Thursday, August 18, 2016

Rewiring update 2

Cello, again!

Back one day later with more updates!

I fixed the issue with the Cello. Well, almost. The issue with broken wire to the volume control was easy to fix. However the signal had another break further down the chain. At the Orchestra section the signal is either passed through the chorus it bypasses the effect. In other words the it's a 2-way switch*. The problem was that the open backed switch for the Cello has had it's contact bent at some point and it was only operating as a one-way switch. The Cello Orchestra (wet) worked but not Cello without Orchestra (dry).  I bent the contact back and it's work okay now. No great, but okay.

Vibrato delay fix

Next, I wasn't sure that the Vibrato was working right so I did some measurements around the LF board and found a couple of broken wires. An earth wire and a control line for the Vibrato Delay. After that fix the Vibrato seems okay too.

The 100Hz Mystery

Yesterday I said it sounded like there was a noise floor of all the voices bleeding though. Last night after fixing the Cello and the Vibrato wires I again turned up the volume to listen to that noise. I can't be sure, but I think it has changed. Now I hear a single tone coming through. Like the stuck note I have on A# on K4 board, but this time much quieter. To be clear maybe my ears were deceiving me previously but now, for sure there is something there at one tone.

100Hz noise

There could be a bleed of everything else under that, but as you can see there's something quite loud. It's around 25mV at 100Hz and crucially it varies frequency with the pitch and detune controls. That means that it must be coming from the voices and not the power supply or some other circuit. I traced it back and it is present all the way back to G1 board.  I started there as it seemed likely that this would be coming from a whatever note is around 100Hz, and that is the lowest frequency board. However, I could also see it on the G4 board outputs. This isn't surprising actually. The outputs are mixed together (crudely without buffers), but I would have expected to see something stronger at the source, probably on G1. I didn't, so I will need to go deeper.

Make like a banana...

Something else I noted is that as I switch the split-gate upwards it lessens. So at split 0 it's loudest and at split 3 it's quietest. It's also present from all sources - Cellos and Violins.The split gate is quite fiddly to describe without a few pictures and tables but some voices are always open to be switched. The lower part the G1 board 16U and 16L are always present. In split 0 G1 16U/L are present as part of the Viola, mixed with the G1 8U/L. At split 1 (and above) G1 16U/L is present (alone) as part of the Cello sound. 

On the down low

If we assume some note in the lower part of the G1 board is the source of the noise then it should go away if only the Violins are switched in and the split is 0. This sort of matches the effect I got when playing around. It didn't go away completely though. I could get a trace of it in G4 so that does make sense. It could still be bleeding back even though the Viola and Violins are mixed separately.

How low can you go? 

Therefore the theory that it's coming from G1 makes sense, except for one thing. If I played through the notes on G1 I should have found the same note somewhere on there. I think I did, actually, but it wasn't quite right. I played note G and got something much louder but sounding the same. But I couldn't get a trace that matched. I think that the lowest G is G3 not G2 and my ears tell me it's more like 200Hz than 100Hz. I must redo this but I think it might just be because that G is a fundamental of 100Hz I'm hearing. G2 is 98Hz and G3 is 196 Hz. I think I'm hearing the first fundamental at ~200Hz because of the natural high-pass filtering in my listening environment from strip lights etc. (and yes, I tried turning them off to make sure it wasn't EMC). What I measure is 100Hz, but what I hear is a mix of harmonics from that pulse trace - I think! 

All very interesting (in a way) but if the fundamental is 100Hz then where is it coming from? The lowest note is C1 at 130.81. If it was the low G and I was wrong about the lowest note frequency it would probably be because the key circuit was broken. But I also shorted that key to -7 and it made no difference.

Pulse, a ting?

Finally the shape of this tone is not 'right'. Firstly it's a pulse, not a square -which is the source of the tones - or the more saw-tooth like shape - from the wave shaping (the subject of another post). If some wave shaper on the G board is faulty and bleeding through the raw oscillator then maybe a pulse is the result. The most obvious fault would be a shorting capacitor, but how that would get to a pulse I don't know.

Enough. Let's see what I find, next time...

* Idea - Replace the switches with a mix control? In fact all the switches could be replaced with such controls. Then instead of switching things in or out they could be balanced instead. Sliders would be faster for a quick switch in/out.

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