Dragon 32

[acsi]

Well I unpacked it today finally.

A couple of dragon joysticks. One got damaged a bit in transit but nothing major.

Allot of books, a few tapes and a cartridge.

Heres a pick of the result once plugged in



The main power is delivered through a 9pin din plug. 28vac (centre tapped 14-0-14) and 8.5vac. These check out ok. But there seems to be an issue with the internal supply as the -5vdc supply is missing at the ribbon cable connecting the motherboard.

Heres a pic of the inside.

[arjoll]

I've found the PSU board circuit at http://dragondata.worldofdragon.org/tec ... SU-MOD.gif

Urgh, looks like they derive the 5V rail from the -18V (more like -20V if it's 14VAC in) through R5 - with the resistor and 7905 dropping almost 15 volts things are going to get warm! I'd check that R5 is within spec and maybe replace the 7905.

I'd almost be tempted to feed in -5V from an external regulated source and see what kind of current is being drawn, then do some calculations to make sure everything is going to be within limits.

[EmulatorCoder]

Cripes, NO -5V

I hope im wrong, the RAMs are 4116 ?

I did some reading on the 4116 last week while trying to get my Apple][s going.

If the -5V is not present at the RAM ICs BAD NEWs (reverse flow bias), according to the APPLE book I was looking at it overloads the RAM.

the screen shot looks almost exactly like my screen shot of my Apple ][ (Yes I know, I shouldnt be comparing Apples with Oranges, pun intended! )

[tezza]

It's good to see some people working with such exotica as the Dragon. I know little about the circuitry but I hope you can get it going.

[acsi]

Yep I found a schematic and parts list (although it's in spanish) on here http://archive.worldofdragon.org/

I'm getting -18vdc on one side of the resistor R5 but virtually nothing coming out. However I think the resistor may still be OK and the regulator may be shorted. Either way both will be replaced today or tomorrow, just got to get to Jaycar.

The ram is worrying me though as they are indeed 4116 chips. Will have to see what happens when I get the -5v fixed.

[EmulatorCoder]

Please collectors.... im not a qualified sparkie or anything but from my personal experience...

Power supplies fail !! all the time.

1. Purchase Vintage computer

2. open case

3. unplug Power supply and test voltages

4. If voltages are correct, plug in and proceed

5. Dont forget the Real Time Clock battery, replace before it leaks, not after!

[acsi]

Good advise but as a registered sparky and knowing a bit about the history of this machine any damage from the missing -5vdc was already done. I purchased this knowing it did not work.

Anyway I have replaced the resistor at R5 (180ohm 1W) and the regulator for the -5vdc. Here is the result.



It's not yet 100% as I probably need to replace the caps and quite possibly the memory (it has it's moments).

But tested with a few basic programs



And the result is promising. Hopefully we will have a fully working new addition to the family (pun intended) although she (do computers have genders?) is around 30 years old.

[tezza]

Excellent!

[EmulatorCoder]

Nice, glad RAM is ok. sorry i didnt know you are a registered sparky, you would know more than me on that topic



Im not having so much luck with my Apple][europlus,
I have purchased more 4116's now I have around 46 of them !

Been cleaning the pins and tried replacing row 1, the picture changed.
Tried fitting a new row, got a different picture again.
Then replaced original row but in a different order and got another different picture

Im off to jay car to get replacement Logic IC's, DIP sockets look mint.

Havent killed it yet, so all good.

hmmmm wonder if I can rig one up to my C=64 for testing, the C=64 runs on the bigger version of RAM , maybe I can read and write to them through the expansion port ?

[acsi]

No worries, Pretty sure the ram just failed. Just get a garbled screen now. Was giving it a soak test making sure it warmed up as someone has already replaced a couple of ram chips by the look of it so I figured the rest wouldn't be in good shape.

Have ordered the replacement ram and sockets from the states. Thought about upgrading it to 64k but decided against it.

Sorry to hear your still having issues. One of the best tools I've found to have is another working machine to test with, not that I've usually got such a device and it does help when the chips are socketed.

[acsi]

Well Murphys Law really struck and yes it was the final chip to be replaced that was faulty (well of the main ones anyway)

The VDG chip (MC6847P) turned out to be faulty. Got the replacement today and she's back up and running.

Now that I've also got a set of tape leads ill dig out the old cassette player and see if these tapes are still good.

[tezza]

Well Murphys Law really struck and yes it was the final chip to be replaced that was faulty (well of the main ones anyway)

The VDG chip (MC6847P) turned out to be faulty. Got the replacement today and she's back up and running.

Now that I've also got a set of tape leads ill dig out the old cassette player and see if these tapes are still good.

Good one. I ran into a similar thing with an Apple IIe once. I knew I had a faulty RAM chip but didn't know where so I started to replace ALL the RAM chips one by one. They were all soldered in. Murphy's law meant that it what the very LAST chip!

[nama]

Great stuff. The Dragon 32 is a nice little machine.

[lizardb0y]

Bloody good job on the Dragon. There can't be too many batting around New Zealand; I bought mine in from the UK a few years ago, along with an Oric-1.

[Gibsaw]

Murphy's law meant that it what the very LAST chip!

Well, logic dictates that it will always be the last one you try (Heh), however I know what you mean.