Korg Poly800/EX800 Users

> very nice indeed. That's a cool little board. Great for someone getting started that already has some know how.
> I have so much 'real' work going right now that I can't really spare much anything for hawk. Though I really wish I could.

> With an online drag'n drop Audio Editor which generates arduino code I had my distortion pedal ready in no time:
> Volca Sample through DIY Teensy Distortion https://www.youtube.com/watch?v=TV6BCIRzNH0

Attachemnts are not possible, but links are, like you did above. There are several useful guys on there, but Folderol, Nathanscribe and Hugh Robjohns are some of the most knowledgable on synth (and other) circuitry.

Hope that helps :)

On Jan 15, 2016, at 8:18 PM, Vincenzo Vincenzo sarvamala@... [korgpolyex] wrote:

Hello Daniel,
begin to check and possibly replace the capacitors ..

in 90% of cases the fault is due to old capacitors.

Il Mercoledì 13 Gennaio 2016 13:07, "Daniel Forró danforcz@... [korgpolyex]" <korgpolyex@yahoogroups.com> ha scritto:

Sorry for OT but I'm sure somebody of clever members here could help me.

I've just got Korg PE1000 from a friend and could repair some problems like not working tones and other usual stuff on the instrument which was not used for long time. As you probably know it's vintage analog polyphonic ensemble from 1976 (full polyphony, each key has its own oscillator), kind of electronic piano or string machine. Very nice.

Now everything works except one vibrato oscillator - LFO.

There are three LFO's, first one controls first three tones on the keyboard, second one the next three notes, third one the next three notes and this pattern is repeated throughout the keyboard. So I have now vibrato on keys 4 to 9, then 13 to 18, etc. Coupler notes one octave higher follow the same pattern but shifted by three notes, because note one octave up use different LFO. Very clever idea, which helps to get more animated sound - all LFO's have the same speed and depth (controlled by pots) but different phase.

So I suppose one LFO doesn't work. By measuring voltages directly at LFO outputs (points 50, 53 and 54 in schematics on the page 7) it was confirmed - while LFO 2 and 3 have changing triangle wave on the outputs and voltage changes between approx. 6.5 and 7.5 Volts, LFO 1 output gives steady 7.11 Volts.

I have attached Service Manual and I'd like to ask more experienced electronic gurus here:

- Which component in LFO circuit would you considered faulty with the highest probability? I don't want to change all of them... Could be only one half of LM3900 working and the other faulty? Or is it transistor? Some resistor or capacitor?

- How I can find which component is faulty? I have digital voltmeter and oscilloscope....  Can I find wrong component by measuring and comparing voltages in different locations of the LFO 2 (or 3)?

Thanks for any hint...

Daniel Forro

P.S.: If attachments are not allowed here, I will send Service manual by PM, or it can be found here for download:

http://elektrotanya.com/?q=showresult&what=korg%20pe-1000&kategoria=&kat2=schematics

> Hi, Vincenzo,
>
> thanks for advice, I usually suspect them... but here are only two
> caps and seven resistors in LFO circuit... That electrolytic 33
> uF/16 on the output is probably OK when some voltage pass through it
> (at least I suppose so). I will check the other one - .047 in
> feedback between "-" input and output of the first op amp...
>

On Jan 16, 2016, at 6:34 AM, Michael Hawkins korgpolyex800@... [korgpolyex] wrote:

Yes, it's far more likely for a chip to fail in one section when it is an analog type device. Even digital stuff can lose a single stage or line. But op amp chips will fail in one stage, not that common for them to fail completely (but that happens too).

I agree with Gordon, and I've said it before, I shall say it again.. replacing cap's is not a good strategy because a) cap's are actually quite stable if manufactured from 1970 onward and b) replacing all caps may actually make the fault worse by confusing what was originally wrong with an error made while replacing all cap's.

Not wanting to offend anyone but only those that don't know much about electronics say that cap's are most likely at fault. In my experience, there are just too many different ways for electronics to fail to be able to blame cap's first. The exception to that is when working on equipment that is pre 1960's (1960-1970 is a transition era which I won't go into). Anything pre 1960 is going to be using older cap designs using some scary PCB's and stuff that oozes out with the appearance of crude oil and will often smell worse. Those cap's are most certainly the most likely problem. Pre 1960's valves, resistors and coils are incredibly stable components from that era with the exception being the capacitors. By all means, the first thing to look at in 1960 and before kit will be the capacitors. And, since alot of that valve kit is high voltage stuff, you damn well better replace those cap's unless you want a seriously nasty chemical fire in your workshop/home/business.

But anything from 1970 onward is just not going to have that kind of problem certainty. So your best approach is to FIND THE ACTUAL FAULT. No blind man parts replacements. Please, I beg of you.

/Mike

> I'd look at the opamps first.
>
> In the thick end of 30 years of repairing electronic equipment, I've
> found maybe about seven or eight genuinely faulty capacitors that
> weren't in cheap shitty switched-mode power supplies.
>
> Whatever you do, do *not* "re-cap" it.
>
> --
> Gordonjcp MM0YEQ

> Hi, Mike,
>
> it was really that opamp. Capacitors and all resistors were OK, so
> I have decided to change opamp. New one sits now in the socket and
> everything works.
>
> Thanks for your time and advice! It was nice from you.
>
> All the best!
>
> Daniel Forro

> On Mon, Jan 18, 2016 at 05:29:17PM +0900, Daniel Forró danforcz@...
> [korgpolyex] wrote:
>> Hi, Mike,
>>
>> it was really that opamp. Capacitors and all resistors were OK, so
>> I have decided to change opamp. New one sits now in the socket and
>> everything works.
>>
>> Thanks for your time and advice! It was nice from you.
>>
>> All the best!
>>
>> Daniel Forro
>
> Glad you got it running! It's a good feeling, isn't it?
>
> --
> Gordonjcp MM0YEQ