ReAmiga 1200 build CIA problem

[Castellen]

Doesn't make a lot of sense, that fault is usually caused by a missing 50Hz VSYNC signal or defective CIA as you say.  From memory, it won't be generating any interrupts in that state, so no activity on INT2 is probably normal at that point.  You should see one strobe on INT2 when you run the interrupt tests however.

If you look at ODD_CIA (the enable line) while the CIA tests are in progress, do you see that strobing low normally?  You should see a lot of short duration active low pulses around 1uS in duration.

Do you see valid bus activity on *all* data and address lines at U7?  Should be there all of the time regardless of what else is happening.

Do the I/O tests relating to that CIA work?  i.e. Does the keyboard work and is the parallel port test successful?

[Castellen]

Those measurements mostly look OK, and while it still sounds like a CIA fault, swapping them and not fixing it doesn't add up.  I can't remember exactly what INT2 does with the DiagROM interrupt test function.  Will have a look when I get a spare moment to set up a test machine.  Or you could do the same thing if you have another machine about, it'll work exactly the same on an A500 for example.

[Castellen]

He said that the INT2 behaviour is not as designed on the latest build of DiagRom, ie there's a bug that needs fixing as I understand it. There's no INT2 changes so that I can't measure them checks out. But this shouldn't affect the test itself though.

I've just had a quick look at DiagROM v1.3 on an A500 and I'd agree that the interrupt test doesn't appear to be working correctly.  INT2 and INT6 stay high all of the time despite the tests showing as passed, which is consistent with what you're seeing.  And I'd agree this won't affect what you're seeing with the timer issue anyway, the timer tests use polled results and don't rely on interrupts.

The most likely explanation at this point is still a bad CIA.  See how things go with a known good 8520.

[Castellen]

Regarding the CIA timer, that's a hardware timer which different software might use.  I don't know exactly what software uses it specifically, your mileage might vary without this timer working as it should.

Regarding the dark video issue, it's likely to be something affecting the video DAC as you've guessed.  I'd start with checking the DC voltages on U30 pins 17, 18, 19.  I've scanned some of my service notes, attached, which includes references from a normally working machine.  Also check you have the impedance matching resistors (25 and 50 Ohm ones) correctly placed on the three RGB outputs.

Failing that, compare voltages and levels with a normally working machine and you should be able to track it down fairly quickly.

[Castellen]

Update on the CIA problem. I've received the sockets and swapped the CIAs and indeed the problem moves

OK, that explains the confusion then.  When you swapped the two 8520s last time, you didn't actually swap them at all, hence the fault remain unchanged.


The RGB test on that monitor looks too dark.  I'd suspect it's something to do with the specific video DAC you've sourced.  I'm not overly familiar with them as they don't usually give a lot of trouble.  But from a quick skim read of the VP101 datasheet, you might be able to fix the brightness issue by changing the scale on the output current.  i.e. Modify the value of R217.  Perhaps change 470 Ohms to 560 Ohms for a start and see what difference that makes.

Or temporarily replace R217 with a 1k potentiometer.  Vary it to see if you can find the optimum value.   After you've found that, remove the potentiometer and measure its value.  You can use a mixture of two different value resistors in parallel to get close enough to the 'perfect' value you need for R217.  If you don't know how to calculate parallel resistances (it's easy enough), there are probably online calculators for this.

I've not spent long looking at it, but see what you make of the VP101 operation:
http://lvd.nedopc.com/Amiga/VideoDACS/vp101.pdf

You'll note that the Vref output (pin 18) is from the internal 1.2V reference, it'll just need the external capacitor connected.  If the DC voltage on yours is much different from 1.2V, it might suggest something wrong with the VP101.  The A1200 main board has the option to supply Vref externally (R215, D215A), though I expect you won't want those fitted.  Maybe there are some variants of VP101 that need Vref supplied externally?  I don't know, you'd need to work it out from the datasheet.

[Castellen]

Image looks too dark on DiagRomRGBBarsTest.jpg?

Yes, compare the colour gradient with the attached photos from my 1084S test monitor, which shows the normal/expected brightness of the pattern.

[Castellen]

Here's the other photo showing the DiagROM test pattern (was over the attachment size limit), ignore the minor glitches in the vertical bars, my test A500 isn't perfect.  The point is that this is the expected image intensity, which is better shown in the gradient bars in my previous post.