BBC B long beep and (mostly) unresponsive keyboard

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chawlings
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BBC B long beep and (mostly) unresponsive keyboard

Post by chawlings » Mon Mar 23, 2020 10:27 pm

Hello All
I am new here. I retrieved my old BBC B from the attic a few weeks ago in a fit of nostalgia. I am having trouble getting it to work.
The symptoms are : booting with a normal boot message but sustained long beep and caps lock and shift lock are stuck on. The keyboard is unresponsive until I press the break key. After this the upper left part of the keyboard partially responds. It will register escape, 1, 2, tab, Q and sometimes W, E and R. Pressing break changes the pitch of the beep and can alter which keys are active.

Things I have tried so far:
Cleaning PCB (issue 7) with IPA, removing all socketed chips and spraying sockets with contact cleaner.
Swapping keyboard for known working one.
Reducing socketed ICs down to minimal config.
Checking large socketed ICs work in a functioning BBC micro (except IC51 which is a different type in the other micro I have access to, so I was not sure if they were pin compatible).
Checked continuity between legs of 6502 and system 6522 and the ends of their socket pins.
Checked for continuity between the keyboard header and various connected vias etc.
Burned and ran Tricky's test ROM: no apparent bad RAM

I have reached a dead end. I thought I might try reflowing the solder around the keyboard header but suspect this would be pointless. Any suggestions would be gratefully received.

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danielj
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Re: BBC B long beep and (mostly) unresponsive keyboard

Post by danielj » Mon Mar 23, 2020 11:12 pm

Check continuity on the OS socket. Check the contacts on the keyboard connector! :)

d.

Kazzie
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Re: BBC B long beep and (mostly) unresponsive keyboard

Post by Kazzie » Tue Mar 24, 2020 5:33 am

Assuming the keyboard connector's fine...

Both the sound chip (IC18) and keyboard connector are interfaced through the System VIA, IC3. You may have tried swapping that chip out already. If so, there's an associated chip nearby that could be to blame. IC32, a 74ls259, buffers several values out of the VIA to use as control lines. These lines control things such as the Caps and Shift Lock LEDs, enabling a scan of the keyboard, and activating the data input of the sound chip. (That matches your symptom list fairly well.)

IC32 is clocked by a D-type flip flop, in half of IC31 (74ls74). It may be worthwhile probing the pins of IC32 and IC31 looking for anything unusual. Let me know if you'd like any further guidance.
BBC Model B 32K issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
RiscPC 600 under repair
Acorn System 1 home-made replica

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DutchAcorn
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Re: BBC B long beep and (mostly) unresponsive keyboard

Post by DutchAcorn » Tue Mar 24, 2020 6:36 am

As Kazzy suggested it is most likely related to the system VIA and related circuits. You will see similar symptoms if you completely remove IC3. Tricky’s test rom does more than test memory, it also checks the system VIA by controlling the caps and shift lock leds and producing sound. Both should occur during the Frogger screen. My guess is that there is no sound and caps / shift lock activity during that test.

The first thing to try (as Kazzy suggested) is booting with a known working 6522 in IC3. If you have a multimeter you could check if IC3 gets 5V (and 0V) and also check if the reset pin (34) has 5V on it.
3213B3EC-A2DF-4BE8-A9B5-412FEA32A853.jpeg
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chawlings
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Re: BBC B long beep and (mostly) unresponsive keyboard

Post by chawlings » Tue Mar 24, 2020 11:33 am

Thanks for the replies everyone. There are no problems with continuity around the OS ROM socket. The system 6522 via is good as is the keyboard cable. I decided to desolder IC31 and check it. Sure enough, it is faulty! So I am keeping my fingers crossed. I will post an update when the replacement arrives.

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RetroBob
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Re: BBC B long beep and (mostly) unresponsive keyboard

Post by RetroBob » Tue Mar 24, 2020 11:37 am

Swapping keyboard for known working one.
What happened when you tried this?

Worth replacing the capacitors in the PSU before it blows.

chawlings
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Re: BBC B long beep and (mostly) unresponsive keyboard

Post by chawlings » Tue Mar 24, 2020 9:05 pm

Good point. I meant to say I had recapped the PSU. With the test rom I get the initial frogger splash screen but the LEDs stay on and there is no sound. Initially I did get the expected sound but these seem to have fallen by the wayside. I have high hopes for the 74LS74 which should be arriving on Saturday.

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Re: BBC B long beep and (mostly) unresponsive keyboard

Post by Kazzie » Wed Mar 25, 2020 4:32 am

Let us know how you get on. :)
BBC Model B 32K issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
RiscPC 600 under repair
Acorn System 1 home-made replica

chawlings
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Re: BBC B long beep and (mostly) unresponsive keyboard

Post by chawlings » Thu Apr 16, 2020 7:39 pm

Sorry for the delay in providing an update but postal delays slowed progress. After some time I received a replacement IC31. This resulted in slightly altered symptoms. I was able to type some characters from the right side of the keyboard, but the general behaviour was unchanged.
A replacement IC32 sorted things and now, after reflowing the solder joints of the keyboard LEDs, I have a fully functional Beeb!
Thanks to everyone for their help.

Kazzie
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Re: BBC B long beep and (mostly) unresponsive keyboard

Post by Kazzie » Thu Apr 16, 2020 9:39 pm

Excellent =D>
Many thanks for getting back to us and letting us know how it went.

It would help future readers if you could add a post to our fault-finding index. It doesn't have to be long, just a quick summary of the symptoms, the fault/solution, and a link to this thread. :)
BBC Model B 32K issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
RiscPC 600 under repair
Acorn System 1 home-made replica

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