Synthacon feedthrough

[-VCO-]

hiya people,

just finished one of my CGS pcbs (woohoo, my first day with my weller since...i can't even remember when! since i started working full time with electronics i've got very little time to build stuff. ironic really...), a cgs35 synthacon vcf. i still have to do some minor adjustments, such as the frequency range and the regular stuff. i did notice that the resonance pot's wiring on the pic on the page was reversed (or i was tired). anyhoo, simple to fix.

HOWEVER, when I hooked it up it seems I get quite a lot of feedthru - i can clearly hear the unfiltered signal in the background, especially when the freq pot is at zero. did i mess up or is there some kind of adjustment you can make? i did change the 2N2907 for a 2N5401 since i can get those through my job, but not the 2N2907. please help, i'd love to have a perfectly working module!

cheers

[andrewF]

You may be right suspecting the 2N5401
I didn't look at the specs too closely, but it seems to be spec'd for lower voltages and currents than the 2N2907.

Assuming all else is correct, it would be a good starting point to swap in another PNP tranny.

[-VCO-]

Hmm, the maximum ratings for the voltages are much higher on the 2n5401 than the 2n2907, however the collector current (Ic) is half the 2n2907's. Maybe that's what causing trouble. Perhaps a BC327 could work? Its Ic value (collector current) is at 0.8A instead of the 2N2907's 0.6A, but all other values are quite similar. I don't want to order just a single transistor from the web with all the shipping and all that, it's just so much trouble for one little component. I can get all the other components through my work.

Are there perhaps any other transistors I could use in the 2N2907's place?

[andrewF]

I didn't mean the max ratings, these circuits get nowhere near such levels - more the off and on characteristics.

my Steiners have A1015 PNPs, so other transistors can be substituted.