My latest eBay purchase was a Sinclair QL fully boxed with manual, power supply and a bunch of microdrive cartridges. This was again listed as not working so was expecting to do another repair.
When it arrived my first test was to check the voltages coming from the power supply. Nothing seemed to be being output so this was definately one of the issues I needed to sort. I didn’t want to make a start repairing this one yet as I am still working on the MSX2. But I thought I’d just open up the power supply and check for any obvious issues.
It all looked clean inside the PSU, so I plugged it back in to check some of the voltage lines only to find they now all looked fine. I checked the voltages at the connector end again and once again everything looked good.
I can only assume a loose connection at this point so I put it all back together for now. I then plugged the QL in to find it all seemed to be working perfectly. I tested both microdrives and they worked fine also.
The only issue I discovered was the keyboard membrane had a break in one of the tracks. This is a common fault with these old membrane based keyboards and luckily brand new ones are still available so I have ordered one and just need to wait a few days for that to arrive.
So, I think I got another pretty good deal with this one. Bit of a shame that it wasn’t much of a repair as I really enjoy getting stuck into that (The MSX2 repair has been great fun in that respect so will post an update about that shortly).
When I recieved this MSX2, I had concerns as it had certainly already had a reapair attempted on it. What if this person had tried everything and discovered that a custom chip had blown and it was unrepairable?
Well, I decided to carry on regardless and work through the issue myself. To start off with I checked the basics on the CPU. Clock signal, reset circuit, data and address buses. They all looked fine.
The video circuit all seemed to be operational, I could see the 15khz sync signal but the composite video output was just black.
I noticed that the CAS signal for the main RAM was missing, which suggested that the CPU wasn’t successfully executing code, even though there was a bunch of activity going on. My first assumption was bad memory. I socketed both the video RAM and the main RAM and swapped chips around but it made no difference.
I decided to add a useful tool to my inventory and purchased a mini DRAM tester off Ebay.
One by one I inserted the DRAM chips and they all tested good. Not quite as simple as a RAM fault then.
After reading through the schematics for the Sony HB-F1XD (The closest MSX2 I could find to my model) I noticed the VDP read and write signals came from a custom IC on the board labelled as a MB64H444.
I checked with my thermal camera and noticed this chip stayed completely cold when powered on. My first concern was maybe this chip was completely dead which would be game over for this computer as they are virtually impossible to purchase.
I found the pinout for this chip and probed every pin with my scope. Everything seemed to be in order.
It was at this point that I put the scope back on the composite video pin and noticed it was showing a signal!
How could this be? I’ve not changed anything!
Even though I don’t have the replacement hic board yet, I hard wired the composite video output from the vdp chip into my little monitor and there it was, a perfect working image.
Although I was happy that the computer was working and that all of the custom chips were good. I don’t like things just fixing themselves as it can always break again.
And that is exactly what happened. But I discovered that by putting some pressure on the board I could break and fix the computer at will.
My next job was to find the bad connection, so I started re-flowing the solder joints on the chips in the area where I was bending the board. I also reflowed the solder on the MB64H444 chip. At this point the computer was non working and bending the board wouldn’t bring it back to life. Back to square one 😞
Only this time, I knew all the components were working and I was looking for a bad trace or solder joint.
I went back over the MB64H444 IC as this was the last thing I touched, and that is when I noticed no activity on the A13 address line. Could it really be that simple?
As a temporary test, I soldered a bodge wire from A13 on the MB64H444 up to A13 on the ROM chip.
I crossed my fingers and powered on the machine which greeted me immediately with the MSX logo. And no amount of flexing the board would cause it to not work.
The entire computer brought to a halt because of a bad trace on one of the address lines to this MB64H444 chip.
I’m still not 100% sure what role this chip plays in this computer. The schematics list it as a speed controller. So I will be doing some more reading up now to see why this issue caused the machine to not boot.
I have alsp sourced a modern replacement for the HIC board now, so once this arrives, I will be able to put this computer fully back together and start using it.
There is something so satisfying when you finally find the issue with these old machines.
If you have anything sat in your loft broken and you fancy donating it, I will do my best to bring any computer back from the dead and give it a good home 😀
My next repair project is this Sony MSX2 computer. Purchased as not working (black screen).
Unfortunately this machine has a bit more to its history than the eBay advert let on. Upon opening it I noticed that the problematic HIC1 board (The video out multiplexer that tends to suffer from corrosion on these machines) was not only missing, but pin headers had been fitted in its place.
This tells me two things. The hic board on this machine had indeed failed. But also, someone had replaced it with the modern replacement hic board but I guess it still didn’t fix the problem so they took it back out and sold the computer.
So, I need to either build or source a hic board for starters. But even then I probably still need to get the actual machine running.
I have already gone over the board with my scope to see if I can see what is going on. For starters it seems the computer tries to start for a few seconds and then comes to a stop. I can see a horizontal sync and csync but it appears the video signal is just a black screen.
I’ve also noticed that the CAS signal for the system memory is not there. Currently I’m assuming a possible memory fault so I will try swapping that out first. If it still doesn’t appear to be running then I will probably look into the ROM chip next.
All of this is under the assumption that the MSX2 should actually run fine even without the hic board present. Unfortunately I don’t have another MSX2 to test this theory.
I’ve had this one for a few months now but forgot to add it to my list. Don’t really know too much about these yet, so will be fun having a play and seeing what I can get it to do.
Apparently they were very popular with reporters and editors as it has a good keyboard and allowed them to write articles easily.
This one is in perfect condition. Think I’ll have to get a Wi-Fi modem hooked up to it and see what it’s like browsing some BBS’s
