exxos's Atari, Amiga & retro forum

stephen_usher wrote: 31 Mar 2022 15:18 Re: PSU.

This may help: http://dragon32.info/info/psu.html

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Power Regulator Board to Main Board Ribbon

                  Power Connector
 _______________++++_____________________
|                                        |
| Modulator                              |
|                            Heatsink    |
|      ^^^^                              |
|______|||||||||_________________________|   (Unconventionally, 9 is the
       987654321                              marked strand.  Numbering
 ______|||||||||_________________________     convention matches the
|      |||||||||                         |    circuit diagrams and PCBs)
|            vvv                         |
|                                        |
|                                        |

 Pin  Direction  Function

  9       ^      Composite Video to modulator
  8       ^      +5V supply to modulator (linked from pin 2 via main board)
  7       ^      Sound to modulator and monitor socket
  6       ^      Composite Video to monitor socket
  5              Ground
  4              Ground
  3       v      -5V (D32) or -12V (D64)
  2       v      +5V
  1       v      +12V

D64 power regulator PCBs have a -12V regulator (7912) fitted for the RS232 port instead of a -5V (7905) originally fitted to the D32s. A small -5V regulator is used on the D64 main board for the video circuit. 

Thanks, I have the schematics from there now too :)

CASDunio works well for loading tape games and making up one of those 9pin joystick adapters is handy. A lot of games are Spectrum ports and most others don't benefit from an analogue joystick.

Higgy wrote: 31 Mar 2022 16:14 CASDunio works well for loading tape games and making up one of those 9pin joystick adapters is handy. A lot of games are Spectrum ports and most others don't benefit from an analogue joystick.

I have a TZXDuino, though considering getting an SVI-CAS as well for my growing band of machines!! I'll take a look at the 9 pin adapter as well, thanks for the info.

Ok, so got round to a bit of Dragoneering this evening.

Popping off the lid, it really reminded me a lot of the BBC Model B, especially the way the keyboard is mounted:


Lots and lots of dust bunnies, but no spiders, mice, hamsters etc... All cleaned out now. There was a white dusty corrosion like residue in a number of places, so I also cleaned that off. It may have been stored somewhere damp, but overall the board was in great condition.

First problem to tackle was the power button, which just wouldn't latch:


A look under the magnifying glass showed the latching pin was sticking and not going into the correct latch on the switch, simply because it lacked lubricant. A dash of very light oil in the rectangular hole where the latching pin is, then a few operations and we were back to normal operations.

Given everything looked ok , I hooked it up and voila:


Sadly the keyboard doesn't work at this point, though the keyboard switches and cable all test just fine. I sincerely doubt it's the keyboard, as aside from the mechanical switches, there just isn't anything to go wrong.

There is a 74LS244 connected to the keyboard, and it does have signs of corrosion on the legs, so I think it'll be coming out in favour of a socket and replacement.

Overall a decent evenings plodding.

Oh, I nearly forgot, here's one for the mobo nerds (You know who you are!!)


Oh, and look at the size of that 74LS783, it's HUUUGE!! Biggest 74LS I've ever seen.

:dualthumbup: :yay:

Ok, so the the 74LS244 is an octal buffer, and going by the schematic, seems only involved with the printer:


But, since if it had failed it could be playing havoc with at least half of the keyboard lines, I'll definitely swap it out.

It also looks like potentially the keyboard could run without it too.

Slightly more concerning is that it could be the 6821 (IC26), but lets see what happens.

Of course there is the other possibility that the computer is freezing after it boots, but I'll set that one aside for now since the other signs are good.

Well, the weather was bloody awful this morning, so I didn't get out for my usual Friday cycle, so I started examining the Dragon a bit more closely to see what I could see.

It turns out there was some sketchy soldering on the 8261, with some poorly formed joints all heading to the keyboard connector

In particular I saw this:


This is the same solder joint rotated 90 degrees.

Clearly this is not good as it also heads off to the keyboard connector. Soldering iron out later for a reflow party I think :)

Might also need to set the clock on my microscope :)

Hopefully that's all it is. :-)

Not having a blinking cursor is a right pain as you don't know if the machine's alive or not.

stephen_usher wrote: 01 Apr 2022 09:10 Hopefully that's all it is. :-)

Not having a blinking cursor is a right pain as you don't know if the machine's alive or not.

Fingers crossed!!

And definitely a yes to the lack of a blinking cursor!!

I'll de-solder and reflow the whole IC as there is other sketchiness like this hole:


Most of the rest look OK, but I'm not sure how much I can trust them.