From the MAKE: Flickr pool
Anachrocomputer shares with us this handmade piece of digital days gone by –
Top view of the 300 baud acoustically-coupled modem that I built in 1984. I made the double-sided PCB myself with an etch-resist pen (Dalo pen) and some rub-down transfers. The main chip is the Motorola MC14412V single-chip modem, but there are also some LM311 op-amps and LM324 quad op-amps. The board is normally installed inside my Compukit UK101.
That’s some well-kept shiny copper for a 25 year old board! Hmm, makes me wonder – what methods(if any) are people using to preserve their hand-etched circuits these days? – 300 baud modem, PCB
10 thoughts on “Homegrown modem from days of yore”
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Thanks for featuring my modem on the Make blog page! I just wish I could remember where I got the circuit from — it’s not the same modem that was described in Practical Electronics shortly after the UK101 (Feb/Mar 1980). That one used a 4046 and a 565 (VCO and PLL), but my version must have been a later constructional article. I think I may have wiped the board with flux after etching it, which may have helped to keep it shiny!
“what methods(if any) are people using to preserve their hand-etched circuits these days?”
“Tinnit” tin-plating solution
I remember modems with circuits like that… the operating frequencies were set by RC time constants and were prone to drift, whereupon your connection would get noisier and noisier. (Remember, there was no error correction!) Using a remote computer in the 1970s was tricky because about 1% of the characters going in either direction would be randomly altered by transmission errors.
And another thing. Back in the 1980s, we all had motley collections of resistors of all ages from 1940 to 1980 back to WWII. I see a wide variety of vintages of resistors went onto that board. Nowadays we all have neat kits of 1/4- and 1/8-watt resistors that all look alike except for color codes.
I used to apply a spray on circuit board lacquer that was designed for the purpose and could be soldered through – it was meant to even act as a flux. It works, but it’s a bit messy.
Now I use “Tinnit”, which works well and gives me some nice looking boards, though I don’t have any of these tin plated boards that are more than about 4 years old so I don’t know yet how well they will last in the long term. I haven’t found an Australian supplier yet, but it can be mail ordered from All Electronics (http://www.allelectronics.com/index.php?page=search&search_query=tinnit&x=0&y=0).