Fault Fix Model B Showing 16K

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ComanderJameson
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Fault Fix Model B Showing 16K

Post by ComanderJameson » Wed May 22, 2019 10:02 pm

Hi it's been a while.

I've spent the evening trying to resurect my three Model Bs after the only working one started smoking and crackling. So I recapped all three PSUs which took a while.

After much keyboard and connector swapping, I managed to get my Issue 4 working again without a problem. One PSU is completely dead. One Issue 7 refuses to boot (I suspect a broken track from memory).

The other Issue 7 boots, but is showing 16k (not 32k) otherwise it works fine. I've checked that S25 jumper is North.

Not sure how to proceed from here? Any ideas welcome.
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AndyF
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Re: Fault Fix Model B Showing 16K

Post by AndyF » Thu May 23, 2019 8:11 am

Would one of the test discs that included a memory test be worthwhile running perhaps ? Possibly a reasonable 'quick and easy' step although I half suspect it might only "try" to test the 16K rather than look further but for the sake of a minute if you can grab a disc its worth doing.

Actually what happens if you try to pick a lower mode that needs more than 16K ? Bad Mode error ?

Code: Select all

10 ON ERROR MODE 7:REPORT:END
20 FOR X=7 TO 0 STEP -1
30 MODE X
40 PRINT '"MODE ";X''
50 FOR A=32 TO 127
60 PRINT CHR$(A);CHR$(32);
70 NEXT A:N=GET:NEXTX:MODE 7
80 STOP
^ That's probably a bit pointless when you could just type MODE2 or something :oops:
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ComanderJameson
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Re: Fault Fix Model B Showing 16K

Post by ComanderJameson » Thu May 23, 2019 9:23 am

Thank AndyF. I get 'Bad Mode' for the lower modes. But mode 4/5 don't show any corruption at all. But I'll give your code a shot this evening.

I did do some further investigation with the S25 jumper:

North: Boots and says 16k, everything works, no corruption at all, keyboard LEDs and all keys work, break key good.

Open: Same as North.

South: Intermittently boots. When it does, it says 16k, keyboard/LEDs working ok. Pressing break repeatedly, occasionally see corruption in characters before it quickly switches into Mode 4 (which is odd), but displays BBC Computer 16K anyway. Pressing break again mostly goes back into mode 7.

I may investigate the motherboard itself and re-seat/swap ICs, but will have to desolder the composite out wires which is a faff.
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Re: Fault Fix Model B Showing 16K

Post by AndyF » Thu May 23, 2019 9:50 am

You're welcome, sorry tis not much help though! :oops:

A test disc would be a good thing maybe as I say, Watford one I think has some memory test on it.

Composite: I ended up once putting a couple of single pins on the board, a bit like the IDC type header pins so I could simply plug/unplug the composite wiring. Not ideal I know!

The 'code' is probably not worth trying really its only a 'each mode 7 down to 0' with a 'show ASCII codes' on each mode + wait for keypress etc between each.
Last edited by AndyF on Thu May 23, 2019 9:50 am, edited 1 time in total.
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Re: Fault Fix Model B Showing 16K

Post by Kazzie » Thu May 23, 2019 11:34 am

ComanderJameson wrote:
Thu May 23, 2019 9:23 am
I did do some further investigation with the S25 jumper:

North: Boots and says 16k, everything works, no corruption at all, keyboard LEDs and all keys work, break key good.

Open: Same as North.

South: Intermittently boots. When it does, it says 16k, keyboard/LEDs working ok. Pressing break repeatedly, occasionally see corruption in characters before it quickly switches into Mode 4 (which is odd), but displays BBC Computer 16K anyway. Pressing break again mostly goes back into mode 7.

I may investigate the motherboard itself and re-seat/swap ICs, but will have to desolder the composite out wires which is a faff.
This behaviour of S25 might be a RAM fault. If you are able to wire up and inverter to S25 I suspect your machine would report 32k RAM, but with corruption onscreen. Here's my reasoning:

The way the Beeb detects how much RAM it has (in software) is by incrementing a counter as it clears the RAM on a hard reset. This happend early on in the boot sequence:

Code: Select all

.LD9E7 

\********** clear memory routine ******************************************

LDX #&04            :\ get page to start clearance from (4)
STX &01             :\ store it in ZP 01
STA &00             :\ store A at 00

TAY                 :\ and in Y to set loop counter
.LD9EE 
        
STA (&00),Y         :\ clear store
CMP &01             :\ until address &01 =0
BEQ LD9FD           :\ 
INY                 :\ increment pointer
BNE LD9EE           :\ if not zero loop round again
INY                 :\ else increment again (Y=1) this avoids overwriting
                    \;RTI instruction at &D00
INX                 :\ increment X
INC &01             :\ increment &01
BPL LD9EE           :\ loop until A=&80 then exit
.LD9FD 
                    \;note that RAM addressing for 16k loops around so
                    \;&4000=&00 hence checking &01 for 00.  This avoids
                    \;overwriting zero page on BREAK


STX &028E           :\ writes marker for available RAM 40 =16k,80=32
OPT FNif(VALversion$>=1.20)
  STX &0284         :\ write soft key consistency flag
OPT FNendif
The value written to address &028E (and &0284) will be &40 for a 16k machine, or &80 or a 32k machine. When S25 is North, the value &80 should be written, but if a RAM fault is causing it to be written (or read) as &40, then the Beeb would assume it only had 16k fitted.

If your Beeb behaves as I think with S25 inverted, then we should be able to identify which bits of RAM are faulty from the display corruption that is shown.
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
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ComanderJameson
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Re: Fault Fix Model B Showing 16K

Post by ComanderJameson » Thu May 23, 2019 5:59 pm

Thanks again both for your help.

The lack of memory corruption was puzzling me. I did a quick memory test and found that address &1800 was being mirrored at &5800! So it had to be address decode logic or some sort. So I took the plunge and desoldered the composite out.

Once I looked at the underside of the of the motherboard, I immediately saw that IC28 had been removed at some point, that explains everything (I hope)! Serves me right for buying things on ebay.

I think a visit to my favourite electronics supplier is in hand.

Now to get my last dead Beeb working, but that's another project.
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AndyF
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Re: Fault Fix Model B Showing 16K

Post by AndyF » Thu May 23, 2019 6:34 pm

Good to read there's been some progress. :) :)

I'm sure it will all be 'as expected' soon.

So true regarding the purchase, same as most things I suppose you could find anything! :D

I found a most bizarre transistor/cap arrangement "floating about" on an Oric-Atmos around the modulator input area that "someone" had thoughtfully (un-thoughtfully?) added, not really sure why , it did not work anyway! :lol: I must get around to sorting that out actually. :oops:
Last edited by AndyF on Thu May 23, 2019 6:36 pm, edited 3 times in total.
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ComanderJameson
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Re: Fault Fix Model B Showing 16K

Post by ComanderJameson » Thu May 23, 2019 7:04 pm

Oric Atmos, is that a rare beast?

I have loads of little projects I start but never finish. I did half build a Pascal 6502 compiler, but that's languishing! Software's where it's at, but it's good to get my hands dirty with spit and solder.

I am starting to think that the random Apex board I have (looks like some sort of ROM/RAM type expansion) is related to the lack of IC28.

I'll have to post up a picture of it once I can find a way to do it. Maybe in a different thread for that.
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Re: Fault Fix Model B Showing 16K

Post by AndyF » Thu May 23, 2019 11:10 pm

Nearest (ish) equivalent I can really think of is a 48K 6502 based (instead of Z80) Speccy, but with a keyboard instead of a membrane rubber mat. Its half decent to type on. Brief info here

There are some slightly odd things with it (generally I think not just mine) but decent-ish selection of software if you hunt. A great shame I think it arrived too late "to the party" as such back in the early 80's oh well.

Anyway something for another topic perhaps, I do want to talk about it at a point though. :)


Back on topic have you checked via Google or here for any info on the Apex board ? I'm thinking here of some kind of fitting instructions where perhaps like some of the rom/ram type boards they say to remove IC xyz and place it into said 'new' board etc. Just a thought. :)
Last edited by AndyF on Thu May 23, 2019 11:11 pm, edited 1 time in total.
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Adventure games ported across to the BBC (in progress) as soon as I can find some time!

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