BBC Micro B Repair Problems

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Kazzie
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Re: BBC Micro B Repair Problems

Post by Kazzie » Mon Aug 13, 2018 4:17 am

That's what I'd conclude. It appears that the RAM chips that provide D1 and D3 (in the relevant bank) are faulty. (Or perhaps the 74LSLS245 bus transciever.) I identified a faulty RAM chip (with slightly different symptoms) in my Beeb through a similar process.

There's a listing of which chips correspond to which bit of which bank in this fine post by 1024MAK.

Well done on your progress so far!
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM (mid- restoration)
Acorn System 1 home-made replica

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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Mon Aug 13, 2018 4:53 am

... and now my PSU just emitted the magic white smoke... I still have one of these cap replacement kits.
Time to use it now. So far, I only had to repair one PSU.

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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Mon Aug 13, 2018 4:53 am

Kazzie wrote:
Mon Aug 13, 2018 4:17 am
Well done on your progress so far!
Thanks! Let's see if I can wrap it up... need to fix my second PSU first though.

cmorley
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Re: BBC Micro B Repair Problems

Post by cmorley » Mon Aug 13, 2018 6:21 am

Are you in 16 or 32K mode? Try the graphics mode again. If that is OK then IC15 (or connections to/from it), the buffer to the teletext chip, might be bad & the RAM fine.

Run MartinB's memory test.

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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Mon Aug 13, 2018 6:35 am

Eureka! Little do I know about the Beeb yet - haven't realized that the thing is INDEED working now in graphics mode!!
Screenshots attached. Thanks a lot!! Will take a look at IC 15 then...
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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Mon Aug 13, 2018 6:35 am

cmorley wrote:
Mon Aug 13, 2018 6:21 am
Are you in 16 or 32K mode?
16 K Mode.

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1024MAK
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Re: BBC Micro B Repair Problems

Post by 1024MAK » Mon Aug 13, 2018 10:29 am

Good progress =D>

Well done :D

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...

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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Tue Aug 14, 2018 6:06 am

I was checking connectivity of IC15, and couldn't find anything wrong. Also, the logic probe showed OK behavior IMHO. Then I thought, what the heck, let's socket and replace IC15 anyway, maybe it has gone "slightly bad". I removed the chip and in that process, I was getting close to the upstanding 1k resistors R98 - R101 to the right of it. I tried to move them a bit in order to get access to the chip to clip it out, and realized that at least 2 of them were broken at the base, in the ceramics! I could just bend them up and lift them of with my fingers. Hence, replaced all of that stuff, and voilá, welcome back to the world, little beeb :D :D

Next, the 32 k mode is still giving me the constant beeb at startup. The 2nd 16 k bank is still bad. However, now I should be able to narrow it down by using the memory test that was mentioned here, right? That should be much easier I hope.

Thank you guys for all the help so far, I wouldn't have come so far without this great help and support - especially the IC15 hint was gold! I'll keep you posted on the progress of the other mem bank repair.
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Last edited by lambdamikel on Tue Aug 14, 2018 6:12 am, edited 3 times in total.

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1024MAK
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Re: BBC Micro B Repair Problems

Post by 1024MAK » Tue Aug 14, 2018 7:33 am

I wonder if a previous owner got a bit too aggressive with an upgrade/expansion, or when working on the board.
Resistors don't normally cause problems in Beebs, let alone break like you have described...

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...

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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Tue Aug 14, 2018 3:29 pm

It's hard to tell what happened, right... maybe I contributed to the resistor problem when I wasn't careful enough, or I might have even broken them / pushed them over the top when I carefully bent them away to gain access to the chip. One problem I see with this way of mounting them (flushing the ceramics body onto the PCB) is that there is not much space then for wiggeling / bending the resistor, and there is a lot of tension and torsion on the ceramics body.

I don't have a disk drive (I have a Turbu MMC old-style, not user port in my other Beeb, but I do not want to uninstall it). Is there a way to put the memory tester into a side way EPROM?

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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Tue Aug 14, 2018 4:18 pm

It seems there a couple of options for RAM test:
- MartinB's prog (either SSD DFS or MP3 or WAV via cassette)
- Tricky's test ROM
viewtopic.php?f=3&t=13973&hilit=memory+ ... m&start=30

Anything else I am missing? I think it might be easier to go with Tricky's test ROM (I don't have cassette or SSD disk set up )

Kazzie
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Re: BBC Micro B Repair Problems

Post by Kazzie » Tue Aug 14, 2018 4:33 pm

There are also some type-in memory test programs you can use. follow the links in my post at the top of the page for some examples.

As small programs they're simple in their scope, generally writing a value to a memory location and reading it back again. This will catch a bit stuck high (or low, depending on the value written), but for some stranger faults, such as in the address decoding logic, you may need to adapt the code slightly, as I did in the first link.
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM (mid- restoration)
Acorn System 1 home-made replica

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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Tue Aug 14, 2018 4:55 pm

Kazzie wrote:
Tue Aug 14, 2018 4:33 pm
There are also some type-in memory test programs you can use. follow the links in my post at the top of the page for some examples.

As small programs they're simple in their scope, generally writing a value to a memory location and reading it back again. This will catch a bit stuck high (or low, depending on the value written), but for some stranger faults, such as in the address decoding logic, you may need to adapt the code slightly, as I did in the first link.
One problem in understanding I have though is the following - the beeb is not booting in 32 K mode. (Constant beeb). Are the test programs, if run in 16 K mode, regardless able to test the disabled, defect 16 KB bank? I thought that was impossible due to the way the address decoders are set up?

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KenLowe
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Re: BBC Micro B Repair Problems

Post by KenLowe » Tue Aug 14, 2018 5:46 pm

lambdamikel wrote:
Tue Aug 14, 2018 4:55 pm
One problem in understanding I have though is the following - the beeb is not booting in 32 K mode. (Constant beeb). Are the test programs, if run in 16 K mode, regardless able to test the disabled, defect 16 KB bank? I thought that was impossible due to the way the address decoders are set up?
No. The test program wouldn't be able to test the disabled bank. Since your machine can boot using one of the 16k banks, then it should be able to boot to 32k, but you would need to invert the upper 16k and lower 16k banks, so that the faulty bank is mapped to the upper 16k instead of the lower 16k. All you need is a simple inverter. Details can be found on this site.

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Elminster
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Re: BBC Micro B Repair Problems

Post by Elminster » Tue Aug 14, 2018 5:49 pm

Mark posted the link in the thread already I think. but it is

viewtopic.php?f=3&t=2190&start=30#p38006

just in case it was a different topic I was thinking of.

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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Tue Aug 14, 2018 5:58 pm

Aha, yes the inverter... now I remember I once watched a video on YouTube about it. Thanks, will try this weekend.

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