Korg Delta... aka The Repair From h**l!!

[HideawayStudio]

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 h**l 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 h**l 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 h**l - 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

[themilford]

Good work man.

Sleuthing out intermittent issues is always a freaking bear.

Congrats on a job well done.

[clubbedtodeath]

Indeed, congrats. And what a sound, too.

[adhmzaiusz]

Congrats, must feel great to finally conquer the damn thing... I know exactly what you mean, I myself have made those "it works!" claims where things seemed good and then a day or week later problems back... I've shared the same long frustrations with repairing synths. Reminds me of the moog opus from h**l where I literally replaced everything on the main board. Months later after aquiring my own opus for reference I had the damn thing working the next day.

Yesterday my Synkey's root oscillator died... opening it up its a mess of wires and going to be a horror story to repair... information is sparse and nothing is identified in the so called "service manual". Not looking forward to it..

[garranimal]

I had gone through the same procedure with the Delta I used to have: narrowed down the culprit to the filter circuit, replaced the LM13600 and highly suspected the caps which checked out fine on the tester. But that was as far as I could get. It's extremely difficult to find intermittent problems like flaky caps, cold solder joints, and frayed wires.

[nathanscribe]

I feel your pain. I have two Lambdas that play up in different ways. I'm crossing my fingers it's not all the unobtanium that's gone...

[rhino]

Thanks! You've added to the knowlege base to the gain of us all.

[Steve Jones]

The first time that I came across that problem it took me all day to find it. I never suspected those caps and tried everything else. Strangely enough two weeks later another two Delta's showed up in the shop with the same issue. They make the VCF cutoff control behave very erratically. I put it down to a bad manufacturing batch of caps that must have gotten to Korg. I posted the information on Electro-music back in 2007 but it is buried in there pretty deep....

To make it even stranger, soon after I had to replace every ceramic cap in my Waldorf Microwave 1, they were shorting out one after the other.

[kingink]

dang. i've wanted one of these for a while. now i kind of don't

[dimebucker1]

I have exactly the same problem!!

Im trying to buy some replacements caps now, they are 1nf right?, but what voltage are they?

Thank in advance

[mjrippe]

I have a Delta with a different problem - C1, C2, C3, and C4 have noises and C5 does not. Power supply is clean. The top octave divider is a replacement from FlatKeys Ltd in the UK and it is working great. The square wave coming out is nice and clean. The square wave coming from the 4013 (IC5) however shows noise on my scope that coincides with the noise I hear - popping or crackling. Again, pin 14 on IC3 is clean, pin 40 on IC1 is noisy. I replaced the 4013 and surrounding electrolytic caps, cleaned the multipin connectors, even resoldered the joints related to the C notes - no luck! Any suggestions would be welcome at this point.

[rhino]

That is why I never opened a repair shop. Strictly buy, fix and sell (or keep!). Some vintage synth problems could bankrupt you with the time they take to diagnose and fix. Sometimes, the fault hides like a gopher in a hole, other times it stands there in plain sight and DARES you to find it!

Good work, and a perfect example of patience and through diagnostics paying off!

[nathanscribe]

I have a Delta with a different problem - C1, C2, C3, and C4 have noises and C5 does not. Power supply is clean. The top octave divider is a replacement from FlatKeys Ltd in the UK and it is working great. The square wave coming out is nice and clean. The square wave coming from the 4013 (IC5) however shows noise on my scope that coincides with the noise I hear - popping or crackling. Again, pin 14 on IC3 is clean, pin 40 on IC1 is noisy. I replaced the 4013 and surrounding electrolytic caps, cleaned the multipin connectors, even resoldered the joints related to the C notes - no luck! Any suggestions would be welcome at this point.

+1 for FlatKeys, I recently repaired a Lambda using one of their TOG units. Good stuff.

I don't have the Delta schematics to hand, but I'd check all the parts associated with shaping that C - caps, transistors, diodes... dry joints, corrosion, etc. Sounds like you're on top of things though.

Similar problems with my old RS-101 as the moment. Nothing seems to bring it back fully every time... sigh.

[mjrippe]

Yes, I'm on top if it, underneath it, beside it, inside it, and I haven't had any luck yet! There are no film caps in the area, but I might try replacing the ceramic cap (C68) even though it tested good when I pulled it. To be clear - I meant the NOTES C1 - C5, not capacitors C1- C5 in my original post. Here is the relevant page of schematics (not a great copy but the only one I have found - the same one on twenty web sites)

[nathanscribe]

Pin 40 on IC1 is noisy? Did you replace the little poly cap there, C68 it looks like at 220pF?

I take it you've also got datasheets for those S10430 divider/keyer ICs. They carry out a combination of dividing down the individual notes and taking control voltage inputs to vary the levels of those resulting notes in the mix outputs.

When you say "C1-C4 have noises, C5 does not" I take it you mean "C1-C4 sound correctly, C5 is silent". Correct me if I'm wrong. Also, you don't say *which* output of the 4013 is noisy.

The TOG chucks out an octave of square waves, and then (because the keyboard has one extra C) it divides the C again with the second half of that 4013 (the first half just divides for octave switching which then feeds the TOG clock). The original C goes to IC3 at pin 14 and comes out in the various footing mix outputs. As for the lower C notes (1-4), if they work, well that suggests the original C output from the TOG is OK and the subsequent 4013 division is OK - so the fault lies after it, or it lies with the envelope generator that feeds that top C control input on IC3 - happily, those components are illustrated and numbered on the schematic - R1, R50, R99, R148, what looks like Di1, Di50, and C1. Basically, the envelope generator is triggered by the keypress, and feeds a variable voltage to the control input for C5 at IC3 pin 13. The voltage there determines the level of the output of the note C5 in the mixed 16', 8', 4' and 2' feeds.

I'd check the key contact, and the envelope for C5.