VCO started taking longer to tune after powering up
2016-06-26 by lauri.saine@yahoo.com
Hi, I'm new to this board. This seems to be a great source of information for anything Polysix-related so thanks for having me.
I could use some assistance with a Polysix I got a few months ago. I just finished pimping this thing up (I installed the Tubbutec Modypoly upgrade alongside with a brand new hardwood case done by a carpenter) so it's unfortunate that I now ran into a problem with my new baby. I couldn't find the exact same issue after browsing around here, so I thought I'd ask.
All of the sudden, one of the six voices started taking approximately 10 minutes after a cold start to get in tune. The other voices still tune up in a matter of seconds - and after a 10 minute warm-up the tuning of all voices is (and remains) perfect. After powering the unit, the tuning returns back to normal smoothly (=not suddenly), so it's: horrible after just one minute, quite close after 5 minutes, barely noticeable after 8 and perfect after 10.
Also - after the issue first appeared, the faulty voice became ever so slightly flat. It was almost not noticeable. I fixed that by internally adjusting the low setting of the voice in question with a flathead screwdriver.
The unit looks super clean inside (it has never been recapped though) with no visible damage anywhere apart from ONE thing: it had indeed suffered from a very minor battery leak earlier on. Any remaining acid was neutralized (I doubt there was much anyway, since as I said, the leak appeared to be quite small compared to any pictures I'd seen online) and there's now a new lithium battery. Still, having said that, there is a darker tint in a couple of the traces right next to the battery (in a very small area though; it's only present right next to the battery and nowhere else) and a couple of the legs of the IC closest to the battery look slightly corroded; two of the legs are not visually quite as shiny/metallic as all the rest of them. Earlier on, I decided to just leave it alone since everything seemed to work great, the darker areas of the traces didn't continue further (as in, under the IC for example) and I didn't want to do any unnecessary tampering with it with my limited knowledge - if it ain't broke..
I'm not really sure how serious the fault is and how common it is for these things to take a little while to get tuned. But it is annoying, since during the first minutes the tuning is very noticeably too sharp (by more than a semitone). Everything else still works as should - the voices are all consistent in volume and there's no difference in how any of the knobs react to the different voices, there are no sudden changes in tune or anything like that, all presets load normally etc.
Do you guys reckon the IC right next to the battery could have something to do with tuning and is something I should take a look at first, since it's the only thing in there that looks a bit sketchy? I'd imagine there are replacement parts to be found for that IC somewhere? I'd hate to start tearing the synth apart any more than I really have to - and I'd also hate to spend any more money on it, so it would be awesome if there's something I can try myself without having to take it fixed somewhere.
While I'm no expert, I do have a soldering iron and am semi-handy with it so I don't think it would be a problem for me to replace that IC, if it would make sense to do so. Thanks in advance for any insights on this.