Socket to 'em! - Torch Triple X - Part 5

At the end of the last installment I had managed to order a spare DMA controller for not too much from eBay. Well, it arrived and I very carefully installed it in place of the existing chip and...

DMA! DMA! DMA!

...no difference. The unit stops at the blue screen, the 68010 fails to start properly and the error beeps for 'main CPU timeout' still ring out from the monitor after 20 seconds or so.

Darnit.

And it was about this point where my replacement ATX power supply started to make worrying buzzing noises. After unplugging it as quickly as possible (and following a quick change of underwear) I put it to one side and decided to come back to it later. Fortunately, I still had my bench supply and I had received a suggestion to use said supply, current limited to about 2 amps to see if I could work out if there were any shorts on the board.

To cut a long story short, the supply went straight into current limit mode and the voltage across the board was exactly the same in every location at around 2.3v. As an experiment I tried to see what current the board would actually be pulling if the voltage on the 5v rail was actually 5v. It draws over 4amps. That's 20 watts for a relatively small bunch of chips. The technical manual suggests that the supply would be drawing a minimum of 2amps but 4amps is worrying.

That's a lot o' amps..

Next thing to try was the capacitors. There are no electrolytic caps on this board but there are about eight tantalums which could fail as short circuit. Perhaps one had failed and was causing a short to ground and the excessive current draw. But after removing them from the board they all checked OK. So then, in a slight fit of frustration (not the last) I removed EVERY 100nf bypass capacitor and checked them all. And there's eleventy billion of the little buggers. I found a bunch of about 20 or so that came out as 70-80nf instead of 100nf. These are clearly out of spec and I will replace them but they're not so far out as to cause any problems. 

Should be 100nf.

Woah! Should be 100nf..

Crikey! Should be 100nf!

Can you guess what? 
Should be 100nf...

Faulty. The lot of you!

After replacing all eleventy billion of the caps and new ones to replace the faulty ones, the result was...

No difference.

Darnit. Again.

Looking over the schematics, and thinking about the various buses in the system it's obvious that the QBUS is working as we are clearly running code from the EPROM. Based on the 'garbage' that used to appear on the screen, the video RAM also seems to be working correctly as this is where the code is loaded before the startup of the 68010. With this in mind I listed out the chips that touch the PBUS and decided to de-solder them so I could test them away from the board - this also means I could install some sockets.

There were a fair few chips in that list. Some of them I could test using my el-cheapo IC tester (but more on that later) but a big proportion were 74LS244, 245 and 373s. These aren't supported by the tester so I had to think of another way to test them.

Enter the Amiga A500+ motherboard! This uses the 74LS244 and LS373 in the data path for its RAM. They are usually the chips that take the brunt of the leaking battery on the A500+ (excluding Gary - poor Gary), and so the A500+ board I have already has them in sockets. It was a simple case of swapping one chip in at a time and checking that the A500+ board booted and didn't generate a green screen. So after a half hour of chip swapping I ended up with a pile of working 74LS244s and 74LS373s.

Ah, the Amiga. 
(Ignore the sockets - I ran out of 20pin ones...)

Next, the 74LS245s. These were a bit more involved and needed me to build a test circuit on a breadboard. Using 8 LEDs, 8 resistors and a bunch of link wires, it was possible to send 8 bits across the chip in either direction based on the level of the DIR pin and check the results using the LEDs as indicators. The upshot was a small pile of fully working 74LS245s.

We're running out of possible suspects on this board...so in the next fit of frustration I de-soldered pretty much EVERY chip from the board.

I thought I had found a problem on the board when a 74LS148 came out as 'BAD' on the chip tester. There is only one of these on the board and its an '8 to 3 priority encoder' which takes eight inputs and turns it into a 3 bit binary output based on the most significant bit on the 8 bit input. The three outputs connect to the IPL0, IPL1 and IPL2 control lines on the 68010. If I have understood correctly, the combination of these lines determines the type of interrupt received by the CPU. My theory was that the faulty chip was causing a non-maskable interrupt that the CPU could not reconcile with a system device causing the CPU timeout. So I ordered some new 74LS148s.

The first thing I did when they arrived was check them on the chip tester. Every one (there were four) came out as 'BAD'.

Oh, no it isn't!

Darnit.

This was annoying for two main reasons. Firstly, I know that the chances of all of the new chips being faulty is slim. This means that this is another case of the chip tester not being accurate. Secondly, the original 74LS148 is probably not faulty either which blows my (rather good I thought) theory out of the water (and I paid for chips I didn't need).. 

The upside of all this though, is that pretty much every chip now has a socket. This will make it much easier to change chips but does introduce a small liability of poor contacts in the sockets. And the hassle of putting them all back into the board again....

So. Many. Sockets.

So. Many. Chips.

After carefully putting every chip back where it should be (foreshadowing), I tried to start it up again. And this time, it was worse. The garbage was back on screen but was not like the previous type. This garbage scrolled down the screen and stopped. No other sign of life. 

So I embarked on a marathon of chip re-seating. Fortunately, using my epic new schematics, I managed to track the problem down to the timer chip. A firm re-seating was all that was needed. The situation improved in that the garbage was as I'd seen before i.e. a series of straight lines moving down from top to bottom followed by a smaller descending rectangle of 'different' garbage. I wasn't happy though as I wasn't getting the beeps out of the monitor to indicate that the CPU had timed out.

Hmmm.

I knew that the audio came from pin 9 on the service processor so it was time for the oscilloscope. And would you believe it..

Bip, Beep, Beep, Be...

So the sound isn't working now even though the service processor is sending out the correct signal. Not a huge problem as long as I know the service processor is still kicking up a fuss about the 68010 not starting up.

But I was still puzzled why I was getting the garbage on screen. Tracing through the video palette schematics I noticed that two chips U26 and U27 should have been a 74F257 and 74F253 respectively but, somehow, they'd been mixed up and actually had a 74LS257 and 74LS253. Interesting.. I swapped U26 and the amount of garbage was reduced. So I swapped out U27 and the garbage was gone! Yay!

But was the board still generating the expected error?

Oh yes. 

I have a habit of uploading the worlds most ridiculous videos. The first 'blip' is the 'beep' it should make when it's turned on. Then, after about twenty seconds or so, the main error beep starts and is 'short long long long', the code for main CPU timeout.

So that means, after all that effort of desoldering and testing almost every IC on the board, buying some chips I probably didn't need, soldering more sockets than any sane person ever should and carefully re-inserting all those chips, I'm basically back to where I was when I started looking at this.

One thing I haven't considered, and which I really should have by now, is if any of the traces on this board have been damaged either by the leaking battery acid from the power supply, or by my removing the chips from the board. I feel a massive spreadsheet coming on..