First ever BBC broken

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Boydie
Posts: 148
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Location: Sunny Wigan

First ever BBC broken

Postby Boydie » Sun Sep 17, 2017 6:58 pm

Hi folks,

This one has been on and off the to-do list for a few years now.
Some years ago, I managed to break the (Issue 7) motherboard from the Model B I've had ever since being a kid. It all goes back to trying to get that Solidisk 4Meg board working, that I recently succeeded in fixing. Having soldered the 4MHz clock lead to IC43 one too many times, I suspect the original problem was that I fried it. From that moment on, it gave the long, sad beep of death.

Replacing IC43 (a 74S04N) failed to cure it, so I may have been wrong with the original diagnosis.

Since then, things have got worse. These days, it doesn't even give the beep. However, Caps Lock still lights up, and pin 40 on the 6502 goes low when Break is pressed (I was determined to use that new logic probe somehow). I can't remember that it is in response to anything specific that I did. I have a recurrent, unfounded, paranoia that I may have inadvertently swapped one 5V and 0V on one of the inputs at some point, but can't specifically remember doing so and in any event all power connectors on the Issue 7 are joined together, so I'm not sure what the effect of reversing one of the inputs would be if the other two are correct (I'm guessing the PSU really wouldn't like it).

I've also replaced the crystal, and the two resistors next to IC43, wondering if I'd fried one of these with careless soldering. No change.

In the end I gave up and replaced the motherboard, but since this board has special nostalgia value I'm keen to resurrect it if possible.

Does the lack of sound mean anything, or is this likely to be a machine that's still trying to give the continuous beep but its sound system has died as well? I initially thought this may be pointing toward something in the clock circuitry?

What I've done so far (with no improvement):

Removed all the usual ICs that can be removed, and keyboard
Confirmed all pins of 6502 have continuity with socket
Confirmed all power connectors have correct voltages
Confirmed 6502 pin 40 goes low with Break

Any suggestions where to look next? Can I investigate further, armed with just a multimeter and logic probe, or should I be asking Santa for an oscilloscope?

Ta in advance.

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DutchAcorn
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Location: Maarn, Netherlands

Re: First ever BBC broken

Postby DutchAcorn » Sun Sep 17, 2017 8:12 pm

Does it boot without the System VIA (IC3)?

Do you have an eprom programmer? Then Tricky's test rom can help diagnosing.

Have you tried the suggestions in the fault finding index?
Paul

Boydie
Posts: 148
Joined: Sat Oct 24, 2015 8:25 am
Location: Sunny Wigan

Re: First ever BBC broken

Postby Boydie » Sun Sep 17, 2017 8:37 pm

Removing IC3 has no effect
I've also tried swapping IC14 - no change
I've been through the fault finding index. Unfortunately the troubleshooting guide doesn't seem to be available for download any more.
I burned tricky's rom earlier tonight. Same lack of response as with normal os rom.

Am I correct in assuming that lack of sound suggests an absence of 4MHz signal, which may suggest absence of proper clock?

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DutchAcorn
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Location: Maarn, Netherlands

Re: First ever BBC broken

Postby DutchAcorn » Sun Sep 17, 2017 8:47 pm

To check for the various clock signals you can see if you can find 1, 2, 4 and 8 MHz on pins 4, 5, 6, and 7 of IC6 (video processor), as suggested by the service manual. If your logic probe can detect a pulse, that is.

An alternative link to the trouble shooting guide is here:
http://primrosebank.net/computers/bbc/documents/Troubleshooting%20the%20BBC%20Micro.pdf
Paul

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1024MAK
Posts: 6798
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...

Re: First ever BBC broken

Postby 1024MAK » Sun Sep 17, 2017 9:36 pm

Yep, in order, things to check first are:
  1. Power supply rails, which you have done. But have you checked the power pins of all the important chips?
  2. The main clock circuits - as suggested above, check the clock output pins on the Videoproc / video ULA. Then check the clock on the CPU.
  3. The reset circuits, if the CPU is in reset, it's not going to do much...

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...

Boydie
Posts: 148
Joined: Sat Oct 24, 2015 8:25 am
Location: Sunny Wigan

Re: First ever BBC broken

Postby Boydie » Sun Sep 17, 2017 9:39 pm

Thanks for the link.

In hindsight, your questions should have been:

1) Did you do something dumb?
2) Are you sure you didn't do something dumb?
3) I bet you did something dumb. Why haven't you found it yet?

On an impulse, I decided to remove the replacement IC43, and check for dodgy workmanship. And I found it. My previous attempt to remove the original broken chip also removed the track between Pin 4 and R91. Unfortunately, this track is so short that I didn't notice it until the IC had been removed. A new socket, a quick link between Pin 4 and R91, and a "borrowed" 74S04N from a Nascom 1 Buffer Board, and now it powers up fine!

Guess I was right about sound being out due to lack of 4MHz, and the absence of clock preventing power-up. Shame it took me so long to realise the underlying cause was my own incompetence!

My (personally, if not chronologically) oldest machine now works again. I'm a happy bunny. Now to break it again re-fitting the Solidisk board and upgrading the ROM sockets (and try to source a 74S04N for the Nascom)...

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1024MAK
Posts: 6798
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...

Re: First ever BBC broken

Postby 1024MAK » Mon Sep 18, 2017 5:54 am

Well, at least you eventually fixed it :lol:

So well done on getting there in the end =D>

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...


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