Korg Forums

[casipai]

Hi,

Since I'm new to this forum, first off about me: I've been playin various instruments since childhood, mostly keyboard/piano and guitar. But for the last year I've finally found the motivation to delve into my true passion production-wise: psytrance/dark psy.

About a year ago I bought an EMX-1 and KP3 second hand, just to get my feet wet in the genre and am enjoying them dearly. But one issue is bothering me ever since and cant find an answer anywhere... lets get right to it.

My setup is following: EMX-1 (master) line-out (L/R) into KP3 line-in, same accounts for MIDI. Headphones are plugged into KP3 headphone jack. Clocks are set: EMX > INT, KP3 > EXT.

Now my problem is, that the sound generated via EMX is only audible with the right side of the headphones and unmodulated, the left is silent when I don't touch the KP3. When I start modulating with the KP3, the modulated sound (EMX & KP3) suddenly is audible on the LEFT headphone and the right still plays the unmodulated sound from the EMX.

I tried to troubleshoot in different ways, first off checking my headphones on other devices, and they definitely work fine. Just to experiment, I plugged the EMX line-out in the KP3 line-out and finally got sound on both h.phones; but couldnt modulate of course. Two weeks ago I had the EMX and KP3 looked at and thoroughly cleaned, yet the problem still remains and my guy assured me there was no hardware defect/damage to be found. So i checked all the Global/MIDI settings that seemed relevant to my problem and even did a factory reset on the EMX, but still no change.

Since I couldnt find any solutions on Korg forums/FAQs or other sites, I'm running out of options here...

Did anyone encounter similar problems? Thanks in advance

[voip]

When the headphones are plugged into the EMX, can the sound be heard in both L & R phones?

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[casipai]

Yep, definitely getting sound on both when plugged into the EMX.

[voip]

So that, together with your earlier results pretty much rules out the EMX as the culprit.

What happens when the KP3 mic input is used? Can sound be heard in both L & R phones? The mic input is mono, but the signal is routed to both channels.

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[casipai]

I agree with the EMX being in the clear.

About testing the mic input: I dont own one at the moment and wasn't really planning on getting one anytime soon either, since vocals are the least of my concern with what I intend to produce.
As I said I'm still in the process of learning my instruments and the field in general - so is there a way to convert my dual 1/4" plugs into a single one? Aka. going from my EMX line-out to the KP3 mic in?

[voip]

Just to be sure, to be sure, check that both EMX line outs are working by connecting your headphones to each EMX output in turn with volume set to low. You will probably only hear output in one headphone, but that doesn't matter for this test, as long as you hear something of similar sonic quality from both line outs.

If both EMX line outs are working, connect one of your cables with 1/4" jacks between one of the EMX outputs and the KP3 mic input, but first make sure the EMX output volume is set to the lowest setting at which it is still possible to hear its sounds. Set the KP3 input switch to Mic, and the trim to minimum and KP3 volume to minimum. Only then should you try listening via the KP3 headphone out and cautiously increase the volume of the KP3 to a quiet "normal level" whilst playing sounds out of the EMX. Alter the mic trim if necessary.

See what happens. Does the "dry" sound still only come out of one KP3 channel?
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[casipai]

Alright, so I tried the line-outs from my EMX with my headphones and I get sound on both L/mono and R. Although pretty much identical in sound quality, when plugged into R the volume is slightly lower than in L.

Next I plugged into KP3 mic like you described... I get the same effect, sound from EMX only on the right and silence/when touched KP3+EMX on the left headphone. If thats what you mean by dry. The same accounts btw, when connecting EMX headphones lineout to KP3 mic.

[voip]

OK, so that probably rules out the line in amplifier stage, which can sometimes get damaged by high input voltages, and the audio out amplifiers are probably also OK from your previous tests. Next thing to check would be the input select switch and input volume control, to make sure the connections to these are good. This will mean taking the cover off. At this point, an oscilloscope would be useful, or some other means of detecting audio signals are present on the input select switch and input volume control. Using headphones as a detector might be pushing it a bit at this stage as the circuit impedances are quite high. An amplifier with line in would be better and the EMX audio in, set to line in could be used at a push, to check if there are signals.

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[casipai]

Just to double check so I don't deal any damage to it myself: I opened the KP3 and separated the circuit boards entirely from the outer shell/cover, whilst still maintaining their connections.

Would a small 8 watt guitar amp work as a means to detect audio signals? My though was, since I need the EMX running to send audio signals through the KP3 in order to detect any audio signals, I probably couldn't use it to receive signals at the same time, right?

Next, you want me to plug one end of a 1/4 " cable to the line-in of my EMX (or amp) and - this is where my understanding fails - touch the circuit board with the other male end of the cable, right where the Input Select switch or Input Volume control are located on the circuit board?

[voip]

Sorry, I wasn't thinking. Of course you need the EMX as a signal source. Yes, the 8-watt guitar amp should be OK with the jack plug lead connected to it. Keep the volume on the amp quite low.

By the way, if you're not sure about doing any of this, please do get the help of someone who does this sort of thing regularly.

The tip of a jack plug is rather a blunt probe and a couple of leads with small insulated crocodile clips (something like this http://www.apogeekits.com/images/alligator_leads_red_black.jpg ) would be handy when connected to the jack plug on the free end of the audio lead to the amplifier. Black croc clip to jack plug sleeve and red croc clip to jack plug tip . Make sure the jack plug can't touch anything. Connect the free end of the black croc clip to a suitable circuit ground on the KP3. The grounds tend to be large areas of copper, mostly covered in green lacquer, but the copper is normally exposed at the screw holes, and there's also a handy "U" shaped screw bracket on the rear corner of the KP3 next to the power switch near the MIDI sockets to which you could clip the black croc clip to. Then use the free end of the red croc clip as the probe. It might be easier to put something pointed made of metal into the croc clip, even a paper clip.

Be prepared for your reflex response if the sound from the amplifier is loud!! If you touch the red croc clip with your hand, you will probably hear some mains hum, but not too much. If the mains hum is loud, turn the amplifier volume down until the hum sounds very quiet.

The places to probe first are the two connections or printed circuit tracks going between the input selector switch and the input volume control. I'm guessing it's on the PCB marked KLM-2734. One connection carries signals for the left channel and the other for the right channel. Put the probe onto where the component is soldered to the connecting track or wire joining the two components. You may need to press slightly, if the PCB has been clear varnished after soldering, in order to break through the varnish and make a good connection.

If you can hear decent audio on both of these connections, then move on to the input volume control wiper output - it's marked as pins 1 and 4 on the schematic, and will be the two tracks going from the volume control potentiometer to the capacitors C14, and C15. These are the small canister-shaped devices which may have a shrink-wrapped label. From what I've seen of the KP3 PCBs, most components are labelled so they shouldn't be too difficult to locate. If you can still hear audio at both test points, and it varies in volume as the volume control is rotated, move onto the other connections from C14 and C15. If there is sound there, move on to PCB KLM-2735 and locate the analogue to digital converter, labelled IC10. Check for audio on either side of capacitors C27 and C29.

If all is well up to here, after this point, the audio goes into the digital domain, and troubleshooting gets a whole lot harder.

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[casipai]

First off, I am impressed by your expertise and patience to walk me through this process step by step, describing every detail along the way. This is the kind of knowledge I am seeking. Your help is more than appreciated, so thank you sir! I meant to save this until we concluded our operation, but couldn't hold back any longer.

Back to business. Since I'm missing alligator leads and hardware stores are closed now, I'll probably get to the probing tomorrow.
Anyhow, I studied your text and found all the areas on the PCBs as you described without hassle. Not to sound arrogant, but since I believe to fully understand what you wrote and have some basic knowledge in electronics, I will attempt this test by myself. Yet, I have a few pointers I'd like to clarify first:

1. On the underside of the KLM-2734 PCB, right where input volume control and input switch are soldered, is a black rectangular rubber sticker covering the connecting tracks. My guess is for isolation purposes. Of course I'll need to remove it in order to do the probing, but do I need to put it back on? Or is this just an unnecessary additive?

2. Both input volume and the input switch have numerous soldering points. Some I can tell are merely to stick the device to the PCB, but I mean actual connectors. They seem to come in sets of 2x3's. Here a rough schematic of the soldering points:

Input switch

OOO
OOO


Input volume

OOOOOO


My question is do I need to probe every point individually or connect 3 at once to produce a reliable test result?

3. The capacitors have their two soldering points located on the top side of the PCB, both on opposite sides of the capacitor. Again, do I need to probe these spots individually or connected?

4. Did you want me to probe the IC10 unit or did you use it as a topographical landmark, in order to find the capacitors C27/C29?

After what you wrote, I am very inclined to ask what to expect when the problem thereafter might lie in the digital domain... but let's take this step by step. I dont want to demotivate myself!

[voip]

1. The black rubber sticker will be there for a purpose, since it adds cost to the manufacturing process, and the manufacturer will be trying to drive costs down. It could be insulation, or to stop something rattling or bending too much, so its worth putting back afterwards.

2. As you say, some of the solder joints are structural, holding the pot or switch body to the PCB. Each single switch and single potentiometer require 3 contacts. The switch because it has a inputs coming one from the line and one from the mic sources, and needs to pass the selected signal down the audio chain, and the potentiometer because it needs an input (from the switch), a ground and an output from the wiper inside the potentiometer. The groups of 3 belong to left and right channels. By all means probe each point individually, and note which points have signal present, and which do not, and whether the signal is affected by switch or potentiometer position.

3. The capacitor solder connections need to be probed individually, as this will help to reveal if the capacitor is faulty. Also, the capacitor will be there to isolate differences in D.C. levels from one side of the capacitor to the other whilst letting the audio through, so best not to short the capacitor leads together. Doing so might damage something.

4. IC10 was mentioned as a landmark. Its component leads are quite small, and one slip withh the probe could short two pins together possibly damaging it. If you feel confident about probing it, you could try looking for audio on pins 1 and 2. Beyond this will need an oscilloscope and logic analyser, so I'm hoping the fault will reveal itself in the signal path before IC10, since that would be easier to fix.

It's worth giving the PCB a good look over, to check for loose or dry solder joints. These have become much more common with the advent of lead-free manufacturing.

[casipai]

About connecting the capacitor solder points, I clearly wasn't thinking straight. By sheer logic connecting the two would of course cause them to short.

Earliest I can get to probing will be after work, so hopefully I'll be able to write you the test result tonight.

[casipai]

So, I did the probing this morning successfully. Just to be sure, I probed every location 5 times. Here are my results:

O = solder connection with audio
X = solder connection without audio (not even a quiet humm, just utter silence)
[ ] = capacitor

Input switch

X O O

X O O


Input volume

O O O X X X


Capacitors

C14

X
[ ]
X


C15

O
[ ]
O


C27

X [ ] X


C29

O [ ] O


Digital converter

IC10

1st pin = O

2nd pin = N/A (too afraid to damage something)


If I understood correctly this is good news right? I.e. the problem is still in the analogue realm?

[voip]

Looking at the results, it looks like the problem is likely to be a faulty volume control potentiometer.

The trouble is getting hold of the same, or equivalent part.

The pot is an Alpha (Taiwan) part: RV112BCF-40-20A-B10K

Korg refer to it as: 510374524027

I think it is this one:

http://www.fullcompass.com/prod/280321-Korg-510374524027

but, as you will see, it is special order.



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