Amstrad CPC464 – switching to 4164 RAM

I got this poor Amstrad sold as seen for a very low price, and I can see why. It’s a total mess, filthy on the outside (greasy too eugh!), even dirtier on the inside and it won’t boot. The board looks like its been hacked at too, with a missing 4464 ram and a socket where it should be. The tape drive looks completely knackered with the reset counter not working and the record button seems twisted and broken.

The first thing to notice was that the on/off switch is broken and only very rarely allows the machine to turn on. Shorting the contacts on the switch with my tweezers brings the machine to life, albeit with a broken blue bordered boot screen:

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This board has 8 slots along the bottom for 64kx1 RAM (4164) and 8 missing capacitors, so rather than waste my time hacking at the 64kx4(41464) area I desoldered the remaining RAM and used my lovely new Antek 760RW hot air rework station to add sockets for the 64kx1 RAM and added some Oki chips in them, and some ceramic 100nf capacitors int he empty spaces below. I guessed the capacitor value by matching the two ceramics near the old RAM.

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Honestly I wasn’t expecting much at all. More than anything I just wanted to play with my new toy as I hadn’t had a chance, and I was just thinking this is a potentially scrap board so why not give it a try. But when I shorted the pins:

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It actually fixed the issue! Keyboard is working great too. Sometime in the future I’ll give this a huge cleanup, recap, change the power switch and have a go at the cassette drive repair. But for now I’m chalking this down as an unexpected win.

 

Sinclair ZX Spectrum +2A RAM repair (eventually!)

I’ve had this Spectrum +2A for some time but it never booted correctly. It took me a while to even get power to it as I didn’t have an original PSU, in the end I made a makeshift adapter for my Amiga PSU that converted from the square Amiga plug to the DIN plug the +2A requires. I’m beginning to hate systems that require negative voltage or an AC supply, makes my life more difficult.

The system would either boot with a blank screen as shown above or with random vertical bars of various colors. This usually points to a RAM fault but with no spare 64k 4bit ram chips available nor any socketed in my other systems I couldn’t immediately test this theory. After ordering a new supply I got to work blindly replacing and testing what I could:

  • The Z80 cpu is always socketed so easy to test this and I have a few of those, but alas no luck here.
  • Next up I replaced the 74HCU04 and the TEA2000 chips socketing both for good measure, no luck here either.
  • I didn’t think capacitors would be causing a problem here but replaced them all anyway as its good practice on these older systems.

In all honesty I knew it was going to be the RAM as the chips looked slightly charred suggesting they had overheated at some point:

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Sure enough when the new chips arrived today and I replaced them, I was greeted with the familiar +2A boot screen. Loading up a game from tape worked successfully if a bit noisily (need to oil the cassette mechanics next). Another machine to add to the collection yay!

BBC Micro Model B RAM repair

I managed to acquire an untested BBC Micro 32K Model B from Ebay, the price was pretty low so I just could not resist. This is the first computer I learned to program on at the tender age of 7 (interesting how my generation started so much younger: report-80s-kids-started-programming-at-an-earlier-age-than-todays-millennials)

After plugging it in to the TV using the RGB cable from my Acorn Electron and turning it on I heard the familiar double beep and saw the boot screen, all appeared well. However when I changed into any screen mode other than mode 7, I got lines of pixels running up the screen and flickering on and off.

My suspicion was one or more of the ram chips and after testing some 8264 ram chips by piggy-backing them onto the existing chips I found IC60 to be faulty.

I de-soldered the old IC and put in a socket and the new 8264 ram chip (drop in replacement for the 4816 ram chips in the BBC). While I was there I replaced the electrolytic capacitors and changed the thermal paste on the ULA heat sink.

Hey presto, the glitch has gone from all modes! I’m now finally a proud owner of a BBC Micro just a mere 34 years after they introduced me to the world of computing. Thanks Acorn!! Now to find a copy of Elite!