Monday, 5 November 2012

It's been a long time since January, but I have been busy since then on many things. They just never made it on to this blog :-)

The Oric Atmos, a lovely little beastie this one.  I bought this from a friend after he inadvertently bought two on eBay by mistake.  This baby was a little sick as video output was not well and audio was very very quiet! I was able to track this down to a faulty LM386N OP-Amp.  A simple process of buying some more and replacing said chip was required.

That's the LM386N right in the centre of the picture, this is the old dead chip.  The replacement looks pretty much the same so no need for another picture.  This cured the low volume problem although anyone who has owned an Oric will testify that the output is very loud and I decided that a headphone socket would be quite a nice idea.

There were further issues, as the Oric kept going off. This was simply a knackered power socket and I took the opportunity here to not only replace the socket with a more sturdy one but it would mean I could use a much better switch-mode supply instead of a wasteful transformer.  In the meantime an RGB SCART cable had arrived and this needed a good regulated supply in order to work correctly!  If you should buy one of these cables on eBay be very careful to use it exactly as stated because in order for the TV to get the correct blanking signal on Pin 16, it needs the power supply fed into the cable, then into the Oric or it won't work!

This is the old knackered power socket!

 To the rear of this picture you can see the headphone socket I installed into the case and to the right is a modification of my own.  The Oric comes with a rather annoying warm-reset button in the base of the case which can only be got at with a biro or some such probe.  Also in order to do a cold-boot requires unplugging the power supply then reconnecting it.  Annoying when trying out different cassette games and such.  The switch I used is quite heavy duty and its a double throw momentary type, wired directly to the original reset and secondly wired to the reset pin on the 6502 Microprocessor.  Why they never used this I have no idea, but I guess they had their reasons.  Now in the other Oric my friend had bought was a version of the cold-boot reset switch I was building.  However it was wired directly to ground from  a rather large capacitor which was keeping the reset pin of the CPU high.  What I have done instead is connect the pin to ground via a 100k resistor thereby pulling the voltage down enough to reset the CPU but gone is the horrid noise from dumping 5 volts directly to ground!  I have seen a lot of resets done that way, for example in the Sinclair Spectrum.  A resistor costs less than a penny, so why not just use one in line with ground from the cap? 

I found a few problems with the keyboard, a number of the keys were not working at all, not even intermittently.  I suspected many things one of which was the ribbon type cable that cures around the mainboard from the keyboard.  You do need to be careful as it can come away easily from the soldered points and you'll have to re-solder them back into place.  It was while checking this out I noticed that there were a number of cold solder joints on the back of the keyboard..That is to say joints that have no solder on them or have come away from fatigue.
If you look closely here you can see in the middle of the picture a joint that has no solder at all.  The previous owner must have been cursing the keyboard for years due to this manufacturing fault. All it needed was to be soldered and the keyboard worked fine.

Here I have a Windows Wav file of "frog hop" for the Oric 1. It works nicely on the Oric Atmos, loading through the cassette port. This can be a little hit and miss due to earthing problems and sound level but will load eventually.  Transferring sound files to an iPod works much better as that runs of it's own internal battery and cured my loading problems.

For fun I added a tiny blue LED to the corner of the Atmos, they are designed to work at around 3 volts not the 5 volts coming from the Regulator. The little LED fits perfectly into one of the small holes top left of the Atmos and is hot glued in place behind the keyboard.  The idea was to compliment the Oric and not add some big "fugly" lights and swtiches.. I wanted it to look as factory spec as possible.