Parts parts parts.. Sure has been a slow crawl towards the target of acquiring all the parts. Currently I’m lacking a coherent set of potentiometers, most of the active parts (4xxx series / opamps) and some of the odd-value capacitors used on the BPF boards. The rest of the passives and misc supplies are sorted out though.
Anyway, returning to the previous etching issues I have a few more close up photos from one of the boards..:
Some neat holes and cuts there! I wonder whether the traces had turned out better if copper pours had been used (or added) to fill up the larger empty areas in the original mask designs? It would’ve cut down the etching time at least, this might’ve saved some of the traces.
Moving on with this project, it was finally time to continue by etching the back planes and a few filter boards. Since the UV unit I use for exposing the photosensitive boards (a low-power EPROM eraser) is very far from perfect for this use, the back planes turned out almost a complete failure. Here’s why:
First, low-powered means that a single board has to be kept exposed for around 30-45 minutes to get decent results. Second, this exposure time depends a bit on the brand of the board (different coating solution and/or process per manufacturer, I assume), so whenever I try out a new brand it’s a bit of trial and error. For this batch I had a new brand, of course. Third, as the UV unit is meant for erasing EPROMs it has only a single small lamp, meaning that large boards (such as the backplanes) need to be exposed in several parts. For large boards it’s not uncommon to have overall exposure times as long as 1,5-2 hours with this UV unit! Just for comparison, a real PCB exposure unit handles boards of almost any size in 5-10 minutes per side.
Upon stumbling across the documentation for Elektor Vocoder circa 1999, this very complex piece of diy made it to my to-do list immediately. I knew I just had to complete one some day! The project took it’s first step forward when around august 2003 I found a Tandberg TR2075 stereo amplifier in the dumpster. The casing, made almost entirely of thick clear plexi glass, was just something I could not pass up on. Its size and style just made me think of just one thing, the Elektor Vocoder. Bring it on then! I decided that since the casing is all dumpster material, maybe the electronics could also be built using as much recycled parts as possible. Make it a real Vocode-A-Trash Deluxe ™!
As the design is rather old (circa 1980), the first issue prior to etching any boards was to check the combined parts list for obscure/obsolete items. The only difficult ones were the Siemens connectors used to interface all the boards together through the backplane. Despite extensive searching, I didn’t come across suppliers or matching replacements either. Lucky for me, there were plenty of excess 3×32-pin DIN41612 bus/backplane connectors at work waiting to be binned; these were easy enough to modify to match the pin spacing on the boards (verifying against paper prints ftw!). The second issue were the etching masks for the boards. The scans floating around in the net were piss-poor quality jpgs and thus cleaning them up took quite a while. Nonetheless, the editing needed to be done so I’d get my hands on some real boards and see how the whole structure would fit the casing. CAD, now wtf is that.