Hello again! Riding the winning streak of fixing my first "as-is" Microkorg, I decided to try my luck again and pick up another unit to try and refurbish.
This time the main problem seems to be with the tracking on the edit param knobs.
A couple observations:
1. the tracking seems to "continue" in the same direction that i turn the knob in, but in a decelerating fashion.
2. Whatever param that the previous knob was in seems to sneak in there. Some sort of short between the leads going to the diodes?
I have the service manual downloaded and am staring at the schematic trying to make sense of it. I know my way around a multimeter a bit, and can solder if necessary.
Any help is appreciated!
Last edited by aphelionz on Tue Mar 21, 2023 1:09 am, edited 1 time in total.
I'm not entirely sure what is going on. Is it like that you turn knob 2 counter clock wise and that if you leave it, it further goes down? At least the value, but not the knob physically itself?
If I understand it correctly, it is only knob2 that sneaks in?
The sneaking in has to do with the fact that the knobs are scanned al the time one after the other, and if their value is sufficiently different than the previous scan, the CPU thinks that there was actually a turn on the knob and will update the corresponding parameter and the display as well. So I think you see the knob 1 and knob 2 values alternating whilst you only turn knob 1.
There are no diodes involved for the potmeters, only the pot, and a resistor and capacitor per knob, where the resistor and cap acts as a low pass filter on the value read out. This helps to stabilize the value and make it less sensible to noise when the pot is not touched. And as the value is stable, the CPU thinks also that the pot is not touched.
So my guess is that the capacitor on knob 2 may be broken or disconnected so that the stabilizing effect is gone. Or the pot itself is worn out maybe?
Good luck.
microKORGXL, Kaossilator Pro, monotribe, SQ-1, volca fm, Kross 88 BK
Alesis SR18, Akai Miniak, Fender Strat, Line 6 Spider II 112, Zoom MS-50G
I'll check the capacitors now - regarding the knobs: I did apply some deoxit and while the knob action is very nice now, the tracking issue still remains.
I'll report back with the capacitors. I'm not exactly certain right now but it looks like the knob 1 value is sneaking in when knob 2 is trying to alternate so I'll just check all the caps.
Let me ask - if C112, the capacitor on Knob 3 was defective, could it potentially cause this behavior?
I just connected my multimeter across the poles of the pots to the far end of the capacitors and they all looked fine to me, except for VR4 -> C112 which stayed at a value of 0L consistently and didn't change at all
I think if one of caps is broken, the behaviour can be seen. Well at least if I could be sure what happens exactly...
Your measurement is not clear to me. Can you indicate the points that you've put your multimeter probes on, referring to the service manual and the value that you measure?
(Being VR?, pin 1, 2 or 3 of the pot, point between the R and C.)
Maybe an experiment. Use edit select 1 and choose Amp Eg.
Then turn all knobs completely counterclock wise (value 0). What happens then on the display? Is it stable (+/-0 or OFF if you turned knob 5 as last one).
Turn now knob 1 to 12 o'clock. What happens then on the display? Is it stable (+/64). Turn knob 1 back to 0.
Repeat the above on the remaining knobs.
Bye.
microKORGXL, Kaossilator Pro, monotribe, SQ-1, volca fm, Kross 88 BK
Alesis SR18, Akai Miniak, Fender Strat, Line 6 Spider II 112, Zoom MS-50G
Mmh. Knob 1 flickering to 127 seems also a bit odd. But maybe you turned knob 1 to 0 before selecting the amp eg mode.
For knob 3, if I understand it correctly, it always tends to go back to 0, but never back up. Right?
Anyway, yes knob 3 is problematic. My analysis:
* If the cap would be shorted, then the value would always read as 0 -> not the case.
* If the cap would be open, the filtering would be broken and the value would track more or less but the value would show some noise, dancing around the set value -> not the case if I saw it correctly.
* If the resistor would be open, then the value would always read as 0 -> not the case.
* If the resistor would be shorted, then the value would track but with some delay, not sure it would be noticable -> not the case.
* A problem with the potmeter? If the parameter always tend to go 0, I would think that the wiper has a bad contact towards the internal resistive path of the pot. When you turn the knob the wiper has decent contact and the value tracks. But if you then release the knob, the wiper loses contact. The value is then determined only by the charge on the capacitor. As that will slowly leak away the value will go down to 0 in the end and stay there. (Till you wobble the knob again.)
So at this moment I tend to think it is the pot itself that is broken or worn out. What happens if you turn the knob and then keep your finger on it? Does it then decrease to 0?
Good luck!
microKORGXL, Kaossilator Pro, monotribe, SQ-1, volca fm, Kross 88 BK
Alesis SR18, Akai Miniak, Fender Strat, Line 6 Spider II 112, Zoom MS-50G
