Extra PCB fitted to Dragon 32

[zephyr]

I have a Dragon 32 with an Issue 5A mainboard (16x16) and a MK3, Issue 4 power board. The mainboard has an extra little PCB which is plugged directly into the socked where the SAM chip usually sits. The top of the PCB is comprised of a socketed SAM chip (SN74LS783N), two smaller chips (SN74LS74AN, SN74LS32N), and a tiny blue 16v capacitor. There is a long thin red wire which goes from the middle of the PCB to pin 37 of the MC6847 VDG chip. As the seals were all intact, this extra PCB must have been fitted at the factory.

Does anyone know what this board is for?

[Rinconete]

Never heard of it before, I have no idea of what can it be

[rolfmichelsen]

Don't know what the purpose of the board is but both additional ICs are quite standard logic gates. SN74LS74 is a pair of D-type flip-flops (basically 1 bit memories) and SN74LS32 contains four 2-input OR gates. My guess would be a "patch" to compensate for an error on the main motherboard, but that will require closer inspection...

[prime]

If you have the ability to take a couple of good quality digital pictures (top and bottom of board) and post them somewhere, I could have a go at tracing the board, which might help work out what it's for.

Cheers.

Phill.

[zephyr]

Thanks for the offer Phill, but I don't currently have access to a decent digital (or other) camera.

[zephyr]

I found this poor quality picture of a Dragon 32 with the very same board fitted on eBay today. It may not be very helpful but at least you now have a visual reference.

[RWAP]

I also have a non-working Dragon 32, listed on ebay with this board - photos are attached (of top and bottom)....

The board is marked Dargon Data Limited, so must have been a standard fit for something!!

[prime]

Right having bought that self same non-working Dragon, thanks RWAP, I didn't realise it was boxed, and though the box was a little scrappy it had a decent set of pollys

It works now BTW, had to supply a PSU board, change the pot behind the power connector and change the standoff on the addon board that was broken.

Whilst I was repairing the add-on board I desoldered the 2 LS chips and traced the board, what it seems to do is two things :

1) Makes sure that DA0 on the SAM is forced high whilst frame sync is low (i.e. in between frames).
2) Makes sure that transitions of horizontal sync only happen on the falling edge of the E clock.

The net effect of these two seems to be to clean up the video signal, on my LCD TV, fed from the monitor socket on the Dragon the picture is much shaper, especially in text mode and pmode 4.

I may try replicating the board and fitting to some of my other Dragons.

Anyway I'll attach the circuit diagram, and incase anyone is interested I have re-produced that board as an Eagle 5.x project complete with board layout if anyone wants it, just ask, or I can post here
if there is any interest.

Cheers.

Phill.

DragonData-PN48200.zip

circuit diagram / eagle files.

(63.49 KiB) Downloaded 348 times

[Alastair]

Phill,
would this board work with Dragons with different issue mainboards, and if so, would that be all issues or just some?

If it will work on other Dragons then this news is sure to please many Dragon owners. So would you, or anyone else on this forum who has the ability, be prepared to make a small number of video patch boards? If so, how much are they likely to cost (please include labour as well as material cost) and what would be the likely p&p to elsewhere in the UK?

[tjewell]

Quite a co-incidence this discovery, given my other thread about display quality! Like Alastair, I'd be intrigued to know which Dragons this would work with. Might it already have been done on the later issue5/6 boards, which is why they have a better quality display? Or is it better still?

I love the fact that new things can be discovered about these machines even after nearly 30 years.