Only the Break key works on Keyboard

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SpudGun
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Only the Break key works on Keyboard

Postby SpudGun » Sun Dec 10, 2017 10:36 am

So there I was on one of my BBC B's happily playing a game of Chuckie Egg when the keyboard stop responding. The caps lock light was on but only the break key seemed to do anything.

- Tried power off / on, no joy.
- Let the machine cool down - no joy.
- Swapped the keyboard over with one from another working BBC B - everything worked fine.
- Put the original keyboard back - back to square one, no joy.

So the Beeb main board appears to be fine, it's just a problem on the keyboard.

Any ideas?

Cheers, SG

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1024MAK
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Re: Only the Break key works on Keyboard

Postby 1024MAK » Sun Dec 10, 2017 11:10 am

The most common problem with keyboards (well, apart from the keyswitch contacts tarnishing and/or getting dirty) is the cable between the main board and the keyboard failing. One or more connections may have failed.

The break key is an independent circuit as it is connected in parallel with the position for a reset button at the back of the main board. All the other keys on the keyboard are part of a matrix. There is active circuitry (IC's) on the keyboard that scan the keyboard matrix. A failure of one of these, or a PCB track associated with this circuitry may well cause a complete failure.

So the first step is to have a good visual look at the keyboard. You are looking for anything that looks damaged or in distress. Check the tracks on the edge of the board, a crack could have broken a track. Also closely look at the solder points. If you can see fine cracks, breaks, rings, lumps, or a dull discolouration, that could be a "dry" joint. If you have a soldering iron and can solder, reflow the joint (desolder, clean the component pin/leg, resolder).

Do you have a multimeter or any other test equipment?

Mark
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vanekp
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Re: Only the Break key works on Keyboard

Postby vanekp » Sun Dec 10, 2017 12:22 pm

The other problem with the keyboard is that pressure gets put on the 3 LED's at the bottom left and the tracks get pushed away from the board and brake, so have a close look at those as well.
Peter.

SpudGun
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Re: Only the Break key works on Keyboard

Postby SpudGun » Sun Dec 10, 2017 3:05 pm

Thanks for the replies.

Yes I've got a decent DMM, I've done some basic checks e.g. visual inspection, confirmed continuity from each pin of the the connector to various points on the points on the underside tracks. I've also randomly checked continuity between various tracks & pads, and confirmed none of the keys are permanently short circuit. The keyboard is in very good condition.

I've not had this machine that long and this was the first extended use of it. The fact that it was working then just stopped mid gameplay makes me wonder if one of the ICs died?

Are there any obvious other tests I can perform or known faults that can occur?

Cheers, SG

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vanekp
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Re: Only the Break key works on Keyboard

Postby vanekp » Sun Dec 10, 2017 4:42 pm

Check that you have 5V on IC2 (top left of the keyboard) pins 8 0V and pin 16 +5v, you can check the other chips as well to be sure.
20171210_172548.png

The keyboard is not that complicated so cant be to hard to find out whats wrong
BBCKeyboard.png

Even my keyboard I have to fix the 5v line by the LED's
20171210_172516.png

If all that checks out then I would think one of the chips is not good.
Peter.

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Re: Only the Break key works on Keyboard

Postby crj » Sun Dec 10, 2017 5:12 pm

I've never personally had to diagnose a keyboard fault on a Beeb, but as I understand it:
  • Mainboard works (with a different keyboard)
  • LEDs work (so +5V present)
  • Break works (so 0V present)
  • Not even a single other key on the keyboard works (not an isolated problem within the key matrix itself, nor a keyswitch issue)
That's got to be either a broken IC or an issue with power or ground continuity within the PCB, right?

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1024MAK
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Re: Only the Break key works on Keyboard

Postby 1024MAK » Sun Dec 10, 2017 5:43 pm

The 1MHz signal and the KB EN signals are very important. If you have a 'scope or logic probe, check these on the input pins of the relevant chips (see the schematic above).

Mark
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Re: Only the Break key works on Keyboard

Postby dp11 » Sun Dec 10, 2017 5:44 pm

I think pin 4 or pin 12 on the cable being broken might cause the keys to stop working.

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Re: Only the Break key works on Keyboard

Postby SpudGun » Tue Dec 12, 2017 8:36 pm

Quick update.

- I've probed around a bit with a basic logic probe and can see evidence of a 1Mhz signal on IC1 Pin2
- I can see the various pins of IC3 go high when I press a key on the appropriate column (e.g. Pin 1 goes high when pressing Escape).
- I can't see any signals at all on pins 12 to 15 of IC3

Probing a working keyboard I see activity on pins 12 to 15 of IC3, and also it's noticable that IC3 1 to 11 are low initially then go high on a keypress, whereas on the broken keyboard they are NOT low initially and just go high on a keypress.

Assuming IC1 generates the signals on pins 11 to 14, is it safe to assume the IC1 is the culprit here?

Thanks, SG

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1024MAK
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Re: Only the Break key works on Keyboard

Postby 1024MAK » Tue Dec 12, 2017 11:34 pm

IC1 74LS161 / 74LS163 datasheet
This is a 4-bit binary synchronous, presettable counter. Synchronous operation is provided by having all flip-flops clocked simultaneously so that the outputs change coincident with each other when so instructed by the count-enable inputs and internal gating. This mode of operation eliminates the output counting spikes that are normally associated with asynchronous (ripple clock) counters. A buffered clock input triggers the four flip-flops on the rising edge of the clock input waveform.

The 1MHz clock from the main Beeb board is connected to pin 2, the counters clock input. Input pins 1 (/CLR), 7 (ENP) and 10 (ENT) are connected to +5V as these are not used here. When pin 9 (/LOAD) is low, at the next clock pulse, the logic levels on in inputs (A to D, pins 3 to 6) are transferred to the outputs (QA to QD, pins 14 to 11). But when /LOAD is high, the chip counts up with each clock pulse.

IC3 7445 datasheet or 74145 / 74LS145 datasheet
These are BCD-to-decimal decoders/drivers. Full decoding of valid BCD input logic ensures that all outputs remain off for all invalid binary input conditions. They have n-p-n output transistors connected as open-collector logic-circuit drivers. So pins 1 to 11 are outputs, but they can only pull down to 0V. They are unable to output a logic high (logic 1). Note that the outputs are inverted, so are normally open circuit, for each valid BCD number on the inputs, one and only one output will go low.

Resistors R4 to R11 pull up the relevant keyboard lines if there are no switches pressed for the relevant line, or if a switch is pressed, but the relevant output of IC3 is not low.

Combined, IC1 and IC3 can scan the keyboard matrix with no help from the CPU on the main board. This is the normal operation.

When a key is pressed and the relevant output of IC3 is low, IC3 pulls down the horizontal line, so both IC2 and IC4 then see a logic low on their relevant input pins. IC2 will output data to the main board when requested. IC4 is used to trigger an interrupt to signal that a key was found pressed during auto scan mode.

I hope this helps ;-)

Mark
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SpudGun
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Re: Only the Break key works on Keyboard

Postby SpudGun » Wed Dec 20, 2017 3:58 pm

Quick update: I've got hold of a SN74LS163AN and piggy-backing the new one on top of the existing one appears to resolve the issue.

I'm not 100% certain replacing the 74LS163 is the final fix as the keyboard does work intermittently, I'll try and get the new one properly soldered in the next few days.

Cheers, SG

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vanekp
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Re: Only the Break key works on Keyboard

Postby vanekp » Wed Dec 20, 2017 4:43 pm

Till you change it you won't know, you could fit a socket when you remove the old chip makes it easier if you have to further fault find things.
Peter.

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Re: Only the Break key works on Keyboard

Postby JannievanZyl » Sun Dec 31, 2017 7:39 am

I had this happened to me recently, perfectly fine keyboard stopped working altogether.

Having read in the Acorn Service Manual that these keyboards are prone to tracks breaking, I resoldered all the joints (except the keys themselves) and that fixed it 100%.

A visual inspection showed no obvious broken tracks. Was likely just a dry joint or two.

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Re: Only the Break key works on Keyboard

Postby SpudGun » Mon Jan 01, 2018 2:14 pm

Replacing the 74LS163 has resolved the issue.

:D :D :D

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vanekp
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Re: Only the Break key works on Keyboard

Postby vanekp » Mon Jan 01, 2018 4:38 pm

Glad you got to the bottom 0f the problem, another BBC restored to a working condition, well done :D =D>

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1024MAK
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Re: Only the Break key works on Keyboard

Postby 1024MAK » Tue Jan 02, 2018 12:20 am

Another fine repair =D>

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