Troubleshooting old korg synth psu - Page 1
Forum section: electronics / repair

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 14, 2025, 01:40:38 am
content: Hi all.After a mistake (wrong power connector) i shorted the power supply in my old korg m1 synth (before any further tests).Disconnected the power supply from the main-board, replaced the fuse and some small shorted components, replaced a burnt r2 with a 6.8ohm 5w one, a burnt r9 and c11. Both r10 and r11. Replaced q1 with a 2sc3310. Replaced also q2 with a 2sc2655. Replaced pc1 optocoupler with a pc817 and also ic1 with a tl431 acz. Haven’t replaced d8 - the strange schotky spq 30 diode.The small signal and the other diodes seem ok, the vr1 is good and all other caps and resistors good (replaced what felt not very good).Now the output is giving double figures.10v instead of 5v on the 5v rail, and +-24v on the +-12v rail. And after a while q1 and r10 and r11 become hot.Load doesn’t matter in that circuit (have tested the supply without load in the past when it was in working condition). The vr 1 trimmer, is for few mv adjustments so doesn’t really count.The transformer T1 is not shorted (but other than that i can’t tell details about primaries - secondaries ratios since none is given), and i doubt it works that way.My guess was that the problem lies before or just around the 4 pins of transformer T1 primary and around q1, or the regulation circuit after the transformer secondary. But now i feel i may be wrong.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 - the european version) on page 11 of 28.https://drive.google.com/file/d/1AP_trewLUwmO_p-bd1tzzaBkPkTsOo8CThanks in advance

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 14, 2025, 06:51:56 am
content: Easy solution is to get another switching power supply that has +12,-12 and 5V outputs.Do keep in mind that I'm no expert on switching power supplies, so carefully weigh everything I wrote.Since it is a switching supply the probable cause of the doubled output voltage is the primary side not getting proper feedback and putting to much energy into the transformer. As it is fully discrete it should be easier to fix and my first thing I would look at is the optocoupler PC1 a pc817, but you already replaced that one.A test that could be performed is to take PC1 out and replace the transistor side with a potentiometer of 1 kilo ohm and see if it is possible to reduce the output voltage. Be careful though, since the primary side works on high voltages.

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 14, 2025, 07:17:25 am
content: Thanks for the reply. Thought about the 2 different supplies, but it’s a matter of space, price and vintage value. I mean, i think my fault is around a small super cheap component.The other solution with the trimmer is not what is expected from the schematic (and i always take precautions on the ac side of things). I believe - even ifi don’t really understand the advantages of their circuit too well, they used such an optocoupler (and not a led and a trimmer) for a reason.

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 14, 2025, 07:47:59 am
content: Replacing the transistor side of the optocoupler with a potentiometer is not meant as a permanent solution, it just to see if it is possible to lower the output voltage.The transistor of the optocoupler is acting like a resistor to control the, most likely, pulse width modulation of the transformer driver stage. For what I know of it, this is how switchers work. They control the pulse width of how long the transformer primary coil is powered to induce the right amount of energy on the secondary coil. With more load on the secondary side, the voltage drops and the feedback makes sure that it goes back up again.That is why I think it is in this part of the circuit that things go wrong.

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 14, 2025, 07:56:06 am
content: If no one else comes with a solution based on the existing components i will try it, but i wonder in case it works, will it show that the pc817 is faulty (seriously doubt it, since i bought brand new 2 days ago from a very trustworthy retailer), or something else?Don’t we already try to find that “something else”?I may be wrong but that’s my train of thought on your suggestion - although i really do appreciate your input.

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 14, 2025, 09:05:50 am
content: The test will either show that the problem is with the circuit surrounding the pc817 on the secondary side, or that it lies in the circuit on the primary side. If it is possible to adjust the secondary voltage, you will need to check if the pc817 is being steered properly. With this I mean the amount of current that flows through the led of the pc817.If it is not possible to adjust the voltage the problem lies in the primary side.This is just diagnostics I would do when faced with a problem like this.But for you I hope others chime in with maybe better advice and solutions.

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 15, 2025, 08:29:49 am
content: Since there is no other option so far, i will try the trimmer idea and i will report back.What trimmer resistance would you think it’s working ok in a circuit like that (highest -lowest values please) and if you please let me know how to connect the three pins (the opto transistor side has 2 pins i guess i have to remove- i don’t think i havebto removebthe led side as well).

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 15, 2025, 09:29:43 am
content: Quote from: tedsorvino on February 15, 2025, 08:29:49 amJudging on the other resistors in the circuit near the transistor, I would say a 1 kilo ohm potentiometer, with maybe a 100 ohm resistor in series, like shown in the picture. A and B are the transistor connections. Might be that the 100 ohm is too high, but experiment with that a bit. The voltage in that part of the circuit should be lower than 80V since that is the max for the pc817There is no need to remove the LED side, but as it is one component it might be the easiest to totally remove it.

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 15, 2025, 02:44:35 pm
content: Thanks for the reply. The voltage is well under 50v on that part of the circuit. All capacitors are 50v.Is the wiper connected to the first pin of the pot? And the resistor goes in series with the third pin?Doesn’t the lack of the led side affect the circuit function?

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 15, 2025, 07:11:27 pm
content: Yes the wiper is connected to the first pin of the pot, and the resistor is indeed on the third pin. The led not being there won't matter, because it is only there for the feedback, but without the transistor side there is no feed back.The whole feedback setup is there to compensate for load variations. With the circuit working properly the amount of feedback sets the voltage to 5V on the output. With load, the voltage drops a bit, and the feedback loop will compensate for this by requesting for more energy.The whole point of this exercise is to see which part of the circuit is at fault, like explained in my post here:https://www.eevblog.com/forum/repair/troubleshooting-old-korg-synth-psu/msg5817511/#msg5817511

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 16, 2025, 08:39:54 am
content: Update.Unfortunately bad results. Connecting the pot + resistor on the transistor side of the opto (opto removed completely from the circuit) took voltage way up. 20v and it could reduce it by half more or less (11v). Then tried to connect the cables the other way around (same side of the opto- transistor side) and that was a disaster. The min. was 20v and i tried to increase it…that was it…. A spark , a blown fuse and hopefully just a shorted regulator.Does this tell you anything?

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 16, 2025, 09:25:12 am
content: The fact that it was possible to adjust the voltage with the potentiometer tells us that the primary part of the circuit is (was) working. You should have stopped at that point.It means that the feedback was not high enough, and more current should be flowing through the LED of the pc817.I did some calculations on the original design with the UPC1093J, but to me it makes no sense. The rule is vref * (1 + ( r1 / r2)) for the output voltage, and with the 3k3 and 3k6 resistors it comes out on 4,78V, leaving no room for the LED when the source voltage has to be 5V.Like I wrote before I'm no expert on this, but you might be able to get it working properly by changing those resistors controlling the TL431 you replaced the UPC1093J with (R24 and R25).

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 16, 2025, 09:33:43 am
content: Thanks for the help. Unfortunately have already done it. The only components around that area i haven’t replaced is c22 and d8, and the coils.Another thing i’ve figured is that the resistance (between pins) around the c22-d8 area towards the 5v out is quite low (around 30-40 ohms). I don’t say that it means something. But i just mention.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: February 16, 2025, 09:58:58 am
content: HiThis is a standard old style switch mode power supply, it is very unlikely to give the correct output voltages without a load on the +5V supply at a minimum, say 5Ohms.DO NOT under any circumstances try to bypass the optical coupler, it is there to provide vital safety isolation between the low voltage side and the high voltage mains supply side, it could kill you if you do.G Edmonds

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 16, 2025, 10:23:18 am
content: Quote from: George Edmonds on February 16, 2025, 09:58:58 amIn itself a good point, but the design already has a 220 ohm resistor to provide a minimal load.Quote from: George Edmonds on February 16, 2025, 09:58:58 amDon't be over dramatic with the warning. There is no suggesting of really bypassing the optocoupler, it is just providing a simple test to investigate the problem, and with it also a warning about high voltages on the primary side.But please do chime in with some useful advice for the OP in order for him to fix his supply.

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 16, 2025, 10:46:32 am
content: Thanks for the input . As i’ve written before. Load is not needed to have correct output voltages, in this circuit. Have tried it more than 10 times in the past when it was in good shape (before the original accident).Now i replaced q1 and q2 again (they were shorted after the test) + i have to replace r12, r23 and r9.

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 16, 2025, 11:10:40 am
content: If you take a good look at the schematic, you can see that there is one high voltage coil on the transformer. On the left side of the schematic, the top coil with pins 1 and 2. The other three are low (ish) voltage circuits. The lower coil on the left side provides the supply voltage for the circuit around Q2 that drives Q1 to drive the primary coil.But it is still on the high voltage side of the supply, so measuring with for instance an oscilloscope can cause health problems for you and the scope. A handheld DMM is not a problem as long as you stay aware of the fact that you are handling a high voltage circuit.How the circuit creates the operating frequency and how the PWM works I don't know. That takes analyzing the circuit, which is, for me, not that easy. All that I can say, is that the feedback seems to be incorrect.Another test I can think of is to bypass IC1 with a resistor to increase the current through the LED of the optocoupler, and see if the voltage on the output drops. Make sure to stay within the max current limit of the LED.

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 16, 2025, 11:29:47 am
content: Thanks for the tips but i don’t bypass anything else on the circuit since things seem pretty vague so far. I will stop testing till somebody who has dealt and solved such an issue on a similar circuit give some definitive answer (if tgere is one).Yes i know the area on the transformer (it’s the one closer to the analog power output side, the +-12v one).And yes it’s the only high voltage side.But I prefer to change all the small things (i mean everything ), when i will be in the mood, than risking more (not so serious) damage. Especially risking scope or valuable dmm - of course without measuring amps there is no real risk.As you initially said, at the end if the day there is always the two separate power supply option.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: February 16, 2025, 11:57:41 am
content: HiFirst may I make it absolutely clear that I have not only successfully repaired hundreds of switch mode power supplies I have also designed them and completely understand them.Across C11 and C12 (no balancing resistor?) there is some 360VDC at very low impedance. THIS IS LETHAL. One side of the optical isolator is at half of this potential, again at low impedance and is also lethal.I hear that this supply in the past appears to have operated without a load, this is NOT normal for this type of supply as R22 will only draw 25mA and is a capacitor discharge resistor ONLY. The rule with this type of supply is that the supply that provides the feedback. +5, must be externally loaded, hence the use of a 5 Ohm resistor load. Use a 25W or 50W metal clad one.Meanwell will have a suitable replacement supply, these are usually smaller and are fully mechanically shielded for electrical safety reasons.Safe working on these supplies requires training and a safe environment equipped with a mains isolation transformer.G Edmonds

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 16, 2025, 12:28:59 pm
content: Thanks a lot for the advice. I do appreciate your input. I trust your experience, so i will follow your advice.Obviously the two “safety” capacitors on that area prove the existence of such high voltage.Regarding readings under load, I’ve made correct voltage readings of both rails (analog and digital )many times before while repairing other parts of the synth without any external load. Maybe it’s super well constructed as a supply.And if course i have no problem to use an external power resistor as you suggest, since my objective is to sort out my problem and not prove me right on something i just can’t explain - i might be lucky in the past and not now. And that’s what i will do, tomorrow.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: February 16, 2025, 03:06:11 pm
content: HiI am puzzled, You show as being in Greece which is a 230V mains country but the 1268 is for 115V mains or similar, is this correct?Please photograph the component side of the PSU board.G Edmonds

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 16, 2025, 05:48:26 pm
content: Doesn’t matter if it’s 1267. Only C1 is different. It’s not 1269, because c16 is 47 nf and not 22nf. Do the small signal diodes matter? Haven’t replaced any as i wrote before. The mains is absolutely 230v. The synth was working well on the power side. I may not be a master of power supplies but these things are out of question.Will post a photo soon just in case you want to see other things.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: February 16, 2025, 06:29:35 pm
content: HiSorry to have to disagree with you, it does matter. The 1267 and 1268 are 115VAC in ONLY whereas the 1269 is 230VAC in only, if the manual you provided is correct. I hope a photograph may resolve this.G Edmonds

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 16, 2025, 07:17:10 pm
content: You may be right on papers. It’s 1267. But the synth is working perfectly (more or less) more than 30 years with this pcb. I guess korg modify their pcbs (as so many big companies do- according to what’s on stock each time). But the basic things are there.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: February 17, 2025, 03:49:00 pm
content: HiI have now spent some time rereading the entire posting, but I need a better photographs of the power supply before I post what I think is the problem.I need one taken at 90 degrees to the PCB, but only of the area of the +5V supply and the optical coupler PC1 with the lighting at 90 degrees to the PCB. I also need one taken at 45 degrees to the board with the optical coupler in the centre of the photograph, if at all possible make this one a macro photograph.What is the value and working voltage of C11?G Edmonds

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 17, 2025, 04:27:59 pm
content: I noticed that on the PCB R9 is not installed. Is that bridged on the solder side? There has to be some connection from Q1 emitter to the null.

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 17, 2025, 08:21:31 pm
content: Thanks for the interest. I do appreciate it.At the moment the pcb is not working at all. I mean it’s cursed.I bought some new components (q1, d8, some large resistors, some electrolytics and a 4.7ohm load)…but apparently the shop employee gave me a 47 ohm load…and now i have to buy a new r2 and q1 (and i ordered some more components -a good load, opto, d8, q2, ic1- just in case).My c11 is a 100uf/250v.I’m a bit confused with the degrees of the photos. I will do my best to send you what photos i understand from your writings.

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 17, 2025, 08:27:58 pm
content: Here what i think are good working pictures

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 17, 2025, 08:33:00 pm
content: R9 was not installed because it was burnt (the previous potentiometer test suggestion that ended up badly). Today there are other components missing.I wish tomorrow or by the end of the week to bring it back to the initial fault of the double voltage values (it was also caused by a short).

author: George Edmonds
author_rank: Frequent Contributor
timestamp: February 17, 2025, 09:18:21 pm
content: HiI need a photograph looking directly at the top of PC1 showing the text on it.G Edmonds

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 18, 2025, 01:41:56 am
content: Here’s what’s now onboard (el 817 445) and what’s originally on it (sharp pc 817 - original late 80s one).The results were the same. I have bought a new one just in case.

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 18, 2025, 09:18:17 am
content: Actually the transistor side of el 817 may have gone, since d3, d4 and r15 are gone. The led side is ok. But the transistor side measures a resistance of 140ohms, where the pc817 measures OL.I’ve ordered a new one so i will be able to compare.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: February 18, 2025, 10:32:27 am
content: HiA failure of the Optical coupler is exactly what I suspected, also I am not certain about the EL817 as it is a Chinese copy/clone of the original PC817 Sharp device available on eBay for very little money in vast quantities.A working PC817 should show a diode on the input with a forward voltage drop of around 1V, the output should show open both ways, both when using the diode test facility on a DMMG Edmonds

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 18, 2025, 11:38:20 am
content: I have the suspicion that the problem is not on the opto. I may be wrong. But I have tried with 2. A pc 817 and el 817 with similar results. Double voltage values.Before burning the el one.As you can read in my previous post, the diode test on both sides is the test i applied to the chips.Of course i will try with a new one as well.Also I have to say that pc 817 is an equally cheap chinese made chip. I mean i can find them both locally for 1 euro vs 1.15 euro. It just handles a bit more voltage. But in my case it’s needless, since i deal with less voltage.Maybe the problem is on the pcb itself or as you initially said, it needs a load.If this time it doesn’t work, i will throw it away and i will buy two different new supplies directly from china - i mean they are cheaper.

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 21, 2025, 07:02:31 am
content: Good news and near success. It was the load apparently.With (5ohm-25w) load i had 4.7v on the 5v rail and +13v on the +12v rail…But i had a small spark and a dead 6.8ohm r2 resistor when i tried to measure the - 12v rail.I used a smaller power rated resistor (2w instead of 3w) but could that be the reason? 1w makes such a critical difference?I dug a bit deeper and all were good (d2-d7-d6 and the relevant resistors and capacitors of those secondary rails. I don’t care too much about the coils as long as the continuity between their pins is good.Was it just an accident (maybe something simple like touching clips etc (i was so surprised with the accident - i mean it was the last pin tested and i was happy, might be a bit careless- that i didn’t really took notice what and where could the damage start from. I have to say that the pcb traces are not in perfect consition any more, so i used a couple of insulated jumpers, But all on the 5v rail.The fuse was intact. That made me vey anxious initially. But…The feedback rail was intact, all transistors and in general the 5v rail seems good. Of course without a new r2 nothing can be tested.So i wonder what could have gone wrong? Just the current flow throught that resistor? So 3w is like a bottom limit.But i feel i’m nearly there and i really thank you for your positive input mr. Edmonds from your initial post. I do appreciate it.

author: tedsorvino
author_rank: Regular Contributor
timestamp: February 21, 2025, 10:05:31 am
content: Feel happy. Replaced r2 with a 5w one. All fixed in papers.i wiil do a last check and will try it in the synthThanks again mr Edmonds.What i really wonder is how before the initial mistake - accident, the power supply was giving good voltages without load. Not that i have to find out, but it’s strange.Synth is working. Properly….not so…but this has not to do with power.