BBC Micro B Repair Problems

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lambdamikel
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BBC Micro B Repair Problems

Post by lambdamikel » Sun Aug 12, 2018 5:50 am

Dear Board,

I am trying to repair a BBC Micro B which I got for cheap. I already replaced / socketed the 16 KB RAM, but it still shows garbage... I believe the RAM is OK. All the connections are as they should be. Also, the "big chips" (IC1, IC2, IC3, ...) which are socketed are all ok, swapped and tested with another working BBC Micro B. So, the problem must be with one of the little LS support chips or related. An idea, anybody? One strange thing is - it seems the previous owner did some fixes to the motherboard. There is a white little jumpercable on the back. No idea what purpose it serves. The other yellow cable is from my RAM socketing messups. So, it seems, it is not the RAM nor the CRT controller nor the ULAs.

S25 is in "south" position. It starts up with a beep, and Caps Lock etc is working, also break.
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Re: BBC Micro B Repair Problems

Post by cmorley » Sun Aug 12, 2018 6:10 am

Have you tried control-G and/or "VDU7" blind typing once it is powered up?

If it beeps then BASIC is running. Then try changing screen mode "MODE0", "MODE1" etc 0-6 and see what image you get.

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Re: BBC Micro B Repair Problems

Post by lambdamikel » Sun Aug 12, 2018 6:19 am

Thanks for the suggestion! Ctrl-G indeed beeps reliably, VDU7 and MODE <n> I cannot enter it seems, du to bad keyboard characters showing up while typing.

EDIT - YES I can enter commands, see below:
Last edited by lambdamikel on Sun Aug 12, 2018 6:38 am, edited 1 time in total.

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Re: BBC Micro B Repair Problems

Post by lambdamikel » Sun Aug 12, 2018 6:29 am

Upps, sorry, indeed it does something!

First, if I have S26 left = west, then it DOST NOT boot, but rather is stuck with a long beep (then second beep).
The pictures I posted above are with S26 = right = east.

If I remove this S26 jumper, then VDU7 beeps, and I can change the modes. I am attaching some pictures of mode3, mode6, mode7.
VDU7 also works with S26 = right (pictures above), and MODEn changes works too, but the screen is completely dark then. Whereas
I can at least see garbage in MODE6 and MODE7 when the jumper S26 is pulled.
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DSC07643.JPG
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Re: BBC Micro B Repair Problems

Post by 1024MAK » Sun Aug 12, 2018 8:46 am

Welcome to our forum :D

You may want to introduce yourself. We are a very friendly community.

Which version board do you have? I can't be certain from the photos you posted due to the black crock clip being in the way.

From what you say, you have two faults. One is definitely a fault with one bank of RAM. But don't worry about that at the moment, better to wait until the other fault is fixed. Note that with only one bank of working RAM (16K bytes only), as far as the machine is concerned, it's a model A, so not all display MODEs are available. For example, any MODE which needs 20K bytes will not be able to be selected.

The second fault is a display / picture generation circuitry fault.

I recommend that you first of all check the socket pins on IC2 and on IC6 in case the problem is simply a poor contact. I assume you have swapped IC6?

Have you got a copy of the schematic/ circuit diagram? If you have an electronic copy, I recommend that you try to get it printed out on A3 sized paper.

I assume you have a logic probe based on the croc clips connected to the +5V and 0V points. Do you have, or have access to, any other test gear?

Mark
Last edited by 1024MAK on Sun Aug 12, 2018 8:47 am, edited 1 time in total.
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Re: BBC Micro B Repair Problems

Post by Elminster » Sun Aug 12, 2018 10:05 am

First, if I have S26 left = west, then it DOST NOT boot, but rather is stuck with a long beep (then second beep).
The pictures I posted above are with S26 = right = east.
S26 is video output? Set to east it should invert the output, when inverted you have a white background (from memory), but that doesn’t tie in with screenshots below.

I am not sure if this is a mistake or a clue.
Last edited by Elminster on Sun Aug 12, 2018 10:06 am, edited 1 time in total.

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Re: BBC Micro B Repair Problems

Post by Kazzie » Sun Aug 12, 2018 10:24 am

Elminster wrote:
Sun Aug 12, 2018 10:05 am
First, if I have S26 left = west, then it DOST NOT boot, but rather is stuck with a long beep (then second beep).
The pictures I posted above are with S26 = right = east.
S26 is video output? Set to east it should invert the output, when inverted you have a white background (from memory), but that doesn’t tie in with screenshots below.

I am not sure if this is a mistake or a clue.
A possible clue. Lambdamikel talks of having S25 south in their first post, so we may have gotten off-course at some point.

I seem to recall that the video inversion with S26 only worked with the early (heatsinked) ULAs (where the ULA as manufactured was accidentally inverted, and needed re-inverting to show correct output). In my issue 7, moving S26 didn't give an inverted display.
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Re: BBC Micro B Repair Problems

Post by Elminster » Sun Aug 12, 2018 10:28 am

I don’t believe it is fitted on later Beebs that had the fixed ula (edit: manual must be wrong if issue 7 still has s26). On my beeb issue 4 with heat sink ula it is west, but as that has broken video I am not sure My results/memory of moving it to east are reliable. When you changed it did you get nothing, screen as before, or garbage?

Edit: cannot see issue in pictures above as croc clip obscures it. Assume 7 as no heat sink on ula, but that could have already been changed.
Last edited by Elminster on Sun Aug 12, 2018 10:32 am, edited 3 times in total.

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Re: BBC Micro B Repair Problems

Post by tricky » Sun Aug 12, 2018 10:35 am

GadgetUK164 recently did some videos where he delved into the ULA and patches etc, I think it is summarised on here somewhere.

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Re: BBC Micro B Repair Problems

Post by Elminster » Sun Aug 12, 2018 10:49 am

if anyone finds that thread then probably needs to be linked to the main faulty finding thread, I don’t remember seeing any mention of it when I skimmed the thread last week.

I shall have a view of gadgets videos (already subscribe but have a backlog of stuff to watch) for my own video issues, unrelated to this post.

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Re: BBC Micro B Repair Problems

Post by lambdamikel » Sun Aug 12, 2018 3:58 pm

1024MAK wrote:
Sun Aug 12, 2018 8:46 am
Welcome to our forum :D
Thanks, Mark and all!
You may want to introduce yourself. We are a very friendly community.
Upps, just did that by now!
Which version board do you have? I can't be certain from the photos you posted due to the black crock clip being in the way.
Issue 7
From what you say, you have two faults. One is definitely a fault with one bank of RAM. But don't worry about that at the moment, better to wait until the other fault is fixed. Note that with only one bank of working RAM (16K bytes only), as far as the machine is concerned, it's a model A, so not all display MODEs are available. For example, any MODE which needs 20K bytes will not be able to be selected.

The second fault is a display / picture generation circuitry fault.

I recommend that you first of all check the socket pins on IC2 and on IC6 in case the problem is simply a poor contact. I assume you have swapped IC6?

Have you got a copy of the schematic/ circuit diagram? If you have an electronic copy, I recommend that you try to get it printed out on A3 sized paper.

I assume you have a logic probe based on the croc clips connected to the +5V and 0V points. Do you have, or have access to, any other test gear?
That's right, a logic probe is connected. I used that to check the RAM that I replaced. Yes, both the IC2 (CRT controller) and the ULA (IC6) have been swapped / replaced with another BBC Micro, and the faulty machine shows the same faulty behavior with IC2 and IC6 from the working BBC Micro B. So I belive that the defect is not caused by the big socketed chips.

Yes I have schematics, thanks.

Thanks for your help, Mark!
Last edited by lambdamikel on Sun Aug 12, 2018 4:00 pm, edited 2 times in total.

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Re: BBC Micro B Repair Problems

Post by lambdamikel » Sun Aug 12, 2018 4:05 pm

Elminster wrote:
Sun Aug 12, 2018 10:05 am
First, if I have S26 left = west, then it DOST NOT boot, but rather is stuck with a long beep (then second beep).
The pictures I posted above are with S26 = right = east.
S26 is video output? Set to east it should invert the output, when inverted you have a white background (from memory), but that doesn’t tie in with screenshots below.

I am not sure if this is a mistake or a clue.
Indeed, it is S26. The machine doesn't boot with S25 = 16 KB and S26 = left / west (well, it goes into permanent second beep tone). S26 is supposed to invert the video. To me, it doesn't seem to do that. The machine only boots to BASIC with S26 = pulled or S26 = right / east.
Last edited by lambdamikel on Sun Aug 12, 2018 4:06 pm, edited 1 time in total.

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Re: BBC Micro B Repair Problems

Post by lambdamikel » Sun Aug 12, 2018 4:09 pm

Kazzie wrote:
Sun Aug 12, 2018 10:24 am
I seem to recall that the video inversion with S26 only worked with the early (heatsinked) ULAs (where the ULA as manufactured was accidentally inverted, and needed re-inverting to show correct output). In my issue 7, moving S26 didn't give an inverted display.
That is consistent with my findings. Doesn't invert.

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Re: BBC Micro B Repair Problems

Post by lambdamikel » Sun Aug 12, 2018 4:12 pm

tricky wrote:
Sun Aug 12, 2018 10:35 am
GadgetUK164 recently did some videos where he delved into the ULA and patches etc, I think it is summarised on here somewhere.
Thanks for the pointer, will watch it. I swapped the ULA with a working one from the other working Beeb, and the result is the same.
But I will try that again and post pictures, just to be sure.
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Sun Aug 12, 2018 4:20 pm

1024MAK wrote:
Sun Aug 12, 2018 8:46 am
Do you have, or have access to, any other test gear?
I am an electronics Newbie, but have access to an oscilloscope and even an old HP logic analyzer.

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Re: BBC Micro B Repair Problems

Post by cmorley » Sun Aug 12, 2018 4:28 pm

A scope is more useful than a logic analyser IMO.

First I'd give it a really close visual inspection looking for shorts - there are some solder flecks in your photos so there could be more shorting some signals.

Next I would scope the address pins to the the RAM & see if there is anything funny. You are getting some display and things are being written to RAM. In a screen mode, that isn't mode 7, can you successfully use the cursor keys & COPY button to copy text? (Press left cursor then COPY it should keep copying the ">"). If it beeps then the RAM is definitely corrupt. If it copies then it is more likely a fault in the video read addressing circuit path.

Edit: There could be multiple faults as Mark suggests but try going after one at a time. There is MartinB's memory test to try too:
viewtopic.php?t=14809
Last edited by cmorley on Sun Aug 12, 2018 4:31 pm, edited 3 times in total.

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Re: BBC Micro B Repair Problems

Post by 1024MAK » Sun Aug 12, 2018 4:40 pm

Link S25 determines how much memory the system can see, and which bank of RAM the system sees at address 0000 hex onwards.

Link S26 has to be correct for the actual chip fitted in IC6 position. There are two different manufacturers, and at least two different revisions of the chip from the two manufacturers. Just to confuse matters further, some boards have a modification wire that has been added on the underside. So it's not really a selection between normal and inverted display, because you will only get a proper display if it is in the correct position (where needed) and that depends on the actual chip fitted. This link ONLY affects the display system, it has no effect on the memory or the CPU.

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Re: BBC Micro B Repair Problems

Post by Elminster » Sun Aug 12, 2018 5:15 pm

Didn’t I read somewhere that you can use s25 to get it to 16k, and then do something with a not gate to make it use the other 16k. I might have imagined that.

Also is it worth swapping the Mode 7 ssa chip (don’t have ic number to hand but next to the ula and below the crt) with know good one, just to rule out it is not 2 different issues effecting mode 0-6 and mode 7.

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Re: BBC Micro B Repair Problems

Post by 1024MAK » Sun Aug 12, 2018 5:32 pm

S25 question - Yes, it's detailed in the fault finding index thread, as well as elsewhere in the hardware section.

The 'teletext' chip, IC5, an SAA5050 may cause a failure of the MODE 7 screen. It is also affected by link S26. I doubt it has failed in the OP machine though, given the problems with the other MODEs. At least, I hope not :mrgreen:

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Re: BBC Micro B Repair Problems

Post by Elminster » Sun Aug 12, 2018 5:49 pm

1024MAK wrote:
Sun Aug 12, 2018 5:32 pm
S25 question - Yes, it's detailed in the fault finding index thread, as well as elsewhere in the hardware section.

The 'teletext' chip, IC5, an SAA5050 may cause a failure of the MODE 7 screen. It is also affected by link S26. I doubt it has failed in the OP machine though, given the problems with the other MODEs. At least, I hope not :mrgreen:

Mark
That is probably where I read it.

No not expecting it to be the SSA5050 but just seemed an easy thing to rule out as it is socketed.

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Re: BBC Micro B Repair Problems

Post by lambdamikel » Sun Aug 12, 2018 6:20 pm

cmorley wrote:
Sun Aug 12, 2018 4:28 pm
A scope is more useful than a logic analyser IMO.

First I'd give it a really close visual inspection looking for shorts - there are some solder flecks in your photos so there could be more shorting some signals.

Next I would scope the address pins to the the RAM & see if there is anything funny. You are getting some display and things are being written to RAM. In a screen mode, that isn't mode 7, can you successfully use the cursor keys & COPY button to copy text? (Press left cursor then COPY it should keep copying the ">"). If it beeps then the RAM is definitely corrupt. If it copies then it is more likely a fault in the video read addressing circuit path.

Edit: There could be multiple faults as Mark suggests but try going after one at a time. There is MartinB's memory test to try too:
viewtopic.php?t=14809
Copy cursor (hey, I thought hat was invented for the CPC!!) works. I am able to type in something like
10 print 123
run
and it works. Of course, the output is not readable. But no error. I can also use copy cursor after "list" to enter the same 10 print 123 line again.
And works.

That also works in mode7.

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Re: BBC Micro B Repair Problems

Post by Kazzie » Sun Aug 12, 2018 6:30 pm

cmorley wrote:
Sun Aug 12, 2018 4:28 pm
A scope is more useful than a logic analyser IMO.

First I'd give it a really close visual inspection looking for shorts - there are some solder flecks in your photos so there could be more shorting some signals.
Or alternatively, if you have access to a multimeter with a continuity test function (a BEEP when there's on resistance) you could check if any of the address/data/etc lines are tied together around the RAM chips.

Choose one chip from bank 0, and check for continuity between all possible pairs of pins. Then the same for a chip from bank 1. (You should only need to do one from each bank.) You may want to reference the 4816's pinout from a datasheet. If you find an unexpected connection, that will give you an idea of which pins to check on all your RAM chips.
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Re: BBC Micro B Repair Problems

Post by cmorley » Sun Aug 12, 2018 7:10 pm

lambdamikel wrote:
Sun Aug 12, 2018 6:20 pm
Copy cursor (hey, I thought hat was invented for the CPC!!) works. I am able to type in something like
10 print 123
run
and it works. Of course, the output is not readable. But no error. I can also use copy cursor after "list" to enter the same 10 print 123 line again.
And works.

That also works in mode7.
The reason for doing it in a graphics mode is that if the memory is corrupt then the copy will fail because the character cell doesn't contain the exact character. So accessing the memory from the CPU works.

Your problem is with the video circuitry accessing the memory. The 6845 generates the address lines then a bunch of logic IC 8-11 generates the RAM rows and columns. Mode7 and the graphics modes use different address buffers so the problem is likely upstream of that.

Check for shorts to ground or 5v on the MA lines of the 6845. Check for continuity with a meter between the 6845 and the address latches with your meter. Could be a damaged 6845 socket or a damaged trace or via around it.

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Re: BBC Micro B Repair Problems

Post by leenew » Sun Aug 12, 2018 7:28 pm

IC39 LS283 looks like a good candidate to check.
I had 2 beebs that showed multiple instances of the start-up message and both turned out to be IC39...

Lee.

EDIT: bloody phone!
Last edited by leenew on Sun Aug 12, 2018 7:29 pm, edited 2 times in total.

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Re: BBC Micro B Repair Problems

Post by lambdamikel » Sun Aug 12, 2018 10:37 pm

Kazzie wrote:
Sun Aug 12, 2018 6:30 pm
Or alternatively, if you have access to a multimeter with a continuity test function (a BEEP when there's on resistance) you could check if any of the address/data/etc lines are tied together around the RAM chips.

Choose one chip from bank 0, and check for continuity between all possible pairs of pins. Then the same for a chip from bank 1. (You should only need to do one from each bank.) You may want to reference the 4816's pinout from a datasheet. If you find an unexpected connection, that will give you an idea of which pins to check on all your RAM chips.
Right, that's what I did after I put in the sockets. That way, I found 2 damaged traces; the little yellow jumper wire and DIN / DOUT connection between pin 2 and pin 14 was required. All the data lines arrive correctly at the 74LS245, and the address pins are chip-wise correclty connected. No shorts between address pins eithers. RAS / CAS is also correct. So, I don't think it the RAM or the sockets that I put in. Actually, the RAM was probably never defect. After replacing all the old RAM I am still getting the very same errors.

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Re: BBC Micro B Repair Problems

Post by lambdamikel » Sun Aug 12, 2018 10:54 pm

cmorley wrote:
Sun Aug 12, 2018 7:10 pm
Check for shorts to ground or 5v on the MA lines of the 6845. Check for continuity with a meter between the 6845 and the address latches with your meter. Could be a damaged 6845 socket or a damaged trace or via around it.
I put the logic probes on MA0 to MA13.
I used this pinout:

http://www.classiccmp.org/cpmarchives/c ... Pinout.jpg

Result: there is a pulse on MA0 to MA10.
However, MA11 = constant HI, MA12 = constant LO, MA13 = constant HI.
Same in MODE7.

Is this good or bad ? #-o
Last edited by lambdamikel on Sun Aug 12, 2018 10:55 pm, edited 1 time in total.

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Re: BBC Micro B Repair Problems

Post by lambdamikel » Sun Aug 12, 2018 11:22 pm

leenew wrote:
Sun Aug 12, 2018 7:28 pm
IC39 LS283 looks like a good candidate to check.
I had 2 beebs that showed multiple instances of the start-up message and both turned out to be IC39...
Here is what it does (logic probe):

www.fdi.ucm.es/profesor/mendias/FC/74LS283.pdf

1 = Sigma2 = pulse
2 = B2 = hi
3 = A2 = pulse
4 = Sigma1 = pulse
5 = A1 = pulse
6 = B1 = hi
7 = C0 = hi
8 = GND = lo

9 = C4 = hi
10 = Sigma4 = hi
11 = B4 = hi
12 = A4 = hi
13 = Sigma3 = pulse
14 = A3 = pulse
15 = B3 = hi
16 = VCC = hi


Btw, IC2 and IC6 are getting quite hot after a while....

So, S1... S3 seem to pulse, and S4 is contantly high. S4 goes to IC28 PIN 3 (which is responsible for selection the 2nd half of the 32 KB mem, so to me it seems fine if this is constanly high given that I am testing in 16 KB mode)
Last edited by lambdamikel on Sun Aug 12, 2018 11:34 pm, edited 1 time in total.

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Re: BBC Micro B Repair Problems

Post by lambdamikel » Mon Aug 13, 2018 12:02 am

cmorley wrote:
Sun Aug 12, 2018 7:10 pm
Check for continuity with a meter between the 6845 and the address latches with your meter.
Did that. All MAx from the 6845 to the corresponding inputs of the 81LS95's are ok. No continuity problems.

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Re: BBC Micro B Repair Problems

Post by lambdamikel » Mon Aug 13, 2018 12:29 am

Next, I checked continuity between the outputs of the 81LS95's - from the schematics, it seems that IC8 - IC13, all pins 11, 9, 13, 7, 15, and 3 (A1 - A6) should be pairwise connected. Using the continutiy tester, I found that pin 9 between IC8, 9, 10, 13, 12, was shared, but no connectivity from this group to IC 9 and IC 11. So I put in a jumper cable and the screen is much cleared now. See picture. So it seems that was a good / right move. Still more problems it seems.
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Mon Aug 13, 2018 1:52 am

I am getting "@@A" instead of "BBC"

Looking at the binary:

Code: Select all

@@A 01000000 01000000 01000001 
BBC 01000010 01000011 01000011 
 
it seems that D1 is getting zero constantly (instead of 1).
In addition, "~" turns into "t":

Code: Select all

01111110 ~ 
01110100 t


So it seems that D1 -> 0, and D3 -> 0.
Consequently, "Z" turns into "P", which seems to validate the point:

Code: Select all

01011010 Z
01010000 P 
Is it possible to conclude something from that? Like D1 and D3 are somehow "bad"?
Last edited by lambdamikel on Mon Aug 13, 2018 1:54 am, edited 4 times in total.

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