Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

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leenew
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Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby leenew » Thu Jul 05, 2012 5:08 pm

Hi,
I guess this has been asked loads before, but I am new to this restoration business and could do with a hand!
Just bought 4 beebs and I am trying to bring them all back to life so I may keep the best bits and re-ebay the rest, all hopefully good and working.
I am gonna try to do them one at a time... so....

BEEB 1
I have pre-emptively changed C1, C2, and C9.
On switch on it gives a long continuous beep.
moving S25 SOUTH allows it to boot as 16K.
removing S25 altogether goes back to a long beep.
Am I right in thinking this has to be a ram fault? or could anything else cause this?
If it is RAM, is there a software program that I could run that will tell me which locations are bad, or even better, which CHIP is bad.
Also, I am not clear that, if I am switching out the bad RAM by moving the S25 link, will the memory test even find the naughty chips??
I am a complete amateur at this, and I am just investing in a new (cheap) soldering iron, and a new (cheap analogue) multimeter, but I have no other test equipment, other than having spare beeb parts to swap.
Thanks in anticipation.

Lee.

RobC
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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby RobC » Thu Jul 05, 2012 5:54 pm

It does indeed sound like you have a fault in the low 16K of RAM. I'm pretty sure you are right in thinking that you want be able to run a RAM test on the bottom 16K with S25 set to South.

I've got a Beeb that's got a similar problem - what I think you'll need to do is rig up a little circuit to invert the signal going from S25 North to the middle of S25. I think this will cause the RAM banks to be swapped so that you can run a memory test on the dodgy RAM (which will now be mapped from 0x4000 to 0x7fff).

Others on here will be able to confirm/deny this!

Hope you get it sorted.

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paulv
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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby paulv » Thu Jul 05, 2012 7:19 pm

RobC wrote:I've got a Beeb that's got a similar problem - what I think you'll need to do is rig up a little circuit to invert the signal going from S25 North to the middle of S25. I think this will cause the RAM banks to be swapped so that you can run a memory test on the dodgy RAM (which will now be mapped from 0x4000 to 0x7fff).

Others on here will be able to confirm/deny this!


A 74LS04 hex inverter is used to do this...

Paul

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leenew
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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby leenew » Thu Jul 05, 2012 10:00 pm

Hi,
Well I can write...
10 Print "Hello"
20 GOTO 10
And now you are speaking of 74LS04 Hex inverters.
It is also a quarter of a century since I studied electronics at college and I have remembered about 0% of it :o

**thinks..... wonder if my friend may help??...**

Is there a simple circuit diagram someone could draw or explain??
I was just about to say I will pop off to Maplin in the morning.. but their search engine hasn't heard of a 74LS04.... can anybody flog me the bits :D
or point me to a supplier.
Gawd I am so out of touch with this!!!
Cheers everyone.
This is much more intriguing than swapping keyboard membranes.......

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MartinB
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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby MartinB » Thu Jul 05, 2012 10:19 pm

If you go to this Maplin page and scroll down the chips you will find the one you seek :wink:

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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby richardtoohey » Thu Jul 05, 2012 10:28 pm

They never covered electronics in my courses, so I'm following your adventures with interest - so keep posting about how it goes and what you have to do (in as plain English as possible ... Low Power Schottky Bipolar Logic ... ulp!)

I've got some BBCs now and I know they won't last forever so I do need to learn some more about what is inside those cases.

Good luck!

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leenew
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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby leenew » Thu Jul 05, 2012 10:50 pm

Yes...
Adventure...
That's what it is!

Maybe it shall be one of the beeb's finest!

Sphinx Adventure... Castle of riddles.... and now...

The Quest for Satan's Ram.

I have followed Martin's scroll of destiny to the land of Maplin....

I indeed have discovered 79 pence worth of treasure! using the secret code... 74LS04N.

get chip
*taken*

This is a strange device to my eyes. I still need to unlock its' secrets.

put chip on board
*done*
The chip pushes into the vero board with a satisfying CLUNK

Can any of you that are well versed in these dark arts tell me how to trick satan's ram into believing that high is low and low is high??

can it be so simple as to.......
hmmm.
maybe... just maybe...

TBC

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MartinB
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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby MartinB » Thu Jul 05, 2012 11:16 pm

I've just doodled this to help....
beeb s25.GIF
(40.21 KiB) Downloaded 830 times

I ran out of creative enthusiasm so note that +5v is Pin 14 and 0v is Pin 7 :wink:

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leenew
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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby leenew » Thu Jul 05, 2012 11:41 pm

Marvellous Martin!
Well as far as circuits go, it pretty simple!
a chip and a couple of bits of wire :lol:
I shall report back when I get results from the various memory checking programs i have found around here.
This may be monday ish as i am extreeeeeeeeemely busy this weekend.
thanks so far for all the info,

thinking into the future a bit....

I am gonna have to probably de-solder and re-solder Ram chips...
I am sh*tting it a bit I must admit!

cheers all.

RobC
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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby RobC » Fri Jul 06, 2012 9:11 am

leenew wrote:thinking into the future a bit....

I am gonna have to probably de-solder and re-solder Ram chips...
I am sh*tting it a bit I must admit!


The soldering bit isn't hard - the removal of the old chip is the trickiest bit. Get yourself a side-cutter to snip the legs of the offending chip and some desoldering braid to clean out the PCB holes.

Once you've removed the bad chip, gently heat each of the holes and remove any remains of the pins (they'll either stick to the tip of your soldering iron or you can use tweasers). Then use the desoldering braid to remove any solder in the holes so that they are clean and you can see through them.

When you think you've done this, make sure that you have a good look at each of the holes with a magnifying glass just to check that there aren't any bits of the pins left in the holes.

Once the holes are clean you should find that it's relatively easy to solder in the new chip from the underside of the board.

Alternatively, if you're anywhere near South Wales, PM me and I'll happily do it for you.

Cheers,

Rob

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leenew
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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby leenew » Fri Jul 06, 2012 9:22 am

Thanks Rob. However, I am in Yorkshire so it looks like I'm doing it myself :D would you not solder a socket in then? Bear in mind I don't think I am the best solderer in the world!

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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby RobC » Fri Jul 06, 2012 11:13 am

leenew wrote:would you not solder a socket in then? Bear in mind I don't think I am the best solderer in the world!


That's probably a very good idea. (I prefer to make repairs look as authentic as possible but that's just an aesthetic thing.)

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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby paulv » Fri Jul 06, 2012 11:36 am

RobC wrote:That's probably a very good idea. (I prefer to make repairs look as authentic as possible but that's just an aesthetic thing.)


Many Beebs are a 50/50 split on socketed RAM. I don't see adding a socket for a RAM chip as detracting from the authenticity of the machine. The added benefits of a good turned pin socket make life easier in the future if further chip swapping is required later too :D

Paul

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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby RobC » Fri Jul 06, 2012 12:28 pm

paulv wrote:Many Beebs are a 50/50 split on socketed RAM. I don't see adding a socket for a RAM chip as detracting from the authenticity of the machine. The added benefits of a good turned pin socket make life easier in the future if further chip swapping is required later too :D


Yes - I've got a Beeb like that (suspect it's an upgraded model A). A good quality turned-pin socket should be easier to fit too.

When I've replaced RAM chips before, I've always thought that having just one of them socketed would look a bit odd but maybe I should've been less OCD about it! :)

Cheers,

Rob

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leenew
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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby leenew » Fri Jul 06, 2012 1:46 pm

I think i'm gonna buy some sockets anyway.... and think about it :D
Where the bugger do I get Japan 4E3 8 HM4816AP-11's from then :?

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leenew
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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby leenew » Fri Jul 06, 2012 5:11 pm

Right, just before I get ready to go back to work (**sigh**)...
I have bought a breadboard and a 74LS04 and wired it up as per the diagram...
beeeee bip.
** screen full of zeros in random places but still seems to operate. Difficult to type as garbage is all over.

switched to mode 0 (as advised on another post)
scrren is now more clear of rubbish, just as the clever people said it would be.
ran this program (courtesy of clever people again)

5 MODE 0
10 ?&4000=0
20 A=?&4000
30 IF A=0 THEN GOTO 20
40 PRINT A

the screen has a flashing cursor and is gradually filling up with pixels.
no answer spat out after 30 minutes though...
so I will try this more thorough one in the morning.

10CLS:VDU23;8202;0;0;0;
20M%=&E00:X%=&8000:E%=FALSE
30I%=1:A%=&AA:V%=&55:S%=42:C%=13
40REPEAT
50?M%=A%:IF?M%<>A% E%=TRUE
60?M%=V%:IF?M%<>V% E%=TRUE
70PRINTCHR$((M%ANDI%)+S%);CHR$(C%);:M%=M%+I%
80UNTIL M%=X% OR E%
90IFE% PRINT~M%-1;ELSEPRINT"OK";

Once again,
you are all amaaaaaaaaaazing :-)

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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby paulv » Fri Jul 06, 2012 5:32 pm

leenew wrote:5 MODE 0
10 ?&4000=0
20 A=?&4000
30 IF A=0 THEN GOTO 20
40 PRINT A


Which thread did you get that code from? It's faulty as it only ever sets and checks memory in one location &4000 and if that location is sound then it'll keep checking it ad infinitum...

Paul

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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby TopBanana » Fri Jul 06, 2012 6:10 pm

It's here Paul

http://www.stardot.org.uk/forums/viewtopic.php?f=3&t=5089&p=46827&hilit=10+%3F%264000%3D0#p46827

I must admit I looked at the code Mark posted and wondered what I was missing as I couldn't see how it worked ...... :lol:

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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby 1024MAK » Fri Jul 06, 2012 7:27 pm

I think it is based on checking one bank only for one type of individual bit fault.
Each DRAM chip is actually a one bit by 16k memory. So to make a 16k byte memory bank you need 8 chips. Each chip being connected to a different data bit.
If one chip is faulty due to a faulty data input or output circuit, this BASIC program would show this.
Mark K.
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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby retroclinic » Fri Jul 06, 2012 8:13 pm

1024MAK wrote:I think it is based on checking one bank only for one type of individual bit fault.
Each DRAM chip is actually a one bit by 16k memory. So to make a 16k byte memory bank you need 8 chips. Each chip being connected to a different data bit.
If one chip is faulty due to a faulty data input or output circuit, this BASIC program would show this.
Mark K.



Spot on. Also, it's only a very simple guide program, you can change the 0 to 255 to check for stuck on bits, or change the address, or use a RND(&4000)+&4000 to check for a range within the faulty bank.

Mark.
Image

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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby paulv » Fri Jul 06, 2012 10:02 pm

Ok, I see it tests the control lines for each bit ad infinitum until a failure condition appears so rather than testing the RAM it's testing the surrounding circuitry. Cool.

Paul

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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby 1024MAK » Fri Jul 06, 2012 11:36 pm

HITACHI wrote:16384-word X 1-bit Dynamic Random Access Memory

The HM4816 is a new generation MOS dynamic RAM circuit organized as
16,384 words by 1 bit. As a state-of-the art MOS memory device, the HM4816
(16K RAM) incorporates advanced circuit techniques designed to provide wide
operating margins, both internally and to the system user, while achieving
performance levels in speed and power previously seen only in MOSTEK's high
performance MK4027 (4K RAM).
The technology used to fabricate the HM4816 is HITACHI's double-poly,
N-channel silicon gate process.
This process, coupled with the use of a single transistor dynamic storage cell
provides the maximum possible circuit density and reliability, while maintaining
high performance capability.
The use of dynamic circuitry throughout, including sense amplifiers, assures that
power dissipation is minimized without any sacrifice in speed or operating margin.
These factors combine to make the HM4816 a truly superior RAM product.
Multiplexed address inputs (a feature pioneered by MOSTEK for its 4K RAMs)
permits the HM4816 to be packaged in a standard 16-pin DIP.
This recognized industry standard package configuration, while compatible with
widely available automated testing and insertion equipment, provides highest
possible system bit densities and simplifies system upgrade from 4K to 16K RAMs
Non-critical clock timing requirements allow use of the multiplexing technique
while maintaining high Performance.
HM4816 pin-out S.png
HM4816 1 bit x 16k DRAM pin-out
(25.41 KiB) Downloaded 689 times

As you can see from the pin-out diagram there are separate data input (Din) and data output (Dout) pins. In the Beeb these are connected together and then to one line of the data-bus.

Mark
Attachments
HM4816 datasheet.pdf.zip
The data sheet in a zip file
(197.61 KiB) Downloaded 77 times
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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby TopBanana » Sat Jul 07, 2012 7:36 am

paulv wrote:Ok, I see it tests the control lines for each bit ad infinitum until a failure condition appears so rather than testing the RAM it's testing the surrounding circuitry. Cool.

Paul


OOoohhh, that's subtle, far too subtle for me :lol: :lol: :lol:

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Re: Display Problems

Postby leenew » Sat Jul 07, 2012 9:21 am

OK,
I have ran the first program again, using 255 instead of 0 to check for stuck bits (as per Mark).
It ran for a while, with the same hypnotic twinkling pixels, and then.....
239
the magic number is 239.
I still don't get the program!!! but 239 it is.
I am going to type in and run the second one now and I will report the results shortly.

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leenew
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Re: Display Problems

Postby leenew » Sat Jul 07, 2012 9:33 am

Right,
tried the second program but I am having probs.
if I type PAGE=&900
then as soon as I type the first line of the program in, the computer hangs.
if I don't type PAGE=&900 then I can type it all in but as soon as I type RUN
I get MISTAKE
then BAD PROGRAM, and the program is lost.
hmmm...
running memory programs on faulty memory machines.... catch 22

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MartinB
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Re: Display Problems

Postby MartinB » Sat Jul 07, 2012 9:44 am

Try typing NEW<cr> after typing PAGE=&900. Just use a simple 10 PRINT"Hello":GOTO 10 to check basic memory viability before typing in the whole thing.

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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby 1024MAK » Sat Jul 07, 2012 10:03 am

Cross reference to other thread >>> viewtopic.php?f=3&t=1871#p49137
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...

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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby 1024MAK » Sat Jul 07, 2012 10:12 am

The result, 239 is a decimal number.
When converted to a binary number, it has more meaning:-
239 decimal = 11101111 binary

Code: Select all

bit number 76543210
result=    11101111
So it looks like the DRAM chip in that bank that provides data bit D4 is faulty and needs to be replaced.

(Data bits are normally numbered D0, D1, D2 to D7).

Mark
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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby Samwise » Sat Jul 07, 2012 11:16 am

OK, I've tidied up these topics a bit. leenew's original posts have been moved to this new topic.

Do carry on ... ;)

Sam.

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Re: Beeeeeeeeeeeeeeeeeeeeeeeee faulty beeb help please

Postby TopBanana » Sat Jul 07, 2012 11:22 am

1024MAK wrote:So it looks like the DRAM chip in that bank that provides data bit D4 is faulty and needs to be replaced.

Mark


Mark, how do you identify which chip that is ?


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