Well after 2 false claims to success and following 20 hours of repair investigation running the poor little synth with it's guts all over the bench I have finally returned the beast to the land of the living for sure and so painful my experience that it's got to be worth spreading the word... or even the truth...
Well if there ever was an excuse to coin a phrase from Sherlock Holmes this is it....
“Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.”
15 hours into the repair from hell I was at least sure that some of the components on my Delta's synth block pcb had become horribly temperature sensitive. Cool the synth pcb down and sweet sounding Delta, apply fan heater and in seconds it descended into a cacophony of loud rustling, pops and crackles. The thing is that I'd already replaced what everyone was telling me to tackle first - ie. the two LM13600 OTAs which, unlike it's modern successor, are notoriously unreliable. But, to my dismay, the same wild popping and cracking noises came back after an hour of heating up.
The problem was that no matter what I ended up heating and then spraying a whole tin of freezer spray all over didn't seem reveal anything other than potentially 4 bright orange polyester metal film 1nf caps in the filter section which, normally, are known for being pretty much bullet proof - even at extreme age.
After another 2 hours of poking around, in sheer desperation, I decided to remove one of these orange critters for a second time but againt didn't find much of interest - they all measured perfectly on a capacitance meter. It was only after I wired one of them across ohmeter, got curious, and stuffed it in front of the same heater I'd been cooking the hell out of the pcbs with that I saw the meter start to dance around like a mad thing! Bingo!!!
Well it turns out that all 4 of these orange caps in the filter section were as leaky as hell - but only once they had been heated and by the time they were hot at least one of them was completely short circuited!!
Well it seems that much like ARP destroyed the relatively good reputation of tantalum caps in old synths, Korg have done the same with old metal film caps - D'oh!
Well it just goes to show that even after 20 years of pulling gear apart there are still lessons to be learnt!
So in short, if you have a Korg Delta that is making weird popping, rustling or cracking noises from the synth section then at least you just might now have a head start! Korg Delta Synth PCB: Filter Section - Before & After:I couldn't help myself... here is the string section through my tube phaser.... http://www.sendspace.com/file/2r3pky