Sick Dragon 32

[Vyper68]

Hi
Thanks sixxie, not to worry i am not giving up on this Dragon. I don't know what that board does except what i have read on the forum. I wasn't expecting it when i opened the case that's for sure. The main chips do seem to run quite hot in this D32 the 6809 was very hot and the VDG and SAM - more than i was expecting is that normal for Dragons? The other thing i was thinking was do people re-cap the D32/64's when they take ownership of them the way you would a ZX Spectrum or C64 for example and also what about heatsinks on the main IC's?

BTW Here is a better shot of the Daughterboard i took at the same time. See it says iss2

[sixxie]

The other thing i was thinking was do people re-cap the D32/64's when they take ownership of them the way you would a ZX Spectrum or C64 for example

I’m sure it must happen... but otoh I’ve never done much at all to my D64 and it’s carried on regardless since the 80s... until a couple of days ago when I think I managed to kill it - hooked up through composite and watched the display fade out and now it won’t start up.

Not investigated yet, but I’m wondering if I’d tweaked the variable resistor on the PSU board a bit low...

[robcfg]

Can you try removing the LM1889N chip and see what happens? It should remain gray, maybe more stable picture.

Do you have another working Dragon at hand? If yes, try swapping the LM1889N chip.

You can also try to take the daughterboard out (without cutting the cable!) and put the SAM chip in its socket.

[bluearcus]

What's this... a SAM daughterboard? Huh?

[Vyper68]

Hi
I have just taken some photos of the video output with the LM1889N fitted and without and it does clear up when it's not fitted. Fitting the SAM directly to the mainboard has no effect on the video.

With LM1889N


Without LM1889N

[robcfg]

Well, definitely it doesn't look like ntsc output.

Any chance to try another LM1889N?

[Vyper68]

Well, definitely it doesn't look like ntsc output.

Any chance to try another LM1889N?

I should have a new one in a Day or so. Until then i will just have to wait. I does seem like the LM1889N or a fault in the Chrominance feed out of the LM1889N. I saw some of the schematics which show cutting the track from the LM1889N gives a Black & White picture. The problem is the schematics are not very clear. Does anyone have any better ones so i can trace the feed through to RV1?

[sixxie]

That kinda does look like something's being modulated onto it. Did you try tweaking the variable capacitor (C7)? (Remember where it was!)

Edit: top tip - you change it by touching it, so adjust and remove screwdriver before deciding what it's done...

[sorchard]

The problem is the schematics are not very clear

They are also for the earlier main board revisions and have some differences. I've never seen official schematics for the MKII and ended up making my own. (Attached) It's just the schematic for the main board and doesn't include the PSU board or any daughterboards.

Your picture showing video with the LM1889 in circuit looks like a chroma signal of some form is present i.e. it's nearly working.

A large percentage of the components in the Dragon are related to video so there's potentially a lot of tracing to do to find the fault. Worth checking the PSU voltages are not too far out and don't have too much ripple, particularly the -5V. Could be a dried up aluminium electrolytic cap somewhere. Old tantalum caps can fail short circuit over the years, eventually getting so hot they burn. We threw away a drawer full of old stock at work with 50% fail rate and 'only' 20 years old. I've even heard of ceramic caps failing short circuit, but that's less likely than electrolytics failing.

[Vyper68]

Hi
Thanks for all the responses they greatly appreciated. I had the main PCB off to check underneath for any obvious bad solder joints, and to try and trace some of the lines for the Video circuit and it does look like i'll have a problem tracing this fault if the LM1889N is not the problem. I have ordered a new RV1 10K and C7 2-22pf capacitors as well.

There was also something i found when i was looking at the PCB. Pin 20 of the VDG which i think is Ground was broken away at the solder joint and had actually lifted up and was not connected ( for how long i don't know ) but resoldering the Pin did not make any difference to the video. Would the VDG carry on with Pin 20 disconnected and if so what would be the effect on the VDG and wider system???