Recapping Korg Mono/Poly question

[redonyellow]

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?

[PacificState]

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.

[redonyellow]

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?

[BugBrand]

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]

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

[KSS]

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]

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).

[Chaotic]

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.

[KSS]

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.