Dragon 32 Repair

[sorchard]

So now Ive gone through inserting one chip at a time back into its socket and reading the voltages at the RAM sockets. Everything is groovy until I stick the 74LS783 back in. The the -5v comes up to -2.1v as before.

Does that indicate that this is maybe where the fault lies?

If -5V is bad with all the memory removed, and the 74LS783 (SAM) in, then the most likely explanation by far is that there is a short between -5V and one of the control lines between SAM and memory.

Take the SAM out, and check each pin of the socket in turn to see where -5v is appearing. This is the line that is shorted to -5V. You need to clear that before you put a new SAM back in. If you're really lucky then your SAM is OK but I would be surprised.


74LS783 turns up on ebay from time to time. Also available here http://www.littlediode.com

[martinjharvey]

Thought I'd pop by and offer an update and a bit of closure to this post, for my sake more than anything

I got to the bottom of the missing -5v. Because it looked dubious, I redid the solder joints on the only RAM socket installed from a previous owners repairs. The new solder had unfortunately bridged a connection and caused a short to the -5v under the socket on the component side. Removed the old socket, tidied up the short, installed a new socket, plugged it all in but I was just back to where I started with the columns of @@@@@ symbols.

All the 4116's were good so I ordered a new 74LS783. That arrived today, so I stuck that in and there was no difference. I've tried a new CPU as I think I mentioned previously, but having tried everything I can think of by way of troubleshooting it, I think I've come to the end of the road now with this Dragon

I'm thinking that this is something to do with corrupted/faulty/damaged ROMS or something as I never managed to get anything from the keyboard when trying the MOTORON (or MOTOR ON) as suggested. I've spent too much time( and money !!) on it trying to get it going again. I think I'm just going to break it up and sell off the bits and pieces to try and recoup some of the costs.

Thanks for the help and suggestions offered here, it's much appreciated.

Thanks

Mart

[robcfg]

Hi Mart,

I think that replacing the roms is such a cheap option that may be worth the try.

I'll take a look and see if I have any spare eproms, and if that's the case, I can program and test them. If everything work, I'll put them on their way to you.

Cheers,
Rob

Edit: I forgot to tell that an easy way to rule if the roms are damaged is trying a cartridge. If the game shows up, then definitely you need new roms. Also check for bad connections or corrosion around the rom chips.

[martinjharvey]

I think that replacing the roms is such a cheap option that may be worth the try

OK Rob you talked me into it Having spent £30+ on replacement parts though I'm at the point where I've got to be sensible and cut my losses If its a cheap option then I'll give it a try before giving up completely on it.

I'll take a look and see if I have any spare eproms, and if that's the case, I can program and test them. If everything work, I'll put them on their way to you.

I get the impression that it had problems with the ROMs before I got my hands on it. IC17 was already socketed and it had two different looking ROMS in IC17 & IC18. Looking at the motherboard pics on this site, IC17 is the type shown in the MKII Issue 5 revision and then IC18 is the type shown in the MKII Iss 6, which is the board that I have. I don't see any motherboard shown with the ROMS from different manufacturers or chip suppliers, or with only one of the ROMs socketed!

Edit: I forgot to tell that an easy way to rule if the roms are damaged is trying a cartridge. If the game shows up, then definitely you need new roms. Also check for bad connections or corrosion around the rom chips.

I've checked for any obvious problems around the ROMS and there doesn't seem to be anything along the lines of corrosion or shorts. I took the time to socket IC18 with the idea of replacing the ROMs, but I only have an old Watford EPROM programmer for my BBC Master and no compatible 21v 24pin EPROMs, so that idea went on the backburner.

I do have a cartridge but its empty with no EPROMs or otherwise in the sockets. With both of the ROMs now socketed, I did try the Dragon without the ROMs in and I got the same @@@@ symbols and columns as I did when the ROMs where inserted.

Mart

[tormod]

I get the impression that it had problems with the ROMs before I got my hands on it. IC17 was already socketed and it had two different looking ROMS in IC17 & IC18. Looking at the motherboard pics on this site, IC17 is the type shown in the MKII Issue 5 revision and then IC18 is the type shown in the MKII Iss 6, which is the board that I have. I don't see any motherboard shown with the ROMS from different manufacturers or chip suppliers, or with only one of the ROMs socketed!

To my knowledge, it is fairly common that one and only one of the ROMs is socketed on the Dragon 32. I wouldn't be surprised if the two ROMs look different either, from the way Dragon Data was rummaging up their parts.