WNG noise toaster

[oram]

Hi all, dunno why but wng's always seem to give me problems. If anybody could help me diagnose and fix I'd much appreciate it.

Anyhow, I have this breadboarded but am not getting white noise sound from Q6-C. In fact no sound. In hz i get random frequencies the most common being 3.3hz (ish). I have swapped in and out transistors etc but to no avail.

9v in (8.9 at test)

Here is the leg readings.

Q2: C = 8.9v. B = 0.56v. E = ground
Q5: C = N/C. B = ground. E = 8.11v
Q6: C = 8.15v. B = 0.56v. E = ground

And here is the schematic.

Thanks all

Darren

[David]

This is a very common circuit for generating white noise, and it relies on the zener-type effect of running the reverse-biased BE junction of Q5 at virtually a constant current of a few micro amps. This is followed by two stages of amplification in Q2 and Q6.

The collector voltage of Q2 looks suspicious - it is at the supply potential, and the presence of R9 should drop the observed collector voltage down a bit. Check all connections, then Q2 and if it is OK, try reducing R10 to turn Q2 on a bit further. Aim for the same collector voltage as Q6.

White noise sounds like a gentle hiss, with a perceived (but not real) pitch of a few KHz - where the ear is most sensitive.