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Messages - Phu

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FOR SALE / WANTED / Re: Memotech ROMs or images thereof
« on: February 28, 2013, 06:46:52 PM »
RCM Guys - Pretty sure we had a PASCAL plug-in ROM for the Memotech kicking around - would that be the one Andy is looking for ??

No chance of finding it right now given state of HQ post move - but there is a PASCAL rom for the MTX in there somewhere.

Yes, we definitely had this, and it was even in a working state.

It might take a while to get it to you though, as it is buried under a mountain of stuff right now.

-- Richard

FOR SALE / WANTED / Re: Memotech ROMs or images thereof
« on: February 27, 2013, 11:01:21 PM »
Your description of Speculator seems right. If I remember the description, you had to load a "helper" from tape for the compatible game you wanted to play. The helper was essentially a program  (ran in it's own RAM, under interrupts maybe?) and was aware of which bits of the screen were actually being updated as per the game you were playing.

Seeing as there is little in that process that can be put into ROM, it wouldn't surprise me if it only had some RAM in there.

-- Richard

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)

The random jap,n64 fighter, who knows what it is called? ???.


(pronounced hee-r'yoo no tswin - with the "o" in "no" pronounced as in "got").

-- Richard

VALUATIONS / Re: Genesis 32x
« on: November 13, 2012, 11:04:47 PM »
Are you offering one? If so, I might be interested at those prices.

-- Richard

« on: October 27, 2012, 01:19:13 PM »
Tried an alternative TMS9929A from another MTX... no difference.
Distortion visible on oscilloscope with no daughterboard.
4116 VRAMS are warm, but I wouldn't have said they were hotter than expected.

Could be one to replace 4116s on... fortunately RCM recently acquired a small pile of these.

Unfortunately, they are not candidate for replacement for e.g. 4464s (single supply/a quarter of the chips) or static RAMs, due to the peculiar D/Q access the TMS9929 uses.

-- Richard

« on: October 24, 2012, 02:19:40 PM »
The fault we keep getting is a banding problem on the video output. A wide band moves down the screen at a steady rate. It starts off as a mild distortion, but as the machine is left on longer it turns into a solid black/grey/white band that starts interfering with sync levels and destabilising the picture.

Generally speaking, it affects the U and V outputs coming from the TMS9929, but not the Y output.

I checked all the power inputs and they are solid flat, so it's not a "wobbly" power issue. I did look at the dry capacitor approach, but the only electrolytics are the power supply caps and they seem fine (flat DC on all lines).

-- Richard

FOR SALE / WANTED / Re: WTD : ATARI 7800 Power Supply
« on: October 08, 2012, 10:34:44 AM »
Incidentally, I checked the pin spacing on the Atari 7800 connector. You can get a standard two-pin connector that will fit it (like the connectors for "HDD LED" and "PWR LED" on PC motherboards). If you're really polarity-conscious, you can glue a small piece of whatever material you have to hand to fill the "notch".

So in fact, it wouldn't be hard to modify a standard power supply to fit the original socket.

-- Richard

FOR SALE / WANTED / Re: WTD : ATARI 7800 Power Supply
« on: October 01, 2012, 07:34:52 PM »
Don't know of any genuine ones, but I might be able to supply a new-build PSU. It's dependent on whether I can make a connection that will fit the unusual input plug on the 7800.

I'll need to check pin spacings etc with with RCM's 7800 on Saturday, but assuming I can build a connector, a new PSU is likely to be in the region of £14.95 (new build, not old/refurbished stock).

-- Richard

I had a look through by mag-o-commodore-chips, but no 901486 m'fraid.

Plenty of 901225 and 901226 if you ever need them though.

-- Richard

HARDWARE/SYSTEMS DISCUSSION / Re: BBC Domesday Computer repair log
« on: September 20, 2012, 11:04:58 PM »
Part II - Modem's revenge.

So the previous fix didn't last long... 3 minutes into running and Beeeeee....e...k..eee......kk...kkrrkckck. Silence.

Clearly, while the battery was uh... "unprofessional", it wasn't to blame for the sound chip dying. Something on the board was killing one or more components.

A consultation with knowledgeable facebook friends suggests replacing the filter capacitor. This didn't work. So that wasn't it. All other answers dried up, so some probing was in order.

A little bit of electrical knowledge regarding capacitors. They will conduct until they are charged, or more specifically until they are charged in the same polarity as you are trying to make them conduct. Then they stop conducting. This is useful, as it allows you to test capacitors as to whether they need replacing.

A capacitor should make your continuity test beep for a short period. The larger the value, the longer the beep. Then it should abruptly cut off. If you switch the polarity then it should do the same thing as the original charge discharges, then the capacitor recharges in the opposite polarity.

Thus a capacitor that beeps continuously is either in excess of 3300uF and the size of texas, or faulty. Guess what? None of the capacitors were faulty. They all pulsed a beep as expected. GRR.

Circuit diagram consultation time. Part of the audio circuit is tapped off to a "modem" connector. The Master 128 is equipped with internal modem support, in the form of a digital bus (which in the Domesday system is where the SCSI board connects) and a 4-prong analog connector for mixing the phone line sounds with the internal speaker.

I did some tests with the pin's connectivity to other parts of the circuit, and by chance happened to touch one of the speaker pins (speaker was disconnected at this point, as from a continuity point of view it forms a deceptive low-value resistor) and got a beep.

OK I thought, that's clearly where the audio signal mixes in in some way, so I test the other pin which I assume to be ground, then I'll get nothing....  silence. Good.

Not good. The pin I touched accidentally was ground, not audio. The "audio" pin of the modem connector was connected to ground by a 10ohms resistance, which it shouldn't be.

Much examination, rechecking of capacitors, etc. later I found myself staring at the modem connector and thinking... I wonder if it's the connector itself? I don't see how, but let's check.

After desoldering it, the resistance went up from 10 ohms to 40 ohms. Curious. Some liberal application of RCM's favourite cleaning agent (nail varnish remover) and the resistance went up to ..... too high for my multimeter to read in that mode ;)

Could this be the fault? I fitted two sockets to the motherboard (one for the sound chip, and one for the remaining amp chip) and installed two sockets. A test program to send every tone to the sound chip has been running for 20 minutes without fault now.

So it seems a conductive "something" got between "audio" and "ground" on the modem connector, and crosswired the entire audio circuit. The "gunk" had a greenish hue to it, so I do wonder if some corrosion transferred atmospherically to the pins from the manky battery (cf. the delights of Amiga batteries for how nasty that can get), though there are no other signs of battery gunk getting in.

At least now the audio works I can put the lid on a test the whole Domesday system.

-- Richard

Your A3010 was gratefully received, along with the other machines and bits and pieces :) Many thanks for those :)

Please do think of RCM rather than dumping stuff... that's what we're here for :)

Panther: you'll have to negotiate with sir Andy of RCM for access to the Arm250 chip ;)

-- Richard

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

COLLECTORS CORNER / Re: martinws' Collection
« on: September 05, 2012, 12:08:48 PM »
You need more macs.....

But then I have more Suns than you (*nod to jimbob*), not that I use them. They're currently forming a very ornate (and possibly expensive) sculpture in the back of the garage....

Aside from the SparcServer 1000E to my right that will eventually become the "RetroServer" for RCM... a sort of generic server for any information to be delivered to other machines... e.g. BBS, Teletext generator, game image server (for remote loading of games into machines), etc...

-- Richard

ARCADE / Re: Massive Williams Defender Repair Log
« on: August 23, 2012, 09:01:51 AM »
*Standing ovation from RCM*

You Sir, are a legend :)

-- Richard

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