Fault finding index

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1024MAK
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Re: Fault finding index

Postby 1024MAK » Tue Jul 19, 2011 12:07 am

I do indeed think that it is a good idea :D .
I do hate reinventing the wheel, and I suspect a lot of faults with BBC machines are what the TV repair trade call stock faults.
So if a fault is common, or various symptoms are caused by one component (or a handful of components like when the regulation feedback loop goes wrong in a switch mode power supply :cry: ). Then if it is documented, it saves having to always start fault finding from scratch :D
The problem is, that as most of the forum members are hobbyists, most will have limited electronic skills, and fewer still will be good at logical fault finding, tracing and repair :shock: :( .
Fewer still will fully document what the exact symptoms were and what they did to fix the fault :cry: .
This does limit the flow of information into to such a database.
Some years ago when the TV repair trade was large, they had the benefit of large numbers of repair workshops which carried out lots of repairs - and as a result, lots of information was collected (and a lot was published in the now defunct trade magazine "Television").
Therefore I think this idea / project will be a bit of a slow burner :-| . But it also needs someone to do a bit of proactive prodding to extract information from members who do post about problems, but who do not forward the required information [-o< .
Once members get used to the information that is required, things may improve.

Anyway them there are my thoughts.

BTW I work for a company where the main part of my job is attending and repairing failures. I work on electronic systems (but not at component level), electrical systems, mechanical systems, computer systems, telecom systems, relay based logic systems or a nice mix of the different technologies :roll: :lol:
Do the company make it easy by having a fault database like described above - NO! :evil: They do have a data base system, but it fills up with so much incorrect and misleading "information" (a "controller" updates the fault entry when you 'phone the details through), it is more of a record keeping system than a useful tool :evil: .

Now then, time to provide some help :mrgreen:...

My Beeb goes Beeeeeeeeeeeeeeeeeeeeeeeee and no longer works, or...
My Beeb only reports 16k when it starts, or...
My Beeb has strange things going on with the screen and it crashes...

All these symptoms are likely to be caused by a memory circuitry fault. Often it is due to either a 74LS245N (IC14) buffer failing (more on this later), or due to a dynamic RAM (DRAM) chip failing.

So first off lets start with the first step. Using a multimeter confirm that the +5V supply voltage is okay (4.85V to 5.15V is good). Test on the PCB terminals where all three sets of the red and black leads from the PSU connect to the PCB.

Next, find and note the position of link S25.

S25 selects if the board is configured as a 16k board, or a 32k board.

Now about DRAM (or How is the RAM arranged in a Beeb?)
The type of DRAM chips used in the Beeb are 4816 types. One of these can only store 16k bits of data. The memory in the chip is arranged as 16384 memory cells that are only one bit wide. So for each address (there are 16384 addresses) the chip can store one data bit. As the CPU used in the Beeb needs 8 bits (one byte) for each memory address, we can use eight of these DRAM chips. Eight bits = one byte. The memory layout is now 16k bytes (16384 addresses, each of which can store 8 bits, where each bit is stored in separate DRAM chips).

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.

Image
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.

The HM4816 data sheet is available in a zip file here

So now you know a bit about DRAM. How do we get the Beeb to see 32k bytes of RAM?
Well, Acorn used two "banks" of 16k chips, making a total of 16 HM4816 type chips. Arranged as 2 "banks" of eight HM4816 chips.

But where does link S25 come in, and how do I work out which chip(s) is/are faulty?

The circuitry on a 32k byte Beeb PCB is designed to select either one 16k DRAM bank or the other 16k DRAM bank depending on the current CPU address. We can use this via S25 to change the RAM bank that is in use. Which hopefully will enable us (assuming the defective DRAM chips are in only one "bank" to persuade the Beeb to start up as a 16k unit.

With a model A /CAS1 controls the only 16k RAM bank, as IC61 to IC68 are not present.

With a model B /CAS0 controls the lower 16k RAM bank and /CAS1 controls the upper 16k RAM bank. /CAS0 is in the memory map from &0000 to &3FFF (0x0000 to 0x3FFF) and /CAS1 is mapped from &4000 to &7FFF (0x4000 to 0x7FFF).

Link S25 is South on a model A (16k) so only /CAS1 is active.
It is north on a model B (32k) and so both /CAS0 and /CAS1 are active.

If you suspect that you have a RAM memory fault, on a model B you can move link S25 South and see if the machine will run with only one 16k RAM bank. The /CAS1 bank will be the active RAM and the RAM controlled by /CAS0 will be disabled.

For testing only, on a model B, link S25 can be removed, then only /CAS0 is active. The /CAS1 RAM bank will be disabled.

If you use a 74LS04N hex inverter to invert the signal from the North pin of S25 and connect the output of the inverter to the middle pin of S25, this will swap the two banks around in memory as far as the MPU is concerned. You can now run a memory test on the suspect RAM from &4000 (0x4000) to &7FFF (0x7FFF). Note that if such a test shows a fault, it is the /CAS0 bank that is faulty.

This is one test program (thanks to Mark/retroclinic)
retroclinic wrote:In 32K mode, invert the signal going to the centre pin of S25 using a 74LS04. The computer will now boot, but you'll see the memory fault on the screen.

Then it's just a case of:

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

(go to mode 0 first if you can't see what you're typing because of the memory fault, the top of the screen will still be usable)

In most cases that should give you a number after a short time, which converted to binary will give you the bad bit - then just change that particular ram out. Sometimes you need to change the &4000, or make it use random addresses between &4000 and &8000 until it shows the fault.

Mark.
This test program 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.
retroclinic wrote: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.
Full thread here :D

Here is another memory test program: (thanks to MartinB)
MartinB wrote:I know there are 1001 different memory testers out there but here's one I wrote for quick first-look testing when there's obvious display symptoms. It tests the whole of user ram from &E00 to &7FFF and by running it from a low (normally illegal) PAGE of &900, it will work fine on tape and disc systems. By using &55 and &AA as the test patterns it will find stuck bits and cross-talk errors and because it creates a visual mode-specific 'test pattern' it also shows cross-chip addressing faults and display circuitry anomalies.

Type it in and save it as per any normal BASIC program. To use, press <Break>, set any screen mode with MODEn <cr>, set PAGE=&900 <cr> and CHAIN"MTEST" <cr> (or whatever name you've saved it under.) The given screen should be completely black during the test apart from the 'twinkling' activity marker in the top left corner until the screen memory for the selected Mode is tested. Then, a pattern will slowly fill the screen from top left to bottom right and there should be no visual deviations from this progressive fill. The pattern will vary according to Mode but will always be constant for that Mode. In the case of Mode7, there will (should) be nothing until the last few seconds when the screen will fill with 'U's. For some modes, you will see that your monitor gets a good resolution and EHT test too! The complete test takes about two minutes.

If a location fails, the program will simply stop and print the hex memory location where the error occurred. If all locations pass, you will just finsh with 'OK'. This type of program specifically tests the CPU<>Memory chain and if it passes but you see random characters, holes or pattern shifts then this points the finger at the display circuitry and thus halves the diagnostic process.

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";


Don't forget though, after saving, use as follows :

MODEn <cr>
PAGE=&900 <cr>
CH."MTEST" <cr>


Good luck!

Martin

Table of DRAM chips vs. /CAS RAM bank and data bits
Data bits - IC numbers

Code: Select all

   /CAS0 /CAS1
D0= IC61  IC53
D1= IC62  IC54
D2= IC63  IC55
D3= IC64  IC56
D4= IC65  IC57
D5= IC66  IC58
D6= IC67  IC59
D7= IC68  IC60

Using a 74LS04N hex inverter to invert the signal from the North pin of S25

Image(Diagram courtesy of MartinB

Also the following connections are needed: +5v is Pin 14 and 0v is Pin 7.

The 74LS04N is a fairly common standard logic chip. Some possible suppliers are ESR, Bowood, Quarndon, Rapid, CPC, Farnell and Maplin.

If you find, or strongly think that a chip is faulty, it will need to be replaced. Don't bother to desolder the old chip as-is. Using some good quality, but fine side cutters / snips, cut each chip leg near the plastic body of the chip. Then remove the chip. Now you can desolder and remove each pin, one at a time. Not only is this easier, it also means there is less risk of damage to the PCB tracks (traces).

Don't solder the replacement / new chip directly to the PCB. Instead fit a IC socket. This means if the chip has to changed again, no desoldering or soldering is needed :D

Sometimes the actual DRAM chips are not faulty and it is some of the logic circuitry that fails. Of the various standard logic chips, the 74LS245N line driver/buffer in position IC14 is the most likely to fail. Test by renewing it ;-).

Mark K.

Edited 2014-12-25 to update links :D
Last edited by 1024MAK on Thu Dec 25, 2014 9:16 pm, edited 3 times in total.
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BeebMaster
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Re: Fault finding index

Postby BeebMaster » Fri Jul 22, 2011 1:10 pm

1024MAK wrote:BTW I work for a company where the main part of my job is attending and repairing failures.
Mark K.


Would you like a busman's holiday round here? Free tea & board provided!!
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Re: Fault finding index

Postby TopBanana » Sat Jan 14, 2012 12:49 pm

Troubleshooting the BBC Micro

This handy guide can be downloaded here

Edit: An alternative link to the trouble shooting guide is here: http://primrosebank.net/computers/bbc/documents/Troubleshooting%20the%20BBC%20Micro.pdf
(Thanks to Dave for hosting it on his web side)
Mark
Last edited by 1024MAK on Mon Sep 18, 2017 6:01 am, edited 2 times in total.
Reason: New link added

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matelot
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Re: Fault finding index

Postby matelot » Mon Mar 26, 2012 9:48 am

would you mind checking that link ? all I get is an Escher type box and error 404.
Bob

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Re: Fault finding index

Postby firthmj » Sun May 06, 2012 8:26 am

Hi,

I would second the request for another link to TopBanana's PDF - I also get a 404 error from the Dropbox link.

If someone who already has a copy is willing to send it to me, I'd be more than happy to shove it on my WWW site and post a link back here.

Also (assuming they're not embedded in TopBanana's document) does anyone have copies of the two files that used to be on Michael Foot's site:

http://homepages.paradise.net.nz/mjfoot/BBCFaults.txt
http://homepages.paradise.net.nz/mjfoot ... Faults.txt

There was a reference on the mailing list archive from 2006 (ancient I know!), but they're not valid now and www.archive.org doesn't seem to have grabbed them.

As you can probably guess, my interest comes from a previously healthy Beeb that's keeled over this morning...

Michael
Had fun at the
Image
Meeting 13th May 2017

TopBanana
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Re: Fault finding index

Postby TopBanana » Sun May 06, 2012 3:50 pm

Fixed the link - download to your hearts content !

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matelot
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Re: Fault finding index

Postby matelot » Sun May 06, 2012 10:53 pm

Thanks for that TopBanana, all working fine now and also contains useful info for connecting beebs together and connecting other acc's.
Very helpful to those of us wanting to experiment.
All I need now is a scope, "Isn't my birthday soon dear" :D


I did notice that firthmj's links go to error 404's #-o

Bob.

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Re: Fault finding index

Postby daveejhitchins » Tue Jul 10, 2012 6:20 pm

Hi All:

This reply is, probably, slightly off topic? However, thought it would assist in repairs and, more importantly, preventative maintenance.

The job that needs to be carried out, on just about any electronic consumer equipment over 10 years old, is to replace all the electrolytic capacitors (those are the one that look a bit like a can see attached photo - These are 'big' ones in my JukeBox power-supply). Why? These components have a 'wet' electrolyte in their make-up and, over time, dry-out. As they dry out their value (capacitance) changes. This change can cause all sorts of problems . . . I've seen a few posts, here at STH, that, probably, were caused by 'dryed-out' electrolytic caps.

If you buy second hand machines the FIRST thing you should do, before turning on (yes, I know all about temptation :lol: ), is to replace all those cans. Could save you a fortune in repairs, let alone the frustration :?

If there's any sort of demand, maybe I could get together a 'kit' for the various machines covered by this site?

p.s. the idea of the fault LOG is brilliant =D> :D =D>

Dave H :D
Attachments
Electrolytic Caps.jpg
Some, large, electrolytic capacitors
(157.04 KiB) Downloaded 795 times
Parts: UM6502CE, GAL22V10D, GAL16V8D, AS6C62256A, TC514400AZ, WD1772, R6522, TMS27C512, AT28C256
Products: ARA II, ABR, ATI, AP6, MGC, AP5 . . .
For a price list, contact me at: Retro Hardware AT dave ej hitchins DOT plus DOT com

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Re: Fault finding index

Postby daveejhitchins » Thu Jul 12, 2012 5:35 pm

Further on the subject of capacitors . . . Tantalums :evil:

I'm not sure if they were ever used in any Acorn products? If they were, they need replacing also. Why? Tantalums get stressed, very easily, with surges e.g. switching on!
If their source impedance is less than 3 Ohms it's recommended to derate the working Voltage by a factor of 0.3. E.g. If you have a 5V supply rail then the working Voltage of the capacitor (Tantalum) should be 17V or higher (5/0.3=16.6666) Actually, I think 16V is one of the 'standard Voltages for these parts, so that would do OK. If the source impedance is greater than 3 Ohms then a derating of 0.5 would be OK. E.g. 5V supply would give you a 10V working capacitor. So if you use these calculations for any replacement Tantalum capacitors you're giving the part the best chance of surviving, probably for the rest of the equipments life. I have to design circuits to have a MTBF (mean time between failure) of greater than a million hours!

I designed the P.R.E.S. Advanced Plus 3 with a single bead Tantalum in it (didn't know about derating then - life's a learning curve :? ). It caused me all sorts of problems. It was a 'belt and braces' decoupling part. Anyone who had a problem (they went short and the Electron didn't like it :- ). I gave customers a choice, if they rang me with this problem, they could either (a) send the unit back for repair or (b) waggle the parts (I guided them to it) until it came free of the board and have an extra years warranty. I seem to remember not having a lot back. :D

Dave H :D
Parts: UM6502CE, GAL22V10D, GAL16V8D, AS6C62256A, TC514400AZ, WD1772, R6522, TMS27C512, AT28C256
Products: ARA II, ABR, ATI, AP6, MGC, AP5 . . .
For a price list, contact me at: Retro Hardware AT dave ej hitchins DOT plus DOT com

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paulv
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Re: Fault finding index

Postby paulv » Thu Jul 12, 2012 6:25 pm

I'm not sure if they were ever used in any Acorn products?


They were certainly used in Archimedes machines. On the original A300/A400 universal boards some of those caps were 100uF 16V on the de-coupling circuit of the 12 volt rail and I know of at least one Archimedes that developed a short across one of those caps causing it to fail to start...

The A400/1 series had a revised spec. using 25V 100uF caps in the equivalent circuit.

Paul

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Re: Fault finding index

Postby CanonMan » Thu Jul 12, 2012 7:27 pm

I'm pretty sure the Model A/B and Masters used the evil little things as well...

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Re: Fault finding index

Postby retroclinic » Thu Jul 12, 2012 7:36 pm

The only Tant I've ever come across that blows regularly is on the -5v line of the BBC Master 128 audio circuit (C14) - I've seen them fail both open and short. Short means little or no sound, plus the cap gets very hot. Open means you get a lot of background buzzing and noise out of the speaker when idle, changing it fixes that.

Of course any cap can go at any time, but that ones seems to be particularly susceptable.

Mark.
Image

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Elminster
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Re: Fault finding index

Postby Elminster » Sat Jul 21, 2012 10:29 am

another one to add

from this thread.

http://stardot.org.uk/forums/viewtopic.php?f=3&t=5312&start=30

Shift keys appear to stop working, but in fact a key is permanently jammed in the pressed position and this messes up shift if this happens when bbc switched on.

Replacing faulty key fixes shift issue

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Re: Fault finding index

Postby stevendt » Tue Dec 18, 2012 2:16 pm

Hi,

I downloaded the troubleshooting document that TopBanana posted a while ago. It looks like a very useful document, but is a bit awkward to use as it is a collection of individual TIF pages. I have combined these into a single PDF document.

I note that the original document was produced by the Microelectronics Educational Development Centre, The Open Tech Unit, Paisley College and was published by HMSO in 1987 and is still "Crown Copyright".

I don't know whether there would be any real issues with making this available some 25 years after it was produced, but until I hear otherwise, I am happy to provide an electronic copy fof the Pdf for anyone that needs it,

regards
Dave

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Re: Fault finding index

Postby TopBanana » Tue Dec 18, 2012 4:28 pm

If it's still copyright then I should take it down :shock: :shock: :shock:

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Re: Fault finding index

Postby stevendt » Tue Dec 18, 2012 4:34 pm

I guess that's the problem with most documentation from the '80s.

Officially, the material is stilll under copyright, but whether anyone really cares any more is another question. Many of the hardware and games companies no longer exist. In the case of this document, as it was sponsored by the UK government, we all paid for it anyway!

Personally, I would be inclined to leave it, unless anyone has any strong objections - or the original copyright owner contacts you - you'll recognise them as they will be the ones in the cars with the blue flashiing lights on top :-)

regards
Dave

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Re: Fault finding index

Postby aerworuld » Tue Dec 18, 2012 5:05 pm

Dave H, you've scared me now! Should I be looking for electrolytic convertors to change in my Elk? :shock:

*edit* C-a-p-a-c-i-t-o-r Stuart, capacitor.

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Re: Fault finding index

Postby TopBanana » Tue Dec 18, 2012 7:50 pm

stevendt wrote:I guess that's the problem with most documentation from the '80s.

Officially, the material is stilll under copyright, but whether anyone really cares any more is another question. Many of the hardware and games companies no longer exist. In the case of this document, as it was sponsored by the UK government, we all paid for it anyway!

Personally, I would be inclined to leave it, unless anyone has any strong objections - or the original copyright owner contacts you - you'll recognise them as they will be the ones in the cars with the blue flashiing lights on top :-)

regards
Dave


Dave, I was joking ...... :lol:

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Re: Fault finding index

Postby nOmArch » Tue Dec 18, 2012 8:05 pm

Nice =D>
Alex

Back up to 1 Beeb again. \o/

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Re: Fault finding index

Postby ctr » Sat Jul 19, 2014 11:46 pm

This thread is a bit stale, but this may help someone...

***********

BBC Model B displays "BBC Computer 16K" at boot.

Ensure link S25 is properly connected in the north position.

***********

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Re: Fault finding index

Postby Pablos544 » Tue Sep 09, 2014 4:08 pm

TopBanana wrote:Troubleshooting the BBC Micro

This handy guide can be downloaded here


This is so useful for someone like myself [-X that I thought I would help others (and me) by converting it to PDF.

You can search it, view it at your leisure, annotate and share it with other people. :D

https://dl.dropboxusercontent.com/u/723 ... CMicro.pdf

I don't pretend to know anything about it but it sounds like a good idea. :D

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Re: Fault finding index

Postby paulv » Tue Sep 09, 2014 4:11 pm

The link appears to be broken but a quick guess fixed it...

https://dl.dropboxusercontent.com/u/72363592/TroubleShootingBBCMicro.pdf

Paul

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Re: Fault finding index

Postby Pablos544 » Wed Sep 10, 2014 11:20 am

paulv wrote:The link appears to be broken but a quick guess fixed it...

https://dl.dropboxusercontent.com/u/72363592/TroubleShootingBBCMicro.pdf

Paul


yes thanks Paul!! I couldn't spell trouble #-o

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DutchAcorn
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Re: Fault finding index

Postby DutchAcorn » Tue Dec 29, 2015 8:30 pm

Beeb does not boot: flashing mode 0 cursor and long beep.

Setting S25 south makes it bootable. Can change mode to, eg. mode 0 and type stuff. Part of the screen is corrupted and the typed characters are mirrored in the lower right corner of the screen. Running a program fails.

issue.jpg


Solution: replaced LS283 at IC39.
Paul

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DutchAcorn
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Re: Fault finding index

Postby DutchAcorn » Tue Dec 29, 2015 8:34 pm

Beeb boots but with a long beeeep. Two or three (ctrl)-breaks and it crashes. Sometimes characters appear typing on the keyboard, but not always the ones typed. Only a few characters can be typed. Sometimes caps lock is off at boot, sometimes it is on. Different results with different keyboards.

Solution: replaced LS259 at IC32.
Paul

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Re: Fault finding index

Postby Pablos544 » Wed Dec 30, 2015 2:26 pm

What is LS283 / LS259? :?

-Pablo

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DutchAcorn
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Re: Fault finding index

Postby DutchAcorn » Wed Dec 30, 2015 2:57 pm

Pablos544 wrote:What is LS283 / LS259? :?

-Pablo

It is short for 74LS283 / 74LS259; the IC types.
image.jpeg
(35.9 KiB) Not downloaded yet
Paul

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Re: Fault finding index

Postby Pablos544 » Wed Dec 30, 2015 3:43 pm

DutchAcorn wrote:
Pablos544 wrote:What is LS283 / LS259? :?

-Pablo

It is short for 74LS283 / 74LS259; the IC types.
image.jpeg


Cheers for the image, Paul.. I don't think I would have found it without it. :lol:

-Pablo

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Re: Fault finding index

Postby Pablos544 » Wed Dec 30, 2015 3:47 pm

DutchAcorn wrote:
Pablos544 wrote:What is LS283 / LS259? :?

-Pablo

It is short for 74LS283 / 74LS259; the IC types.
image.jpeg


Excuse my naivety but what do these actually do? :?

-Pablo

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Re: Fault finding index

Postby DutchAcorn » Wed Dec 30, 2015 4:01 pm

Pablos544 wrote:
DutchAcorn wrote:
Pablos544 wrote:What is LS283 / LS259? :?

-Pablo

It is short for 74LS283 / 74LS259; the IC types.
image.jpeg


Excuse my naivety but what do these actually do? :?

-Pablo

Here you go: https://en.m.wikipedia.org/wiki/7400_series. Enjoy! :D
Paul


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