ilovefuzz.com

I've got two (non DIY, aka not my fault) PT-2399 delays that exhibit the following behaviour.

Every now and then they will pass the dry signal only. Doesn't matter if I turn on and off the pedal with they bypass switch, change the delay time or anything else. It's just clean. If I pull the 9V power and plug it in again everything is jim dandy.

It is extreeeeeemely intermittant - happens maybe once every month or two.

Any ideas what could be going on?

Firstly, check that pin 3 AND 4 are both grounded. Apparently some chips have a very low value resistance built in between these pins and some don't. If you just jumper these pins (pin 3 is grounded on all PT2399 schematics I've seen) it may well sort it out: I do this routinely, and haven't had a problem.

I've also read that if the resistance to ground from pin 6 is too low when the chip is first powered up, some chips will lock up and even get hot: If connecting pin 3 to 4 doesn't sort it out, you could try adding a tiny daughter board that connects to pin 6 and keeps the resistance high briefly whilst the chip powers up: A layout for this is here:

https://dl.dropboxusercontent.com/u/967 ... %20Fix.gif

You could also google "PT2399 lock up" for wider reading on the subject, it doesn't seem that rare.

It seems mostly to be caused, as Nocentelli mentioned, by having very low resistance on the delay time pin (6) during the first couple hundred milliseconds after the IC is powered up. Not all 2399 have this issue, and after it's done with the power-up routine, you can then lower the resistance to essentially nothing without a problem. It's just during the half second after power up that the problem happens.

If you don't need for the resistance to be super-low, you can avoid all the problems by keeping the minimum resistance on pin 6 at about 1K or a little higher. I often use 2K2 because below that the delay time is so short as to be almost useless for most echo applications. The little lock-up circuit posted above was designed for Rick Holt's "Little Angel" chorus circuit, which requires the resistance on pin 6 to be ultra-low. If you're making an echo/delay, I wouldn't bother with it.

Oh thanks a bunch guys. I'll crack em open on the weekend and take a look. Will report back!

I had some problems with excess distortion and no delay. I just replaced the chips and keep a couple of extra around now. I don't know why the chip fried, but it was in a seppuku mind warp if that makes a difference. The effect was stronger after changing the chip (and Jero said there is some variance in the chips).

I've been having the same problem with a pitch vibrato circuit I built - Pitch Pirate type. I was trying different photocells and the cell goes from pin 6 to ground, with a resistor and pot from pin 6 to ground, in parallel with the photocell. Trying different cells of different resistances would cause what the OP was talking about.

I never had the issue before with prior builds of this circuit. Maybe I wasn't experimenting with the photocells like I am now.