Model B Fault finding help please :)

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Bat
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Model B Fault finding help please :)

Post by Bat » Tue Mar 13, 2018 6:53 pm

Hi,
Excuse the rambling nature of this post but I'll try to give all the info I can....
I purchased an issue 4 with disk drive and monitor. When powered up it buzzes from the speaker and the 2 lock LEDs are on.
I swapped all the disk interface ICs into my iss 7 where they work fine.
I've have 2 ROMs left:
HN613128PB04 Acorn computers, which I've put in IC51
HN613128P Acorn Basic, which I've put in IC52
I've tested the power supply whilst connected to the main board, readings are -5.3V and 4.8V, I assume these are close enough?
Doesn't make any difference if the ROMs are fitted or not.
Pressing BREAK will change the LEDs status and even pull in the tape relay.
No video output on BNC connector with a known working monitor.
Reading through the fault finding guide my symptoms stretch across more than 1 section.
Asking the audience as I'm new to BBCs, but have some electronics knowledge.
I can try parts from the working iss7 machine if it won't cause a disaster which leaves me with 2 wonky machines!
Can anyone give me some pointers please?
Thanks for reading!
Cheers,

Gavin :)

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marcusjambler
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Re: Model B Fault finding help please :)

Post by marcusjambler » Tue Mar 13, 2018 7:03 pm

Hi Gavin

I've sorted a couple of Beebs out with Tricky's test ROM
viewtopic.php?f=2&t=10268#p139467

Marcus

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mlouka
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Re: Model B Fault finding help please :)

Post by mlouka » Tue Mar 13, 2018 7:13 pm

Have you tried swapping the keyboards (with the ribbon cable) to rule out a faulty ribbon cable (or poor connection) as that can result in some weird I/O related behaviour.
BBC Master 128, BBC Model B i7, Watford Electronics Solderless Sideways ROM board, PMS B2P-6502 2nd proc., PiTubeDirect (both internal and external), RetroClinic Multi-OS Selector, Sundby 256k RAM/ROM card, MMFS, ++

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DutchAcorn
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Re: Model B Fault finding help please :)

Post by DutchAcorn » Tue Mar 13, 2018 7:37 pm

There can be many different causes for the symptoms. Relatively simple to find are issues with the system VIA and with memory.

You can try setting the S25 jumper from north to south and see if there are any changes. This will change the bank used for lower memory. It is not a decisive test; if there is a fault in the other memory bank it will still not start and components in the memory circuit other than the memory itself can have failed.
You can also try removing the VIA (IC3) and see if it "boots". It won't really be functional and you will still hear the continuous beep, but it can eliminate the VIA (and the keyboard cable) as a cause.

See here for a "map" of the BBC hardware.

Good luck!
Paul

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Re: Model B Fault finding help please :)

Post by flaxcottage » Tue Mar 13, 2018 8:16 pm

As Dutch Acorn says about the 6522 VIA but try swapping them as well. That sometimes works. Also try 'wiggling' all socketed ICs. Sometimes they are slightly corroded and 'wiggling' breaks the corrosion and makes a good contact.
- John

Why do I keep collecting Acorn gear? I'm going to need a considerably bigger man-cave. :?

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Re: Model B Fault finding help please :)

Post by Bat » Tue Mar 13, 2018 8:26 pm

DutchAcorn wrote:There can be many different causes for the symptoms. Relatively simple to find are issues with the system VIA and with memory.

You can try setting the S25 jumper from north to south and see if there are any changes. This will change the bank used for lower memory. It is not a decisive test; if there is a fault in the other memory bank it will still not start and components in the memory circuit other than the memory itself can have failed.
You can also try removing the VIA (IC3) and see if it "boots". It won't really be functional and you will still hear the continuous beep, but it can eliminate the VIA (and the keyboard cable) as a cause.

See here for a "map" of the BBC hardware.

Good luck!

We have a winner!!!
I tried swapping and removing VIA chips into IC3 to no avail.
However the S25 jumper has now given me a functional 16K model B :D
Can you give me some pointers as where to look/test next please?
Thanks Paul :)

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Re: Model B Fault finding help please :)

Post by Bat » Tue Mar 13, 2018 8:53 pm

Hi,
I've pulled/refitted all the socketed RAM which is the non-working sector, but no change.
Is there any way to test or is it just replace?
Is there another jumper to enable this bank?
Cheers
Gavin :)

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Re: Model B Fault finding help please :)

Post by Bat » Tue Mar 13, 2018 11:34 pm

Hi,
More reading up told me that IC14 is a common problem http://www.stardot.org.uk/forums/viewto ... 416#p43416
However swapping them between machines proved the chip to be OK.
Cheers :)

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1024MAK
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Re: Model B Fault finding help please :)

Post by 1024MAK » Wed Mar 14, 2018 12:07 am

Hi Gavin

Read this post. It gives details of how the memory in a Beeb works, more details on extra tests you can do, including some BASIC programs.

See how you get on, and shout out if/when you need more help.

Mark
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DutchAcorn
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Re: Model B Fault finding help please :)

Post by DutchAcorn » Wed Mar 14, 2018 6:14 am

Bat wrote:Hi,
I've pulled/refitted all the socketed RAM which is the non-working sector, but no change.
Is there any way to test or is it just replace?
Is there another jumper to enable this bank?
Cheers
Gavin :)
Have you tried to boot with the S25 link removed?

If that configuration also boots to 16K the CAS selection circuit is a likely suspect.

If not, it is best to double check on the memory chips IC61-IC68. If you have the skills you can use a 74LS04N hex inverter to invert the signal from the North pin of S25. This will swap the lower and higher memory banks and will let you do fault finding from a semi-functional BBC. See Marks post for instructions and explanation.
You can also try to feel if some of the memory chips get hot.
Paul

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Re: Model B Fault finding help please :)

Post by Bat » Wed Mar 14, 2018 8:41 am

Hi,
I was looking at the hex inverter last night, will have to go down that route if needs be.
Glad you reminded me about the no jumper test. I'll try that later when wifey puts the heating on :lol:
Cheers,
Gavin :)

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marcusjambler
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Re: Model B Fault finding help please :)

Post by marcusjambler » Wed Mar 14, 2018 8:54 am

If you can burn Eproms then using the test OS ROM will tell you exactly which bits in the RAM memory field are stuck.
It replaces the OS ROM and cycles through several tests. It does not require working RAM for it to operate.

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Re: Model B Fault finding help please :)

Post by Bat » Wed Mar 14, 2018 12:26 pm

Hi,
Unfortunately Marcus I don't have eprom equipment, although I need some doing for some software with missing ROMs :(

Anyway things have got worse this morning :x Composite video has gone belly up, see attached :roll:
IMG_20180314_111359[1].jpg
I'm waiting for a RGB cable to arrive so I can't test anything else to see if it's just composite.

Cleaned up on a video processor it looks like this...
IMG_20180314_122133[1].jpg
Would this tie in with the memory problem? I know the two are linked but don't know if it would cause this fault as I still have some form of output ...
Cheers,

Gavin :)

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Re: Model B Fault finding help please :)

Post by tricky » Wed Mar 14, 2018 12:54 pm

Could be if it is causing the crtc to be miss programmed.

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Re: Model B Fault finding help please :)

Post by marcusjambler » Wed Mar 14, 2018 1:00 pm

Try the RF out with S25 south To see if it still boots to 16K

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Re: Model B Fault finding help please :)

Post by Bat » Wed Mar 14, 2018 2:28 pm

Hi,
Looks like IC14 hadn't made a good connection when I pushed it back in late last night?
I'm back to 16K operation which I'll give a good test, whilst I wait for the Hex inverter and breadboard to arrive.

S25 results... North or off completely gives the same results, no boot and wonky video output.

Thank for the continued assistance! :D
Cheers,
Gavin :)

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1024MAK
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Re: Model B Fault finding help please :)

Post by 1024MAK » Wed Mar 14, 2018 2:59 pm

With the Beeb, the CPU has to run the code from the ROM in order to correctly set up the CRTC chip. Now, the code in the ROM is the Acorn OS. This was written assuming that the RAM between 0x0000 and 0x3FFF is fully functional. If the RAM is faulty (in any significant way), the CPU will be unable to correctly set up the required parameters in the registers of the CRTC chip. The result is either no display, or a mix of unsynchronised lines with little or no actual screen data.

The inverter can be any moderately fast logic chip that can invert a TTL logic signal. So 74LS04, 74LS14 (both inverters or NOT gates), 74LS00 (NAND gates), 74LS02 (NOR gates), or the 74HCT versions of these. Note that this list is not exhaustive and there are other chips that will also work.

When testing, it pays to keep notes of what you have tried, and the results.

Then when you get stuck, you can type up your notes and post them here. We can then give more useful advice :wink:

Mark
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KenLowe
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Re: Model B Fault finding help please :)

Post by KenLowe » Wed Mar 14, 2018 7:22 pm

If you've got any spare memory ICs, or a known good one you can borrow from another machine, then one other trick you can try is to piggy back the good IC on top of one IC from the suspected faulty bank. Cycle through all 8 and see if your Beeb kicks into life. This doesn't always work. It really depends on how the faulty IC has failed. I've had success on at least one occasion using this trick.

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Re: Model B Fault finding help please :)

Post by Bat » Wed Mar 14, 2018 7:34 pm

KenLowe wrote:If you've got any spare memory ICs, or a known good one you can borrow from another machine, then one other trick you can try is to piggy back the good IC on top of one IC from the suspected faulty bank. Cycle through all 8 and see if your Beeb kicks into life. This doesn't always work. It really depends on how the faulty IC has failed. I've had success on at least one occasion using this trick.
Thaks Ken :)
Sadly no spares, but happily it seems the bank with the failure is all socketed on this iss4 board :)
My other beeb is iss7 no sockets on the RAM :(
Just need to order some caps now for my snap, crackle & pop power supply! #-o

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Re: Model B Fault finding help please :)

Post by KenLowe » Wed Mar 14, 2018 8:24 pm

Bat wrote:Sadly no spares, but happily it seems the bank with the failure is all socketed on this iss4 board :)
Well, that should make testing a bit simpler. If you do need replacement RAM, then I've bought replacement from here before. Good service:

https://www.ebay.co.uk/itm/8x-MB8118-10 ... SwImRYE6LJ

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Re: Model B Fault finding help please :)

Post by Bat » Thu Mar 15, 2018 8:29 pm

Hi,
While waiting for stuff to arrive, my iss7 machine has started to chuck a wobbly! #-o :x
After it's been on for a while the picture breaks up and bounces around the screen before going black and white only.
Games play with the top half of the screen all messed up but in colour.
Any ideas please?
The video ULA is a little warm to the touch, are they supposed to have a heatsink?
Cheers,
Gavin (The guy that thought BBCs only had PSU faults!)

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1024MAK
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Re: Model B Fault finding help please :)

Post by 1024MAK » Thu Mar 15, 2018 11:29 pm

Most Ferranti video processors (ULAs) need a heatsink. Very late versions don't need a heatsink. VTi types don't need a heatsink.

Which type is fitted?

Mark
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Re: Model B Fault finding help please :)

Post by DutchAcorn » Fri Mar 16, 2018 6:19 am

Using coolant spray or a can of compressed air (used upside down) you can try cooling the video processor when the issues arrise - or other components when that has no effect.
Paul

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Re: Model B Fault finding help please :)

Post by Bat » Fri Mar 16, 2018 10:21 am

Hi,
Thanks for the replies guys :)
I've found out this morning that it works fine on my 40" monitor but the problem is with the 24" setup.
Sorry for the lack of testing everything, it's not easy to move something that big!

Cheers :)
Attachments
IMG_20180316_101633[1].jpg
IMG_20180316_101430[1].jpg
IMG_20180315_234033.jpg

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1024MAK
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Re: Model B Fault finding help please :)

Post by 1024MAK » Fri Mar 16, 2018 12:38 pm

On some LCD TVs you may have to go into the menus/settings and manually select PAL 50Hz mode.

I have a LCD TV that when it is set to AUTO mode, it can't lock on to the video signal :(

Mark
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Re: Model B Fault finding help please :)

Post by Bat » Fri Mar 16, 2018 10:38 pm

Hi,
Thanks Mark. The 40" monitor has the yellow rca on it, plug in and away it goes.
The 24" is a HDMI monitor with an external Lumagen video processor. The processor is a high end bit of kit that's designed for clean signals, I think the beeb is a bit rough for it lol. I'm making an RGB cable to feed it next week, hoping for an amazing picture...
Any way ....
IMG_20180316_221014[1].jpg
I've got my bits together and hooked up the hex inverter. I've used the test program from MartinB to get this...
IMG_20180316_222311[1].jpg
You can see where it's going to crash before it gets there as there's already a faint pattern on the screen before it starts. When the two pattern meet boom, it stops at 4000. Can some kind person tell me with chip relates to 4000 please?
Cheers :)

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Re: Model B Fault finding help please :)

Post by MartinB » Fri Mar 16, 2018 10:47 pm

That’s not ‘a chip’ as such, it’s failing at the start of the second 16k. You need one of the short bit test proggies that are around to test for a specific chip - I’m away from my stuff just now so someone will doubtless pop up with the appropriate code and chip reference.....

This will be ok but you’ll still need the chip cross-reference....

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

Try it as shown or, if no failure is printed, escape and change the zeroes in lines 10 and 30 to 255

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Re: Model B Fault finding help please :)

Post by Bat » Fri Mar 16, 2018 11:22 pm

HI,
Thanks Martin :)
That spat out 20
Cheers :)

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Re: Model B Fault finding help please :)

Post by MartinB » Fri Mar 16, 2018 11:29 pm

Ok, so that’s Bit 5 and is the 6th chip in the second logical 16k bank (which might be the first physical bank if you have used the S25 inversion?) so just need someone with the schematic to hand to identify the IC number..... Might be either IC66 or IC58 from memory (depending on S25 inversion or not) but that could be wrong so wait for confirmation!

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Re: Model B Fault finding help please :)

Post by Bat » Sat Mar 17, 2018 6:44 am

Hi Martin,
Yes I used the hex inverter on S25 to get that result as the machine won't boot at 32k without it.
Schematic is no problem got some I've downloaded here :)
Cheers :)

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