The latest broken beeb repair

discuss both original and modern hardware for the bbc micro/electron
Colday
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The latest broken beeb repair

Post by Colday » Tue Jul 28, 2020 6:56 pm

Evening all.

So this beeb is an issue 7 and is quite standard apart from the fact that the 6502 processor is mounted in a daughter board that plugs into the socket for the 6502.

There are a few other ic's on the daughter board and then a bloody long ribbon cable that goes off and plugs into an external sideways rom case/board.

I have posted pics before but here it is again.
6502 and board.jpeg
But, before I get to this, I want to talk about the power supply issue.

I recapped this PSU when it arrived here a while back, yesterday while testing it I noted that the +5v supply is fine, however the purple -5v line is sitting at -4v (almost exactly)

1st off, is that an issue?

Secondly, when I plug the vcc lines into the board, I then note that the +5v have dropped to about +1.4 to 1.8v (give or take a few hundredths)

So, lets start with that.

I have some spare PSU's so it's not a problem to test with a known good one, but I've not come across this issue before and wondered whether the drop in voltage once attached to the board is synonymous with the low voltage on the -5v line?

Once again I doff my hat in your general direction and look forward to your solutions in words of may 2-3 syllables or less.
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

Colday
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Re: The latest broken beeb repair

Post by Colday » Tue Jul 28, 2020 7:23 pm

Right, deffo the power supply as have replaced with a known good unit and now I get the long beep.

CAPS lock LED for about half a second.
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

Colday
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Re: The latest broken beeb repair

Post by Colday » Tue Jul 28, 2020 8:01 pm

Ok, done some faffing.

Removed all extra ROMS and it now boots but with a corrupted screen.

When my son gives my phone back from the vitally important chat he is having with his friend (he's 9, how earth shattering can it be?) I shall take a pic.

Moving S25 to south I get a choral constant noise and the screen flashes with white corrupted squares.

Removing S25 altogether I get 16k showing.

It appears I have no basic ROM installed however... and the screen is slightly corrupted with some lines still. (although this could be the TV as it's an original analogue flat screen)

I shall follow up with pics in the near future. Hopefully.
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

Colday
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Re: The latest broken beeb repair

Post by Colday » Tue Jul 28, 2020 8:22 pm

Ok pics...
IMG_4576.jpg
S25 North
IMG_4577.jpg
S25 removed
IMG_4578 (1).MOV
S25 South
(4.03 MiB) Downloaded 18 times
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

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DutchAcorn
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Re: The latest broken beeb repair

Post by DutchAcorn » Tue Jul 28, 2020 9:21 pm

Can you post a photo of the row of large chips under the keyboard on the right side? Preferably with the keyboard removed? That should tell if there is a BASIC rom installed.

About the corruption; it looks like you have a faulty IC53 (one of the memory chips).
Paul

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Colday
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Re: The latest broken beeb repair

Post by Colday » Tue Jul 28, 2020 9:47 pm

I can... but I can tell you there is only a PB04 installed.

I've been doing a bit of swapping and have put a known good 04 and 05 IC into the sockets and still get the same message.

I have currently reverted to the original 04 IC that was in it.

Does it require an 04 and an 05 to run?

I've not tried that out yet.

I am quite pleased to have got any response to be fair as this is with the 'processor' installed on the daughter board.

Yes, I think RAM is a culprit and it seems to be on CAS1 as with S25 removed I get the 16k message, S25 south is just junk and with S25 North I get the vague possibly 33k message. Lol.

Until I can get the BASIC ROM to show up I can't try Mark's Marvellous Memory Mapper (I couldn't resist)
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

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1024MAK
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Re: The latest broken beeb repair

Post by 1024MAK » Tue Jul 28, 2020 10:35 pm

A ROM with a code ending PB04 is the OS ROM and has to go in the left hand socket (IC51)
A ROM with a code ending PB05 is the Basic II ROM and can go in any of the other four ROM sockets
See this photo

Mark

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Diminished
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Re: The latest broken beeb repair

Post by Diminished » Tue Jul 28, 2020 11:26 pm

Colday wrote:
Tue Jul 28, 2020 8:22 pm
IMG_4576.jpg
I have nothing to add, but this picture is hilarious.

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DutchAcorn
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Re: The latest broken beeb repair

Post by DutchAcorn » Wed Jul 29, 2020 5:52 am

Colday wrote:
Tue Jul 28, 2020 9:47 pm
I can... but I can tell you there is only a PB04 installed.

I've been doing a bit of swapping and have put a known good 04 and 05 IC into the sockets and still get the same message.

I have currently reverted to the original 04 IC that was in it.

Does it require an 04 and an 05 to run?
Depends on what you define as "run" but the OS ROM (04) is needed for basic functioning and the 05 ROM is needed for BASIC to function 8) .
04 needs to be in the left socket but the BASIC ROM can go in any of the other sockets so you could try different sockets to see if the issue is caused by a bad socket.
Colday wrote: ...
Yes, I think RAM is a culprit and it seems to be on CAS1 as with S25 removed I get the 16k message, S25 south is just junk and with S25 North I get the vague possibly 33k message. Lol.

Until I can get the BASIC ROM to show up I can't try Mark's Marvellous Memory Mapper (I couldn't resist)
You mean the one in this post?

From the corruption in the startup screen you can see that the faulty letters are always 1 step off in the alphabet. That indicates an issue with bit 0, in CAS1 as you already suggested. Using the table from the same post you can find that IC53 is the likely culprit:

Code: Select all

   /CAS0 /CAS1
D0= IC61  IC53
D1= IC62  IC54
D2= IC63  IC55
D3= IC64  IC56
D4= IC65  IC57
D5= IC66  IC58
D6= IC67  IC59
D7= IC68  IC60
There could always be more issues with the beeb, but I'd start there (and getting a BASIC rom to work).
Paul

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Re: The latest broken beeb repair

Post by Colday » Wed Jul 29, 2020 10:48 am

Thanks muchly.

I have tried a known good PB05 in the 2nd slot (with known good PB04 in 1st slot) with no joy. I will try PB05 in the other slots later on this afternoon.

If that works then I will look for dry solder etc.

I did have a look for that anyway and it all looked fine but will check.

Are there any other reasons why the PB05 wouldn't be picked up?

I need to get my head round the whole CAS1 D0 thing but I'll take your word for it.

Do we have a resource where I can buy the PB05, or are there alternatives?

I also need to buy the IC53 chip. I shall go have a look in a while at what is needed and get it all bought.
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

Colday
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Re: The latest broken beeb repair

Post by Colday » Wed Jul 29, 2020 12:48 pm

So, in my IC53 I have an: HM4816AP-11

Does the -11 after the main number denote anything? As I have found some -7 ones for sale (as well as -11) but was curious if they are the same thing)

Thanks
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

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1024MAK
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Re: The latest broken beeb repair

Post by 1024MAK » Wed Jul 29, 2020 1:37 pm

Colday wrote:
Tue Jul 28, 2020 6:56 pm
I recapped this PSU when it arrived here a while back, yesterday while testing it I noted that the +5v supply is fine, however the purple -5v line is sitting at -4v (almost exactly)

1st off, is that an issue?
The -5V line only feeds analogue circuitry, so it will not affect any of the logic circuitry, so the computer should still start-up and run. However-4V is out of spec. so something is wrong.
Colday wrote:
Tue Jul 28, 2020 6:56 pm
Secondly, when I plug the vcc lines into the board, I then note that the +5v have dropped to about +1.4 to 1.8v (give or take a few hundredths)
That PSU is not happy.
Colday wrote:
Tue Jul 28, 2020 6:56 pm
I have some spare PSU's so it's not a problem to test with a known good one, but I've not come across this issue before and wondered whether the drop in voltage once attached to the board is synonymous with the low voltage on the -5v line?
Most likely, there is a fault with that PSU.
Colday wrote:
Tue Jul 28, 2020 7:23 pm
Right, deffo the power supply as have replaced with a known good unit and now I get the long beep.
So that PSU is in need of repair, make sure you attach a note to it so you remember what the symptoms are.
Colday wrote:
Wed Jul 29, 2020 10:48 am
Are there any other reasons why the PB05 wouldn't be picked up?
Yes, including but not limited to damaged contacts in the chip sockets, incorrect configuration links or an open circuit fault with one of them (check S20, S21, S22, S32 and S33) faulty ROM selection/decoding circuitry.
Colday wrote:
Wed Jul 29, 2020 10:48 am
Do we have a resource where I can buy the PB05, or are there alternatives?
PB05 ROM chips sometimes come up on eBay, and it’s likely that Mark of RetroClinic may have some. Alternatively, the required data can be programmed (blown) into a suitable EPROM (new one time programmable or OTP EPROMs are still being manufactured) by a member here.
Colday wrote:
Wed Jul 29, 2020 10:48 am
I also need to buy the IC53 chip. I shall go have a look in a while at what is needed and get it all bought.
It is sometimes possible to find suitable DRAM chips on eBay. Or you can ask Mark of RetroClinic for one. Alternatively with a minor alteration, a 64k bit DRAM chip (which are easier to get) can be used.

The reason that I keep referring to RetroClinic is because although you pay more, he will sell you good working chips. Whereas on eBay, it’s a bit of a gamble.

There are other suppliers and other repairers if you want a wider choice. If yes, I can put links up to some of them.

Mark

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1024MAK
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Re: The latest broken beeb repair

Post by 1024MAK » Wed Jul 29, 2020 2:23 pm

Colday wrote:
Wed Jul 29, 2020 12:48 pm
So, in my IC53 I have an: HM4816AP-11

Does the -11 after the main number denote anything? As I have found some -7 ones for sale (as well as -11) but was curious if they are the same thing)
Yes, it denotes the official access time (speed) rating. A copy of the datasheet is here.
Alas, this datasheet does not include the HM4816AP-11, and I can’t find a datasheet that does specify the access time for the HM4816AP-11. It may be the case that Hitachi changed their classification system.

A HM4816AP-7 DRAM chip has an access time of 150ns.
The service manual specifically says use HM4816AP-3 which have an access time of 100ns.
A lower access time is better, a chip with a longer access time may not work.

Mark

Colday
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Re: The latest broken beeb repair

Post by Colday » Wed Jul 29, 2020 2:49 pm

Thanks for the help Mark.

For some reason I had you pegged to be the Mark from RC. I guess due to huge knowledge.

I've bought a few bits from RC Mark previosuly, but ideally I was looking for new items.

It seems the -3 ones are still available. I was considering ordering from a company in the states (as this seems to be where they all are).

Was going to get perhaps 10x so that I have some spares.

Was looking here: https://www.utsource.net/itm/p/1710882.html?digipart=1

Thoughts please?
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

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1024MAK
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Re: The latest broken beeb repair

Post by 1024MAK » Wed Jul 29, 2020 5:13 pm

Colday wrote:
Wed Jul 29, 2020 2:49 pm
Thanks for the help Mark.

For some reason I had you pegged to be the Mark from RC. I guess due to huge knowledge.
No, I’m definitely not RetroClinic :D

There are at least four members here called Mark, maybe more!
I’ve been err, tinkering with electronics and computers since 1983. But that’s not my paid work. Hence when my paid job keeps me busy, I go silent on the forums for a bit.
Colday wrote:
Wed Jul 29, 2020 2:49 pm
It seems the -3 ones are still available. I was considering ordering from a company in the states (as this seems to be where they all are).

Was going to get perhaps 10x so that I have some spares.

Was looking here: https://www.utsource.net/itm/p/1710882.html?digipart=1

Thoughts please?
I’ve used UTSource via eBay in the past. Normally they are okay. Only problems I’ve had so far (that I can remember at any rate) is one EPROM out of a box of 50 (not used all of them yet, not by a long way...) did not program correctly. I just picked out another one and that worked fine. And one time they messaged me to say that an item that I had ordered was not available, so they were cancelling the order and giving me a refund. Which they did.

Mark

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vanekp
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Re: The latest broken beeb repair

Post by vanekp » Wed Jul 29, 2020 5:19 pm

I posted this awhile back when I was having problems trying to understand the BBC RAM layout and to try to make it simpler to track down a faulty RAM Chip maybe it will help you.
viewtopic.php?f=3&t=12920#p166721
Peter.

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Re: The latest broken beeb repair

Post by Colday » Wed Jul 29, 2020 6:39 pm

Thanks Peter.
Right, I've worked it out.

That daughter board attaches itself by a jumper cable to S21

When I was looking at the service manual it suddenly occurred that is the ROM socket chooser (in laymans terms)

I nipped up to the bedroom and grabbed those sideways ROM boxes, guess what IC is in IC15 on each? Yup, a PB05.

Plugged it into the beeb via the ribbon cable and I got a BASIC and a DFS on 16k when I switched it on.

So, no need to buy any PB05 as it seems I have plenty! Lol.

I'll order the IC53 replacement and we can then go from there.

Edit: However, now that I have done that (and broken one of the vcc connectors off the board) I only get the beep of doom and only flickery out put to the screen.
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

Colday
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Re: The latest broken beeb repair

Post by Colday » Thu Jul 30, 2020 9:04 am

Ok, thanks Mark (not RC)

So, the ones from the link are all good? Then I will order 10 I think, always of use to me (and others sometime maybe).

In addition I have now ordered some of the male vcc connector spades as I broke one (and noted one was also a little loose)
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

Colday
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Re: The latest broken beeb repair

Post by Colday » Thu Jul 30, 2020 1:58 pm

I checked with Mark at RC, he has no spare beeb RAM currently so I will be making an order with the United States of Murica!

I'll open this up, does anyone else want to tag along with my order?
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

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Re: The latest broken beeb repair

Post by rmbrowngr » Thu Jul 30, 2020 4:06 pm

It may be useful contacting Dave Hitchins (Retro Hardware) on this forum to see if he has the RAM you need.
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Re: The latest broken beeb repair

Post by daveejhitchins » Thu Jul 30, 2020 4:23 pm

Sorry, no . . . I have Electron DRAM only, at the moment.

Dave H.

Colday
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Re: The latest broken beeb repair

Post by Colday » Thu Jul 30, 2020 4:31 pm

Well, thanks all round.

I've found a couple of UK suppliers but you have to submit a request for quote before you can get a price and order.

I begrudge paying the eBay prices but I'm considering buying 8 for £18 just to get me going and then will order a load more from the suppliers.
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

Colday
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Re: The latest broken beeb repair

Post by Colday » Fri Jul 31, 2020 10:57 am

Ok, I bought the 8 for £18 off eBay. But not arriving till next week.

I've PM'd Mark, but does anyone know how long it should take for his little basic program to check the RAM.

It's been running for 10 mins so far with no output, just a flashing cursor.
I'm presuming this is too long?
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

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Re: The latest broken beeb repair

Post by vanekp » Fri Jul 31, 2020 11:25 am

this basic program :-
5 MODE 0
10 ?&4000=0
20 A=?&4000
30 IF A=0 THEN GOTO 20
40 PRINT A

it only stops if there is an error all it does is write zero to location 4000 and keeps checking if it still = zero
its not realy a memory test, it's just a check if there is a bit stuck high to then locate wich chip does that bit.
Peter.

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Re: The latest broken beeb repair

Post by DutchAcorn » Fri Jul 31, 2020 11:28 am

Are you running this in 16K mode? Or with S25 North?
Paul

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Re: The latest broken beeb repair

Post by Colday » Fri Jul 31, 2020 3:44 pm

I tried it with both S25 North and removed.

The machine doesn't run at all with S25 south.

We are 99% sure it is IC53 that is the culprit and once the new RAM arrives next week I will continue with it.

I was curious on the run time as with S25 north it doesn't seem to end, then with S25 South it doesn't run at all. (I can't remember the error, maybe bad mode at 5 or similar)
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

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Re: The latest broken beeb repair

Post by tom_seddon » Fri Jul 31, 2020 4:18 pm

Colday wrote:
Thu Jul 30, 2020 1:58 pm
I checked with Mark at RC, he has no spare beeb RAM currently so I will be making an order with the United States of Murica!

I'll open this up, does anyone else want to tag along with my order?
Might do, I should probably get some spares as these don't seem particularly widely available. Feels like 10 should be plenty. But how much will it end up per chip once you've accounted for shipping? (And VAT?)

--Tom

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Re: The latest broken beeb repair

Post by DutchAcorn » Fri Jul 31, 2020 4:21 pm

Yes, mode 0 is not available with S25 removed or South, it needs > 16K of screen memory.

You could try this variation (S25 north). If you first change to MODE 0 you can see what you type.

Code: Select all

 5 MODE 0
10 ?&4000=255
20 A=?&4000
30 IF A=255 THEN GOTO 20
40 PRINT A
I would expect it to return 254.
Paul

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Re: The latest broken beeb repair

Post by Colday » Fri Jul 31, 2020 4:28 pm

@tom_seddon

Well, I have managed to locate a place in the UK that has some in stock, I am waiting for a quote.

However the place in the US has noted that I put stuff in a checkout basket but didn't go through with the order.

They have promised to bring the shipping down to US$10 whih seems far more reasonable, however I am now negotiating with them to send them from Spain or Italy to avoid the custom trap.

To be fair, if I order from them then I will swallow the postage anyway for ones I sell on to you guys. I need to pay to have them delivered anyway. You'll just pay UK postage costs from me to you (plus the chip prices) but we are not talking huge money when ordered from them.
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

Colday
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Re: The latest broken beeb repair

Post by Colday » Fri Jul 31, 2020 4:31 pm

@Dutch_Acorn

It doesn't seem to matter what would be in the byte part of the program, when the S25 is north and I run the program I never seem to get a result, just a flashing cursor.

Or would changing to 255 make a difference in this case?

I'll go reconnect that beeb up a bit later and give your version a go.

I suspect there is a separate issue somewhere but stand to be corrected on that.
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

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