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Topics - Phu

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CODING AREA / Aliens for AndyRCM
« on: December 08, 2012, 11:18:36 AM »
If you were at the RCM HQ event last Sunday, you might have seen me with pen and paper trying to convert Andy's aliens program into Z80/ZX Spectrum machine code.

If you hung around long enough, you would have seen me fail. Pen and paper is not the way.

On the other hand, the assembly listing and TZX file attached do the job nicely :)

Load the TZX, then RANDOMIZE USR 32768 to begin.

-- Richard

(Due to forum filetype restrcitions, rename aliens.doc to aliens.tzx to load into your favourite emulator)

HARDWARE/SYSTEMS DISCUSSION / BBC Domesday Computer repair log
« on: September 09, 2012, 05:48:27 PM »
Those in attendance to Snibston in August 2012 will have witnessed the BBC Master Turbo driving the Domesday system being somewhat uncooperative. After making crackling noises from the speaker, it flat refused to start.

On getting it back to d'Workshop, the machine was more co-operative. Starting, but being confused. It's CMOS RAM was suspiciously corrupt. It also wouldn't beep. Or sometimes it would beep, sometimes it would try to beep and make crackling noises, and other it would just sit silently.

Now all of this is part of one subsystem in a BBC Micro, called the "Slow Peripheral Bus". Essentially, one of the 6522 chips acts a slow-access device controller. It has two latchable I/O ports which are used as address and data buses. The "B" port of the 6522 is connected to a 74259 chip called an "addressable latch". To explain simply, this has 8 output lines to which you can "write" a 1 or 0 to, and the value sticks. One of these is connected to the 76489 sound chip.

To summarise: CPU -> 6522 VIA -> 74259 latch -> 76489 sound -> amp -> speaker. Got that?

So I figured that perhaps the addressable latch was faulty. I initially ruled this out because the other things it controls (screen start address, CMOS RAM access) were working fine (while CMOS was corrupt, it did still work - more on that later). But putting an oscilloscope on it showed no drop on the sound write enable line.

A swap out from a spare BBC B board later and..... no difference. ???

Ok, lets concentrate on the crackles. There's no familiar low-tone on startup, so perhaps the sound chip is faulty? Swapped out and... no difference. !!!!?!

OK, if it's not the sound chip screwing up, it must be one of the amplifier chips. The LM324 is the pre-amp and is connected directly to the sound chip. Since I'm seeing crackles there on the scope, it would be logical to believe that's the faulty chip. I think you know what happened when I swapped that.


At this point I'm somewhat confused. Everything logical seems actually to have been intact. Looking at the circuit diagram shows the LM386 amplifier only connected to the sound circuitry via its input, not its output.... unless...

This 8-pin amp chip was backfeeding noise into the sound circuit and making it appear at the sound chip. On top of that, it wasn't doing its job and was creating silence/crackles. Once I replaced the LM386...


Turns out that WE line *was* going low, it was just doing so so quickly that the scope could barely see it. If I looked closely I could see a dot for a blink of time at the 0 line during sound writes.

So what caused this mess? Well, the battery pack in this machine was less than professionally put together. Three energizer AA batteries, taped together with insulation tape, and then the wires to the motherboard taped on too. This was used to connect batteries 2 + 3 together:

I should point out that blob of solder was just "there" - it wasn't being used to hold it to the batteries - that piece of metal was just taped on.

Unfortunately, during the movements of the machine to various events, the positive contact slipped a little from battery 3, and cut through the plastic covering of both itself and battery 1. This is the impact mark on battery 1, where the casing (negative) of battery 2 contacted with the negative from battery 3, causing a loop through batteries 2 and 3:

At some point it scraped itself, as can be seen on the negative edge of battery 3:

I shall be building a somewhat better battery pack with fresh batteries to restore the system :)

-- Richard

OFF TOPIC / Happy Birthday to the Esteemed Leader
« on: August 03, 2012, 08:58:19 AM »
All rise for the right honourable (honourable?) Andrew C Spencer (AndyRCM), Retro Computer Museum Proprietor and molestor of ZX Spectrums and Commodore 64s, for he gaineth another year of age today!

-- Richard

RCM ANNOUNCEMENTS / Website updates
« on: April 15, 2012, 08:45:13 PM »

At around 9pm BST we're going to be uploading some changes to the RCM website. They're not a huge amount of changes, so we'll be doing them "in place" without putting the site into maintenance mode.

So that you're not lost afterwards, the changes are part of a move to separate the RCM website into two parts: a customer-facing site with a simpler layout that will provide information on who we are, what we do and what services we can offer. This will be for the casual visitor who doesn't need all the technical information and the immediate in-your-face retro nostalgia :)

The other half will be the traditional "members" area that you're all familiar with. This will have the forums, wiki, blog, systems pages, etc with the current layout. You can access this from the front pages via the "Forums" link on the right-hand end of the link bar.

We've made a few minor changes to the members area to try and tidy things up a little. The left-hand bar links have now all moved to the top link bar, with the exception of the calendar and messages buttons which are two smaller buttons at the top of the left bar. The top link bar has been widened to accommodate this.

The forum info has been moved to the side bar under the shoutbox.

The music player now pops out as a separate window so that playing isn't lost when navigating to a different page.

The shoutbox now has a swear filter built in to curb any outbursts ;)

All of this is part of a bigger over-time move to improve the website for everyone. We will be adding an online shop (as time and resources allow) for RCM merchandise, and possibly a full-blown classifieds section to replace the current FOR SALE & WANTED forum, should time allow.

Do report any issues you spot to an RCM administrator.


-- Richard

FOR SALE / WANTED / Wanted: tiny telly
« on: February 06, 2012, 05:42:29 PM »
I use a small 5" B/W television on my workbench for testing video output. This works well, but doesn't exactly allow for checking colour output.

Does anyone have (free or cheap-cheap) something similar, but with a colour output?

It needs to be CRT, as LCDs don't deal with old computers.

A/V connectors (e.g. phonos for video/audio or SCART) preferable, but not essential.

These very often arrive in homes as free gifts that a lot of people don't have a use for. Anyone got something like this they'd like to donate to a good cause?

-- Richard

RCM ANNOUNCEMENTS / What resolution do you browse at?
« on: October 11, 2011, 09:33:02 AM »
Over the next couple of months the RCM website is being rearranged a little to make information easier to find for casual visitors as well as retro computing enthusiasts.

To this end, I'm curious to get a representative sample of what sort of resolution people's monitors are at when browsing RCM. The site is designed to "stretch to fit", which works well on 1280x1024 and above,  but does anyone use any other resolutions?

-- Richard

FOR SALE / WANTED / WANTED: Cheap Colour Inkjet
« on: August 12, 2011, 11:01:12 AM »
I'm on the sniff for a colour inkjet that won't snap my wallet. I know they're phenomenally cheap these days anyways, but even at £20 its a lot (at the moment) for something I won't use that often.

So if anyone has a working colour inkjet printer going for free or incredibly cheaply, let me know.

There's no urgency at all, so if no-one happens to have one I'll probably end up buying one through the usual channels anyway. So this is more of an "if anyone happens to have".


-- Richard

FOR SALE / WANTED / Wanted: DreamCast part
« on: July 20, 2011, 08:12:11 PM »
This is a highly optimistic post, but you never know.

I finally managed to find a circuit diagram of the audio circuit for the DreamCast. Using an oscilloscope I've determined that the audio DAC is dead. Thankfully the ARM7 audio processor still works (yay!) but there's nothing to convert the bitstream to analog audio.

Before I go and investigate possible alternate parts/circuits, would anyone happen to have an otherwise dead DreamCast that they'd be willing to extract a chip from?

Specifically it is IC303, located in the top right corner of the mainboard. It may have a SEGA part number or a more commonplace PCM1725U part number. It is a 16-pin surface mount part and is probably located very near to IC304 - an 8-pin surface op-amp.

If any of the soldering-savvy members out there think they might be able to extract this, let me know ;)

-- Richard

FOR SALE / WANTED / Free: enormous colour laser printer
« on: July 07, 2011, 01:49:30 PM »
I have (and want rid of) a huge HP Colour LaserJet 4500N. You should note the following facts:

1) It is (guesstimate) 2.5ft wide, by 1.5" high by 2 ft deep. It is huge and needs a wide area to sit.
2) It weighs a metric fsckton. It generally takes two people to lift it.
3) It has colour toner contamination like nobody's business. If you want to use it you'll have to strip the thing down and clear out all the toner.
4) The imaging drum cartridge is on its way out, and has a damaged cover. It will need to be replaced.

It comes with the parallel port card and suitable connecting cable, but I do not have the network card module for it. The three toner cartridges are reasonably full, however I don't know if they are also contaminated (see above).

Due to size and weight if you want it you have 3 options:

1) Come and get the thing.
2) If you live nearby (Portsmouth) I might deliver it to you (ask me first).
3) You pay for a suitable cardboard box and pay for a courier to ship it - note that this will cost you the Klopman Diamond to pay for.

Otherwise, its free to whoever wants it and has the time to service it. Mechnically and electronically its all sound aside from the damaged imaging drum and the toner contamination.

-- Richard

FOR SALE / WANTED / Wanted: Archimedes keyboard
« on: June 22, 2011, 09:08:03 AM »
I'd like to acquire on the cheap an Acorn Archimedes keyboard. The keys themselves don't have to be in brilliant order, but the encoder electronics do need to be working.

If anyone's got one spare lying around they want rid of, send it my way :)

-- Richard

« on: June 01, 2011, 10:51:03 PM »
I thought I'd treat you to a Womble-style repair log on this fairly simple fix.

If you seen our PET at the recent RCM Weekender, you would have noticed it was a little ill. Its screen looked like this:

By itself this might not have been a problem, unfortunately Scramble reads the screen back to see if you collided with anything. Which meant unless you kept your finger on FIRE, you collided almost instantly.

Before showing the chips that failed, you might like to know how PET screen memory works. There are a set of 2114 RAM chips (1K x 4 bit) that are dedicated to the screen. The PET not only uses these to display, but because of the "type anywhere" nature of input, it uses it as an input buffer too. Which means when what you typed shows as "doad" the PET thinks you actually typed "doad" - which doesn't help.

The 8032 has an extra quirk. To maintain some sort of compatibility with 40-column PETs, the screen memory is in two sets. One set manages the even columns, and the other set manages the odd columns. If you look at the screenshot, you can see that the fault is at a seemingly predictable position. In fact, the faults are on exact multiples of 32 characters.

Remembering the odd/even nature, this means that the fault is possible every 16 bytes. We can also work out which half (thus chip) and which bit is faulty, however (as will be seen later) this academic. The code for * is 2A hex, where as the code for " is 22 hex. 2 and A differ by one bit... the "8" bit, or the top bit of a 4-bit block. Thus, bit D3 of one of the 2114s is faulty.

Looking at this image, you can see where this fault occurred before on a previous chip (and caused no end of lopd'ing fun):

Since this is the second chip to fail like this, and since spares are available I decided to go the whole hog and replace all three remaining 2114s. Incidentally, the chips have a manufacture date code of March 1980... they did well for 31 years of service (they are the same age as me!).

I used a hot air gun and solder sucker to desolder the chips. Unforunately, on a board this old a couple of other things occurred:

The capacitor above one of the chips snapped under heat stress. This is only a decoupling capacitor designed to smooth out any voltage ripples that may have arrived at the chip. Its marked 104M, which is a 100nF part. I also managed to knacker a nearby via that seems to have buckled under heat stress.

Some wire and a spare 220nF part later (value is not vital here) along with 3 18-pin sockets and 3 spare 2114 chips, the board looks good again:

The rest of the board seems to have held up well, though you can see the adapter board I had to make in its first repair to replace the character ROM:

It required a transistor/2 resistor NOT gate on it in addition to the socket adaption as Commodore used a 2364 mask ROM and activated it with an active-HIGH signal... something that 2764 EPROMs don't support.

Powering the old dear back on gives a nice clear display, and we see the DRAMs are still in fine working order:

I just need to take the keyboard apart and clean the contacts now so that we can type LOAD"",2 in less than a minute!

-- Richard

FOR SALE / WANTED / Wanted: Composite to RGB converter
« on: April 05, 2011, 12:23:52 PM »
This is a pre-emptive wanted, in that I'm considering setting up a good old RGB CRT monitor alongside my 19" TFT for some of my retro machines, to get that proper screen glow effect :)

Commodore 64s however are not noted for their RGB output tendencies. Does anyone have or know where I can find a composite to RGB converter on the cheap? Not VGA (I have one of those) but RGB @ 15kHz. I've seen some for in excess of £60, I was hoping for something a little cheaper, since the process is just colour separation.

-- Richard

OFF TOPIC / Upgrading Windows
« on: March 10, 2011, 12:48:10 PM »
Ever wondered what would happen if you had just upgraded Windows all these years from the first version 1?

I was amazed that applications installed in Windows 2 were still in the start menu of Windows 7....

-- Richard

COLLECTORS CORNER / Phu's collection
« on: March 07, 2011, 12:26:08 PM »
I also have a modest (ish) collection:

Sega Master System II - modded with RGB/Audio out and 50/60Hz switch
Sega Megadrive - modded with 50/60Hz switch
Super Nintendo - modded with 50/60Hz switch
Playstation 2 with modchip
BBC Micro B with Econet
BBC Micro B with Econet + Disk
BBC Master 128
Home-made ZX Spectrum
Home-made ZX81
ZX Spectrum 128K Heatsink
Commodore VIC-20
Commodore 64
Commodore Amiga 500+
Commodore Amiga 1200
Pentium 166 DOS PC
Acorn Archimedes A3000
Acorn Archimedes A4000
Acorn Archimedes A5000
Acorn Archimedes A3020
Acorn RiscPC 700 with 200Mhz StrongArm
Numerous Macs of varying vintage
Numerous Sun Machines (*cough*)
SGI Indy

... but then, doesn't everybody? ;)

OFF TOPIC / Phantom of the Floppera
« on: February 16, 2011, 01:39:00 PM »

@scarlettkitten: It accepts MIDI input....  think you can handle it? ;)

-- Richard

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