Since I was dabbling around the VCF/VCA chip's diagram (IC 1/NJM2069)... comparing it to a CEM 3372 and other CEM VCF/VCA's, it dawned on me that you may have a problem with either the Noise Generator circuit itself, IC 1, or a bad signal going into IC 1 at pin 9 (the CPU's Noise Level Control).
Here's where to look at the schematics:
If you look at the schematic for the main board, you will see sections of the schematic blocked around. The area of interest right now is at the lower right, near IC 1. It is block section #29 (29 is circled).
That's the Noise Generator circuit. You will notice there is a variable resistor inside that block. VR3.
You might try turning that VR all the way down and see if it reduces your noise level.. that VR should be the first stage of NOISE level... it's independent of CPU control.
If you follow the circuit, you will see that section's final ouput from the IC2 (m5223 Dual OpAmp) goes into PIN 7 of the IC 1 chip. This is IC1's "VCF SIG in", or source of HISS you get (the hiss is generated by transistor Q5). It is IC1's 3rd VCA signal input.. the other two VCA signal inputs are at PINs 20&21.. they are for DCO1 and DCO2.
PIN 9 is how the CPU controls the Noise Level (your parameter settings, etc.), I believe it is a CV input and is affected like this, "voltages at this pin are 0v = 0% volume, 3.83 volts = 50% volume,
and 7.65 volts = 100% volume."
So it may be possible to check the voltage level at Pin 9. and see if you have a high voltage level there when your parameter setting for NOISE is at 0. If you're parameter NOISE level is 0, that pin should be 0V or very close to it.
If it's say, 2-8V.. then something has gone wrong further back in the control line.
Just my WAG.
If you want a cleaner schematic to look at, check this link for a MII Service Manual.
http://www.synfo.nl/servicemanuals/Korg/POLY-800II_SERVICE_MANUAL.pdfBEWARE tho... things are different from a M1 to a M2. But I like to keep a copy of a M2 pdf handy for areas I have difficulty reading on the M1 pdf since most of the core circuitry is the same.. such as the Noise generator circuit section and most of the main board.. minus the alterations done to slip in the expansion board for a M2 to have Digital Delay.
There are also a few errors in the M2 schematic that I wont go into right now.. the point is that you can use that pdf to get a clearer view of an area that is smudged up on the M1 pdf.. for the most part.
Here is a link to a discussion about the IC1/NJM2069.
http://sounddoctorin.com/synthtec/korg/njm2069.htmThey are discussing the use of IC1 in a different Korg. The DSS-1. Korg's first Digital Sampler.
The application in the DSS-1 is a bit different than a Poly 800, but I figured it was handy info to have.
The diagram you want to stick to of IC1 is on page 30 (pdf page#, not manual page) of the M1 pdf.
IC1/NJM2069 is a very difficult IC to find for sale on ebay, etc. It was Korg's very own chip (perhaps their own version of a CEM chip). So if it's gone bad on you, there's not much that can be done unless you get it from another Korg synth that has one in it.
But.. there might be a possibility to retrofit a CEM VCF/VCA into the Korg. Yea.. some may laugh or snear or perhaps even cry BLASPHEMY! at me for even thinking of doing this to a Poly 800.. but CEM chips are a damn good sounding chip. They are in SC Prophet 5's, Oberheim's and a slew of other classic sounding synths.
Here's a link to a CEM 3372 datasheet. You'll see how somewhat similar it is to Korg's NJM2069
http://www.synthtech.com/cem/c3372pdf.pdfAnyway.. I'm getting off the subject here.
Give a go at VR3 and see if turning it DOWN will help reduce your noise level. If it does (and it should). then we can try to move on to other areas to look for problems.. which is probably the input CONTROL signal at PIN9 of the IC1 chip.
Maybe Mike can chime in here a bit on this too.. I've had to be corrected by him several times before and sure to be again, LOL.
This HISS problem is a recurring issue with this synth. Maybe if we collaborate on understanding the synth's circuitry, we can help others with problems like yours in the future.
I got a feeling your problem is an issue that is related to your Q1 having been removed. Usually Q1 goes bad because someone shorted it out.. and if Q1 gets shorted.. it could have botched up another component that controls the Noise Level.
HTH's
-Blaine
--- In korgpolyex@yahoogroups.com, "mcdonaldtavish" <tavishmcronald@...> wrote:
>
> Hi, just made a trade for a broken poly 800. At first glance this is what I have noticed about my broken unit. Let me start by saying I have yet to open this thing up. I tried to reload presets and this had no effect on the board or its problems, I did it multiple times, it said "good" then I switched back to tape disable and it sounded the same, nothing changed at all. Unit powers on and seems to respond to parameter changes to an extent, for instance I can change deg1 attack value, and the display shows the parameter number I chose and the value I'm changing. it kinda effects the sound but it is clear that the sound is not right in the first place (it is like that with most of the parameters except the noise, dco levels and vcf functions don't seem to do much of anything), so some of the parameters seem to half work, I can hear them change but the overall sound just isn't right at all and has a loud hiss to it. In the background you can hear the sound playing but over that is the hiss. it's as if the noise generator is at full blast, the dcos are turned down (it doesn't sound like a ground hum) and the filter is open and at hi resonance. I can usually tell when chorus is on or off, the tune fader seems to work. The bend joystick doesn't seem to do anything. The headphone jack doesn't work. When it first powers on it wont make any sound, after about 30 seconds to a minute it starts to make a sound that starts with the filter cutoff low but slowly rising, I can take my finger off the key and it will stop, when I press back on the key it starts from where it left off on the filter cutoff envelope and eventually fully rises and stays that way. The sequencer seems to work, it just plays the messed up noise, I can change speed and I think I missed around the with steps. When the sequencer is on the display flashes, the flashes speed up when the sequence speeds up. All the keys function and you can hear the pitch of each key you press, but here's the weird part, you press a note and then you take your finger off and the note stops, then you play another key and it plays both notes together and so on. It's hard for me to explain most of this in a coherent way so my apologies, but hopefully you guys are getting a feel for the issues I'm having with my poly 800. Any suggestions would be great appreciated, I have a lot of solder experience but I don't know how to troubleshoot circuits or anything like that.
>
> Thanks guys.
>