Korg monopoly problems.

sam · Post by **sam** » Sat Oct 21, 2006 2:30 pm

BTW Thanks for the help....I have checked the the continuity from the jack plug to the pcb and many other points on all the boards. The phones out doesn't work either..All the contacts have been traced to the amp board and seem ok.
All the voltages check out on my meter.
This is a strange one but we shall crack it..

Post by **Maschinengeist** » Sun Oct 22, 2006 7:24 pm

I read into the schematics that there is a OFF/LOW/HIGH selector (S7) for the output, have you checked it? And I presume you've read the values of the resistors in the output circuitry?

Next step, the VCA

sam · Post by **sam** » Sun Oct 22, 2006 7:26 pm

Update....I think the problem is on the power supply.

The 5v transistor is reading ok.
The 15v is reading 50v on the middle pin.

My schematics don't show this.

sam · Post by **sam** » Sun Oct 22, 2006 7:29 pm

Maschinengeist wrote:I read into the schematics that there is a OFF/LOW/HIGH selector (S7) for the output, have you checked it? And I presume you've read the values of the resistors in the output circuitry?

Next step, the VCA

Yes I have also checked this section..I will double check though.

Post by **Maschinengeist** » Sun Oct 22, 2006 7:55 pm

You've got the service manual, right?

For the power supply adjustment (KLM-376)

1) Connect meter to J11 (J12=Ground): -15V
2) Check J50: +15V
3) Check left side of R54 (J39): +15V. Adjust KLM-376 VR2 if necessary

sam · Post by **sam** » Sun Oct 22, 2006 8:54 pm

Hey thanks for that, This one is quite a challege for me but I feel there could be more than 1 fault although I am getting a clearer picture on how this synth works.

Cheers

sam · Post by **sam** » Tue Oct 24, 2006 3:52 pm

update....I have located faults on the -15 v line.
I will replace R12 and R16 resistors and the C14 and C13 caps for good measure.

Anything else I should check or replace..?

This board is the klm 376 a

Post by **Maschinengeist** » Tue Oct 24, 2006 4:35 pm

Depends, what were the reading of the caps + resistors? Do they measure to their value? And what was the reading on the -15V line? Note that a variation of +/- 0.01V is normal.

Usually, the resistors and caps are prone to problems and are usually the instigators of trouble.

But have you adjusted VR2?

sam · Post by **sam** » Tue Oct 24, 2006 5:54 pm

There was a very small reading on the -15 v line and the resistor R16 is not correct.
I always replace the psu caps if I am in there.

The vr2 controls the 5v line on mine..?

dougt · Post by **dougt** » Tue Oct 24, 2006 9:47 pm

Actually resistors hardly ever fail (unless they burn up which you can usually tell by looking at them)... You also must remove them from the board to measure them... I would guess it is either the -15 voltage regulator or bridge rectifier...

sam · Post by **sam** » Tue Oct 24, 2006 9:54 pm

dougt wrote:Actually resistors hardly ever fail (unless they burn up which you can usually tell by looking at them)... You also must remove them from the board to measure them... I would guess it is either the -15 voltage regulator or bridge rectifier...

Yes ..I agree your suggestion will be the next step.

Post by **Maschinengeist** » Tue Oct 24, 2006 10:32 pm

Resistors fail in vintage synthesizers, not as frequently as capacitors though. Not entirely by burning up but by temperature aging. Or maybe is it just a Canadian thing?

I asked about the resistors' value to know if they read correctly, if so... we go on. If not, and if it's not temperatur damage, then something has caused damage to it. As Dougt suggested, more upward is the bridge rectifier. But before that, you noted the resistor R12 and the 2 caps C13 + C14...

Wait, why do I give myself all that trouble for? Follow the service manual. Test and change everything till the knobs and screws.

sam · Post by **sam** » Wed Oct 25, 2006 8:09 am

Maschinengeist wrote:Resistors fail in vintage synthesizers, not as frequently as capacitors though. Not entirely by burning up but by temperature aging. Or maybe is it just a Canadian thing?

I asked about the resistors' value to know if they read correctly, if so... we go on. If not, and if it's not temperatur damage, then something has caused damage to it. As Dougt suggested, more upward is the bridge rectifier. But before that, you noted the resistor R12 and the 2 caps C13 + C14...

Wait, why do I give myself all that trouble for? Follow the service manual. Test and change everything till the knobs and screws.

Synths can throw up the most unusual problems. I feel this is the right path to follow.

sam · Post by **sam** » Wed Oct 25, 2006 9:36 am

It get's worse....My meter shows 80v across the diode bridge.

This should show below 30v although it doesn't say so on the schematics...?

Post by **Maschinengeist** » Thu Oct 26, 2006 3:54 am

Do you get a pulse (pulsating direct current) out of the bridge rectifier?

And what reading do you get from the transformer's outputs?