Keyboard Question....

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bmrw64
Posts: 2
Joined: Sat Feb 10, 2018 11:58 pm

Keyboard Question....

Post by bmrw64 »

Hello all...

I recently bought a Dragon 32 from e-bay in which the seller said was fully working ha ha ha and of course there is a problem with it! When I press buttons 6 and 7 and letters f, g, n, o, v, w along with the space bar nothing happens. From checking the keyboard I can not find a problem with it. I see from the circuit diagram it has a MC6821E chip that seems to show that it connects the keyboard to the CPU. Is it possible that this chip has failed?? Any advice would be nice..

Thanks...
pser1
Posts: 1655
Joined: Sun Mar 25, 2012 7:32 pm
Location: Barcelona (SPAIN)

Re: Keyboard Question....

Post by pser1 »

Hi,
so, you are having problems with the keys that are connected to bits PB6 and PB7 of PIA0 ($ff02)
The first try should be opening the Dragon case and re-seating the keyboard connector
on the mainboard, for maybe it could have changed position and so these two contacts are failing ...
If this does not cure the problem, then it could be a fault on that PIA (MC6821) this would
be a bit awkward because, usually, they are not socketed, so changing it will be an extra work :-(
I think it might be IC26, but not sure if it is the same number on the various motherboard versions
that exist.

Good luck!
pere
sorchard
Posts: 530
Joined: Sat Jun 07, 2014 9:43 pm
Location: Norwich UK

Re: Keyboard Question....

Post by sorchard »

Hi,

Another possibility is the keyboard cable. These are not very flexible and can break where they connect to the keyboard pcb. I've had a few fail like this.

The PB6 & PB7 signals just happen to be the last two connections on the right hand side of the cable. If you're lucky you can diagnose it by applying gentle pressure to the cable to see if the keys will start working again.

Alternatively, disconnecting the keyboard and shorting pin 1 to pin 16 (far left and far right) should type a '7' if the main board is OK.

Otherwise it does sound like a faulty PIA IC26 or possibly faulty 74LS244 printer port buffer IC24 which is also connected to those lines.
Stew
bmrw64
Posts: 2
Joined: Sat Feb 10, 2018 11:58 pm

Re: Keyboard Question....

Post by bmrw64 »

Hello....This is what I thought it may be. I found that the CPU programmes the PIA when first switched on so this also could be a suspect. So I have ordered the CPU along with the PIA and IC24 connects to PB6 and PB7 so this is also on order just in case. They should arrive this week. IC26 is double sided and I found long ago never try to take the chip out in one go as the tracks break up, so I will cut out the chip with a mini saw and then solder out each leg at a time then drill out the holes. I will let you know if all is well..Thanks.

Brent
prime
Posts: 266
Joined: Fri Apr 10, 2009 1:40 am

Re: Keyboard Question....

Post by prime »

bmrw64 wrote:Hello....This is what I thought it may be. I found that the CPU programmes the PIA when first switched on so this also could be a suspect. So I have ordered the CPU along with the PIA and IC24 connects to PB6 and PB7 so this is also on order just in case. They should arrive this week. IC26 is double sided and I found long ago never try to take the chip out in one go as the tracks break up, so I will cut out the chip with a mini saw and then solder out each leg at a time then drill out the holes. I will let you know if all is well..Thanks.
Noooo!!! don't drill out the holes, also a mini saw would be a very bad idea, too much chance of damaging other components. Way to go is :

Get a fine pair of cutters and cut each pin, individually where it enters the chip.
Then heat up the solder on each pin and remove it one at a time, forceps help here as they stop you burning your fingers :)
Once the pins are out get some solder wick** and clean the holes out, note to get this to work you may actually have to *ADD* solder to get a good thermal contact to the solder in the hole.
Once the holes are clear fit a chip socket so if the chip fails in the future swapping it is much easier.

** Solder wick that I use : https://www.rapidonline.com/servisol-62 ... 0m-85-0624

But before you do any of that, get a multimeter on the low ohms range, better still the continuity beep and check each track between the PIA pin and the keyboard connector, you may find you need to re-flow the solder on either the PIA or connector, do this by heating up the solder till it melts, and letting it solidify again. Again sometimes adding a little new solder helps.

Also remember that a machine as old as the Dragon will use lead/tin solder so please do not use lead free as mixing types of solder can be problematic.

Cheers.

Phill.
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Vyper68
Posts: 35
Joined: Thu May 25, 2017 4:51 pm
Location: Hurworth-On-Tees

Re: Keyboard Question....

Post by Vyper68 »

I have just bought a Dragon 32 which has been sold as having some keys not working. Going off memory of when i owned a Dragon as a child the Keyboard is fairly robust so i would imagine the problem lies further down the trail to the PIA. It has yet to arrive but it was also listed as never having been opened, owned from new and never modified.
Is there a formal set of troubleshooting steps to isolate the fault before ordering new chips on the off chance the fault is repairable ?
Thanks in advance.
The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails.
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