Falcon 700015003 Rev B

[Steve]

@jwd I probably should have mentioned before you did the re-cap, but don't forget there are 2x capacitors you need to install in the opposite orientation to factory/stock.

You wouldn't have noticed with the originals since they don't bulge. But new caps will start to bulge/malfunction after some usage unless you swap their polarity.

This:
https://exxosforum.co.uk/forum/viewtopi ... =17&t=5989
And This:
https://exxosforum.co.uk/forum/viewtopi ... =17&t=1698

[Rustynutt]

@jwd I probably should have mentioned before you did the re-cap, but don't forget there are 2x capacitors you need to install in the opposite orientation to factory/stock.

You wouldn't have noticed with the originals since they don't bulge. But new caps will start to bulge/malfunction after some usage unless you swap their polarity.

This:
https://exxosforum.co.uk/forum/viewtopi ... =17&t=5989
And This:
https://exxosforum.co.uk/forum/viewtopi ... =17&t=1698

Steve, Exxos or anyone else notice the power on the KB ACIA is opposite the IC spec? I draw power from it to power a 500KHz oscillator and noticed it.

[Steve]

Are you saying the VIN and GND are opposite from the datasheet to the actual chip on the board? I guess, even if that was the case, it's nothing for any Falcon owner to worry about.

[Rustynutt]

Are you saying the VIN and GND are opposite from the datasheet to the actual chip on the board? I guess, even if that was the case, it's nothing for any Falcon owner to worry about.

Think that's what I'm saying
I'm a transmission mechanic, so look at everything as positive and ground
But know what you mean.
I socketed the ACIA's on this one Falcon because in the past, pins were clipped from the main board and used by the Nemesis (and Power Up 2) to add a switch function. Apparently they can be programmed to indicate HI/LOW, or pull voltage on those pins HI/LO (whatever term works here), and the speeders made use of that. The other "lifted" pins were to change where the 500KHz was picked up from, and feeding it via the Nemesis/Phantom.
That was the reason I'd socketed them in the first place, instead of just soldering new IC's direct to the board.

I needed a +5Vcc to power a 500KHz oscillator to feed to the ACIA after speeding up the bus. The Nemesis supplied that signal before.
That's when I discovered it. Not just the KB ACIA, but the MIDI as well.
So yes, the data sheet for the IC, and polarity on the board is different. But they obviously work.
It's just if following the data sheet for power (as I'm doing), polarity is flip flopped. 5Vcc os picked up off the pin listed on the datasheet at ground.
How you managed to say that with so few words amazes me

[HomeLate]

@jwd I probably should have mentioned before you did the re-cap, but don't forget there are 2x capacitors you need to install in the opposite orientation to factory/stock.

You wouldn't have noticed with the originals since they don't bulge. But new caps will start to bulge/malfunction after some usage unless you swap their polarity.

This:
https://exxosforum.co.uk/forum/viewtopi ... =17&t=5989
And This:
https://exxosforum.co.uk/forum/viewtopi ... =17&t=1698

I recapped mine some time ago and I replaced C6 in the same direction as the stock one. I measured yesterday and I got a positive reading So I'm not sure if this applies to all revisions. I'll check the revision of my Falcon later today.

[mikro]

Nice, one of the last Atari-produced PCBs.

[jwd]

@jwd I probably should have mentioned before you did the re-cap, but don't forget there are 2x capacitors you need to install in the opposite orientation to factory/stock.

You wouldn't have noticed with the originals since they don't bulge. But new caps will start to bulge/malfunction after some usage unless you swap their polarity.

This:
https://exxosforum.co.uk/forum/viewtopi ... =17&t=5989
And This:
https://exxosforum.co.uk/forum/viewtopi ... =17&t=1698

Yes thanks, i saw the need for C24 which i have replaced in the way round in the post but measuring the voltages
(nothing connected to either video port or RF) i get ~260mV across C24 with positive nearest outside of board. So not only different voltage to post but different polarity too :

20230825_140112.jpg (277.62 KiB) Viewed 1391 times

Could this be due to later revision of board, so maybe this revision had it the right way round originally?

Hadn't seen about C6 before have measured mine and it is indeed -12v on the positive marked leg of the cap, will swap - thanks for the pointer.

[Steve]

@jwd I think it might be worth plugging a TV into the composite out. Perhaps it doesn't show it's real polarity unless it's got a load.

[jwd]

@jwd I think it might be worth plugging a TV into the composite out. Perhaps it doesn't show it's real polarity unless it's got a load.

Good point @Steve thanks

Found my old Toshiba CRT TV in the loft (still works ) got a picture of the desktop up fine. Measured across C24 again and similar voltage (177mV) with still opposite polarity to this post :

https://exxosforum.co.uk/forum/viewtopi ... =17&t=1698

Unless anyone has any other explanation it seems this last Atari revision of PCB must have fixed the C24 being the wrong way round and it should be as the PCB is marked/installed at factory. I will put it back the original way round and re-test just to confirm.

weird

[Steve]

That is interesting to know I wonder why the voltage you get is so low though. Exxos measurement was 3.5v wasn't it?