Hello and help me fix my beeb

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CommanderCoder
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Hello and help me fix my beeb

Post by CommanderCoder » Wed Nov 06, 2019 8:36 pm

Hi. Just joined after reading some of the helpful posts about fixing BBCs. I got a BBC back in 1982, during the original preorders when a BBC B was £335 before it went up to £399 at the end of 1981. (£335 is about £1300 in today's money - wow!). I'd pestered my Dad for some kind of computer since about 1980. Didn't get a ZX81 because it looked too much like a calculator. Wanted a EACA Video Genie but then my Dad heard about the BBC so we ended up getting that. (I did get a Video Genie in about 2002 though.. it used to work back then but it's been in the loft ever since)

I recently got out my old BBC Master and recapped it and rebatteried it, and it works fine. I sold my original BBC in about 1988 to help me pay for an Atari ST for university work. I picked up an Issue 7 BBC from a school that was throwing them out and it has sat in my loft for 18 years too. I got that out recently and recapped it and rejointed some of the keys as they weren't working. It almost works!

The BBC B comes on and there is a continuous BEEP (after initial BURRR). If I press BREAK the beep stops but there is no cursor (just '32k' and 'DFS'). If I press T & BREAK, then it goes to tape and works. If I remove the DFS rom it works.

I've read the 8721 could be the cause but before I try to source a new one, is there any way I can be sure it is broken. I have an oscilloscope I've probed the pins. Some of the pulses to the DB pins are pretty dodgy (see picture), but the clocks are pretty good. The INT pin is constantly low making the NMI constantly high.
Screenshot 2019-11-06 at 20.27.40.png
Screenshot 2019-11-06 at 20.27.40.png (50.49 KiB) Viewed 466 times
Well before I bore you any more, I'd love some help in getting it fixed.

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Elminster
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Re: Hello and help me fix my beeb

Post by Elminster » Wed Nov 06, 2019 9:44 pm

Welcome to the fourm.

I think I have had similar issues before that were fixed by reseating the 8721, or from memory you can just take it out to see if that is the casue of the issue.

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flaxcottage
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Re: Hello and help me fix my beeb

Post by flaxcottage » Wed Nov 06, 2019 9:45 pm

Welcome, Commander. :D

That sounds like an 'interesting' problem. It could be an easy fix. Someone with the knowledge will be along soon. [-o<
- John

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Multiwizard
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Re: Hello and help me fix my beeb

Post by Multiwizard » Wed Nov 06, 2019 10:22 pm

Hi,

welcome to this great Forum... :D


Greetings from my little Dutch Atomic Attic, Wim... :-)


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davidb
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Re: Hello and help me fix my beeb

Post by davidb » Thu Nov 07, 2019 12:04 am

Welcome! :D

Kazzie
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Re: Hello and help me fix my beeb

Post by Kazzie » Thu Nov 07, 2019 7:49 am

Hello, and welcome.

My Beeb is currently away visiting friends, so I can't open mine up and experiment on mine myself at the moment. But here are a few thoughts...

There's a thread here of an individual who had a similar prompt displayed at boot; their issue was that link 4 on the keyboard was made (which reverses the effect of pressing break/shift-break), meaning the machine was attempting to boot from floppy disk. Try holding shift (or escape) when powering the machine on, to see if there's any difference.

It may be easier to swap out the DFS ROM rather than seek a replacement 8271 for testing. BeebMaster here has recently bought a job lot of 8271 chips, and is selling them here (see the for sale section) and also through outlets such as eBay, if you find you need one. If you don't have the ability to blow/write an EPROM, one of us may be able to do so and drop it in the post for you.
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
Acorn System 1 home-made replica

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CommanderCoder
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Re: Hello and help me fix my beeb

Post by CommanderCoder » Thu Nov 07, 2019 7:17 pm

Removed the FDC but no change.

Looked around the keyboard for wiring but nothing seems to be linked anywhere.

As I said, T-BREAK puts the machine into tape mode fine, and the DFS appears to be responding to a *HELP and *HELP DFS command. *DISK causes it to hang.

I'll wait for a few more ideas and then get a new FDC from beebmaster.

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Re: Hello and help me fix my beeb

Post by Kazzie » Sun Nov 10, 2019 5:20 pm

Is your DFS ROM an EPROM or a mask ROM?

It's possible that your ROM may be partially corrupted, such that a bit somewhere in the executable code sends the machine hanging when it tries to talk with the 8271, but the strings in the ROM that are queried by the *HELP command aren't affected. That'd be more likely to have happened with an EPROM, mask ROM failures are much rarer.

Some other thoughts that may be of use:
  • Have you tried placing the DFS ROM in a different ROM socket?
  • You expressed some concern about the signals on the DB pins (which are connected to the system data bus). A continuity check between the 8271's pins and another chip on the data bus may help prove whether there's a dodgy connection.
  • The NRDS and NWDS control signals for the 8271 are generated by IC77 (74LS00). Only one other device on the board uses these signals (as far as I can recall): the ADC chip (IC73). A failure of IC77 would only affect those two elements of the machine.
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
Acorn System 1 home-made replica

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CommanderCoder
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Re: Hello and help me fix my beeb

Post by CommanderCoder » Fri Nov 22, 2019 7:11 pm

Thanks for you suggestions. I tried finding any obvious faults in the various components and connections but I couldn't detect anything.

I've started a new thread (viewtopic.php?f=3&t=18280) about a SMART SPI problem since this is more of a hardware thing, than an introduce myself thing.

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daveejhitchins
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Re: Hello and help me fix my beeb

Post by daveejhitchins » Fri Nov 22, 2019 8:56 pm

Welcome to the Forum, CommanderCoder . . . Enjoy . . .

Dave H :D
Parts: UM6502CE, GAL22V10D, GAL16V8D, AS6C62256A, TC514400AZ, WD1772, R6522, TMS27C512, AT28C256
Products: ARA II, ARA III, 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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1024MAK
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Re: Hello and help me fix my beeb

Post by 1024MAK » Sat Nov 23, 2019 2:00 pm

Welcome on board CommanderCoder!

:D :D :D

Enjoy the forum :D

You can make your confession here :mrgreen:
Put yourself on the map by sticking a pin in it at this link :P
You can list your Acorn machines vitals here

I’ve also posted in your other thread...

Mark

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