Saturday, May 21, 2011

Juno-60 up and running

Time to pop the hood
When I came back home from work yesterday I decided it was time to open this thing up and see what it looked like inside... Perhaps also, if possible, repair any faults I find so it becomes useable.

Ground on the loose!
First thing I noticed when I opened it and I took a look at the PSU...

Well that could have become interesting! :)
Ok, I assume a dangling ground cable is the least dangerous thing. Better than one of the live ones... Anyway, it's not really good since it could have touched something and shorted it, and also all the grounding is all gone, which is a risk in itself...

I didn't find any loose screw or washer inside the Juno so someone must have removed it earlier, maybe when trying to figure out how to open it up? (It's only 2 screws on the sides of each of the wooden cheeks btw)
After looking through my small box of "screws, washers and nuts" I found a suitable pair of screw and nut and screwed the ground cable back to the case.

I also found the compulsory dead spider on the inside near the ventilation slits.

I then removed the two panel boards from the panel and looked at them.

Hm, what's that white-green stuff between the capacitors where those wires run??

I moved the wires a bit and took a closer look...

Yuck! Ok, they've gotta go. The electrolytic looked like it had leaked onto the board and the two ceramics next to it. It was a 47uF 16V but I only had a 47uF 25V at home which I guess is fine, if not better. The ceramics were 100nF and were replaced by 2 much smaller compact ones.

Alien battery
I knew I would have to replace the battery because the patches were all just garbage. When I found it I was convinced it needed to be replaced...

It looks like some acid attack from alien blood, doesn't it?
I didn't even have to use the soldering iron to remove it. I pulled it a bit and just had to wiggle it a bit to get it out. The solder joints were either very poor from the beginning (looked like a bad job when replacing it) or the acid had just tried to liberate the battery from the tin and lead shackles, planning an escape.

I cleaned the area around the battery as well as the area around the leaking capacitors above with some isopropanol.
I'll put a new battery in at a later time but I'll use wires and put the battery on a small separate stripboard, away from all the precious components :)

I also noticed a very professional "repair". I guess the connector holding the power supply wires was bad and instead the wires were soldered directly onto the board connector...

I found a similar one on another place (The CPU board I think). I just left them the way they were...

Yeah.. Time to hunt down whatever is causing the Juno to go nuts when I push the panel down a bit.
After doing the fixes above I put the panels back, with only a minimum amount of screws, in case I needed to remove them again.
When I started the Juno up, it was just chaos constantly. I noticed that it turned normal when I pushed some sliders sideways. In particular, if I pushed the VCF Envelope slider to the right it seemed easy to fix it. So I started looking there.
With the unit unplugged from mains power I measured between the voltage rails and ground and some solder joints on the slider in question.
I noticed that sometimes I'd get several kilo ohms of resistance between the ground rail and what was supposed to be ground on the slider. Pushing the slider made it go to zero... Hm? Problem with the ground?
I did some continuity testing along the ground trace at different places and found this interesting spot:

How is that even possible?! And of course this is just underneath the VCF envelope slider. There was probably a crack in the PCB trace that was impossible to see and the joint didn't move at all. The brown stuff you see on the photo was just on the screen and came from the solder that attached that capacitor. It was not the problem because it looked fine when I removed it and cleaned it. The crack was somewhere where the black probe is.

The trace above the black probe was not connected to the trace below it.
I tried reheating it so see if the joint was just cracked, so that should fix it, but the problem was still there. Hm..
I then bypassed that place with a wire so that at least both sides of the trace had ground and one of them were still connected to that solder joint.

That did it! Apparently that trace also provides ground to a lot of other components on the right panel board. So of course, when they all lost ground it's no wonder that the whole thing just went crazy!

After wiggling all the sliders and checking for cracked solder joints I found another at HPF slider, which just needed to be heated again.

There were two more, namely the noise slider, which had the entire pad had broken loose from the board and I put a small wire to connect it with the trace again.

Finally the Release slider in the ADSR Envelope had several broken solder joints and also a pad that had been broken off of the board so I fixed it with some more wire.

So the Juno is playable now. Everything is working though there are some final things needed.
I have to put a new battery on a stripboard and place it somewhere in an empty space so I can save patches.
I have to find replacement switches for the three broken ones.
I have to find a pitch bender.
I need to calibrate it, in particular the filter since two of the voices won't self-oscillate :(
I get a audible distortion when I play chords and have the Chorus enabled so perhaps calibrating output levels from the voices or something in the chorus part is needed.

Oh, and I've bought a second-hand CHD Elektroservis Juno-60 DCB-MIDI converter! :)
Hopefully I'll get it before next week-end (today is Saturday)!

Anyway, this Juno-60 is now removed from the "Defective" category and placed under "Equipment" since I think it's in a good enough state to be used. Yay! :)
(And there was much rejoice...)