[DaKi][er]

A while back, I posted a thread about some weird problems I’d been having with a few mints I’ve been making, well after having more time to sit down and fiddle with them I’m still no closer to fixing them but I worked out what it happening.

So I got a number of mint boards, using all standard vishay resistors, ad8620, 470uf FC caps ,1uf input caps, 1.2ma dc bias, 1k buffer bandwidth resistors Panasonic EVJ-C20 (P2U4503 from digikey)

So now the problem is I’m getting bad oscillations, but this is only when the pot is at a certain value. So that it sounds 100% fine from 0 to about 70% of the final volume and then very suddenly when you get to a certain point it kicks in and it seems to be oscillating.

I’ve only got a multimeter for measuring, but measuring dc voltage from out to ground shows 200-500mV, swapping the polarity of the meter around I still have a positive dc reading, switching to AC shows around the 200mV.

I’m powering them with 2*9v fresh batteries and I’ll also check with a regulated psu as well, but I don't see this fixing the problem.

I’ve also checked some of the pots outside of the circuit (I got a bag of 50 of them so I got a few spares

) and they seem to track fine and don’t have any strange resistance readings where the oscillations cut in.

They’ve got me stumped on what’s causing this, and I really want to get these mints done.

 

[Pars]

Have you tried pulling the DC bias and the bandwidth resistors to see if the problem is still there? Just a thought... Also, are you using CRDs for the bias, or something else?

 

[DaKi][er]

just the standard CRD's as per tangents recommendations

i'll pull them and see what happens