Pot and button boards tested

I finally got around to testing the potentiometer and button breakout boards yesterday. They seem to work just fine.

Button board with integrated pullups and diodes on top, potentiometer board (six pots per 10p connector) on bottom

breakout board for buttons, making it easier to connect 8 dupont connector buttons to a single 10 connector

While the buttons worked straight away, the pots gave me a bit of a headache. First time around, nothing worked. Turns out, the connection between the potentiometer and the 10p IDC connector is not particularly good. Pushing on the pot made it work, but releasing it made it lose connecton again.

After comparing with the unmounted connectors I have in my drawer, I realised there are two types. One of them has the sprung contacts further out into the hole:

 

Top: Connectors from the drawer, the contact fills half the hole

Bottom: Connector that failed. The spring fills less of the hole - also, it looks like the edges of the pot legs, being slightly to wide, have dug themselves grooves at the side of the connector, meaning they may not be able to touch the contacts.

Pot inserted into the working connector

Pot inserted into the non-working connector. When pushing the pot down, the legs are pushed up slightly inside the connector, making contact.

pot inserted into the non working connector (slightly more rounded than the working one).

All things voice card related ordered

 ...well, almost - the digital parts remain.

But still - tonight I placed a huge order with JLCPCB, 5 copies of every card needed for a voice:

VCO

Pre-filter FX

Bus mixer

Bus mixer angled connector

Juno filter

Moog + JP6-filter

Waveshapers

Mainboard

Ordering 5 instead of just 2 copies is kind of a gamble as I don't know if everything works yet. But the cost (excluding shipping) is only 50% more for five than for 2. I guess I just have to wait and see if the gamble pays off. 

I have yet to sign off on everything, but I expect that part to go smoothly. So while we wait, here are some cool board shots

3D model

I've used Fusion 360 to check that everything fits together and that we have the clearances between cards that we need. 

I COULD have converted the eagle designs into something that Fusion can read, but this is the next best thing:

All the cards making up the voice. To the left is the (yet to come) digital board.

The narrow board closest to the camera is the bus board. To the left is a combined Moog/JP6 board and to the right a dual waveshaper board. Top left is the bit crusher (the only PCB I am not redoing!), below it is the FX board. Below the Waveshapers is the VCO

Under the Moog/JP6 filter board is the Juno filter. I really wonder if this will work out, the spacing is very narrow and it may get hot in there. Also note the vertically mounted board connecting the bus mixer to the mainboard.

On to the PCBs

A panelized board containing two waveshapers, the FX board (ring mod, distortion and noise) plus a special board that will make it possible to mount the bus mixer board parallel to the mainboard but 3cm away from it

Another panelized board, this time it's the VCO, Juno filter and bus mixer boards

The third board is the dual filter PCB, containing the moog-style ladder filter and the extended JP6 state variable filter.

The CV board - made on a four layer card to minimise noise (hopefully). This one has 56 output CVs!

Finally, the mainboard. I've spent six months or close to 200 hours on routing this, I REALLY hope it works.

Stacking PCBs

To have enough room on the voice card PCB, I think I will have to stack at least the Juno and Moog filter PCBs on top of eachother. But I don't want to pay for double sided boards and I don't want to solder the connectors myself.

Here are some options that I've come up with. All of them requires the filter PCBs to be mounted with their parts facing downwards:

This is probably the best option. It has a clearing of 5mm between the middle and lower board, and 4.4mm between the top and middle. Further more, we can increase this to 5.9mm by not covering the whole connector of the middle board, this is necessary as the Moog board has some 5.7mm high capacitors.

It also has the smallest possible space needed on the bottom board, 1.28cm (at least if parts on the bottom board are not higher than 2.5mm). 

It would be possible to put the middle board closer to the bottom, but this only leaves 2.5mm for parts and we need more space without parts to be able to slide the card in. It does leave a lot of space between the middle and top though.

This is a slightly more stable version of the first, as it uses a double sided connector on both cards. This will however steal more than twice the space from the bottom board as we need a lot of space to slide the card into place.

Waveshaper bug

I finally got around to testing the waveshaper boards last week. Most of it work flawlessly though I get some clicking when doing large frequency jumps for some of the waveforms. Also, saw and sine waves are not as loud as the others at the same amplitude (which is to be expected I guess, I just need to consider if its ok).

For some reason though, two of the TL074 op amps got really hot and the sub oscillator didn't work. After some probing I found that the -2.5v source output 13.5v instead. I had my wife solder a second board but the same happened there.

I couldn't figure out what was going on, but it suddenly dawned on me when I tried to fall asleep yesterday - I have mixed up the op amp inputs when trying to make a voltage buffer :(

The fix is very easy in CAD, but on the boards I have to cut two lines and solder a new wire:

Cutting the input, it goes to the negative terminal of the op amp instead of the positive. I've also scraped away the solder mask to be able to add a new wire to the positive terminal instead. I will see if I can find a wire tiny enough to fit in the via (which is 0.3mm...). I also have to add a solder blob between the negative terminal and the output.

Cutting the feedback loop between the positive terminal and output. It should of course have been between the negative input and the output.

The board is smaller than it appears btw, here is my finger for reference.

UPDATE:
My wife managed to solder this just fine, after that the filter works flawlessly! The tiny diagonal wire is a single copper strand from a networking cable!

Vocoder analysis/synthesis boards arrived

I picked up the vocoder analysis/synthesis boards yesterday. They look good as always, though I noticed some differences. Nothing to be alarmed about but I get why they are a bit cheaper than other boards:

- The silk screen is a bit misplaced on some boards
- The holes aren't always dead center on the pads
- One of the boards isn't completely flat, i.e. the fibre glass is slightly bent.

These are just very tiny deviations and well within what is acceptable, they just aren't perfect.

I'm very excited to get started on the build, to see if I've managed to get the circuit and layout right :)

Etching trials

Today I finally had time to do some photoresist developing and etching to try to figure out a sweet spot before doing all the vocoder cards.

I got some AA16 cards from C.I.F at Farnell and got to work.

After reading a lot about various methods, I landed on using Sodium Metasilicate as the developer. Incidently, I had some laying around. More specifically, I had a 25g sample of the Mega Electronics universal developer, but after reading the datasheet I figured it is what I wanted.

For etching I used "Etching power for PCBs" from Scan Kemi that I bought last time I did any cards. It seems it is mostly sodium peroxidisulfate (sodium persulfate).


I tried various exposure times as well as developer strengths and development time. Three of them failed and three were closer to success

The failed ones:

1) 2 minutes of exposure, developed in a 13g/2.5dl solution. This was not long enough, the developer had a hard time doing much at all.

2) 3 minutes of exposure, developed in a 25g/2.5dl solution. I left the card in the developer for three minutes. When etching, the resist floated away.

3) 4 minutes of exposure, developed in a 13g/2.5dl solution. Some of the traces were too thin and broke.

The better ones:

4) 3.5 minutes of exposure, developed in a 13g/2.5dl solution for 5 minutes. When etching some of the tracks got too thin and some were not etched enough

5) 3 minutes of exposure, developed in a 13g/2.5dl solution. The development time was a bit on the short side so some resist still stuck and was etched away later. The result was probably the best one of all the trials.

6) 3 minutes of exposure, developed in a 25g/3.5dl solution for 1m 30sec. The solution was a bit warm as the additional water was hot when added. This turned out almost as well as the previous one but with slightly thinner traces and one short circuit.

My conclusion is that 3 minutes of exposure seems perfect, and that I should probably retry the 13g/2.5dl developer solution but leave the cards in a little longer.

The etching times was around 10 minutes, depending on the heat of the water. I did the etching in a small plastic box in a bath of hot water which started out at 44 degrees celcius. The recommended temperature is 50 degrees.

Dry film photo resist

I tried a new technique for making circuit boards yesterday - dry film photo resist.

I already own a (huge) UV-light box that I haven't used for a number of years. My previous UV experiments have all been under a large bulb, so I didn't what to expect.

The way the film works is this:

It has three layers. A bottom plastic layer that must be peeled away (this is on the inside of the curl when the plastic comes on a roll), a middle layer that will form the resist, and a top plastic layer that is peeled away once the film has been exposed.

The film is negative, meaning that any parts not covered by something when it is exposed to light will be "cured" and turn into resist. The remaining parts that were covered during exposure can be washed away during developing.

A friend of mine had already printed some templates on transparent plastic. Though they are meant for positive resist, they work well for the experiment. I put two copies of the same print on top of each other to better block the light.

Start off by sanding the copper clad board with a very fine sanding paper or a scotchbrite sponge to make stuff stick a bit better. Clean off with some alcohol or acetone or something similar to remove any grease.

To remove the bottom plastic, I used a trick I found here - put a bit of sticky tape on each side and rapidly rip the sides apart (it is important to do this quickly). I turned down the lights in the room while doing this, remember, you're working with photo sensitive film.

Then cover the copper side of the board with the film, protective plastic up.Try not to touch the film with your fingers to prevent greasing it up.

The film must be fastened to the board using an iron at a very low temperature (or the plastic will melt and warp) or idealy a laminator. I tried an iron with ok but not extremely great results.

Once the film sticks, put the transparent foil face down on the UV light box, put the board on top and something heavy like an old mathematics book on top of that to keep everything nice and flat.

I tried various exposure times from 2 to 6 minutes. 2 minutes turned out best, the others had cases of light leakage - ragged edges, thinner lines than on the foil etc. My UV light box has 10 powerful tubes and is very powerful so times will be different with other equipment.

After exposure I put the cards into what I think is a 1:100 mix of sodium carbonate and water (but I got it in the same package as the dry film from a vendor in china, so I'm not really sure). The development time was three minutes. After two minutes I started brushing the non-exposed parts with a coarse paint brush to rub off the film.

I haven't tried etching the boards yet so I don't know how well the film sticks. It looks and feels very sturdy though.

Exposed for 4 minutes

DAC and S&H boards arrived

The DAC and sample and hold PCBs arrived from oshpark today. They look great as always, I am very excited about them and wether they'll work or not. 

I will start ordering parts today too, I have two populated shopping carts, one with Farnell and one with Rs components and will press submit as soon as I get an answer from Farnell about the value of the Tempco resistor they offer - the data sheet indicates 1k but their page says 100Ohm... 

Oh well - some errors are to be expected

While trying to tune the oscillators I discovered two errors. They are both easily fixable but important. First, I had used 100k instead of 10k pots for the wave balance pots, which means it was impossible to get a good balance around 0V. I replaced the pots and it worked very well.

Second, both this quad oscillator and my stock yusynth oscillator seemed to have a very high-pitched response to 0V CV. Only after reading through the text on the yusynth.net site did I realize that the coarse tune pot is not centered around 0. Instead, the maximum counter clockwise setting is 0 and the max clockwise setting is 10 (not sure if that means 10 octaves though). This means that in order to function "normally", I have to add a 270k resistor connected to -15V to my design as I have no coarse tune pot. It's not a big problem but one that will have to be adressed in a version 2 of the oscillator. I should also concider adding either an octave rotary switch or at the very least an octave/coarse tune input. As I have run out of space on the PCB, I think the pot will have to affect all oscillators, not only one, meaning all four oscillators will be tuned to the same range. Two four-oscillator cards could of course be tuned differently though. I may even be able to squeeze in two inputs so that groups of two oscillators may be tuned differently.

One final note: I have used 100k resistors for the wave amplitude amplifier input. Mr. Usson states that one may have to use an 82k resistor here and I can see why. My saw wave output has a +/-4V amplitude instead of the expected 5V, swapping the resistor may help.


UPDATE: Fixed!

Four 270k resistors soldered directly to the back of the board, with a common wire to -15V. The molten plastic gunk is clearly visible too. What a shame.

Quad oscillator oscillating!

After a false start this morning, I managed to get my quad saw oscillator card up and running tonight. At first, it seemed like nothing worked, but after switching to another set of oscilloscope probes and probing from the start to the end of the circuit, it turned out it was working after all. Great success! :-D

The unpopulated PCB

My first ever try at soldering surface mountable components. It wasn't hard at all, just put some solder on one of the corner solder tabs (on the PCB), hold the part with SMD tweezers and solder the first corner. Then do the same to the diagonally opposite one. When they are done and the chip is adjusted to it's correct location the rest is easy.

The completed PCB. I tried cleaning the board with isopropanol, with disasterous results. The cleaning left white residue all over the card. In retrospect I suspect that the residue is in fact from the toothbrush I used for cleaning...

Oh... and to round up the post - of course an electronics enthusiast needs an electric car, so I bought this amazing vehicle, the Tesla Model S :-D It's a beast!