Author Topic: SN76489 reincarnation  (Read 200 times)

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Offline DrG

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SN76489 reincarnation
« on: March 29, 2020, 11:56:12 pm »
BLUF This is just a silly little report - hopefully entertaining to a few....(and forgive the shoddy photography).

Covid-19 NPI is taking its toll on all of us. Having the infection is, to be sure, a much bigger deal than the quarantines and restrictions. Still, there is the tendency to cogitate. To fall into a rut. Get up and look at the data sites - World, Country, Locality....look at the trends (with my calibrated eyeball). Friends and relatives calling and you can guess what the conversation is about. So too, the email and texts. Turn on the TV and it is more of the same, of course.

I have been trying to intentionally limit my cogitation. I do have several good projects in progress, but, frankly, it is hard to get motivated. So, I decided to go into some boxes with old stuff and I came across this, which I built sometime in the 80s.


I can barely remember it, but the heart of it is a TI SN76489A sound chip. That chip was an "early" sound chip and was quite popular in the day. I think, but am not sure, that the US Radio Shack chain had them at some point and that is likely where I picked up it. The board has an amplifier and is meant to interface with a TRS-80, as I remember. I could probably find the schematic in some file somewhere.

Seeing that SN76489A reminded me of something that happened only a few years ago. I was doing some reorganizing of a parts cabinet (typical set of sliding plastic drawers). I came across a few chips that had, mysteriously, completely corroded - even though other chips in the same bin had not. It was discussed with a few speculations and I just forgot about it (although I isolated those few chips). One of those chips was an SN76489A. At the time, I thought about testing whether the chip still worked. Then I just forgot about it, until now.

Here is what the corrosion looked like after a little brushing to get some "fir" off.


After scraping off all of the crud, I was left with very little of the legs left and for several pins, only a stub (the part outlined in red). After trimming all the pins down, I set out to see if I could solder the chip onto a carrier (below). I learned that you need a lot of heat when soldering stubs  :).



OK, not exactly neat, but it seemed that I did get them all attached.

Now, I had to go figure out how to get a test circuit together. To my surprise, there is a great deal online about this old sound chip. I found a nice Arduino library https://github.com/MajicDesigns/MD_SN76489 Which has a good deal of documentation https://majicdesigns.github.io/MD_SN76489/. Particularly nice was that you could use an UNO to provide the clock - it's a nice library - kudos. Also, since I new that I had another chip (from the board described earlier), I could test out whether the corroded chip was bad, but the hardware connections and software were working.

So I slap together a few connections, an amplifier and speaker....


...S O U N D

Ran several of the example programs in the library and everything works fine.

Sweet

(edited 10000 times to get the attachments right - it's like I need to be trained every week)
« Last Edit: March 30, 2020, 12:25:57 am by DrG »
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