So this custom ROMs business is pretty slow progress I am afraid, but be patient I am working on it as fast as I can but this is a spare time project and I haven't much spare time right now. My first 808/909 kit worked but was had so much wrong with it that it was faster to start again from scratch. So I moved on to a sequential circuits pack and I am much happier with the results.
I am using soundforge to do all my editing. I take the original ROM sample, zoom in as far as you can go and place markers at the zero crossings closest to the beginning of each sound. Each new sound has to be the same length or less than the original sound. The samples need to be conditioned before you put them in - they need to be of a similar output level, monoized, sample rate/bit depth converted, pitched up, normalised then overwritten in place of the original sample before being converted back into a BIN file for burning.
Why pitched up? well the R-50 and R-100 both allow you to repitch the sample within the drum machine, but the sample is stored on the EPROM at its highest pitch/fastest rate. So 0/midpoint is actually the sample pitched down by half an octave. And so the Kawai specs of 12 bit/32K sample rate is actually a bit of a lie, it would only be 32K when all samples were at their highest pitch. But it is probably this sample rate reduction that gives the Kawai R5/100 so much punch! It is also this part that is currently knocking me down.
You see when I repitch things they sound wrong, there is a metallic sound which I am putting down to aliasing. Now when I am doing my sample rate conversion within soundforge I apply an anti-aliasing filter, but this is for 32K. What I THINK I need to do is to resample it at a (yet to be determined but presumably 16K if the tuning range is an octave) lower rate first, to filter out the offending frequencies that cause the aliasing when the samples are pitched down within the drum machine.
At the moment pitched stuff like kicks and toms sound pretty good while crashes and rides sound awful - I am putting this down to the frequency content. Barely anything can get aliased in the kick drum because the offending frequencies found in the crashes simply aren't there. If the 16K theory doesn't work then I think I can figure it out with FFT or spectrogram of the original samples.
Annoyingly though this is only guess work. I may not figure it out at all, this is all very new to me. It means I am not yet able to offer custom ROMs. If I do crack it then I will charge £25 per custom ROM or £15 if it is one I have already made. I will do some kind of discount if you buy the ROM switch and ROMs together, but I have not yet figured the price out for that yet. If you are interested in buying any of the factory ROMs now, email me at analogmonster at live dot com
Wednesday, 11 September 2013
Polivoks LFO faulty - reply to Ott Suurtee
My reply to Ott Suurtee's questions about the faults with his Polivoks (see comments) has got so long it deserves it's own post, so here it is.
Right, just to be sure we are working from the same point (I am assuming like me you don't read Russian Cyrillic) the LFO board is Y7, which connects to the main board at X7. From what you are describing the LFO is not oscillating, so the modulation knobs for the VCOs, VCF and VCA will do either nothing or very little - at most it would add a constant voltage to the other CVs (envelopes and knobs). I expect the VCA pot being bumpy will be fixed with a hell of a lot of wiggling to remove the layers of dirt.
It is going to be difficult to exactly pinpoint the problem without me being there, so I am going to try my best to help you understand the circuit so you can figure it out for yourself. I am going to work backwards, telling you what I think isn't causing the problem so you can hopefully figure out what is.
looking at the schematics, the top section is noise generation and sample and hold - A1, A2 and A3 are just conditioning the noise generated by the transistor V1 to be at the correct voltages. When a pulse is sent to the transistor V3 the voltage from A3 is held for the length of the pulse, with A4 being used as a unity gain amplifier to buffer the signal.
The pulse for the V3 transistor comes from the V4 transistor, which is converting the positive portion of the square wave at A5 into 0.2ms pulses. The square wave at A5 is also driving the LED driver transistor V5. The square wave at A5 is passing through the V6 diode to remove the negative portion, with A7 being used as a unity gain amplifier to buffer the signal.
The rest is the section you need to pay attention to, which is much easier now we have reduced the number of parts to look at. What we have here is a comparator based square wave generator. This is an RC (resistor capacitor) circuit - it sounds like you might want to do a little reading on the basics of RC circuits, op amps and comparators.
If you take a look at this square wave generator example perhaps you can see the resemblance to our schematic - C is C7, R1 is R14, R2 is R13 and R is the combination of R15 and our frequency pot. The capacitor charges and discharges through R, creating a triangle wave. The variable value of R (thanks to the pot) dictates the rate of charge/discharge and hence the frequency of the triangle wave. A6 is being used as a unity gain amplifier to buffer the output of the capacitor. A5 looks at the triangle wave and outputs a positive voltage when it is above a certain threshold and a negative voltage when it is below a certain threshold.
My guess is it is probably NOT a component failure, its far more likely to be the connection between boards as this is a vulnerable point. Do all of this without power applied. I would remove the Y7 board and put your multimeter on continuity test. Check that at the X7 socket on the main board, pins 4 and 5 have continuity to the R107 pot. While the board is out, clean the contacts on both Y7 and the main board using isopropyl alcohol. Leave them apart for a few hours to be totally dry. Put board back in and check continuity between the R107 pot and pins 4 and 5 ON THE X7 SOCKET ON THE Y7 BOARD.
Beyond this you are looking at tracing tracks in case any have broken, or replacing components. The point is that without the RC relationship there is nothing to periodically trigger the rest of the circuit. Seeing as we have already checked the connections of the R part, the first one I would replace is C7 - easy enough, find an electrolytic capacitor of the same value. Folowing that I would replace A5, but that is where things might get awkward - how are you going to get a replacement russian part? It is just a single op amp being used as a comparator, so I think a TL071 would be fine but I can't be sure.
When I first got my Polivoks the LFO was working, but only some of the time. The rest of the time it was doing something similar to yours. My board was sorted with a damn good clean, but only after a day or two of head scratching, looking for something to fix that wasn't broken.
Please let me know how the exploring goes, I hope you figure it out. Good luck!
Right, just to be sure we are working from the same point (I am assuming like me you don't read Russian Cyrillic) the LFO board is Y7, which connects to the main board at X7. From what you are describing the LFO is not oscillating, so the modulation knobs for the VCOs, VCF and VCA will do either nothing or very little - at most it would add a constant voltage to the other CVs (envelopes and knobs). I expect the VCA pot being bumpy will be fixed with a hell of a lot of wiggling to remove the layers of dirt.
It is going to be difficult to exactly pinpoint the problem without me being there, so I am going to try my best to help you understand the circuit so you can figure it out for yourself. I am going to work backwards, telling you what I think isn't causing the problem so you can hopefully figure out what is.
looking at the schematics, the top section is noise generation and sample and hold - A1, A2 and A3 are just conditioning the noise generated by the transistor V1 to be at the correct voltages. When a pulse is sent to the transistor V3 the voltage from A3 is held for the length of the pulse, with A4 being used as a unity gain amplifier to buffer the signal.
The pulse for the V3 transistor comes from the V4 transistor, which is converting the positive portion of the square wave at A5 into 0.2ms pulses. The square wave at A5 is also driving the LED driver transistor V5. The square wave at A5 is passing through the V6 diode to remove the negative portion, with A7 being used as a unity gain amplifier to buffer the signal.
The rest is the section you need to pay attention to, which is much easier now we have reduced the number of parts to look at. What we have here is a comparator based square wave generator. This is an RC (resistor capacitor) circuit - it sounds like you might want to do a little reading on the basics of RC circuits, op amps and comparators.
If you take a look at this square wave generator example perhaps you can see the resemblance to our schematic - C is C7, R1 is R14, R2 is R13 and R is the combination of R15 and our frequency pot. The capacitor charges and discharges through R, creating a triangle wave. The variable value of R (thanks to the pot) dictates the rate of charge/discharge and hence the frequency of the triangle wave. A6 is being used as a unity gain amplifier to buffer the output of the capacitor. A5 looks at the triangle wave and outputs a positive voltage when it is above a certain threshold and a negative voltage when it is below a certain threshold.
My guess is it is probably NOT a component failure, its far more likely to be the connection between boards as this is a vulnerable point. Do all of this without power applied. I would remove the Y7 board and put your multimeter on continuity test. Check that at the X7 socket on the main board, pins 4 and 5 have continuity to the R107 pot. While the board is out, clean the contacts on both Y7 and the main board using isopropyl alcohol. Leave them apart for a few hours to be totally dry. Put board back in and check continuity between the R107 pot and pins 4 and 5 ON THE X7 SOCKET ON THE Y7 BOARD.
Beyond this you are looking at tracing tracks in case any have broken, or replacing components. The point is that without the RC relationship there is nothing to periodically trigger the rest of the circuit. Seeing as we have already checked the connections of the R part, the first one I would replace is C7 - easy enough, find an electrolytic capacitor of the same value. Folowing that I would replace A5, but that is where things might get awkward - how are you going to get a replacement russian part? It is just a single op amp being used as a comparator, so I think a TL071 would be fine but I can't be sure.
When I first got my Polivoks the LFO was working, but only some of the time. The rest of the time it was doing something similar to yours. My board was sorted with a damn good clean, but only after a day or two of head scratching, looking for something to fix that wasn't broken.
Please let me know how the exploring goes, I hope you figure it out. Good luck!
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