Hi!
So i have a strange?(i hope just for me) issue. The power board does what it should, it generates the 15V -15V and the 5V. The synth powers up, but when i connect the VoiceBoard KLM508, the voltage drops to the tenth of its original, and slowly decreases, until it converges around 1.3…V. The fuses are fine, everything else looks OK. Powering the other board does not really produce any difference.
If anyone have some advice, thank you!
Capacitors? either on the PSU or on the voice board. Any capacitors close to hot components (regulators on PSU for instance) will fail faster due to the heat.
Thanks for the quick reply. So dead/dry capacitors can cause this kind of problem?
Ill start with the PSU then. Thanks!
Capacitors either cause shorts or they stop doing their job, which in a power supply is to convert a full rectified AC wave into a DC wave. If you have capacitors in the PSU not working then you get ripples under load. A volt meter will often give you an RMS figure. You need to get it on a scope to see what is really going on.
A PSU with capacitors which have lost capacity will look fine doing a voltage test with just a meter as your meter is drawing hardly anything.
If the power supply consistently puts out the correct voltages with the voice board unplugged, and then drops like that when the voice board is plugged in, that tells me that something on the voice board is drawing down the power supply - like a short or something failing. Is anything on the voice board getting noticeably hot?
Hm, could not sense anything. They are all cold/room temperature.
I had a similar problem on my Korg M1. Turned out to be a bad solder connection on the small choke coil on the power supply board. The voltage at the power supply regulator IC was correct but the voltage to the voice board was drifting all over, causing the mute circuit to activate randomly. (The choke coils are green, look like a cap but are wired in series with the load rather than parallel as a cap is.).
Check for oxidized solder connections on the power supply board. voltages will look good with the high resistance of a digital voltmeter (11Mohms) but fail with any reasonable load.
Thanks, I was sick for a couple of days, and was only able to look at the synth from my bed, but I’ll try to debug things out.
Well, i think/hope i found the problem. There was a B941 transistor, and as I moved the board around, it just got loose. Their legs where completely worn out. The PSU still operates the same way before, even i completely removed that transistor. (now i just have to find an equivalent replacement in my local stores)
Capacitors are like batteries in your car… when faulty they can drain the voltage slowly or quickly away… Yup faulty caps… First thing to consider, or second counting the battery, in an old synth…
And BTW the 3/170DK 3.6V battery will leak and cause corrosion on the CB… Replace if not already done… 
So, did this fix the issue? I have the same exact issue. I have replaced caps with no change, and was thinking transistors. If this did work, can you tell me where you got the replacement transistor? Thanks 