Not to worry, I'm here again now with a fresh batch of information!
The power supply has been a resounding success. I built the new version of the board into its shoddy case after considerable faffing about with dodgy connections and misjudged cable lengths, to produce this:
...Told you the case was shoddy.
Looks aren't important in this case though, as it's only going to live under my desk to power my experimentations. Inside the box is the power supply circuit depicted in the last post, with the indicator LEDs taken on wires to the front panel (such as it is) and also the the rather large 60VA toroidal transformer I picked up on eBay. The transformer takes the 230v AC and gives two lots of 18v AC nd a 0v connection. These go to the input pins on the power supply, which converts the 2x18v AC to +15v, +5v and -15v, all DC.
I had issues deciding on connectors for the power output, as the sexy 4 pin mini XLR type connectors I like are rather expensive, and cheap standardised power connectors don't seem to be an easy thing to find in relatively small quantities. In the end I settled on Molex power connectors as used for PC hard drives etc, mainly because I still had the cable from the PC power supply I was using and because they're pretty easy to get hold of. couldn't find a panel mount one anywhere though, so this supply just has a Molex cable trailing out the back of it.
With that job out the way I got back to thinking about my VCO. all my attempts at combining designs to make something that could potentially in some way be said to be mine had met with abject failure, so I decided to use someone else's schematic and PCB layout. My first inclination was Thomas Henry's VCO-1, but I didn't like the idea of using the completely obsolete CA3080 chip. The LM13700, which is more cheaply and readily available, is effectively two 3080s with bells and whistles. I toyed with the Idea of using this instead but I had no idea what to do with the other half, and it seemd a waste to not use it all.
Thomas Henry to the rescue! For he has designed another VCO with this in mind, the LM-VCO. unfortunately the schematic for this module is not available online, you must do as I did and purchase his book "An Analog Synthesizer for the 21st Century" which contains schematics for various modules which can be put together to form a fully featured modular synth. The book is available from Magic Smoke Electronics here, along with some others by the man himself.
I found a layout for the module here on the electro-music.com forum. I don't know if I already mentioned it, but this place is a goldmine for information. The people there are all pretty cool, and the forum just has masses and masses of helpful posts, layouts, schematics, theory stuff, the lot.
I decided at this point that it was time to up the ante in the PCB production department. I had been ironing on toner transfers to produce my boards, but I felt like having a go at using the UV photoresist method. With this technique the transparency is placed over a special pre-sensitised board and UV light is light is shone on it for a certain peroid, then the board can be developed like a photograph, with the UV-exposed portions of the coating dissolving away leaving nice clean tracks.
I didn't have an expensive UV exposure unit, so I decided to give it a whirl with my mum's facial tanner - You know the sort of thing... like a mini sunbed for your face.I wasn't sure about what distance away to have the light or how long to do it for, so I just placed the transfer on the board with a sheet of glass over it to keep it held flat, then just stuck the tanner face down on top of it with the bulbs a couple of inches away from the board. I had a smoke and got the tray with the developer ready, took maybe 5-8 minutes? Miraculously it worked perfectly first time. The only slight problem is that my original transparency had little pinholes in it and these came out on the final board. Some of the traces are quite thin and looked almost broken withthe tiny holes, but I pushed on regardless, as is my way, and soon I had a fully etched, drilled and populated VCO board!
...It didn't work of course.
Well it did, just not very well.
The problem was that I didn't have the 5 pin monolithic transistor pair that the layout was expecting, so as botch I just used two random transistors with the emitters tied together. This resulted in an asymmetrical wave with no temperature compensation whatsoever. Even my breath on the transistor pair was enough to make the pitch fly about wildly!
A fix was needed, so I came up with this:
It's a simple little adapter board that allows half of the THAT 340 quad monolithic transistor array I have to fit the 5 pin design the circuit called for. Once the board was etched and drilled and the chip was in place, it was just a matter of solering some leads I snipped off LEDs into the end row to connect into the main board. It doesn't fit very well and I've had to mount it on a bit of a jaunty angle, but it does the job! The sine and triangle waves now look perfect and the 1v/oct tracking is good for about 4 octaves (nearly). At this point I connected the pulse and ramp pins to try out the other waveforms and found they didn't work. Hours of troubleshooting revealed a couple of errors in my soldering, but I'm still no closer to fixing them. I think there's either a break in the circuit somewhere that I can't find, or that general shoddy workmanship and the fact that some ofthe pads are half missing cos I used too big a drill bit for the holes is causing it te not work. Either way, I don't think it's getting fixed any time soon.
It's progress though! and in the name of progress I then moved on to another modules I've been meaning to get built properly; the sequencer!
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