Vintage Synth Explorer Forums

Hi guys,

Just got a non-functional non-powering Korg Poly-800 Mk1 and did a bit of research and this is apparently quite common.

The symptoms are that it is completely dead, plug the power in, turn it on and nothing. I'm guessing it's a dead component not making continuity, but I've thoroughly inspected the units' internals visually and it seems perfect. No broken traces, odd-looking caps / burnt diodes or resistors - all fine.

I guess it's now just a case of going over the circuit component by component to see what's going on, but that could be a big undertaking.

Just wondering if there's any tech minded people 'round here with any ideas?

Thanks people.

I assume you have checked that the power supply is delivering juice?

Tried batteries in it yet?

meatballfulton wrote:I assume you have checked that the power supply is delivering juice?

Tried batteries in it yet?

Yeah, like I say I have checked the obvious - connected a test power supply to the unit both from the DC plug in and on the battery terminals and still nothing.

I also checked the continuity of the power switch, and it works as it should. So basically, the power is going in ok, it's getting switched on and off via the power switch fine, it's going to the first PCB fine, and then it goes to the second board which I haven't checked yet - I reckon it could be a component on that board which annoyingly isn't obviously broken from a visual inspection.

Ahh, don't know what to do...

Thanks for your input tho

never owned, played or worked on one... so not much help. but...
IIRC someone posted about a blocking diode in the power input line that will blow if a reverse polarity power supply is used. I will try and find a schematic and get back to you.

first, check D2 for short circut
then replace C1 abd C2 (10uf@16v)
then C4 (100uf@16v)
all thes (alas, and more) could have killed by a incorrect power supply being used (even for a second!)

keep us up,

Hey thanks so much rhino! I actually was heading towards that conclusion myself, I mean I checked and know for a fact that the power was getting to the circuit boards' power inputs and the switch and power plug was all fine. So I think you're dead right, it's got to be that diode.

I'll let you know how it goes, got to restore my MS-20 too. I'll probably do that first, got lots of jobs on at the moment

Thanks guys

Hey! I know this is wayyyy late but what diode type is the D2 diode? I got one of these synths from a old school giveaway and it would not work from the dc power and cracked it open to find the diode torn in 2 aparently they used a 9v AC power on it instead of DC. Any ideas?

If you're asking what the original part was, I believe it's an SR1K-2 diode. If you're looking for a replacement then I think anything rated over 1A, 600V should suffice, so... if you've got any 1N4005, 1N4006, 1N4007 or NTE116, etc. around then that would probably do the trick -- about as garden variety as it gets.

Yes. I was just wondering what diode I could use in replacement. I'm hoping I only have to repair this diode though non others looked broken neither did the transistors. Oh I also ment that they used a AC power Supply that had the polarity switched. Would a 1N4001 function as a replacement? Thank you for your time.

Alienanthony wrote:Would a 1N4001 function as a replacement? Thank you for your time.

As far as being true to the original circuit, you can answer that yourself just based on my last post and then looking up the specs for the 1N4001. But... I don't know that the original actually needed to be that high voltage, so it depends how experimental you want to be.

So it would suffice, but in the long run a higher ranked diode would be safer? Is that what I should be getting at? Or is it that you mean that the Keyboard its self may get a overdose amount of power to work in general? I can always buy another diode I just had a few of the 1N4001 around.

Alienanthony wrote:So it would suffice, but in the long run a higher ranked diode would be safer? Is that what I should be getting at? Or is it that you mean that the Keyboard its self may get a overdose amount of power to work in general? I can always buy another diode I just had a few of the 1N4001 around.

I'm saying that it's a limit of my knowledge how "small" of a diode you can use and still have the circuit operate the way it was intended to. While some random diode may work in a practical sense, I'd say it's a much better idea to just get the proper diode and fix it once and be done with it, rather than just trying to use whatever you have lying around -- the voltage ratings are drastically different between the original diode and a 1N4001, so all bets are off.

It's not as if the part is rare or expensive, so even if there's no electronics supply close to you, it's worth $5 (or less) and a weeks wait to do it correctly, in my opinion. Even if you want to be so neurotic as to find actual SR1K-2 diodes specifically, they're still available and not too pricey.

I totally agree that it is worth it! Ive just had a little trouble finding the original. To be honest I dont want to risk it so I intend to purchase one. thanks.

Could be because I'm stupid and made a typo... it's supposed to be SR1K-2. I accidentally replaced the R with a T in the initial post.

https://www.ebay.com/itm/4pcs-UNIZON-SR ... Sw8d5ZUQPx