I like Michael have repaired quite a few SDS9's in my time and fully agree with what he is saying. 95% of the random triggering issues I have dealt with were the result of a faulty 4051's the others were due to faulty address decoders (IC's 58 and 61) or dud bypass caps associated with the demux circuit (creating noise spikes)
Here is my take... Replace (ic socket fitted at same time) IC19,IC64,65,66,67 and 68. Replace ALL 100nf decoupling caps for these IC's also (you have already done this I see) The parts are cheap and once this has been done you will know for certain that the demux section can be ruled out. We can then go from that point to narrow down the fault.
As to recapping...well that is a subject that stirs up a hornets nest every time it is mentioned. To state that capacitors "wear out" is a bit of a false statement. They simply don't. BUT there are types that can and will deteriorate over time to a point that they no longer function as a capacitor (acting more like a resistor or going dead short) or there rated value at time of manufacture is way out.
Ceramics, mylars and the like are generally quite robust and don't need replacing (unless they have been stressed beyond ratings due to PSU fault or similar)
Tantalum caps usually fail dramatically releasing magic smoke in the process. They are notorious for failing and in any "vintage" gear I replace them...THEY WILL FAIL< IT"S JUST A MATTER OF TIME.
Electrolytics also fail on a regular basis. The electrolyte dries out over time causing drift of value and voltage piercing through between the plates (dead short)
I speak from experience, 35 yrs as an electronics engineer and technician working with gear from the simplest up to highly accurate and sensitive lab gear in universities. I can say without exception any equipment I have recapped has benefited greatly from the work carried out. Gear that once was way out of spec would become usable once again. Yes the process of electronic repair can insert new faults, but in due care is taken during the process this is minimized. It's not rocket science but a bit of knowledge and understanding certainly goes a long way.
Richard
In
Simmons_Drums@yahoogroups.com, "Michael Buchner" <buchnerelectronics@...> wrote:
>
> Start with the 4051s. In most cases...
>
> Best
>
> Michael
>