Another amiga 500+ black screen (well another two)

[arkleyjoe]

Hi,

 

I have a couple of faulty A500+ Rev 8A boards both with battery damage. One is a total mess the other had 2 /3 broken traces which I have fixed, but I need some advice as what to try next.

 

What I have done so far:

• On both boards, I have tested and fixed any continuity issues, focussing on the usual area around Gary,
• Replaced Gary's socket.
• replaced the 74LS244 in U12
• Swap tested all of the ICs with known good chips / board.
• Tested clocks / power.

 

So this is the current state of play with each board.

 

Board 1 (the lightly damaged board):

I get black screen and the power light stays dim.

There is a sync signal to the monitor albeit no pic.

I have tested with a diagrom. No difference still black screen.

I have scoped around and there does not seem to be any activity on the address lines.

Reset lines on CPU / Gary seem to behave normally low for a moment and then stays high.

 

I am not sure what to try next on board 1?

 

Additionally, Agnus is welded in to its socket on this board. Later I plan to replace the socket with a better one but can not get it out. For now I plan to leave it alone unless it is likely / potentially part of the fault?

 

 

 

Board 2 (the horrendous board that now has so many botch wires it looks like a breadboard project :

This board amazingly is getting to a green screen which I know indicates a ram problem.

I tried piggy backing the ram but this did not change anything.

 

My question here is simply I am not sure if I should start desoldering the ram OR is it more likely to be the other 74 logic chips near the damage?

 

 

Joe

 

 

 

[Daedalus2097]

For board 1, if all the traces are 100%, the next thing to look at could indeed be the Agnus chip. If you have issues with the 74 logic in the RAM bus, you'll normally get a green screen, but this sounds like the CPU's not starting. You don't even get a very short burst of bus activity, no? I would check the ROM socket, including the alignment (the ROM socket should have an empty row at the pin 1 end. What is the halt line doing? That should be similar to the reset lines, briefly going low before staying high. If it stays low, the CPU won't try to run. IIRC, Gary also connects to the halt line, so double-check continuity all the way across. If halt, reset and CPU clocks are all ok, you should be getting bus activity as it tries to read the ROM.

 

I've found that some Agnus chips are stuck into their sockets with a double-sided foam pad. Why that is, and others aren't, I've no idea. But I've been able to remove them by pushing two chopsticks through the holes on the underside of the board until it eventually works loose. Not for the faint-of-heart though! Corrosion can and does reach the Agnus socket to cause problems, so it needs to be checked.

 

As for board 2, if all the traces are ok, I'd sooner suspect the 74 logic than the RAM chips. They can get damaged internally by corrosion that creeps up their legs, even if they're cleaned up. In particular, U10-13, 32, 34 & 35 can all be affected by leakage. Also, I once had one that had a damaged trace right by one of the legs of one of the U10-13 chips. It was such a fine fault that it wasn't visible, and pushing the multimeter probe down onto the chip's pin was enough to close the break and make it look like the continuity was fine. Had me stumped for a while until I happened to put pressure on the pin a different way and found an unreliable connection.

 

DiagROM should be able to help you pin down the bad bits on the bus, which you can then check with a scope while the memory test runs to get an idea of where the issue is.

[arkleyjoe]

Hi Daedalus2097, thank you very much for your help. Sorry for the slow reply.

 

So in reply to your specific points:

 

You don't even get a very short burst of bus activity, no?:

No nothing at all that I can see.

 

I would check the ROM socket, including the alignment (the ROM socket should have an empty row at the pin 1 end. :

Rom socket and alignment it in seems fine. Tested the rom in another machine, all good.

 

What is the halt line doing?

My halt line is stuck low. Not even a twitch. Continuity is fine between Gary and the CPU on the halt line. What is the IIRC?? I am pretty sure if I can work out the cause of the low halt line it will point to my cause. Any tips on causes of that?

 

My Diagrom does nothing but I guess that it is not even attempting to read that?

[Daedalus2097]

Heh, IIRC stands for "If I Remember Correctly", so it's not a part But with absolutely nothing happening on the data or address lines, the DiagROM isn't going to run. Halt being stuck low could be a pull-up issue on the board, a fault with either Gary or the CPU, or a fault detected by Gary or the CPU that is so early the bus activity is stopped before it starts. With the power off, check the resistance between Halt and ground, and between Halt and +5V. It should be around 4.7K ohms in both cases. A little different from that won't matter, but if it's below 1K or above 10K on either side, that could cause an issue. Too low on the ground side or too high on the 5V side will cause Halt to stay low, preventing the CPU from starting.

 

The pull-up resistors are a resistor pack that's near Gary I think, and could be affected by corrosion, a broken track in the area, or could simply have failed.

 

If the pull-ups are ok and you try to boot with Gary removed and the Halt line stays constantly low, then the CPU is potentially the issue, though if you've tested it in a known good machine then that shouldn't be the issue.

[arkleyjoe]

Hey, IIRC lol I have been scanning schematics for that

 

This one is turning in to a real puzzle. So I did have some broken traces around that resistor pack near Gary and I have repaired and thoroughly tested again all of the traces right though to their final destination. Gary, CPU and ROM have been tested in a known good machine.

 

I get close as damn it 4.7k ohms between halt and ground / 5v. That resistor pack did have a little corrosion around it but I suppose if I am getting 4.7k ohms on Halt it is functioning correctly?

 

I have also reflowed the CPU socket just incase there is an issue there and still no joy

[Keatah]

Double-sided tape/foam is used to hold the chip in place. Some sockets are designed to grip the chip differently. Some will "press" it in further, others will press it out, under vibration. All depends on where the curves of the chip pins and socket pins interact. Just think of pushing two round objects together, they'll tend to slip one way or the other.

 

Quality sockets will have a metal strap that goes across the chip. But that costs about 15 cents more. So you won't see that often.

[arkleyjoe]

Double-sided tape/foam is used to hold the chip in place. Some sockets are designed to grip the chip differently. Some will "press" it in further, others will press it out, under vibration. All depends on where the curves of the chip pins and socket pins interact. Just think of pushing two round objects together, they'll tend to slip one way or the other.

 

Quality sockets will have a metal strap that goes across the chip. But that costs about 15 cents more. So you won't see that often.

 

Hey, I have since managed to remove the agnus and I have replaced the socket with a new better quality one.

[arkleyjoe]

Heh, IIRC stands for "If I Remember Correctly", so it's not a part But with absolutely nothing happening on the data or address lines, the DiagROM isn't going to run. Halt being stuck low could be a pull-up issue on the board, a fault with either Gary or the CPU, or a fault detected by Gary or the CPU that is so early the bus activity is stopped before it starts. With the power off, check the resistance between Halt and ground, and between Halt and +5V. It should be around 4.7K ohms in both cases. A little different from that won't matter, but if it's below 1K or above 10K on either side, that could cause an issue. Too low on the ground side or too high on the 5V side will cause Halt to stay low, preventing the CPU from starting.

 

The pull-up resistors are a resistor pack that's near Gary I think, and could be affected by corrosion, a broken track in the area, or could simply have failed.

 

If the pull-ups are ok and you try to boot with Gary removed and the Halt line stays constantly low, then the CPU is potentially the issue, though if you've tested it in a known good machine then that shouldn't be the issue.

 

Reference: try to boot with Gary removed and the Halt line stays constantly low, then the CPU is potentially the issue.

 

If I boot with Gary removed the HALT stays low. However halt also stays low in my known good machine with Gary removed.

[Daedalus2097]

Yeah, if you can measure roughly 4.7K (it will never be *exactly* 4.7K, just needs to be the same ballpark) between the Halt pins on the CPU or Gary and 5V, then the resistor pack is fine.

 

Hmmm, there should be a brief blip where the Halt goes high, like the Reset line. Can you see the blip as it goes high on the known good machine? You've tested the CPU elsewhere too I take it?

[arkleyjoe]

@Daedalus2097, I am pretty sure there is no brief period where it is high. I see nothing more than a tiny flicker on the line but more like low level interference rather than activity by design.

here are the are some behaviours of the halt line in different scenarios.

1) gary / cpu in place = halt low.

2) gary out / cpu in = halt low.

3) gary out / cpu out = halt high.

4) gary in / cpu out = halt high.

So I think this rules out a short on the halt line between Gary and CPU??

But both chips are known good. So as @Daedalus2097.suggested previously maybe the CPU is halting because it finds a fault elsewhere?

[arkleyjoe]

So some progress. I have removed and socketed the 74LS logic chips near the battery. I do not have replacements on hand, have ordered them today. However if I remove u13 the 74LS373 the halt line comes up and I see activity on the other lines.

So my hope is that when I replace this it will be ok, or alternatively it will be a problem with the ram.