After I finally got everything up and running from my last post, I figured I would try out a game of Lemmings. It loaded up great but the sound was completely awful. So, time to take it all apart again…
When I ran Amiga Test Kit I really should have run an audio test. All the higher pitched tones were either missing or completely flat and the rest just sounded “wrong”, it was also rather quiet.
The first thing to check is the audio amp and filter. The unique thing about this is that it requires a -5v to generate some of the waveform. Guess what? -5v wasn’t there, instead it was +0.75v. That would probably explain a few things.
Tracing it back the power supply was at fault here, and that may explain the audio chirp I heard every time the system was powered on.
The Amiga 1000’s power supply basically generates a rectified 8v which passes into an 7805 regulator which is wired backwards to generate a -5v (I’m guessing 7905 were more expensive?). It was kind of a pain to do because I had to completely dismantle the entire Amiga, including removing the motherboard, to get the power supply out again. But the replacement was quite easy. This one didn’t have any thermal paste for its heat sink, but I added some to the replacement, just to get a better heat dissipation. Not that it should get too hot as the power usage/dissipation for this regulator should be quite low.
So, hopefully for the last time, I tested it and put the whole thing back together again.
The audio sounds correct now!
I’m trying to get my hands on some replacement rubber feet, as they are all missing, and a custom made cover to replace the missing front plate. Then this Amiga will be complete.
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You know what i'd love?
If you made YouTube videos that showed how you found out about the -5 Voltage requirements for the audio and then how you traced it and then went about repairing it.
Now "that" would make for not only for an educational but also "super geek" food video on YouTube for us nerds and fans!
LOL! Maybe one day, apart from my phone I don't really have the equipment to record proper audio/video.
Phone would be fine :-) I love the content you do put up. I check weekly ;-)
It’s fairly obvious when you look at the schematics that the audio stage opamp uses a negative power supply in addition to the positive one. This is fairly common, although usually the voltages are symmetric !
It’s a bit of an odd design choice that they specified -5V rather than -12V as the later Amigas did… particularly because these voltages are also used by the RS-232 serial driver chips. +/-12V was the proper standard for RS-232 so I’m not sure why they decided to use +12/-5 !
I did have to double check I was reading the specifications right for the voltages for that exact reason.
Yeah that threw me at first.. i was like "Huh, -5V not -12V? Weird"
-5V was common in the days before memory and such had VBB generation though.