Dragon 32 with DRAM removed, video output?

[wayland]

My Dragon had a shorted tant capacitor on the 12v line preventing it from booting. It was blowing the 12v F1 fuse on the Dragon Plus PSU. I've sorted that problem by removing the cap and all the DRAM and the RAM buffer chip. Yeah, I'd chopped the RAM chips out before discovering it was the capacitor, doh!

I believe I should be seeing some video output though. I am seeing nothing. Before I go and put the RAM back (I still have 16k I can use) I want to make sure this is a working board.

Can someone help by telling me if I should expect a video signal when I have no RAM?

Cheers,
Wayland

[robcfg]

What do you see on screen?

The VDG scans the ram to display the picture, so it wouldn't show anything at all.

Do you get a stable black screen? Any border?

[wayland]

I am on composite cable, yellow phono with the TV on the correct AV input.
I don't even get a flicker when I turn the Dragon on.
Obviously I'm not expecting characters but I would expect a sync.

So the VDG should at least be scanning the empty RAM space and outputting blank?
Maybe I need an oscilloscope?

[sorchard]

I would have expected some kind of video display i.e. something with a border. The vdg will generate sync signals with just a clock input, except when something is pulling the CHB output down to ground activating a test mode. I've seen this happen once due to a bad ceramic cap.

The video circuit requires both +5V and -5V to work properly. Worth checking these.

[robcfg]

It may be that the 6847 is also fried.

I diagnosed a CoCo2 of mine which also showed a clear screen and it was a broken VDG. Check this thread.

[wayland]

I found another tant cap, it's on the -5v line which is only delivering -3.8v. I will remove it and try again. The video circuit uses -5v.

[Azerpy]

On a DragonPlus power card, the fuse of 100 mA on the -5V line may be 20 ohms. In this case, there is a fall of voltage on this line used for video.
You could try a 1A fuse for test.

[dublevay]

I now supply the required replacement fuse with the kit, so Wayland has the 125mA fuse which has a lower impedance.

[Azerpy]

Super !

[wayland]

Yeah, I replaced all the fuses with 3 amp.