Thos RCA CMOS chips gave me headaches many times. Every two weeks one of them broke in an OB-1. So in the end I had to replace most of them, even those that were still "OK". Now the synth works fine since more than 10 years.
I really appreciate your well-produced videos like this one over the ones in which you're poking around in a synth and saying lots of "ummmms" and "errrrrs" trying to think of what to tell us.
There weren't very many OB-Xs made, I think the number usually quoted is 800. Cool that you have two of those reunited there! I'm lucky to have one, and it's a keeper. It was supposed to be fully serviced when I got it, but still does silly things like resets itself now and then. I'm thinking bad CMOS, but haven't had a good look yet. BTW, is that a cracked tropical fish cap @6:39, next to the IC 64? I had a 1977 Minimoog with a strangely behaving filter. The cause was just one of those, hiding in plain sight.
Just replace all CMOS, luckily they are socketed. Had some intermittent self-resetting issues on my ob-x as well, replacing all the chips solved it. Normally I'm not a fan of this "shotgun repair method" but I wanted it to last for a decade and not have to open it again in a couple of months until the next RCA chip would fail. Make sure you get the right versions, 74LS/HC/HCT makes a difference. Some are interchangeable with each other, some are not. Most of them are still newly made available. I have a list of all chips inside the ob-x if you need it.
Another great video! I'm (just!) old enough to remember this generation of polysynths when they came out new, and after seeing your recent video on the Juno 106 and now this, it's made me realise just how close in terms of design philosophy they are to modern synths. What I mean by this, is that I didn't really think about just how much microprocessor control and early SOC's these synths had going on inside them. Okay so the electronics are now surface mounted and have a lot more dedicated SOC's and faster RISC based processors which make things a lot more stable - but fundamentally they're the same thing. Much appreciated that you pass your knowledge on!
Love the level of detail in this video. Couple of questions: how did you learn so much about these synths - did the manufacturer make the schematics available? And second, how do you know what contemporary parts are suitable replacements for the vintage parts?
Thanks for this excellent tour Synthchaser! For the envelope generators I'd like to throw in something and a question: It's true that in the SEM modules the EG's where discrete still. However The programmer module for the 4- and 8-voice SEM poly's contained two IC's for the EG"s. Could those be some of the first CEM EG chips? Because the ADSR stages can all be voltage controlled (from memory) at that chip. Other thing: there are quite some problems with the bender box at the left side of the keyboards in the X and SX models. Could you eleborate on how to fix them, preferably in a video, or did I miss it somewhere? Kind regards from Holland :-)
