whats making my BMP go splat?

[lightningfingers]

i built a BMP yesterday, and i get no sound from it, exept at the highest sustain setting there is that infamous mis-biased splat.
i had to do some part subs to build it, could any of these cause the splat?
1st clipping stage 560pf cap instead of 470pf. used zener diodes.
2nd clipping stage 1000pf cap instead of 470pf. LEDs
tone stack, used a 0.0047uf instead of a 0.0039uf.
output stage used a 330k instead of a 390k.
i used JD Sleeps layout and schematic. could any of these changes have caused it to not work or is it time to look at the transistor voltages?

thanks

[niftydog]

zener diodes?!   :shock:  If the circuit is reverse biasing them above their breakdown voltage you've got problems.  Seems unlikely from the schematic, but I'd change them all the same.  I've never encountered discussion on the AC behaviour of zeners that are reverse biased so I just don't know!

LEDs are probably ok.. but depends on what type of LEDs.  They have vastly different forward voltage specs.

That 330k sub for the 390k could be stuffing the biasing.

I would doubt the caps are doing much, but a 1000pF for a 470pF!?  That's a big difference!

[sir_modulus]

Put the two 1000 pf's in parallel, as you'll get a closer value. As niftydog said those are some "interesting" diodes. take them out for now and put in normal ones for testing. The output cap is not all that important, but add like 50K to it (tolerence will make it act like a low 390K). If nopne of this works, transistor voltage testing time.

[Rodgre]

Put the two 1000 pf's in parallel, as you'll get a closer value.

wait a minute.... I always thought that capacitors in parallel ADDED together, so 1000pf would equal 2000pf. It kind of makes sense, if you think of a capacitor as a device that's storing a charge. Put two in parallel and you're storing double the charge.

Resistors don't work that way, but I always thought caps did.

Roger

[lightningfingers]

yes you were right, i just audio probed it and it DOES go all crappy around the zeners. but why? i have zeners in my muff fuzz clone and they work fine.......wierd.  :?

[niftydog]

what type of zeners?

If you have like 43V zeners, then it's highly unlikely that you're going to avalanche them.

But if they're more like 2V zeners, then you're closer to having a problem.

[sir_modulus]

Muff Fuzz is a kinda crappy circuit in my opinion, but BMP has lots of transistors and runs off the same supply as a Muff Fuzz, + BMP has a lot of other components. If the Zeners are ones close to the limit, only sometimes will there be enough voltage left to activate them. Or they might drop voltage and misbias transistors. Yes Rodgre is right I forgot. Put two 1000Uf's in series to get around 500 uf. And the leds will clip "differently. Best choice is to put switches to decide about clipping, as Led clipping is an "acquired" taste. I personally don't much care for it.

[R.G.]

It's always time to read the pin voltages.

"Splat" is almost 100% caused by misbiasing way too cold or hot, and that is an indicator that you're going to find some voltages that do not match the indicated guidelines in the Debugging Page.