With Great Power... - Torch Triple X Part 2

Quick re-cap. The Torch Triple X is uber rare, and has a slightly whacky power supply which allowed for 'soft start' and 'soft shutdown'. Unfortunately, to achieve this, amongst other things, the power supply actually had a battery in it.

Curse you generic battery supplier!

The leaky battery has very seriously damaged the PCB for the supply. In some spots the track has just gone completely and several resistors just disintegrated during cleaning of the board. So I replaced the resistors and the RIFA cap. I also bridged the broken tracks and reflowed as many joints as I could. Battery leakage is a real pain as, apart from the destruction of tracks it also combines with solder to give a grey, hard crystalline lump that is almost impossible to solder. This can be corrosive and loses conductivity so it is worth trying to sort out.

After several hours of work, I was ready for the first switch on. Following advice from Twitter (now 'X') I took an incandescent bulb - actually an old spotlight with screw cap - and soldered a couple of wires to it so that I could add it in series with the live wire. The idea is that if there is a fault, then excess current flows through the bulb making it glow bright. A dim bulb means that there isn't excess current - but no guarantee that there's no fault...

Good job I still had those bulbs...

So, what should happen is that when I switch the power on, there should be nothing from the supply until a pair of pogo pins have a small capacitance - provided by a finger - applied across them. This signals the board to start up and provide 5V to the main board, whereupon the main board will then send a 5V signal back into the supply to signal it should stay on.

For my test, I'm just intending to poke the pogo pins, check the supply starts up and then wait until it shuts itself down after two seconds (it does this automatically if the 5V from the main board isn't seen after two seconds). Here's a quick video of the first switch on:

Yikes.

So, unsurprisingly, I decide against poking the pogo pins with my finger.. I tried switching on again a few times with similar results. It doesn't get any better in slow motion..

Something is very wrong with this supply and, despite my best efforts, it may be beyond my skill to repair. The bulb going bright and then dim is fine as this is likely the capacitors charging. However, the loud cracks and arcing from the relay and the extra arcing from somewhere else on the board are not fine.

Ohhh - extra unexpected arcing. Spicy!

But fear not. After some careful thinking I came to the conclusion that the 'soft touch' feature could probably be bypassed if I used a straight PC ATX power supply and fed it straight into the motherboard. So I removed the connectors from the original supply and soldered some wires to the back of them allowing me to use my Heath-Robinson bench ATX supply. 

What could go wrong?

Going for broke and putting it all together..

So, it came to the first switch on. And I got nothing. And I mean, NOTHING. Hmmm.

After poking around the main board for a while I came to the conclusion that there must be a short to ground on the 5v rail. My reasoning was that the 5v was showing 0.3v. The resistance between ground and 5v was about 12 ohms (foreshadowing).

And now, dear reader, I made an error. One that may haunt me but I hope not. I decided to inject some voltage into the board using my bench power supply to check if there was indeed a short. I started at 4.8v with a current of 100mA. The voltage immediately sagged to virtually nothing and the current limit kicked in. So I increased the current to 500mA. Same thing - and I also realised that nothing on the board was getting hot or even warm. Increase to 1A. Same. 2A and I got the same. At this point I stopped as the current was going somewhere but I could not work out where.

Back to basics, I had another look at the connectors. But this time I took the time to take out all of the conductors from the plugs (relatively simple to do - poke the retaining hook with a sharp pointy thing and gently pull them out). And they were not good.

So I spent about an hour carefully dipping the conductors in white vinegar. The immediate 'fizz' made me realise that they were still heavily contaminated with battery guts.

Removing conductors - sharp pointy thing
in hand.

Then I took some very fine sandpaper and gently brushed the inner part of the female conductors and the outer part of the male conductors for extra cleaning. Putting them all back together I tried again, but this time with just the board and the 10inch monitor connected.

And this was the result:

It's alive! Somewhat..

This is a great start as it shows that there is still some life in there. As a separate exercise I also tried the clean cables on the floppy drive and hard disk 'ring'. Both sprang into life immediately where previously they had appeared to be dead. Most importantly, the hard disk started with no real complaints and made all the right noises for an old but very much working hard disk.

So the blue screen confirms that the video output circuitry is working. But how do we get any further?

After reading the technical manual, which is crap by the way, I realised that the old power supply would hold the NMI signal on the 6303 processor low for two seconds to start up. Grounding the correct pin on the main board connector would achieve the same thing. If the computer is switched on and at the blue screen then holding the NMI to ground for a couple of seconds will generally switch it off i.e. the display goes blank. 

This is also positive as it shows that the 6303 processor is doing its job and sending the right signals out to shutdown the system. 

After a couple of hours of probing and fiddling I noticed that after a certain amount of time the display would change. It would show a definite rectangle of garbage at the top followed by lines running down the screen. Sometimes these would start out black and then change to a dark yellow. I tried to take a photo and quickly realised how difficult it is to take a decent picture of a CRT! The 'best' pics are below:

Garbage.

More garbage.

From this display I am going to take a punt and say that there could be a RAM problem here. What the system should do when it starts up is copy the 'Caretaker' program to RAM and run it. This is what checks to make sure it knows you have the 'key' disk and then starts loading up from the hard disk (if present). I'm speculating that there is an issue with the RAM that is preventing this from happening or there is an issue with the video RAM that means I can't see what's happening. The video RAM on this is an odd chip I've not seen before and that is not supported by my RAM tester either - HM48416 which are 16384 word x 4 bit chips i.e. they have 4 data bits per chip. 

Another quick meander through the documentation showed that the system should beep to indicate startup and also provide patterns of beeps to identify issues which would be really helpful at this point. But this machine has been silent. I think that this is because the power supply does not have -5v. I had assumed that this was for the serial ports only but it looks like it may also be used in driving the sound signals that get piped through the monitor. Or should I say that there are + and - connections labelled 'speaker' in the video connector so that may be an assumption on my part.

To get -5v I could modify the -12v output from this power supply. I first tried a straight voltage divider but this didn't seem to work and I ended up with +0.3v on the -5v rail.. I may have to look for an LM337 or LM137 negative regulator.

But my money is on it being RAM in any case. More soon.