Here was a really tedious, un-fun job. Initially it was blowing one of the heater fuses. A mystery since my client said he changed the speakers to nice Blue Bells and it simply quit working. I was hoping for a quick fix and as it turned out it was a simple problem: one of the chassis nuts came loose upon re-assembly and it shorted out the heater.
But then I figured while I'm in here I'll recap the thing. It is 30 years old after all. I'm glad I did. The first two caps were leaking badly so I did them all. The client also wanted me to modify the tremolo and take that stupid 3 position "all of them are too fast" switch out and replace it with a 1 meg pot. Not an easy task, I needed to unsolder the little PC board that has the trem speed and type switches, carefully saw it in half, put the type switch back in and then hard wire a 1 meg pot into place.
Done. Re-assemble.... and...... damn. This thing sucks.. It's un-usable. So much hum, hiss, RF bollocks going on. I remembered hearing about these issues but other than re-tubing a handful in the past this was the first time I've ever really had to sit with one.
It was bad...... really bad.
I recently did some work in two OG AC30's for my man Sonic Dave in Vermont. I undid some sloppy tech work and tidied it up. There's no such thing as a quirk free AC30, they really aren't that good in a lot of ways other than the awesome sound they produce, but both were easy to deal with and had no objectionable hum.
I remember when these Korg reissues first came on the scene. They were touted as the first reliable AC30, which after the nightmare versions of the 70's and 80's, was objectionably true. They are great road dogs. I wouldn't consider taking a real one on the road, they're fragile.
What I find astonishing is how the good people at Korg simply chose to ignore decades of amp building wisdom and, excuse my language here but I'm pissed...... how did the fuck it up so badly?
The entire circuit board is a ground loop to begin with. It needs to be chopped up into 3 sections and that's just the beginning. I'll provide a path to a video from a fine tech for your viewing pleasure and I hope he doesn't mind. I will not show my own work here just like the tech in the video because if you can't understand exactly what he's talking about then you have no business doing this yourself. It's a big job and PC board amps are not like working on a good ol' tweed Champ. Cutting solder traces sounds easier than it looks.
I will give you this piece of information however:
Last night I tossed a coin to see if I should add a hum balance pot on top of everything else. It came up positive 2 outta 3 tosses which means hellyeah!
I'm glad I did. When you turned all three channels down it hummed and buzzed. If you turn up the normal channel that goes away. So I turned them all down and adjusted the hum balance. It's as close to dead quiet as it can possibly be without changing the power transformer which has a mechanical hum.
Anyway, search this on Youtube:
