Recapping Korg Mono/Poly question

From circuitbending to homebrew stompboxes & synths, keep the DIY spirit alive!

Moderators: Kent, luketeaford, Joe.

Post Reply
User avatar
redonyellow
Common Wiggler
Posts: 55
Joined: Tue Oct 14, 2014 7:54 pm
Location: San Francisco

Recapping Korg Mono/Poly question

Post by redonyellow » Wed Feb 17, 2021 11:13 pm

I am planning for to replace the capacitors on a Korg Mono Poly.

Background, I came across an old m/p that was sitting in storage since 1986. I am a newb doing synth repairs but I have experience soldering micro components.

As I read the service manual and I see that there are two different kinds of capacitors listed under the electrolytic cap section.

Example;

A16v 6.8uf
B25v 4700uf

What is the difference between Axx and Bxx?
Attachments
29F986B4-CBFD-4C26-9A3C-9F76434532C3.jpeg

PacificState
Common Wiggler
Posts: 119
Joined: Wed May 01, 2013 10:19 am

Re: Recapping Korg Mono/Poly question

Post by PacificState » Wed Feb 17, 2021 11:37 pm

I'm not entirely sure - but when I replace capacitors, I generally verify them on the board physically and check dimensions + lead spacing. This then goes into the Mouser/Digikey search box to make sure there's a reasonable match (as modern caps are generally smaller than they were 40 years ago).

That said (and I hate to ask!) is there a particular reason to replace the caps at all? It's often not worth changing much more than power supply caps or old tantalums (particularly in 70s ARPs, which often fail short) unless there's an obvious problem.

User avatar
redonyellow
Common Wiggler
Posts: 55
Joined: Tue Oct 14, 2014 7:54 pm
Location: San Francisco

Re: Recapping Korg Mono/Poly question

Post by redonyellow » Thu Feb 18, 2021 2:54 am

Thanks for the input!

My reasoning is that the synth has not been played for the last ~35 years. I assume that some of the capacitors are dried-out. I keep reading about the tantalum caps and I see four of them on the m/p. Can I just replace for regular electrolytic caps?

User avatar
BugBrand
Knowledge of Bugs
Posts: 7577
Joined: Tue Jul 15, 2008 11:59 am

Re: Recapping Korg Mono/Poly question

Post by BugBrand » Thu Feb 18, 2021 4:16 am

Please consider practising on a less valuable machine ;)
We all go through the learning stages with the temptation to dive in deep, often on presumption.
Really, much will be fine within the MP - do the minimum & proceed slowly. Be wary of 'assume'!
I've got one myself and the main issue has been the keys not triggering - info online about that.

Yes, tants can be bad as they go short-circuit when they fail. But depends *where* they are - I guess in the MP they might be for envelope gen time constants etc. & that's not a typical place of failure - compared with where they're used for power line decoupling & definitely DO need to be replaced [eg. I saved Powertran Delay, Lexicon Primetime, etc in such cases]

You *might* need to recap the PSU but test things first & think twice (or more) before beginning such repairs!

PS - The Peak ESR70 would be a great investment for you - super little unit for testing such caps:
https://www.peakelec.co.uk/acatalog/esr ... meter.html

amonti
Learning to Wiggle
Posts: 47
Joined: Thu Aug 04, 2016 10:22 am
Location: Canada

Re: Recapping Korg Mono/Poly question

Post by amonti » Thu Feb 18, 2021 12:27 pm

You may also want to consider joining the Vintage Korg mailing list on groups.io. They have very in-depth knowledge with Mono/Poly and PolySix synths.
https://vintagekorg.groups.io/g/Maillist

User avatar
KSS
Super Deluxe Wiggler
Posts: 5290
Joined: Mon Jan 25, 2016 7:28 am

Re: Recapping Korg Mono/Poly question

Post by KSS » Thu Feb 18, 2021 2:31 pm

A and B are probably axial vs radial. Notice the large values are the B's.

Agree with nearly everything BugBrand said. Please don't use a MonoPoly as a learning device.

Don T
Super Deluxe Wiggler
Posts: 1364
Joined: Sun Feb 02, 2014 3:11 pm

Re: Recapping Korg Mono/Poly question

Post by Don T » Thu Feb 18, 2021 5:18 pm

redonyellow wrote:
Thu Feb 18, 2021 2:54 am
Thanks for the input!

My reasoning is that the synth has not been played for the last ~35 years. I assume that some of the capacitors are dried-out. I keep reading about the tantalum caps and I see four of them on the m/p. Can I just replace for regular electrolytic caps?
1. If you know someone with a device called a "Variac", I would suggest bringing the synth up slowly on one. It can help re-form the caps if they haven't dried out. A Variac is a variable transformer that can go from 0VAC to slightly higher than line voltage. Open the synth, switch the synth on with the variac at OV, then Slowly turn the voltage up over the course of several minutes to half an hour. Keep an eye on the fuses as you go, if any of them blow on the way up to line voltage, switch everything off and call a tech. If the fuses keep holding, on occasion check the temperature of the PSU filter caps as you progress upwards in voltage. If they start getting warm to the touch, they are leaking current. Switch everything off and replace them, then go through the variac process from scratch after they are changed.
If the synth makes it all the way to line voltage ok, lower the lid and check all the functions. You may be able to get by without changing anything. I've managed to resurrect much tube gear this way, and even a Hammond M3 organ that hadn't been switched on in MANY years.

NOTE TO OTHERS: Do not do the above with anything that has a switch-mode PSU. It will explode!

2. Yes, you can replace tantalum caps with electrolytic of like value and voltage (or slightly higher voltage).

User avatar
Chaotic
Veteran Wiggler
Posts: 564
Joined: Tue Jul 16, 2013 11:38 am
Location: Oslo, Norway

Re: Recapping Korg Mono/Poly question

Post by Chaotic » Mon Apr 05, 2021 5:23 am

I say recapping of the electros is the way to go with old machines. My priority is the psu and the power line. I always look at the schematics and find out where they are connected to the power line.
Not all of them look like this, but quite a lot. Of course, there are many of them looking good and have acceptable measuring. But if I´m servicing a synth I do them all. If the owner wants me to do it.
The original MonoPoly PSU have a reputation to fail. So, if you have not powered it yet, do it without the pcb´s connected.

This is not from the MonoPoly I’m repairing now, but I found similar there too.
caps.jpg

User avatar
KSS
Super Deluxe Wiggler
Posts: 5290
Joined: Mon Jan 25, 2016 7:28 am

Re: Recapping Korg Mono/Poly question

Post by KSS » Mon Apr 05, 2021 11:53 pm

Chaotic wrote:
Mon Apr 05, 2021 5:23 am
I say recapping of the electros is the way to go with old machines.
Depends on the machine. It's crazy to make it an always do it kind of thing. See below.
[quote[
The original MonoPoly PSU have a reputation to fail. So, if you have not powered it yet, do it without the pcb´s connected.[/quote]
This. Agree 100%. There's the real truth. *Some* synths have known and repeating issues with bad elcaps. Others don't. Like at all.
caps.jpg
Just seeing that exact purplish color in a Korg product from that time period is enough to want to change the caps. <--Because *those* are known to be prone to failure.

Details matter.
Treat utility modules as stars instead of backup singers.
Treat power supplies like Rockstars instead of roadies!
Chase magic sound, not magic parts.

Post Reply

Return to “Music Tech DIY”