Beeee fault

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kengr1
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Beeee fault

Post by kengr1 » Fri Oct 25, 2013 2:36 pm

Hi
I have a series 7 board with econet and disc interface fitted. The power supply has been repaired (c1,c2 & c9) and works normally. On power up I get:

1 loud beeeeee sound

2 No LEDs lighting (but they do work if powered independently)

3 blank screen

Ive followed the check list in the service manual (pages 39 and 40). Everything seems fine, except the IRQ line stays in the low state.

So does this mean the 6502 is caput, or does it point to some other device(s) associated with the IRQ line?

Cheers

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roland
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Re: Beeee fault

Post by roland » Fri Oct 25, 2013 10:16 pm

As the IRQ is an input of the 6502 the cpu cannot pull it down. If the Econet, fdc or any other device generates an interrupt and the cpu is not handling it, the irq line stays down.

Can you check with an oscilloscope if the cpu starts running, e.g. by monitoring the address bus or data bus or the r/w pin?

It might be the cpu, but a defective rom causes problems, and if the first kb of ram is inaccessible or not writable, the cpu cannot initialize its stack and the vectors that are needed for the os. So no running at all.

I would start examining the ram ...

Roland
Last edited by roland on Sat Oct 26, 2013 6:24 pm, edited 1 time in total.
FPGAtom: 512 KB RAM, Real Time Clock and 64 colours
MAN WOMAN :shock:

kengr1
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Re: Beeee fault

Post by kengr1 » Sat Oct 26, 2013 1:26 pm

Thanks Roland

Pin 34 seems to be ok. As does the signal on pin 10 of IC 33 (as per the service manual). Ive tried afew different os roms, no change. Whats the best way of checking the first 1K of RAM? Tried changing S25 but no help.

ken

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MartinB
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Re: Beeee fault

Post by MartinB » Sat Oct 26, 2013 10:15 pm

I think I'd stay simple just yet and remove the keyboard ribbon cable (both ends), give everything a good clean (especially the motherboard fixed vertical pins), re-fit and try again. I find this to be quite a common cause of a flat-lining Beeb :wink:

kengr1
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Re: Beeee fault

Post by kengr1 » Sun Oct 27, 2013 1:45 pm

Hi Martinb

Thanks, tried that but no help. As I was 'scoping the memory chips this am, the board started making a peculiar clicking noise. traced this to the relay, presumably something to do with the cassette interface? Anyway this stopped after about thirty seconds. Still no joy with the board. Would appreciate some help in testing the RAm, what should I be looking for?

cheers

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1024MAK
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Re: Beeee fault

Post by 1024MAK » Sun Oct 27, 2013 2:36 pm

There is a thread on RAM chip faults here.

Have you tested the +5V output from the PSU?

Also check the OS ROM. If you have another Beeb, try the OS ROM in the good Beeb.
Also try the machine without the BASIC ROM fitted.

Mark

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1024MAK
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Re: Beeee fault

Post by 1024MAK » Sun Oct 27, 2013 2:42 pm

kengr1 wrote:As I was 'scoping the memory chips this am, the board started making a peculiar clicking noise. traced this to the relay, presumably something to do with the cassette interface?
Yes, the relay turns the cassette motor on/off. If this is clicking then (assuming the +5V supply is okay) the CPU is likely to be processing random junk.

So it sounds like a memory or bus fault.

Mark

kengr1
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Re: Beeee fault

Post by kengr1 » Sun Oct 27, 2013 2:56 pm

Thanks Mark

The 5volt rail is good. Ill try a known good OS rom with basic removed. As i said before ive moved the S25 link to the south position with no luck. Does this make a memory fault less likely?

Ken

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1024MAK
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Re: Beeee fault

Post by 1024MAK » Sun Oct 27, 2013 3:12 pm

Hard to say. Could be that you have faulty RAM in both banks, or the chip that has failed, has failed in such a way, that it is messing up the data bus. Also it is possible that part of the support circuitry has failed.

If you have a schematic (and if not, download a copy), take a look and study it.

First though, do the easy stuff, which is eliminate the main socketed chips from the list of suspects.

Mark

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Re: Beeee fault

Post by kengr1 » Mon Nov 04, 2013 9:03 pm

Hi Thanks

Ive eliminated all the easy socketed chips. The OS rom has been changed, still no luck. Question? Many faults seem to keep the caps lock LED working. Mine isnt (althought the LED does work if powered). So which faults one the main board would stop the keyboard LEDs lighting?

Thanks

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retroclinic
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Re: Beeee fault

Post by retroclinic » Mon Nov 04, 2013 9:18 pm

Ignore the LEDs. Remove both 6522s, the 8271 (or other DFS dude), the 68B54 for Econet, the 7002 ADC, Serial ULA, 6850 (if socketed) and all ROMS apart from the OS. Then try and get the board working to a "Language?" prompt from there with no keyboard attached.

The usual suspect on these boards is RAM, but there can be a general TTL failure anywhere.

Mark.
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kengr1
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Re: Beeee fault

Post by kengr1 » Mon Nov 04, 2013 9:30 pm

Hi Thanks Mark

Done that. No luck, so likely Ram. I have a scope and a working bbc, which pins should i look at on the RAM, (or is it obvious ie all off em? :))

Ken

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Re: Beeee fault

Post by kengr1 » Mon Nov 04, 2013 9:32 pm

Hi again

So a working Beeb should boot with no keyboard attached?

Ken

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retroclinic
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Re: Beeee fault

Post by retroclinic » Mon Nov 04, 2013 9:35 pm

kengr1 wrote:Hi again

So a working Beeb should boot with no keyboard attached?

Ken
With the System VIA removed, yes. With it in place, a 32K machine should have a Mode 0 cursor flashing at the top left of the screen. A 16K one will have a Mode 4 cursor.

Mark.
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kengr1
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Re: Beeee fault

Post by kengr1 » Mon Nov 04, 2013 9:43 pm

You're a great help Mark

I suspect Ill be wrapping this board up and sending it to you !

Ken

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richardtoohey
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Re: Beeee fault

Post by richardtoohey » Mon Nov 04, 2013 10:06 pm

kengr1 wrote:I suspect Ill be wrapping this board up and sending it to you !
It probably won't be as much "fun" but would be a lot quicker! :D

kengr1
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Re: Beeee fault

Post by kengr1 » Mon Nov 04, 2013 10:11 pm

I'll send it to you, you fix it, i pay the published cost. You tell me what was wrong and how you diagnosed it, I pay you an extra 10% for the knowledge?

good deal for me

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retroclinic
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Re: Beeee fault

Post by retroclinic » Mon Nov 04, 2013 10:18 pm

Hi.

What I do to fix it I'm happy to share, no extra charge! Yes, send it on over (with all socketed chips replaced), PM me if you need the address details.

Thanks, Mark.
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