Tuesday, 16 May 2017

BBC Micro Repair

Here we have quite a nice looking Issue 7 BBC Micro. It's nowhere near as yellow as it looks in this photo.
BBCs are a bit like Commodore PETs, they often contain all manner of upgrade boards and add ons. You never know what you are going to find until you open one up. This is one of the more boring ones, but still a nice machine. A tape only issue 7 model B with no expansion ROMs, disk controller, speech synth, ECONET or any other goodies.
Before I go any further, I've replaced the power supply mains filter capacitors, and finally got around to writing a blog post on BBC Micro Power Supply Repair.
With the power supply taken care of, time to turn it on and see what's wrong.
Not a bad start, a single beep, and a flashing cursor. This is a common fault, and is normally caused by a faulty system VIA (the 6522 to the left of the CPU). Handily, the user VIA (the 6522 below the CPU) can be borrowed to test this as it is only required for access to the user and printer ports, and the BBC will boot without it.
Swapping the VIA made a difference, so we can probably conclude the original was faulty. There is now a BASIC prompt, but the beep is now permanent, and the keyboard is not responding.
Fitting a known good 6522 gave the same results, so the orignal VIA was faulty, but there is something more. The VIA is necessary for boot because it controls a lot of the internal devices, and when it's not working, the boot sound isn't cleared and the keyboard won't respond and various other issues with screen mode etc. can occur. Tracing the control circuit out, the VIA drives a 74LS259 addressable latch. All the outputs of this were static, they should be moving around at boot, which was confirmed on a working beeb.
I removed and tested the chip, and it failed in the IC tester. I didn't have any new LS259 to hand, but I found a used chip which was a better match to the surrounding chips than the original was.
With that in place, I got the familiar double beep at startup, and the keyboard started responding.
The usual tests looked good, I'll leave that to soak test for the moment whilst I decide which disk interface (or rather disk interface replacement) I am going to fit.

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Monday, 15 May 2017

BBC Micro Power Supply Repair

If the wonderful BBC micro computer has an Achilles heel, its the power supply.
And the Achilles heel of the power supply is the Rifa brand mains filter capacitors. This is a rare photo of one which hasn't failed. Yet.
These have a tendency to fail and go bang and create large clouds of noxious fumes. It's not pleasant when it happens. And it will happen. These are all failing. If it hasn't gone bang yet, it will do. 
There are similar problems with mains filters on Commodore PETs and the same capacitors on the monitor in an ICL One Per Desk. Here you can see the cracks in the side of the case where the magic smoke escaped from.
This one has been removed from the case, you can see where it has bulged and shattered.
My standard practice whenever a BBC comes in is to replace these capacitors before I even switch it on. The one time I deviated with 'just a quick check before I do the power supply caps' resulted in one going bang as soon as power was applied, so now I stick to the rule.
There are two X class capacitors filtering the mains input, a 0.01uF and a 0.1uF. X class capacitors are rated to be fitted across the mains supply (as opposed to Y class, which is rated to be fitted in series with the mains supply for capacitive droppers etc.). This means they have mains voltage across them, so when they fail, they really fail.
The set on the left are from one that must have been quite loud, the centre set just a minor bang, and the ones on the left, the modern X2 class replacements. Also shown is an electrolytic capacitor which sometimes gives issues with slow or intermittent power supply startup. I normally replace those at the same time.
You can see the gold band on the new electrolytic capacitor in the centre of the picture, and the grey boxes of the new X2 caps. The modern 100nF cap is physically smaller, but there are sets of holes in the board for either size.
These power supplies are a little tricky to work on as you need to remove several earth bolts, push out the floppy drive power connector and mains switch in order to remove the board.
A tip here is to push out the switch (by pushing flat the retaining tabs) before removing the spade connectors.
All refitted, ready to begin testing the rest of the system.

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Thursday, 11 May 2017

ZX80 Keyboard Overlays

When the ZX80 was released in 1980, it came with a version of BASIC in a 4K ROM chip, known as 4K BASIC. Note this has nothing to do with RAM (the ZX80 has 1K, upgradeable to 16K), it is the size of the ROM chip.
When the ZX81 was released in 1981, Sinclair offered ZX80 owners the option to upgrade to the same BASIC as the ZX81. The upgrade kit consisted of a new 8K ROM chip (so this version is known as 8K BASIC), a ZX81 user manual and a keyboard overlay.
The overlay is the same as the ZX81 keyboard, and contains many new keywords, and moves some of the existing keywords around. So if you want to use 8K BASIC, you really need the new style keyboard. Software for the 4K ZX80 was not compatible with the 8K  BASIC, but an 8K BASIC ZX80 could load some ZX81 software (anything which didn't use the 'slow' mode display).
To use these, you opened up your ZX80, ideally without snapping the plastic or losing the rivets, although that is tricky.
The new 8K ROM replaces the old 4K ROM, and the keyboard overlay sits on top of the original.
You then end up with a much better and more capable version of BASIC, but a ZX80 that just looks wrong.
This is wrong, do not do this to your ZX80! You don't see many ZX80's like this these days, as most people prefer the classic look with the blue keyboard.
With the Minstrel ZX80 clones, I have been using ZX81 replacement membranes.
These only come in the version with the grey keys and 8K BASIC keywords.
For the ZX80 builds, I got some overlays printed which stick over the ZX81 membrane to make it look like a ZX80.
You then get the same problem, if you want to use the 8K BASIC, you can't use the nice blue keys, you need to use the ZX81 grey keys, so when I have been building 8K BASIC versions, I used the ZX81 black and grey colour schemes.
I have now designed some new membrane overlays, with all the new 8K BASIC keywords, but in the colour scheme of the original 4K BASIC keyboard.
This now gives you the choice of 4K or 8K BASIC, but keeping the look of the original ZX80. This can be stuck over a ZX81 replacement membrane to make a ZX80 8K BASIC keyboard.
This contains all the keyboard markings of the ZX81, but with the yellow, blue and white colour scheme of the ZX80 keyboard.
You can even fit one to your original ZX80 so you can use 8K BASIC without the embarrassment of having a ZX81 style keyboard fitted.
However, I wouldn't recommend modifying an original ZX80, you should buy a Minstrel clone instead!
The clone kits are now available with a set of overlays for either 4K or 8K BASIC, see my Tindie Store for more options.
I have also added complete built units to the Tindie store, so you can buy a white ZX81 cased ZX80, now with a choice of keyboard overlays.

My thanks once again to Grant Searle, these were adapted from his ZX81 overlay artwork.

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