My NTSC A500 Plus journal

[TjLaZer]

So after reading and seeing how rare the Amiga 500 + is in NTSC form, I was always curious and on the lookout for one. I even took the plunge and got a PAL one from Airey36 on eBay this summer as I wanted to mess with one.

Well I just found one on eBay that was NTSC so I grabbed it. The seller had two, one with severe but cleaned up Battery leakage, and the other, with none. Both were tested working and showing a screen shot of ShowConfig on the auction. I decided to get the non leaked one for a reasonable "USA" price.

Board arrived and it looked great. There seems to be no battery leakage on it, battery was snipped before it was too late.

But I was NOT happy so see that it had 3 broken pins on the second joystick port! I guess I can solder in a new DB9 connector, shouldn't be too hard... part ordered.

Fired it up, and the Kickstart 3.1 insert WB screen came right up!

Next, tried to boot my WB 3.1 disk. It seemed to struggle booting it and crashes right before WB would show up (black screen) and reboots. I got a GURU 8100 00FF.

Turned off pressed down on all socketted chips and tried again. Same thing. Tried a few more power cycles, was getting some GURUs on power on (like it sometimes does on other Amigas when powering on too soon?)

Booted some other disks, seemed to work ok.

Tried WB disk again, ok finally booted.

Then randomly it would crash to back screen and reboot to GURU.

Well over the course of the night and next day, it seems like its getting better and more stable but still crashes.

I hooked up a slingshot with A2091 and it seems to be working ok, left it on overnight with DoPus 4 running and when I woke up it had rebooted.

What can I try next?

---------- Post added at 10:14 ---------- Previous post was at 09:59 ----------

Next I ordered an A500+ top and bottom case from airey36 on eBay. Since these are truely rare as hens teeth in North America!

Now to find a keyboard and floppy drive for it.

 

[quarkx]

@Tjlazer
I have been looking for years also, but I gave up hope of ever finding one
I even went to the extreme to believe that even though it was said that NTSC plus was made, there was never any proof in the mater.
Now, you got one. Dare I ask if there is any indication on the motherboard that this is a genuine NTSC machine? I am not doubting you of course, I am just skeptical when stuff like this appears.
I am wondering if the seller knew this, and was trying to pull a fast one and convert a PAL unit to NTSC, and that is why the system is so unstable.

 

[TjLaZer]

OK how can I tell if this is an original NTSC board? The agnus on it is 8375. Part #318069-11

---------- Post added at 16:23 ---------- Previous post was at 15:29 ----------



Update, swapped the ROM for a 2.04 and no more crashes from what I can see!





---------- Post added at 17:08 ---------- Previous post was at 16:23 ----------

I noticed a slight buzz coming from the speakers, what can cause this? Sound seems to be ok.

 

[quarkx]

I was wondering if there was anything silkscreened into the motherboard. Remember, the Agnus can be swapped out very easily.
It looks like there is a cap missing right behind the battery under the Audio filter stuff. this could be causing the buzz, but its hard to tell in the photo.

 

[TjLaZer]

I think that's normal.

 

[roy_bates]

is it a ntsc model?

 

[BLTCON0]

@roy
Do the terms PAL and NTSC wholly apply to the A500/+ ? Supported video out is either RGB or monochrome composite and AFAIK PAL differentiates itself from NTSC in the colour and audio encoding areas, which is irrelevant on these systems. I think it should be more accurate to talk of PAL-timed or NTSC-timed machines, if we are to somehow incorporate these terms.
Which implies that swapping the Agnus and the oscillator for their "NTSC" equivalents is all that's needed to get a "genuine" "NTSC" (NTSC-timed) A500/+ board.
Other than that, the only measurable "genuine-ness" factor I can see is some sort of proof that certain serial numbers were meant for the North American / Japan / other NTSC-using areas.

@quarkx
As long as there are no special PAL markings on A500s, I don't think there would be any NTSC ones as well :smile:
On a side note, there's a white checkmarks sticker on the motherboard which seems to read "ICT", wasn't ICT the service contractor for Commodore in the UK sometime around the early '90s?

 

[rkauer]

Not quite right: the NTSC machines have a 28.63636MHz crystal instead the 28.37516MHz PAL units have.

Difference in behaviour is minimal, but when hooking up a A520 modulator the difference will be noted as instability in the video output.

 

[BLTCON0]

Not quite right: the NTSC machines have a 28.63636MHz crystal instead the 28.37516MHz PAL units have.

Of course.

...swapping the Agnus and the oscillator for their "NTSC" equivalents...

Difference in behaviour is minimal

So what exactly is this minimal difference?
(Of course we are talking about the difference in behaviour between a, say, "native" NTSC A500+ board, and a "PAL" A500+ board which has had its Agnus and oscillator swapped).

but when hooking up a A520 modulator the difference will be noted as instability in the video output.

But that's external hardware on which both colour and audio encoding occur according to the specific (PAL or NTSC) format, and of course there ARE differences there. I was pretty clear about the bare A500+ board having nothing to do with these factors:

Supported video out is either RGB or monochrome composite and AFAIK PAL differentiates itself from NTSC in the colour and audio encoding areas

As this kind of encoding doesn't happen anywhere in the standard motherboard, the terms PAL and NTSC can't be considered to apply in their whole sense. I trust that if one plugs an NTSC A520 modulator in a "PAL-to-NTSC" converted A500+ board, video/audio will be fine.

 

[TjLaZer]

Not quite right: the NTSC machines have a 28.63636MHz crystal instead the 28.37516MHz PAL units have.

Difference in behaviour is minimal, but when hooking up a A520 modulator the difference will be noted as instability in the video output.

Aha! Mystery solved!

I have a 28.37516MHz Crystal! icard

@roy
Do the terms PAL and NTSC wholly apply to the A500/+ ? Supported video out is either RGB or monochrome composite and AFAIK PAL differentiates itself from NTSC in the colour and audio encoding areas, which is irrelevant on these systems. I think it should be more accurate to talk of PAL-timed or NTSC-timed machines, if we are to somehow incorporate these terms.
Which implies that swapping the Agnus and the oscillator for their "NTSC" equivalents is all that's needed to get a "genuine" "NTSC" (NTSC-timed) A500/+ board.

So would just swapping for the NTSC oscillator make this a true NTSC unit? No other mods?

Seems to function like a NTSC unit though so I am happy. I just have to unsolder the 2nd joystick port and install another one. UGH.

 

[DutchinUSA]

Zoomed in on the other one he has listed on Ebay and it looks like that one also has the 28.37516MHz Crystal .. in that description it also says "This is RARE NTSC system"

 

[roy_bates]

@roy
Do the terms PAL and NTSC wholly apply to the A500/+ ? Supported video out is either RGB or monochrome composite and AFAIK PAL differentiates itself from NTSC in the colour and audio encoding areas, which is irrelevant on these systems. I think it should be more accurate to talk of PAL-timed or NTSC-timed machines, if we are to somehow incorporate these terms.
Which implies that swapping the Agnus and the oscillator for their "NTSC" equivalents is all that's needed to get a "genuine" "NTSC" (NTSC-timed) A500/+ board.
Other than that, the only measurable "genuine-ness" factor I can see is some sort of proof that certain serial numbers were meant for the North American / Japan / other NTSC-using areas.





yes,thats why i was asking because it looks to be pal timing by the oscilator.




but does it show as ntsc when first turned on through a tv,my set can tell if its pal or ntsc.as far as i know pal and ntsc are both colour units as long as the set supports both.

 

[quarkx]

Thats why I ask, because it's too easy to swap out the Agnus and say it is a "Rare" NTSC model, if you are a "shady" seller on Ebay, especially, when you can get PAL motherboards form Airy36.
There is no "KNOWN" 100% verified NTSC one floating around for it to be compared too.
As far as I can tell, the Canadian Assembly plant only pumped out Rev.5 -A500's Motherboards for the North American Market. Knowing how cheap Commodore was, I don't believe for one moment that they would have imported Motherboards from the UK and Europain Plants for the North American market, when there virtually was no North American market by then. Mexico- Maybe.
Like I said before, I think maybe you got taken on this,- I hope not, but with the availability of PAL parts, mixed in with greed on ebay, and the hope of getting one of these, the temptation is just to great to scam the market.

 

[roy_bates]

Thats why I ask, because it's too easy to swap out the Agnus and say it is a "Rare" NTSC model, if you are a "shady" seller on Ebay, especially, when you can get PAL motherboards form Airy36.
There is no "KNOWN" 100% verified NTSC one floating around for it to be compared too.
As far as I can tell, the Canadian Assembly plant only pumped out Rev.5 -A500's Motherboards for the North American Market. Knowing how cheap Commodore was, I don't believe for one moment that they would have imported Motherboards from the UK and Europain Plants for the North American market, when there virtually was no North American market by then. Mexico- Maybe.
Like I said before, I think maybe you got taken on this,- I hope not, but with the availability of PAL parts, mixed in with greed on ebay, and the hope of getting one of these, the temptation is just to great to scam the market.

true,very true there are a few opertunists on ebay that cant wait to fleece the public.

with that aside for a bit,wasent there a jumper for pal and ntsc at startup on the motherboard?

 

[quarkx]

I can't see the auction here, because the seller has "Blocked" Canada, but it would be the same if all the sudden NTSC Commodore C116 started to show up, IIRC there were only 4 NTSC "Prototypes" made, even though the unit was actually designed in West Chester.
With all the research I have done, and the fact that not even the Computer Museum has one, and with all the Videos floating around of the Commodore expo's (like Ami-West) and such, NO one has ever come forward showing a NTSC A500+-it kind of sets off all sorts of red flags and alarm bells. I don't think that even Brian Bangnel's book "on the Edge" even mentions an NTSC A500+

 

[BLTCON0]

So would just swapping for the NTSC oscillator make this a true NTSC unit? No other mods?

Yes, because no colour or audio encoding is performed anywhere according to either standard. So all that's left to cater for is the oscillator and the Agnus.
Actually, I cross-checked with the A500+ service manual: There are exactly two components designated as either "PAL" or "NTSC" (guess which ones).
The remaining stuff is identical.
Furthermore, the service manual implies the PAL units are specified as PCB ASSY 312812-01 and the NTSC ones as PCB ASSY 312812-02, but the -01 or -02 suffixes doesn't seem to be etched on the motherboards. So they just represent a configuration coding rather than a true hardware difference on the PCB.

Seems to function like a NTSC unit though so I am happy.

I believe the "NTSC" Agnus will need the "NTSC" oscillator in order to provide accurate timing for the rest of the system, as according to the specs the various clocks (which are provided by Agnus) are sync-ed to the master clock.

In more detail:
According to the H/W manual, each scanline (measured in colour clocks) can be thought of as exactly 227.5 colour clocks (it's very easy to derive why). So some code (for whatever reason, most commonly beam-synchronised tasks) may exploit this fact to "know" that it'll have finished its job in a given number of scanlines.

If you feed 28.37516 MHz into an NTSC Agnus, the math would be:
28375160 / 29.97 = 946785.45 ticks/field, and 946785.45 / 525 = 1803 ticks/scanline, so each scanline would be about 225.42 colour-clocks long (the colour clock runs at 1/8th of the master clock).

So in such a configuration, less clock-time is provided per scanline. Just slightly less, but less nonetheless (pun intended ), which breaks an assumption provided in the H/W specs.

This is no different from earlier rev motherboards equipped with the 8372A Agnus, which is hardware switchable via pin #41. The set mode should match the oscillator for a "true" system.

 

[TjLaZer]

Looks like I need to find a 28.63636Mhz oscillator.

Composite NTSC video.

---------- Post added at 16:17 ---------- Previous post was at 16:12 ----------

Zoomed in on the other one he has listed on Ebay and it looks like that one also has the 28.37516MHz Crystal .. in that description it also says "This is RARE NTSC system"

Yep it looks like I was had. Oh well I'll just suck it up and keep it and not complain. I should of known better, hens teeth ? Lol

 

[rkauer]

Pick one from a dead A500 motherboard you have floating around.

 

[bdb]

Ads1500 on *bay has a full A500+ and a MB (NTSC) listed