Troubleshooting old korg synth psu

[pcprogrammer]

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.

[tedsorvino]

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.

[tedsorvino]

Here what i think are good working pictures

[tedsorvino]

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).

[George Edmonds]

Hi

I need a photograph looking directly at the top of PC1 showing the text on it.

G Edmonds

[tedsorvino]

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.

[tedsorvino]

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.

[George Edmonds]

Hi

A 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 DMM

G Edmonds

[tedsorvino]

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.

[tedsorvino]

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.

[tedsorvino]

Feel happy. Replaced r2 with a 5w one. All fixed in papers.i wiil do a last check and will try it in the synth
Thanks 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.