K5synth

Hi,

   I've had my K5 two days and have decided to look over the power 
supply. The board is very straightforward, just a couple or three 
78xx voltage regulators, smoothing and decoupling capacitors and the 
nichia inverter (mine reads around 105V on my fluke 77 btw when the 
backlight ELS is disconnected - the fluke's made to measure AC 
voltage at 50 or 60Hz probably, so it's only an indication).

   The pcb is in pretty good shape, but has the usual Japanese dry 
joint syndrome. Several joints were remade including those looking 
like they'll go someday. You can spot these as the solder joint 
appears to have a 'doughnut' like alloy problem around the lead of 
the component itself. Imagine heating and cooling a solder joint over 
years, and this is the result - the alloy gets annealed.

   Removing all the solder on those joints is the best way to service 
the board. Japanese solder from this era is at best dubious and 
contains some levels of impurities that will make the problem 
resurface again over time, even if you just add new solder. I use 
desolder braid (chem wik, easy-braid, whatever as long as it's a good 
one that mops up solder well (avoid servisol ones as they're not good 
at mopping up solder). After all the solder has gone (and I'd work 
one lead at a time on parts otherwise they'll fall out of the board), 
re-solder with a good solder, don't dwell with the iron too long, and 
make sure each joint looks good and shiny - if not, mop off the 
solder and try again. It's important to check the lead of the 
component before soldering to ensure it looks shiny (is tinned), if 
not, clean it.

SOLDER WITH A SMALL IRON FOR SENSITIVE MICROELECTRONICS, NOT A POKER!
(Get google up and do a search under Antex, Weller brands like that)
(15 or 18W Iron with a medium to small tip is just fine            )

    One potential 'upgrade' to the PSU board would be to install 105 
degree rated electrolytic capacitors in place of those already on. 
The existing ones are 85C ones, and the PSU being upside down gets 
warm, and over time this can cause the electrolytics to dry out. 
Replace them and you'll not need to worry about them again for the 
next 5,000hrs or so at least, but buy a good brand of capacitor - 
don't skimp, your K5 is worth every last penny spent on keeping it 
good :-)

    If your board looks messy after resoldering you can clean it 
using isopropyl (isopropanol) alcohol and an old toothbrush to get 
all the dark coloured flux residue off. Sometimes this is available 
in an aerosol form too (isoclene) The pcb has a coat of varnish over 
the tracks, and this may come off, but it shouldn't matter unless 
you're operating the synth in a sauna. The proper term for this 
varnish is 'comformal coating' and can be re-applied if you're a 
purist when everything's done. This should make the solder side of 
the pcb impervious to moisture again.

The usual caveat, delve inside at your peril, or pay a tech to do the 
job for you!

Have fun

Adam