Fault finding index

[pmul128]

Reporting on a Master 128 with an unusual fault, and a surprisingly easy fix.
Today I fixed a Master 128 which, up to now, on boot, did this:

I tried R-Power on to reset the CMOS RAM, did not help. The shift-lock LED still toggled on/off, but the caps-lock LED did not.
I checked the PSU, all OK, I looked in dismay at the vast field of soldered and difficult to find ICs, there being only two chips socketed, the ROM, and the ADC. On a whim I tried replacing the ROM with a spare I had, and it booted fine after that. I went back to the original ROM, cleaned the pins, sanded them down in case of any oxidation, used contact fluid, etc. The original ROM still always booted to the messed-up screen, and the replacement, always boots normally. Looks like the original ROM is a goner.
Anyway, maybe this will be of use to someone: a duff ROM can produce a behaviour that looks more like a video fault.

[SanguineBrah]

I have a couple to add (issue 3).

First was marginal voltage hovering around 4.7v at VCC3 but close to perfect voltages at the other terminals. I thought it might be a bad connection at the spade terminals or a loose crimp but actually the problem was at the other end. The solder joint connecting the VCC3 lead to the PSU board had a slight crack in it. Reflowing that joint fixed the problem.

Second, the system seemed normal at first, but then I started to see graphical static only in modes 4 and 6 and only when a key was pressed. The system had recently passed MartinB's ram test, so I didn't assume a ram problem at first but moving S25 to south led to increasingly erratic behaviour, indicating a problem in the upper ram. With S25 at north again, I piggybacked a known good ram chip on each of the upper 16k until the static went away, which revealed IC60 to be the culprit. That chip replaced, and all was right with the world again.

[SanguineBrah]

And another one. My Cub 1431 started showing a visible blue raster when no image was displayed, and when hooked up to the beeb it displayed nothing but a red screen with retrace lines. After very carefully noting the position of VR932 on the neckboard (the one with the hole through the back), I squirted some contact cleaner into it and exercised it thoroughly before setting it back to its original position. Now it's back to normal. Very glad I tried this first before pulling and testing components unnecessarily.

[Simon]

Symptoms: Continuous first tone, no LEDs, no keyboard response. screen that looks like this


Diagnostics, 16k Jumper, north, south, off. Switching VIAs, running minimum config, test roms all no effect on the issue. Voltages test good.

in the thread it was suggested to try swapping the OS rom socket, did this and it resolved the problem.

[pmul128]

Nice to see it helped someone! Seems it not only applies to Master Systems.

[SanguineBrah]

Symptom: Issue 3 model B would not cold-boot as long as a TMS5220 was installed but worked fine after CTRL-BREAK.

Solution: The fix was to follow the track modifications listed in the service manual for installing the speech upgrade. It seems like this was because during power-up, the TMS5220 sends a signal down the INT line, which goes to VSPRDY of IC3 on an unmodified issue 1-3 and interrupts the boot process.

[1980s_john]

Machine - BBC Model B

Before powering on I removed PSU and replaced C1 & C2 (X2 capacitors). Checked C9 and found very low capacitance so replaced. Put PSU back in, machine didn't turn on. Checked 5V voltage and measured 100mV. Powered off and removed PSU, measured resistance between 5V red and black wires of around 1 ohm. Compared to working PSU 120 ohms (*) so PSU shorted. Removed SCR and C17 + C19 but still shorted. Removed D7 and 5V wires now measured high resistance. D7 was C10P03Q (rated 11A and 30V), three pin TO220, two diodes back-to-back, one of which was shorted and other was fine. Replaced D7 with SBL1040CT (rated 10A and 40V), refitted SCR and C17, fitted new C19 (old one would probably work).

PSU refitted and worked fine.

Regards,
John
(*) - Resistance varied with different multimeters, and if reverse red & black.

[1024MAK]

Machines: BBC Model A and B.

There are some details about the OS ROM (IC51) socket and it’s connections in this topic. This is helpful if you are trying to use a EPROM or EEPROM in place of the Acorn OS mask ROM chip, or if you are wondering about any modification wires on the underside of the board in this area.

Mark

[roganjosh]

Model B

Fault: Intermittent glitch after anywhere between 2-90min.

Typically it would appear as though a [return], or several characters + [return] had been typed into the input buffer. Some of the characters would often spell out BASIC keywords. Inconsistent bit corruption of displayed characters happened more infrequently as did graphic patterns in Mode 0. Sometimes continuous scrolling of BASIC error messages occurred ("Bad program", "Mistake" etc). At other times it appeared that the computer switched to displaying a screen of previously typed text. If the severity of symptoms wasn't excessive then the machine would still be responsive.

After several days a new symptom appeared such that, after a glitch, CTRL-BREAK would produce a "BBC Computer 16K" start message.

Solution: Faulty IC45 (74LS139).

[GoodToGo!]

If you have the misfortune to have the 500mA fuse blow on a Wong's electronics WA1020 Beeb power supply, then check the two schottky diodes D12 and D14 for a short circuit. See this thread
While you are in there, change the startup cap C20 (which is C9 in an ASTEC) and double check all the caps in the secondary side of things.
Lucky for you, there are no smoke machines, aka RIFA's installed.

Cheers!
GTG!

[marcusjambler]

One of my Issue 7 Model Bs ( OS1.2 , BASIC 2 , 8271 with 0.9Acorn DFS ) developed a "Disk Fault 18 at 00/00 " recently.
So out with the Trouble shooting manual from Paisley College page 43 Disk Interface.
Tested each IC and it was IC87 74LS123 dead...
Replaced with new running fine.

[RobC]

An issue 7 Beeb was reporting "Language?" on startup even though a known good BASIC ROM was installed at IC52. Trying the BASIC ROM in the other ROM slots gave exactly the same behaviour.

Andre's test ROM showed that the RAM was all good and so it looked like a problem with the ROM selection hardware. I modified Andre's test ROM to cycle through the ROMs after the RAM test. Putting a scope on pin 9 of IC76 showed that ROMSEL was going low periodically as expected. The databus inputs on pins 3-6 were toggling as expected too but the outputs on pins 13 & 14 were fixed low indicating a fault with IC76.

Replacing IC76 with a new 74LS163 fixed the problem.

[Simon]

Machine BBC B Issue 7.

Symptoms, random jumbled characters on mode 7 odd random pixels in graphics modes random colours in mode 2, in Gem under dos the screen would get completely scrambled when you move the mouse, WHITE LIGHT crashes after the title screen.

Diagnostic steps:

Jumper S25 16k mode would reduce graphics issues.

Martin B's memory test in any mode will pass many repeats, in the graphics modes the coloured pixels are visible.

Fix:

Replace IC14 74LS245N

[Gelpack]

Well Bob, apart from a very few responses, I broke my spade digging through walls of apathy so it's kinda on the back burner. In fact, it's probably fallen off the back of the stove

Perhaps my thinking was a little perverse in that those who would most like to see such an index are those who are least likely to be able to contribute in the first instance. Conversely, those who could contribute probably aren't fussed coz they can do the stuff anyway

I might give it another go when my 'other stuff' goes quiet but the index I had planned was very much community contribution based and without that input it's a bit of a non-starter. That's not necessarily a criticism, it's just the way things can go in communities such as ours. (I've certainly no complaints about the level of support I receive in other projects )

That's unfortunate.

I've just read this and it saddens me a little actually. If you decide to take it up again. I can look back over my posts and solutions few though there are and assemble them, or even do the same for other posts. The information is harvestable and I would be willing to have a go, maybe once it gets going then the inertia would be slowly reduced.

[frankc]

In response to the post immediately above. (Strictly IMHO) It's possibly just the format - a long thread(s) designed for discussions is not perhaps the ideal format for organising a fault finding system which perhaps would be better organized in some sort of indexed (database) system. Which is what brought me here in the first place.......

Some where ... here ... someone ... on some thread - posted regarding running a motherboard with the minimum necessary parts on said motherboard.
That is - what parts can be stripped away and the board still usefully operates? ie networking, disk drives, are not strictly necessary for "normal" operation. Cassette control and AD converter are also not (strictly) necessary. How far can one take this? - user VIA? etc.

If any one could point to that post I would be grateful.
Alternatively - In case I just hallucinated the whole thing and said post never happened - could someone care to take a pass at this.

Thank you for your time.
Frank C.

[hoglet]

If any one could point to that post I would be grateful.

This one?
viewtopic.php?f=3&t=16006&p=220996#p220996

[frankc]

If any one could point to that post I would be grateful.

This one?
viewtopic.php?f=3&t=16006&p=220996#p220996

Bullseye!
Major Help.
Thanks
FC

[shifters74]

BBC Issue 4

Unable to read disks with Disk Error 18.

Floppy would spin, the head would seek but not read any data and after 15 seconds or so give Disk Error 18 (on known good disks and drives)

Turned out to be bad IC82 - replaced (as it was socketed) with a spare and it sprung to life.

cheers

shifters

[Gelpack]

BBC Model B Issue 7

Three faults. A lot of chips were changed during this awful experience, partially through desperation - this took at least two weeks to sort out.

Crashing All the time literally unstable as hell
=============================================
A single RAM chip was to blame.

Mode 7 Video seemed Jittery in the horizontal And was incorrectly positioned Vertically
==================================================================================
This all worked fine on an old Samsung with a video input but did not work properly on my RGB to HDMI Converter ( but I only found out that way down the line).

After much going around the house on this, it turned out that The S31 Polarity Reversed. Once remedied, Mode 7 Worked a Treat. But Mode 0-6 had no output. did not.

Mode 0-6 not generating any Output.
=====================================
As well as no output, Probing Pin 8 on IC6 with a Rigol 300Mhz Scope which turned the screen into a snowstorm and also caused crashing ( Completely locking up the computer as well sometimes ). This action was tested on another BBC and did not have the same affect.

This turned out to be IC40 It tested good when previously changed but was not, when I took it out and tested it again, it was failing the chip tester. Having said that it was an LS00 not an S00, but a replacement using another LS00 worked fine, but I have some National Semiconductor 74S00's on the way to ensure prolonged success.

What Did I learn
==============
Don't change anything unless you have a high degree of confidence that it is faulty.
Take notes,
Take Scope Shots and compare if you can with a good machine
The experts on Stardot, are an experienced helpful bunch and really know what they are talking about.

Further Reading
==============
viewtopic.php?f=3&t=25214

[48kRAM]

BBC Master 128 boots to continuous multiple speaker tones overlapping. Keyboard is non-functional except when a shift- or caps-lock key is held down. Hangs after the Acord ADFS prompt. Configuration changes (*CONF.) are not saved to the CMOS RAM - even before a power-cycle. Shift- and Caps-Lock LEDs are constantly lit, regardless of key state.

Solution: Replaced 74LS259 at IC 10

Background: viewtopic.php?f=3&t=24825

After testing and socketing both VIAs, I suspected the LS174 at IC 4, but this tested fine as well. I next suspected the CMOS RAM chip at IC11 but replacing this did not resolve the issue. Ultimately, I traced the issue to the chip select lines driven by IC10 and replaced that chip. System now functions properly.

[Gelpack]

On a reset around one in 10 times, the following image appeared rather than a basic prompt OR the screen was going completely blank except for a wide cursor. Once a reset had successfully occurred, it appeared to work normally, play games run diagnostics, etc.

After much work, it turned out to be IC20 74LS139 which provides the final part of the chip select logic for the ROMS/EPROMS

Further Reading : viewtopic.php?f=3&t=25592

Typically After Failed Reset

[linker3000]

BBC B, No startup beeps. No sound in games.

Issue 7 motherboard (but applies to others).

Apart from no sound, the machine worked OK in all other apparent respects.

There was a small amount of digital noise audible from the speaker, which implied the LM386 amplifier chip (IC19) was probably OK. This left either the Programmable Sound Generator (PSG) IC18: SN76489 or the sound mixer Op Amp IC17: LM324 as the probable issue.

A multimeter was used to confirm that the power rails on IC17 and IC18 were as expected.

Using a crystal earpiece (https://en.wikipedia.org/wiki/Crystal_earpiece) to listen* to the output of the PSG (IC18) - pin 7, the power-on beeps could be heard, so this narrowed down the fault to probably IC17.

To expedite fault finding and not have to probe the Op Amp and surrounding components in-circuit, the body of IC17 was removed by carefully cutting its pins and then using a desoldering station to remove them from the PCB. A turned pin socket was fitted and then the LM324 Quad Op Amp was replaced with a pin-compatible LM6144 just because one was at hand.

On power up, the beeps were back and game audio was working again.

See: viewtopic.php?p=374445#p374445

*A crystal earpiece has a very high impedance and will pick up audio frequency sounds very easily. To use one to debug a circuit, it is sufficient to touch the tip of the earpiece's audio jack on a point of interest while simultaneously touching the other jack's (sleeve) connector with one's finger.

[maniacminer]

Blank screen, continuous tone, cassette motor LED lit, relay rattling on/off especially after BREAK.

Checked clocks, all good. Checked 6502A data & address bus activity, odd shapes, checked 6502 - duff - replaced.

Problem remained. Checked IC selection logic, removed IC14 to disconnect 6502 from memory, bad waveforms still on data and address bus, suggests contention. Checked IC26 (74LS139) and found more than one output was low, this is impossible and must not happen as it allows more than one device on the bus to talk at once, essentially shorting each other out. IC71 (74LS244) and IC72 (74LS245) were getting hot, removed those.

Replaced IC26 and then checked IC20 (74LS139) for more of the same faults and also found more than one low output, replaced IC20 also. Cassette motor LED went out and no more relay rattling. Checked IC14 (74LS245) and found to be duff, replaced it.

On powering up, the screen was now white and there was no activity on the HS/VS on the 6845 and the clock input to it was non-existent traced to a broken track from pin#28 of the VIDPROC and a broken track from pin#27 of the VIDPROC to S26.

Machine works after these repairs, but there's no MODE7. Checked output of IC15 (74LS273) and it's duff, replace it. Now all working (except VIDPROC is damaged in its palette memory)

[maniacminer]

Symptom: BBC Issue 4 - disc and Econet fitted, disc interface not showing up, Econet hangs when issuing any command that requires access to the network and there is presence of "clock".

Procedure:Checked NMI, held low. Removed IC78, made S9, Econet now works. IC78 replaced, S9 removed, now disc and Econet both work.

Cause:Faulty Intel 8271 FDC holding NMI low.

[dankcomputing]

On my model B I had what felt like a thermal intermittent failure somewhere. The system would lock up (with the cassette motor light inexplicably turning on) after running for 5-15 minutes. Sometimes I'd get garbage on the screen; it would get worse the longer I kept the system powered on. When it got really bad there wouldn't be any startup at all or random garbage/text from the DFS ROM onscreen. I swapped out the CPU and all the major ICs that might have caused it and couldn't get the problem to go away. After swapping a bunch of TTLs out I found the bad one - a 74LS138 at IC24.

[maniacminer]

Another Beeb another problem, this time the Beeb was mostly working apart from the PSU. I tend to leave them running for a day to make sure it's stable. It crashed after about an hour of running. I used the thermal camera to see if anything was overly hot and yup, IC14 was roasting. I desoldered it and replaced it with my big stock of 74F245 and it runs cool. Beeb has been running for a day now without issue.

[maniacminer]

Yet another Beeb, again it crashed solid after around 20 mins and needed switching off and on. Continuous tone. Wait a few minutes and worked fine for around 10 mins and then froze again. I took the lid off and it worked for about an hour before locking solid again. Got the thermal camera out and had a look at the board. IC45 was glowing desoldered it, replaced with socket and a new 74LS139 and now the Beeb runs all day even with lid on.

[e668ecp]

Fault: BBC B running slow when using OS commands / BASIC / doing I/O operations but runs fine when playing assembly written games

Thread discussion: viewtopic.php?f=3&t=26025

Solution: IC20 had been socketed by a previous owner and pin 15 was touching a track which runs around it which is linked to the IRQ signal on both 6522's pin 21. Unsoldered the pin, cleaned it up, resoldered and problem fixed

[mikeh_nz]

BBC Model B - Series 7

First issue Symptoms:
- Continuous beep on power on.
- Blank screen.
- No cursor, and no Caps lock led lit. (no leds lit)

Cause:
- Link S21 wasnt pushed on fully (so it was only NE-NW, and not both NE-NW & SE-SW)


Second issue Symptoms:
- Continuous beep on power on. No screen displayed. Caps Lock led is lit.

Cause:
- Bad IC14


Third issue Symptoms:
- Could hear sound after the boo..bip completed (like the sound chip didnt get told to shut up)
- Changing to Mode 0 showed some white dots on the screen (like I had corrupted memory and the memory wasnt fully zeroed out)
- Wrting a simple 10 PRINT "HELLO WORLD": GOTO 10 would crash after a bit of scrolling of the text (I was in mode 7)
- Removing Link S25 would fix the crashing and dots on the Mode 0 screen, but sound continued.
- Just by typing on the keyboard, the sound chip would sometimes change the tone playing

Cause:
- Bad IC32 (that controls the selection of sound chip, keyboard, and memory offset for hardware scrolling)


Corresponding thread:
viewtopic.php?f=3&t=26003

[maniacminer]

An Econet connected BBC B (Issue 4) that hangs when booting, if set to auto-boot via DIP switches 1 & 5, will play a continuous second tone (of the boo-bip) and when selecting Econet with *NET then asking for a catalogue (*.) the machine does nothing and eventually emits a "Net Error" message. The machine worked fine on its own and also with many others, but when a particular machine was powered, this station would freeze.

I checked the network ID of the station that causes the original station to fail and found the station ID was identical (finger trouble on my part!) I changed the network ID and the problem disappeared.