BBC B beep of death

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ChadsArcade
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BBC B beep of death

Post by ChadsArcade » Sun Sep 09, 2018 5:39 pm

Hi all, I've got a BBC B I'm trying to revive and could use some help.

I have a logic probe, logic comparitor and multimeter, and I know the hot end of a soldering iron :)

Symptoms: beep of death, single continuous tone on boot up. No video (I'm using an RGB - SCART lead, tested good on another BBC).
PSU had been recapped and measures good.
Board is issue 7.

I noticed NMI was stuck low - checked IC78 and found the INT pin floating. Removed all DFS chips and made jumper S9 closed.
The board has at some time had the Econet chips fitted, there are sockets but no chips except a 74Ls123 at IC87. Jumper S2 is closed.

Are there any other jumpers I need to check?

NMI is now high on CPU. Still no dice. Checked all socketed chips in another BBC and they check out ok.
This was: both VIAs, CPU, 6845, IC7 ULA 2C199E, IC73 D7002C, IC6 Video processor.
I've also tried another keyboard.

Burned tricky's test ROM, removed all other ROMs barring BASIC PB05 in IC101 (right most socket)
Boards shows activity on the Address and data bus, but still no video. Still a continuous tone, but the cassette LED lights when you press break (you can hear the relay click) and goes out a few seconds later. Is the Test ROM running and doing this or is it just part of the reset sequence? No activity on the Caps or shift LEDs. No other sounds being played as per the test ROM video on YouTube.

I've changed the buffers IC14 (245), IC15 (273), IC71 (244), IC72 (245), IC96 (244) but no change.

It makes no difference if the memory jumper S25 is N, S or n/c.

Any ideas where I should look next?

Thanks for any pointers... :)

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Elminster
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Re: BBC B beep of death

Post by Elminster » Sun Sep 09, 2018 6:55 pm

Have you tried removing everything that is optional? Wasn’t quite sure from post. I.e. you can remove both 6522, 6845, disk controller chip and some of the serials (forget which off top of my head) and the machine can still boot. I.e. you tested in another Beeb, but did you just try leaving them out. There is a link to the full list somewhere in the sticky fault topic.

Edit: assume you are using topic viewtopic.php?f=3&t=2190&hilit=Faulty

Edit2: have you checked the reset circuit, the 555 and swapping the keyboard as well, another thing that stops machines booting.
Last edited by Elminster on Sun Sep 09, 2018 7:00 pm, edited 3 times in total.

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Re: BBC B beep of death

Post by ChadsArcade » Mon Sep 10, 2018 12:36 pm

Thanks for the reply! Yes, I've been working through that thread and the links referenced.

I did try removing all the unnecessary chips but still no joy. I may try this again though. When it says you should still get a banner, does this mean the "BBC Computer" text on screen should appear, even though the computer itself may not work any further?

The reset cct looks ok, CPU pin 40 goes low when you hold the break key and high again when you release it. I did change the 555 just in case but unfortunately it made no difference.

I've tried a second keyboard and reflowed the keyboard connector too.

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Re: BBC B beep of death

Post by Kazzie » Mon Sep 10, 2018 1:00 pm

I note you've tried S25 north and south, but have you tried fitting an inveter (e.g. 7404) to swap the memory banks? As built, toggling S25 enables and disables the upper 16K, but if you have a memory fault in the lower 16K it could still be stopping your machine from booting.

There's a detailed explanation in the fault finding index thread.
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Re: BBC B beep of death

Post by philb » Mon Sep 10, 2018 1:16 pm

Kazzie wrote:
Mon Sep 10, 2018 1:00 pm
I note you've tried S25 north and south, but have you tried fitting an inveter (e.g. 7404) to swap the memory banks? As built, toggling S25 enables and disables the upper 16K, but if you have a memory fault in the lower 16K it could still be stopping your machine from booting.
Isn't that what removing the link altogether does? South for high bank only, north for both banks, removed for low bank only.

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Elminster
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Re: BBC B beep of death

Post by Elminster » Mon Sep 10, 2018 2:14 pm

philb wrote:
Mon Sep 10, 2018 1:16 pm
Kazzie wrote:
Mon Sep 10, 2018 1:00 pm
I note you've tried S25 north and south, but have you tried fitting an inveter (e.g. 7404) to swap the memory banks? As built, toggling S25 enables and disables the upper 16K, but if you have a memory fault in the lower 16K it could still be stopping your machine from booting.
Isn't that what removing the link altogether does? South for high bank only, north for both banks, removed for low bank only.
Yes.

You only need to build the inverter if you want to invert the memory but still want 32k. This then allows you to use the working 16k and run a program to test all the memory.

So if no position works with S25 (north, south, completely removed) then I dont see there would be benefit in building the inverter, but happy to be corrected.

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Re: BBC B beep of death

Post by Kazzie » Mon Sep 10, 2018 3:30 pm

Apologies, a memory slip there.
BBC Model B 32K issue 7, Sidewise ROM board with 16K RAM
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Re: BBC B beep of death

Post by ChadsArcade » Mon Sep 10, 2018 5:53 pm

Thank you for the suggestions. I've tried S25 N, S and removed. I think the test ROM should run to a degree without working RAM anyway, if I understood it correctly.

All I seem to be getting from the test ROM is the cassette relay and led switching on and off... Though I suppose this shows the CPU is at least running?

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Re: BBC B beep of death

Post by 1024MAK » Mon Sep 10, 2018 8:33 pm

Elminster wrote:
Sun Sep 09, 2018 6:55 pm
Have you tried removing everything that is optional? Wasn’t quite sure from post. I.e. you can remove both 6522, 6845, disk controller chip and some of the serials (forget which off top of my head) and the machine can still boot. I.e. you tested in another Beeb, but did you just try leaving them out.
Removing the 6845 CRTC and running a Beeb is unwise, as even if the DRAM is okay, without the 6845 CRTC, it will not get refreshed, and hence will end up with corrupt data.

Mark

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Re: BBC B beep of death

Post by 1024MAK » Mon Sep 10, 2018 8:58 pm

For running Tricky's test ROM, all other ROMs should be removed. The DRAM configuration should be in it's normal state for the first run. Then later, you can try with S25 in the different configurations.

You have a number of symptoms, some of which may be related to one another. The first is the continuous beeeeeeep whenever the machine is powered on, the second is the caps lock and shift lock LEDs not responding, and of course the lack of display.

The continuous beeeeeeep is due to the OS ROM code either not running (for whatever reason) and not being able to tell the sound chip to shut-up (the sound chip registers starts up in a 'random' state). Or the 'system' 6522 VIA (IC3) or the 74LS259 (IC32) being faulty. These also control the caps lock and shift lock LEDs on the keyboard.

When running with the OS ROM chip fitted, a DRAM fault, or many other faults, can prevent the correct set-up data from being loaded into the 6845 CRTC (IC2). Without the correct settings, the 6845 does not know what type of display to generate.

When running Tricky's test ROM, it can run without needing working RAM for the CPU, but if there is no working RAM for the screen, then this may result in no display.

So next steps, first, set-up for a normal DRAM configuration, remove non-essential chips. Then check the power supply pins on chips across the PCB. Test between the Vcc and 0V/GND pins of a wide selection of chips. Test the between the pins of each chip, so one meter probe on that chip's 0V/GND pin and the other meter probe on that chip's Vcc pin. You don't have to do every chip, but do test all the big chips and at least every third or fourth of the other smaller chips. This is to test to ensure that the power rails are okay across the board.

Mark

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Re: BBC B beep of death

Post by Elminster » Mon Sep 10, 2018 10:36 pm

1024MAK wrote:
Mon Sep 10, 2018 8:33 pm

Removing the 6845 CRTC and running a Beeb is unwise, as even if the DRAM is okay, without the 6845 CRTC, it will not get refreshed, and hence will end up with corrupt data.

Mark
Oops typo should have been 6850

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Re: BBC B beep of death

Post by DutchAcorn » Tue Sep 11, 2018 12:59 pm

Is pin 34 (_reset) on the system VIA (6522) high?
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Re: BBC B beep of death

Post by Kazzie » Tue Sep 11, 2018 9:03 pm

ChadsArcade wrote:
Sun Sep 09, 2018 5:39 pm
Burned tricky's test ROM, removed all other ROMs barring BASIC PB05 in IC101 (right most socket)
Boards shows activity on the Address and data bus, but still no video. Still a continuous tone, but the cassette LED lights when you press break (you can hear the relay click) and goes out a few seconds later. Is the Test ROM running and doing this or is it just part of the reset sequence?
Revisiting your original post, the triggering of the cassette relay is NOT part of the regular startup, but I believe it is part of the functionality of Tricky's test ROM. So that suggests that your Beeb is indeed showing some signs of life.
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Re: BBC B beep of death

Post by ChadsArcade » Wed Sep 12, 2018 12:43 pm

Thanks for all the info.

I checked the voltage across many of the TTL chips last night and they are a consistent 5.05v.

I then turned my attention to the 40 pin sockets as I've seen single wipe sockets fail a lot on arcade boards. I've replaced 3 so far with dual wips, some pins showed tarnish/corrosion but none were broken. I'll replace the remaining big sockets next.

I ran out of time to complete further tests last night, for clarity is the system VIA the one under the CPU, or the one next to the keyboard connector?

Knowing that the CPU is running is encouraging, there are some signs of life then! Just need to persuade it to talk to the rest of the system.

Thanks everyone!

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Re: BBC B beep of death

Post by DutchAcorn » Wed Sep 12, 2018 1:31 pm

ChadsArcade wrote:
Wed Sep 12, 2018 12:43 pm
...
I ran out of time to complete further tests last night, for clarity is the system VIA the one under the CPU, or the one next to the keyboard connector?
It’s the one just north of the keyboard connector. I expect it will currently behave the same if you remove the system VIA.
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Re: BBC B beep of death

Post by Elminster » Wed Sep 12, 2018 2:44 pm

ChadsArcade wrote:
Wed Sep 12, 2018 12:43 pm

I then turned my attention to the 40 pin sockets as I've seen single wipe sockets fail a lot on arcade boards. I've replaced 3 so far with dual wips, some pins showed tarnish/corrosion but none were broken. I'll replace the remaining big sockets next.
I dont think I have ever had a socket fail on any of my 9 machines I have. One has a slight dodgy disk IC socket but that is about it. I know people have issues with the rom sockets where they get plugged in and out a lot. Failures have always been IC for me. (Your mileage may vary).

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Re: BBC B beep of death

Post by 1024MAK » Wed Sep 12, 2018 3:12 pm

Good, the voltage test indicates that power is getting to the whole board.

Focusing on the keyboard (including LEDs) and sound chip problem. I know you have tried various different chips in various positions. I recommend putting a known good working 6522 VIA in position IC3 (the system VIA) that ideally has been tested in the same position on a working BBC B. Then test using Tricky's test ROM. If this does not change the symptoms of the caps lock LED, shift lock LED and sound chip, then the next step is to remove the existing IC32 (74LS259), solder a socket in, and then try with a new 74LS259 chip. Then test again using Tricky's test ROM.

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Re: BBC B beep of death

Post by ChadsArcade » Thu Sep 13, 2018 6:34 pm

Firstly, thank you to everyone for the suggestions, it's greatly appreciated.
DutchAcorn wrote:
Tue Sep 11, 2018 12:59 pm
Is pin 34 (_reset) on the system VIA (6522) high?
It's high. It doesn't appear to go low when I press break, checking the schems shows it's connected to /RSTA which is basically a pull up resistor so this looks OK? /RST on the other VIA stays low as break is held in, goes high when break released.

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Re: BBC B beep of death

Post by 1024MAK » Thu Sep 13, 2018 6:40 pm

The RSTA reset to the system VIA (IC3) is only low very briefly at power up. As capacitor C10 charges via resistor R20, RSTA goes to logic high.

Mark

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Re: BBC B beep of death

Post by ChadsArcade » Thu Sep 13, 2018 6:42 pm

1024MAK wrote:
Wed Sep 12, 2018 3:12 pm
Good, the voltage test indicates that power is getting to the whole board.

Focusing on the keyboard (including LEDs) and sound chip problem. I know you have tried various different chips in various positions. I recommend putting a known good working 6522 VIA in position IC3 (the system VIA) that ideally has been tested in the same position on a working BBC B. Then test using Tricky's test ROM. If this does not change the symptoms of the caps lock LED, shift lock LED and sound chip, then the next step is to remove the existing IC32 (74LS259), solder a socket in, and then try with a new 74LS259 chip. Then test again using Tricky's test ROM.

Mark
Thanks for the ideas Mark. VIAs swapped with my good BBC B. It still boots OK, this one still does not.
IC32 / 74LS259 replaced. No change.

In fact, I'm not seeing the cassette LED switch on and off now, just a "beeeeep" from switch on. No LED activity (the middle one lights very briefly on power on), no video.

It looks like the CPU is still running though, there is plenty of activity on the Test ROM pins. After pressing break, if I probe one of the higher address lines I get a flurry of activity at first, then it settles into a slow toggling on/off. I think this is as it moves from the sound/LED tests to the memory tests?

The EPROM socket for IC51 (where I have the Test ROM) has a mod on the reverse of the board. The track to pin 1 is cut, and pin 1 is jumpered to pins 27 & 28. My working BBC B is the same, perhaps this is a mod for using EPROMS rather than Mask ROMS or a fix of some kind?

Thank you,

Chad

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Re: BBC B beep of death

Post by ChadsArcade » Thu Sep 13, 2018 6:58 pm

1024MAK wrote:
Thu Sep 13, 2018 6:40 pm
The RSTA reset to the system VIA (IC3) is only low very briefly at power up. As capacitor C10 charges via resistor R20, RSTA goes to logic high.

Mark
Thanks for the explanation. Checking this again with my logic probe, it appears to be flickering slightly. It's not going logic low but it's certainly not a steady red high as if you probe a power rail. I see the same on my working BBC B though so perhaps this is normal.

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Re: BBC B beep of death

Post by 1024MAK » Thu Sep 13, 2018 8:35 pm

ChadsArcade wrote:
Thu Sep 13, 2018 6:42 pm
The EPROM socket for IC51 (where I have the Test ROM) has a mod on the reverse of the board. The track to pin 1 is cut, and pin 1 is jumpered to pins 27 & 28. My working BBC B is the same, perhaps this is a mod for using EPROMS rather than Mask ROMS or a fix of some kind?
on a 27128 or 27C128 EPROM:-
pin 1 is Vpp (should normally be +5V for normal operation)
pin 27 is /PGM (should normally be +5V for normal operation)
pin 28 is Vcc (+5V)

On a 28 pin ROM:
pin 1 can be either N/C or A15 address line (128K ROMs)
pin 27 can be either CS1 or /CS1 or N/C depending on the actual device and specification required by the customer, or A14 (32K or larger ROMs)
pin 28 is Vcc (+5V)

Above information from the JEDEC standards.

BTW, I've just checked my "everyday" Beeb, and it has a 27128 EPROM as the OS "ROM" chip in IC51 position. Pins 1 and 27 both being connected to +5V.

Mark
Last edited by 1024MAK on Thu Sep 13, 2018 8:52 pm, edited 1 time in total.

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Re: BBC B beep of death

Post by 1024MAK » Thu Sep 13, 2018 8:38 pm

ChadsArcade wrote:
Thu Sep 13, 2018 6:58 pm
1024MAK wrote:
Thu Sep 13, 2018 6:40 pm
The RSTA reset to the system VIA (IC3) is only low very briefly at power up. As capacitor C10 charges via resistor R20, RSTA goes to logic high.

Mark
Thanks for the explanation. Checking this again with my logic probe, it appears to be flickering slightly. It's not going logic low but it's certainly not a steady red high as if you probe a power rail. I see the same on my working BBC B though so perhaps this is normal.
Keep in mind that R20 is 1M ohm, so only test gear with a very high input impedance will correctly show the level or voltage. I recommend testing with a electronic multimeter that has an input resistance / impedance of 10M ohms minimum.

Mark

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Re: BBC B beep of death

Post by 1024MAK » Thu Sep 13, 2018 10:23 pm

There is also the test ROM as detailed in the service manual. See this thread. May be worthwhile giving this a try. In the linked to thread (my post dated today - 13th September 2018) there is a zip file that contains the "raw" binary EPROM image file.

Mark

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Re: BBC B beep of death

Post by Elminster » Fri Sep 14, 2018 9:48 am

1024MAK wrote:
Thu Sep 13, 2018 8:35 pm

BTW, I've just checked my "everyday" Beeb, and it has a 27128 EPROM as the OS "ROM" chip in IC51 position. Pins 1 and 27 both being connected to +5V.

Mark
I am sure I saw a post on here from Mark Retroclinic that despite what the schematic said pin 1 was NC rather than +5v. But you are saying pin 1 is definetely getting +5v? Should(n't: typo fixed) make any difference as it is NC internally on 27128, will have to find that post and/or dig out one of my beebs to check.

Probably only of interest to me as I am working through the schematic as an academic exercise.
Last edited by Elminster on Fri Sep 14, 2018 10:42 am, edited 1 time in total.

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Re: BBC B beep of death

Post by MartinB » Fri Sep 14, 2018 9:59 am

Yes, rom socket pin 1 is actually floating in an unmodified B, the schematic lies about it being tied to +5v.

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Re: BBC B beep of death

Post by 1024MAK » Fri Sep 14, 2018 11:36 am

Just to be clear here, in the posts dated 13th September, we were talking about the connections to socket IC51 for the OS ROM, NOT the sideways 'ROM' sockets. Further, this is in relation to BBC B issue 7 boards.

So, I've checked one of my issue 7 boards. This computer is a computer purchased directly from a school, so would not have been subjected to DIY alterations. It may have been serviced by Acorn or an approved Acorn dealer or service centre. Or other service agents. But is only likely to have had Acorn recommended alterations (if any) done, or appropriate repair work.

Also, as I said in a post above, the chip in the OS socket (IC51) in this computer is actually an EPROM (part no. HN4827128G-25). It has a date code of 8442 which ties up with the date codes on the rest of the chips. This EPROM chip is what was in it when I bought it.
IMG_7817.JPG
Underside of BBC B issue 7 board showing ROM socket area
IMG_7818.JPG
Underside of BBC B issue 7 board, detailed view of IC51 OS ROM socket
This 'modification' looks very much like a manufacturer production alteration.

So to summarise:-
For IC51, the OS ROM socket:
Pin 1 connected to +5V and NOT to A15 as shown on the schematic.
Pin 27 connected to +5V and NOT to A14 as shown on the schematic.
All other pins as per the schematic.

For the sideways 'ROM' sockets (IC52, IC88, IC100, IC101):
Pin 1 open circuit and NOT connected to +5V as shown on the schematic.
Pin 27 connected to +5V.
All other pins as per the schematic.

For the record, for 2764 and 27128 EPROM chips, pin 1 is Vpp and most manufacturers datasheets specify that for data read, it should be connected to Vcc (in other words, +5V). But as Beeb owners have found, most EPROM chips don't appear to mind if pin 1 is left unconnected.

Mark

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Re: BBC B beep of death

Post by Elminster » Fri Sep 14, 2018 3:52 pm

Interesting. Slightly off topic, but do we know when/why they changed? I am assuming issues 2 or older etc might be as per schematic.

I wonder what else differs. I remember they being a change list somewhere for what changed between issues of Beeb. Will look to see if it mentions pin changes.

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Re: BBC B beep of death

Post by 1024MAK » Fri Sep 14, 2018 4:25 pm

Maybe it was Acorn getting carried away with themselves... thinking about a possible "mega" like ROM in the future, but forgetting that there is no point in routing CPU address lines A14 and A15 to any of the ROM sockets. As this is pointless due to the address decoding circuitry mapping RAM to the bottom 32K and ROM to the top 32K (both A14 and A15 would always be high during OS ROM access anyway).

Also, I'm not sure 64K byte ROMs would have been practical from the point of view of cost and availability.

The other option is that during production machine development, this ROM socket was used for test purposes (remember the first OS was in multiple EPROMs and fitted in what we now call the sideways 'ROM' sockets). And nobody thought to change the connections as the PCB went through the various versions...

But we digress...

Mark
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Re: BBC B beep of death

Post by 1024MAK » Fri Sep 14, 2018 5:11 pm

See also the conversation at this link.

Mark

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