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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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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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Fri Aug 17, 2018 8:22 am

Update - I was lucky enough to find the remaining RAM problem in the 2nd 16 KB bank without RAM test or inverter diagnosis tool.

Using the continutiy tester and logic probe, I found the remaining problem with the 32 KB mode. This time, IC 63 was bad, and also a couple of traces were broken and needed to be bridged. After the RAM being fixed, I fitted in my DFS disk upgrade kit, and installed the 2 game ROMs I had purchased from Retroclinic a while ago. Also, my second PSU was fixed with the cap replacement kit. Now I am a very happy and proud owner of two 32 K BBC Micros B's. Next I will install the MMC solid state disk which should arrive soon.

Thank you all for the great help!! :D This is a great community! =D> =D> =D>
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Elminster
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Re: BBC Micro B Repair Problems

Post by Elminster » Fri Aug 17, 2018 8:26 am

Well done. Must have put a lot of time and effort in to fix that many things from not much previous knowledge.

Sounds like someone went to town on your machine, seemed to have a large amount of faults.

Has motivated me to fix one of my machiens that broke 3 years ago :)

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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Fri Aug 17, 2018 8:35 am

Elminster wrote:
Fri Aug 17, 2018 8:26 am
Well done. Must have put a lot of time and effort in to fix that many things from not much previous knowledge.

Sounds like someone went to town on your machine, seemed to have a large amount of faults.

Has motivated me to fix one of my machiens that broke 3 years ago :)
It is 1.30 am in Palo Alto now, and I am going to bed now - I will probably hear the beeping of the continuity tester and humming of the logic probe pulses in my dreams :)

Without the help from you guys and the schematics (I had purchased the "ACORN BBC MICRO NEW UPDATED CONCISE SERVICE MANUAL & SCHEMATICS" from Retroclinic a while ago - so glad I did, even if it is just for the large large schematics fold out, so much better for fault finding than a monitor and PDF!) this would not have been possible.

Indeed, not much BBC knowledge here at all - but enthusiasm. Can't stand broken micros. This one was a real bargain now, I think I only paid 60 $ or so for it on Ebay - of course it was being sold as "untested"). Fixing it was about 25 hours of work I suppose.

I hope that I can also help somebody on this board here one day.

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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Fri Aug 17, 2018 8:36 am

Elminster wrote:
Fri Aug 17, 2018 8:26 am
Has motivated me to fix one of my machiens that broke 3 years ago :)
Congratulations to fixing yours, too!! The beebs are becoming more and more precious. Let's keep them around for some more decades.

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Elminster
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Re: BBC Micro B Repair Problems

Post by Elminster » Fri Aug 17, 2018 8:41 am

Good night. Morning here, all bushy tailed.
lambdamikel wrote:
Fri Aug 17, 2018 8:36 am
Elminster wrote:
Fri Aug 17, 2018 8:26 am
Has motivated me to fix one of my machiens that broke 3 years ago :)
Congratulations to fixing yours, too!! The beebs are becoming more and more precious. Let's keep them around for some more decades.
[-o< :)

I have "ACORN BBC MICRO NEW UPDATED CONCISE SERVICE MANUAL & SCHEMATICS" from Retroclinic as well IC26 looks like it says IC25 (in poor lighting anyway). Had me scratching my head for ages why the VIDPROC out pin was not working but all the VIDPROC in pins were. Till I realised I was looking at the wrong chip.
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leenew
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Re: BBC Micro B Repair Problems

Post by leenew » Fri Aug 17, 2018 9:27 am

Good work.
Very well done =D>

Lee.

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KenLowe
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Re: BBC Micro B Repair Problems

Post by KenLowe » Fri Aug 17, 2018 11:16 am

Looks like removing RAM ICs was a bit of a challenge for someone, but great to see another Beeb saved from landfill! Well done! =D> =D> =D>
lambdamikel wrote:
Fri Aug 17, 2018 8:22 am
Next I will install the MMC solid state disk which should arrive soon.
The MMC is a good solution, but it would also be worth considering the Gotek FDD emulator - particularly since you already installed the necessary DFS hardware.
You can buy the basic Goteck unit from ebay relatively cheaply, and can then add on some very useful upgrades if you want. You will also need to flash upgrade of the base firmware. Full details can be found here: https://github.com/keirf/FlashFloppy/wiki
Alternatively you can purchase a fully upgraded Gotek from Retroclinic (or from some others on ebay). These pre-built ones are quite expensive though, and you can probably build yourself a fully upgraded one for about half that price if you shop around for the parts.
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Elminster
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Re: BBC Micro B Repair Problems

Post by Elminster » Fri Aug 17, 2018 1:37 pm

Agreed.

The gotek makes internal SD solutions like MMC (or in my case gosdc) much more usable, as no faff with taking off case to extract SD card. You can just use the gotek to copy it over. Life is sooo much eaiser now....

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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Fri Aug 17, 2018 2:07 pm

Elminster wrote:
Fri Aug 17, 2018 8:41 am
I have "ACORN BBC MICRO NEW UPDATED CONCISE SERVICE MANUAL & SCHEMATICS" from Retroclinic as well IC26 looks like it says IC25 (in poor lighting anyway). Had me scratching my head for ages why the VIDPROC out pin was not working but all the VIDPROC in pins were. Till I realised I was looking at the wrong chip.
Ohh?! That can be a source of confusion for sure! I myself struggeled a bit with distinguishing "Dn" and "On" lines on IC14 and IC15... after all, of course there is only one way that makes sense, but still... requires a magnifier, and even then, the tiny O and D characters are almost indistinguishable on the paper.
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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Fri Aug 17, 2018 2:11 pm

Elminster wrote:
Fri Aug 17, 2018 1:37 pm
Agreed.

The gotek makes internal SD solutions like MMC (or in my case gosdc) much more usable, as no faff with taking off case to extract SD card. You can just use the gotek to copy it over. Life is sooo much eaiser now....
Ahh, now you got me hooked again... damn :D More money and time and fun :D

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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Fri Aug 17, 2018 2:21 pm

KenLowe wrote:
Fri Aug 17, 2018 11:16 am
Looks like removing RAM ICs was a bit of a challenge for someone, but great to see another Beeb saved from landfill! Well done! =D> =D> =D>
Thanks you :D Yeah desoldering chips I must say I am really not good at... eventhough I almost got clean PCB holes after desoldering this time. My procedure is to first cut out the chip with a small precision electronics wire cutter, then use twezzers and the iron to desolder the IC pins / leads. After that, there is still residue solder in the PCB holes, so I am trying to get that out with desoldering wick, a little desoldering pin that fits through the hole (if heated) to push out stuck residue solder, a vacuum sucker, and even a can of compressed air. Sometimes, it takes a couple of iterations and all these tools to get all the residue solder out. If somebody has a better procedure to that, please let me hear it :) The problem is that the PCB is quite delicate... sometimes, in the process of cleaning the PCB hole, the solder ring comes off due to heat or mechanical influence. Maybe I have the wrong iron for that. At least this time, I was amazed by the "super wick" desoldering braid I was using - it works great (unlike some braid I had used previously, which was basically doing NOTHING), but STILL it leaves some residue. It's a painful process.
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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Fri Aug 17, 2018 2:31 pm

... after showing it off all naked to the world :oops: , let's dress it up and make it more presentable, shall we? Yes, it also has a case :D

Hmm, you guys got me thinking. Now that I have 2 working Beebs, how about setting up an econet between the 2, then they could chat :) Is the econet option being sold in a similar way as an upgrade kit somewhere? Retroclinic doesn't seem to have it. Would it make sense?
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lcww1
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Re: BBC Micro B Repair Problems

Post by lcww1 » Fri Aug 17, 2018 3:00 pm

lambdamikel wrote:
Fri Aug 17, 2018 2:31 pm
Now that I have 2 working Beebs, how about setting up an econet between the 2, then they could chat :) Is the econet option being sold in a similar way as an upgrade kit somewhere?
Check out this thread - viewtopic.php?f=8&t=15086


You’ll need a second processor for one of the beebs to act as an Econet server, but you can use a Raspberry pi with a level shifter adapter board for this upgrade as described here: viewtopic.php?f=3&t=11325

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lambdamikel
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Re: BBC Micro B Repair Problems

Post by lambdamikel » Fri Aug 17, 2018 5:01 pm

lcww1 wrote:
Fri Aug 17, 2018 3:00 pm
lambdamikel wrote:
Fri Aug 17, 2018 2:31 pm
Now that I have 2 working Beebs, how about setting up an econet between the 2, then they could chat :) Is the econet option being sold in a similar way as an upgrade kit somewhere?
Check out this thread - viewtopic.php?f=8&t=15086


You’ll need a second processor for one of the beebs to act as an Econet server, but you can use a Raspberry pi with a level shifter adapter board for this upgrade as described here: viewtopic.php?f=3&t=11325
I've seen the PiTube, amazing...

One more question - how difficult is it to get the speech chip and ROM? Are they being sold somewhere? If anybody has one to sell, please contact me.

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