808 Clap-Reverb starts too late

[Oli]

Hello,

i've got a problem with my 808 Clap. The reverb of the clap starts too late. It's the same problem like discussed in the MB 808 Manual:

"It was all right finally or so I thought, after a some chatting with Jeff, I found out the reverb delay bug on my box is still there, how to reproduce it/find out if you have it too:

Start a pattern with some CP hits, the first step (some times more than one step, up to four in my case) don't have any reverb on it, some hits later the CP reverb starts to sound fine.

Stop the pattern and wait some time, go play it again and the bug comes back.

It sounds like it has something to do with C14 (original schematic). Just like it needs more than one CP trigger to get charged and once charged stays at a certain level until it is stopped for too long.

Moogah: I've been tracing this problem on one of the units I have here, and what I've found is exactly what Etaoin describes (except I think he meant C76, which is the 47n green cap near the 2 1M resistors). Watching this cap on the scope, the thing that really bugs me is that the voltage settles to -10mV (?!) and I can't figure out why it's able to settle to a point below ground.. and we know from basic transistor theory that the 945 which creates the envelope here won't turn on until the voltage is above 60mV. When I worked out the RC time constant equation for this it made perfect sense that the cap needed a bit of "pre-charging" before there would be any appreciable envelope voltage above the 60mV cutoff point. My thoughts about this are: either the transistor should be biased closer to the 60mV turn on point, or there is a problem with the ground plane in this area which is causing the voltage to be lower than it should be....

Perhaps this problem is solved easily by reducing the 1M resistor which the trigger pulse comes in through, this would allow the cap to charge faster without affecting the discharge rate (I think..)

By the way, I just came across a post on the Dutch synth forum where someone mentions (about a real TR808) "you shouldn't hear the reverb on the first clap, only on the subsequent ones" and someone else agrees. So at least some TR808's exhibit the same behavior."

Does anyone know how to fix the problem?

Thank you!

[sneakthief]

Why don't you try the idea suggested in that paragraph - stick a 1M linear pot instead of the 1M resistor and see if trying different resistances helps?