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Troubleshooting shorted power supply. Old Korg M1.

Profile picture for user tedsorvino
Reaper
Submitted by tedsorvino on Wed, 02/05/2025 - 08:37

Hi all.
After a stupid mistake I've made (wrong power connector) i found a shorted power supply in my old korg m1 (before any further tests).
Disconnected the power supply from the main-board, took it out and started testing it. Replaced the fuse and after some trial- error (some small shorted components) the first problem came down to a totally shorted q1. Replaced it with a 2sc2335 i had around (good equivalent of the original 2sc3570 according to thus guide https://alltransistors.com/crsearch...400&ueb=6&ic=5&tj=125&hfe=40&caps=TO220&cf=on). I’m not certain if the transistor is in perfect shape.
All seemed ok, but i realized that the secondary was giving double figures. 10v instead of 5v on the 5v rail, and +-24v on the +-12v rail.
The transformer is not shorted (but other than that i can’t tell details about primaries - secondaries ratios since none is given).
I guess the problem lies before or just around the 4 pins of transformer T1.
Has anybody any specific ideas what the problem can be and which components could cause such a problem.
I send a link to the m1 service manual from my google drive. The pcb i’m fixing is klm 1267,1268 (it’s 1268) on page 11 of 28.

https://drive.googl…

Thanks in advance

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Comments

Boswell
Boswell

You are not allowing access to your Google Drive for us to look at your copy of the service manual. No matter, I found it elsewhere on the net.

I would assume that the feedback path through the opto-isolator is not making it all the way back to Q1 to control the switching current. Try measuring the d.c. volts across the LED diode of PC1, which should be in the region of 1.2V to 1.5V. If it's much less than this, measure the volts between the anode of PC1 and the anode of IC1. If this is near +10V, check the volts across the pot VR1 and the resistors R24 and R27. If VR1 were open circuit, the feedback loop would be permanently demanding higher output, which is what you report.

Another explanation is that whatever took out Q1 also damaged Q2. If you can't find a problem with the feedback circuit, I would try replacing Q2.

Wed, 02/05/2025 - 10:31 Permalink
Profile picture for user tedsorvino
Reaper
tedsorvino

Thank you once more Boswell for the input

The link problem is sorted thanks for mentioning it. 

Actually i replaced q2 as well as part of the initial repair. Of course it might be damaged once again during testing. 

Pc1 is an ic (sharp pc 817) so i can’t be very certain what pins are the “led” ones. I have to check the datasheet pinout.

I will do the tests and report back.

 

Wed, 02/05/2025 - 11:07 Permalink
Profile picture for user tedsorvino
Reaper
tedsorvino

So here are the results of my testings.

Voltage across pc1 led =1.6v

voltage between anode pc1 and anode ic1 (pin 2) =5v

voltage between r23 (typo on the manual it’s r25 on the board) on the junction ic1 (pin1) and vr1= 0v

voltage between the other pin of vr1 and r24 (it’s not r27) =0v.

The trimmer doesn’t alter the overal voltage more than +-20mv.

do you think vr1 is gone? It seems like an easy explanation.
But the resistance on the trimmer is good. 850ohm on a very clean 1k trimmer.

 

 

ps. R10, R11 and q1 (the untested transistor) seem like they are getting realiy hot. Especially the resistors. Near burn condition.

 

 

 

Wed, 02/05/2025 - 12:22 Permalink
Boswell
Boswell

Something a bit odd going on. If the volts between the anodes of PC1 Led and IC1is +5V, and the nominal 5+V output rail is +10V, it means you have 5V across R23, a 22 Ohm resistor. That implies a current of around  220mA through R22, but where can it be going? The PC1 Led would burn out if it were going through there, and the other route has the VR1 pot plus KOhm resistors in series.

I'm not surprised if the components in the Q1 area are heating up as they are trying to respond to the demand for ever more output voltage.

I would double-check the voltages around IC1/PC1 (Led only), and see if there really is 5V across R23. If R23 has gone high in value (e.g. to a few KOhm), then all is explained.

Thu, 02/06/2025 - 03:14 Permalink
Profile picture for user tedsorvino
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tedsorvino

Voltge between anodes the same as before.

voltage across the 22ohm resistor is -5.4v (i guess polarity doesn’t matter) and the 3.6k resistor in line has 1.8k resistance across it. Off line 3.6k normal. The 22 ohm resistor online stays precisely on 22 ohm.

I wonder why are we measuring voltage between ic1 A and pc1 led anode, when they are not connected.

Ic1 K (cathode i guess) is connected with pc1 led cathode. Am i wrong?

Thu, 02/06/2025 - 05:33 Permalink