[sdiy] Pin 14 on the 3340
tom at electricdruid.net
Wed Jul 18 21:02:29 CEST 2018
I have to say, for repairs to vintage gear that originally used the CEM3340, I’ve been recommending the Rev.G. The Alfa AS3340 is a fine chip for new production, but it’s not a drop-in replacement (yet) for circuits that originally used the CEM3340. I expect most of this is down to the circuit being designed around one's set of quirks and not the other’s. I suspect in time Alfa will correct this if possible since they’re aware of the issues.
Synth & Stompbox DIY
> On 18 Jul 2018, at 17:23, Oakley Sound <oakleysound at btinternet.com> wrote:
> RS > Maybe the capacitor at pin 9 (soft sync) to GND is broken...
> Unfortunately not. I've got several voice cards that show the same problem and some of them have new components. I have found out a little more. The instability does still occur in positions nearer the power supply, however, it is less obvious. Some specimens of AS3340 do not exhibit the problem. It only happens when the pulse width is less than 50% increasing at the positive excursion becomes thinner.
> Adding 100nF to each of the +15V and -5V pins does decrease it but not get rid of it. Removing the pin 14 decoupling capacitor seems to help. But adding thicker 0V connections to the board and to the offending VCO does not help. Both VCO1 and VCO2 have the same problem. Using a new issue rev G CEM3340, or original CEM3340, does not exhibit the problem at all.
> Time to buy ten more rev G CEM3340s I think.
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