell since the slew time is so short.)

Now, I've noticed that I used BF245B instead of BF245C. I realize the B has a lower gate-source voltage range and a lower zero-gate-voltage drain current than C. But, I'm not sure how / if I can adjust the circuit to utilize the BF245B...or if it is even related to the problem.

Any ideas or suggestions?

[yusynth]

Since they share the same cap it is normal that the LIN slew is faster than the EXP slew.
The RISE/FALL switch definitely should give a different behaviour, check that your diodes are not dead (short-cut) and correctly polarized.

[Gypsum Fantastic]

I'm really very sorry to resurrect such an old thread, but this part of your post (below) is an exact description of the problem I'm seeing. I think my dual slew works fine apart from this one issue.

If you're still there, did you ever work out a solution? Anyone else have any suggestions?

[gabbagabi]

could you provide a link to the refering schematic?

[Gypsum Fantastic]

[gabbagabi]

Hi,
i would beginn with shortening the two contacts used on SW2-2,
i would do this to test if Q1 is shortening out P1 permanently,
in adition i would disconnect SW-1 just to make shure there is no extra source of trouble

if Q1 is shortening out P1 permanently, the lin and log mode now both have very short rise and fall times

happy troubleshooting...

[Gypsum Fantastic]

OK so I had a bit more time to troubleshoot the problem.

I'm slightly embarrassed to say that I think the module was working the way it's supposed to work all along.

Even with the slew direction switch in the "up only" position, you still can hear a pronounced portamento effect in the down direction (and vice versa), especially with the crazy extremely long settings I was using to troubleshoot.

I think it's because of the forward voltage on a 1N4148 is comparatively large and this voltage mainly dissipates via the 1MΩ rate-setting potentiometer. Or something like that. *shrug*

I replaced D1, D2, D5 & D6 with a lower forward voltage type (BAT43 Schottky) which has improved things.

Main thing is it's working as intended, I suppose.

Thanks to everyone who contributed.