Amiga 2000 video problem

[Pat the Cat]

Put the black probe on part of the Amiga you know is at ground, 0V.  Then put the red probe to the fuse. While the equipment is turned on,  which is why some probes have clips, so you can turn on from a distance.

If no voltage is detected, that means the fuse is blown. Or part of the  circuit behind the fuse has blown - unlikely, nearly always the fuse is sacrificed first. If you measure the resistance across a  fuse, disconnected (unsoldered in this case) then a good fuse has 0 Ohm  resitance, or close to. A blown fuse has near infinite resistance.  

I must admit, I had no luck ever getting an A2000 to work with an A520  in the UK. I suspect there is a subtle difference between A2000 and A500  video outputs on PAL Amigas. Then again, I had no schematics to fault  find with. I was very much working in the dark.

I'll dig out the Hardware Reference Manual   and see if that has any light on the issue.

EDIT:  Only difference is that A500s don't have a -12V rail, they have a -5V  rail on video pin 21. So yes, does look like the +12V supply isn't  getting to the modulator (I presume is is needed, as other people have  reported changing that fuse has let them use PAL A2000s with PAL A520s).

The  reason the fuse is there is to protect the power supply from blowing up  when the +12 rail gets shorted out. Very easy to do when fumbling with a  big D connector round the back of an Amiga. Replacing it is no big deal  if you have the skills, if not, see if you have a local Makerspace or  Hackspace to help out.

[Pat the Cat]

The fuse for that device is good. the 12V is being interrupted else where.

No. If it's showing 0V (is measuring voltage) that means that no volts are getting through the fuse.

If a fuse is showing 0 Ohms (is measuring resistance) that means the fuse is good, in that it has no resistance, and should pass current when connected into a circuit.

It is very easy to have a meter set to measure the wrong thing that you are looking for.

[Pat the Cat]

EDIT: I think I figured this out actually... would it make sense that the NTSC models have 3 and 4 as the switch positions (to equate to channel 3 and 4 in the US) and the PAL models have the L and H instead?

Dumb question... is there a way to tell if an A520 is PAL or NTSC other than hooking it up and seeing what you get?  Come to think of it, how do you tell which default an Amiga is?

The two variants are almost identical, externally and internally.

Here's an NTSC one (pretty sure it's NTSC, from Brazil). Notice lack of any German markings on the back (Germany was CBMs first choice for making PAL Amigas).

http://produto.mercadolivre.com.br/MLB-818535956-amiga-a500-a520-modulador-rgb-to-rf-ou-video-para-amiga-500-_JM

This one is more likely PAL;-

http://scacom.bplaced.net/Collection/520/520.php

So it's not easy to tell which is which without plugging it in.

As for the mode an Amiga is currently running in, you look at the screen. PAL Amigas have more horizontal lines - NTSC have fewer. So if the image looks "stretched" or "compressed", it's running in the wrong mode for that application.

Generally speaking, a lot of Amiga games prefer NTSC because they can run a little faster (on a real Amiga), wheraas a PAL desktop gives you a bigger canvas to work on. Not always true, some games insist on PAL, and some applications insist on NTSC.

If you can get a page of text displayed, an NTSC system will have 200 or 400 / 8 = 26 or 52 lines of text, A PAL system can display 256 / 512 lines of text = 32 or 64 lines of text. Assuming no overscan, just a straight window with a border on your display. The larger sizes are interlaced, and will tend to shimmer like crazy on most displays (without a flicker fixer fitted).

[Pat the Cat]

Yes, notice the second one has no FCC ID#, not a US product at all.

Must admit, having seen plenty of Amiga kit with FCC numbers, I never noticed the lack of an FCC on on PAL A520s. Mind you, I almost never examined a real one... I guess I was spoilt for choice of genlocks and monitors at the time.

Well spotted.