ScuzzBlog: Diaries February 2021

Entry 12th February 2021: Post 1: Acorn Electron - ULA failure.

Acorn Electron - ULA failure.

One of my Electrons bit the dust. Seems that the ULA finally gave
up with a number of fractured pins. Never mind, I have others.

The Acorn Electron is the brainchild of Chris Curry and Hermann
Hauser of Acorn in the UK. You will be familiar no doubt of the
ARM technology which populates most telephones I am told. No idea.
Hot on the success of the BBC Micro Chris Curry was keen to get
a wider audience for the Acorn and so came up with a cut down
version of the BBC. They basically took a Model B and used a hack
saw to cut it in half. I can't back up that last bit.

Sadly it came at the very time the games boom was over and they
missed the Christmas seasonal period and so the machine kinda
bombed. The ULA was responsible for up to 25% failures in the
early days and they moved from soldering the chip on to sticking
it in a socket... which for me was a bad move. Just saved them
costs replacing faulty parts.

When you open up the Electron and negotiate unplugging the blue
keyboard connector you can see that the board is in two parts.
The one houses the power side of things with the main board to
the left. Power is provided as 18 volt to the expansion port with
the main board taking the 5V.

RAM is in the form of 4 chips at 32KB ROM/ 32KB RAM and sadly
because of the arrangement the feed is slow to access. Whilst the
machine runs technically at 2MHz there needs to be access to the
four 64kb dynamic RAM devices. Cus of the two cycles needed
within the 500ns period of the 2MHz clock, and cus the ULA and
CPU have to take turns accessing RAM this cripples the computer
to just 1MHz. The electron has no video memory. [ Please don't
quote me on any of that as its just my take on things with what
I have read from various forum posts].

Dominating the board is the Ferranti ULA and the 6502A CPU at
2MHz but constricted. The gate has a 68 input/output line and
makes for one busy chip. I understand that the ULA is a TexTool
268-5400 68-pin socket. The ROM has built in BBC Basic and the
operating system.

So what went wrong with mine? Looks like over the years half a
dozen of the small leg supports on the ULA have corroded. It is
difficult to know when the corrosion set in. These legs are very
fragile and slender. I was moved to remove the chip which sadly
broke these pins/legs. Bit like the AGNUS on the 500. I have had
a kind offer to fix the legs. This means de-soldering the chip
which I guess may take place in another life, but not mine.

Like I say I do have other machines but as a 1982/83 vintage it now
falls into the mothballing bracket. I am choosing now never to
switch on any computer older than 1986 and will simply store them
so future generations can pimp them up. I believe that by using
the machines unnecessarily I am more likely to do bad than good so
its time to retire these little babies. Wish I had done that prior to
opening her up. Never mind.

The Acorns are generally pretty sound I have to say. This model
however has got a design weakness that in my case took 39 years
to finally kill the computer. Not that they started corroding in
1982, just that the notion of that chip seems a little on the
problematic side. Bit like a battery on an A500 Plus or the
expansion connector on the ZX81.

Going now.... I'm gonna mail Chris Curry with the word [*Help]
and see what comes back. It's an in joke for AcornMOS fans. Kinda
hoping he can lend me a hand... If not an arm will do. Tee hee !!

Acorn Electron - ULA failure.

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Last updated 12th February 2021

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