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View Full Version : A4000T PSU exchange - could issues with a CV64/3D SD be related to missing -5V?



skyfish
31st March 2015, 20:28
Hello,

Here's a question that might be trivial or not, but I was unable to find any tangible information on the topic on the net... perhaps some of you recognize this?

Some days ago I thought I'd replace the AT PSU in an A4000T with a modern ATX unit before the old breaks down. Currently in the Amiga is a Cyberstorm PPC, Cybervision 64/3D with a scandoubler module and a X-surf 100 with the USB addon.

After having replaced the PSU including adding an adapter cable to convert the ATX connector to the AT P8/P9 + switch, all seemed well and I booted the computer up.

Everything works - except no picture coming out of the scandoubler. It seems to send a signal to the monitor, and the monitor does react when the screen mode changes are to occur - but the image is black. The picture directly from the CV64/3d is fine. I tested using the old PSU again and then the scandoubler works as it should, so the scandoubler itself seems fine.

After some quick checks, I tend to now believe the issue with the scandoubler is that it is using the -5V line coming out of the AT PSU of the A4000T and fed directly to the slots (in the A4000D, the -5V is instead generated on board (...?)). The -5V output is missing in modern ATX PSUs.

My questions on this matter would be:
- Is it a plausable explanation for the scandoubler not working with the new ATX PSU (missing -5V)?
- What devices actually use the -5V? It is available in ISA slots, zorro slots as well as video slots, however at least the rest of the equipment (Z3) installed seem fine without it.
- What would be the easiest way to add the -5V? Using a 15W 12V-5V DC/DC in combination with -12V?

Best Regards,
Chris

hooverphonique
31st March 2015, 20:43
normally, no "pure" digital logic accessory would use -5V, but it's perfectly possible that the analog section of the scandoubler could do so...

I would just whack a 7905 in there and generate -5 from -12... As you mention yourself, that's what the 4000D does, and if in doubt, you can use its schematic to see how to hook it up.

skyfish
31st March 2015, 21:10
Thanks! Makes sense, I'll see if I can give it a try - the schematics looks a bit complex for the task (diode, resistors, capacitors), will have to check whats really needed and give it a whirl

hooverphonique
31st March 2015, 22:24
It's not that complicated, and the capacitor values dont' have to be spot on. the 10ohm/2W resistor (2x20hm in parallel) is there to protect -12V in case something goes wrong with -5V. The diode you can disregard.

page 4 of this (http://www.amigawiki.org/dnl/schematics/A4000_Rb.pdf)used as reference.

skyfish
20th April 2015, 22:04
Just in the remote case someone would be in a similar situation (A4000T, replaced PSU with modern ATX variant, with addon cards that actually use -5V)...

While looking into this, -5V is used for Zorro slots, ISA slots, Video slots and the RGB port on the AV module. Using the RGB->VGA adapter to display normal AGA video without the -5V had no issues for me though. In terms of Zorro/Video slot cards, out of what I tested only the Cybervision 64/3d scandoubler had issues when -5V was missing.

I modified an ATX extension cable to derive the -5VDC from the -12VDC (using identical circuitry as found in the A4000D MB) instead of taking it from the directly from the PSU (since -5VDC isn't available for any remotely modern ATX PSU).

The solution works great, and the Cybervision 64/3d with scandoubler display nicely, while using the new PSU.

Thanks for idea and inspiration, hooverphonique!

hooverphonique
21st April 2015, 08:34
you're welcome, glad to hear it worked out :)