View Full Version : A600HD display garbage when mouse is connected

April 14th, 2009, 11:39 AM
I have kind of a weird problem with my A600HD.. If I have the mouse connected when I start it up, the display just shows garbage. I've tried waiting for it to load workbench and all, but it never recovers. Other than not being able to see what's going on it's not hanging or anything. It seems to load workbench just fine judging from the HD activity.
This only happens when I have the mouse connected when booting, if I connect the mouse afterwards, everything works fine. If I disconnect the mouse after booting, still garbage.
I tried with several different mice, same for all.

I'm using a SCART adapter straight from the display port to the TV, no A520 or anything in between.

Any ideas as to what's wrong?

April 14th, 2009, 10:42 PM
A wild guess would be a partial failure in one of the 8520 CIA chips? Something that causes the machine to freeze at power on but it will work if you hotplug the mouse later.

April 15th, 2009, 05:59 AM
Ok, I guess it's worth a shot :)
Will chips from an A500 or C64C fit/work? I obviously need something to test with and I know my 500 boots fine with a mouse attached.

April 15th, 2009, 07:09 AM
I'm not sure, but I think the 500 and 600 use mostly the same chips except for obvious differences like graphics chipsets. However if you're unlucky those chips are not socketed which makes replacement much trickier, in particular as long as you're not sure that is the actual fault.

April 15th, 2009, 07:19 AM
The A500 used a lot of 0.1" DIP parts but IIRC everything but the ROM on the A600 is surface mounted...

April 15th, 2009, 07:41 AM
Yea, if they are not socketed I'm not going to risk ruining 2 Amigas just to find out if that particular chip might be the problem. I don't have that much confidence in my soldering skills ;)
I'll fiddle around with it and search the net a bit more.

April 15th, 2009, 08:00 AM
Have you tried to connect a regular joystick to the same port? Does the computer boot up then, both if you wiggle the joystick and not? There may be some smaller logic along the same circuit, but I'm not Amiga expert.

If you soft reboot the computer with Ctrl - Amiga - Amiga, will it boot up with mouse plugged in so it only happens on a cold boot?

If I disconnect the mouse after booting, still garbage.
Hm, I didn't notice this sentence before. So you're saying that also pulling the mouse makes the screen corrupt?

April 15th, 2009, 08:43 AM
If I disconnect the mouse after booting, still garbage.
What I meant with that is that removing the mouse after booting doesn't fix the display. I have to start it up without the mouse connected, what I do afterwards doesn't seem to make any difference.

Warm boots work fine with the mouse connected. Or a joystick for that matter. That is, if I had nothing connected to the mouse port when I first started it up.
The results are pretty much the same with a joystick connected to the port, wiggling or not it seems to boot fine, just doesn't display anything. Sometimes the screen goes completely black for a bit, and then it shows garbage again.

The garbage looks like this, btw..

Forgot to mention, I can connect whatever I like to the other port, that doesn't affect anything.

April 15th, 2009, 11:12 AM
The garbage looks like corrupted video RAM or it could be that the wrong part of memory is being displayed.

Just had a skim over the schematics out of curiosity. The mouse/joystick ports go to a 74LS157 which passes the signals straight to Denise (the main video chip, hence the funky garbage effect you're seeing).

You should be able to test that '157 (U34) to some degree in-circuit. If the inputs look odd (compared to the other port with a mouse connected) it could be bad resistors or capacitors between there and the mouse port. If the outputs are bad, the 74LS157 itself may be to blame. If all check out, you need to start looking around Denise herself.

I've not found the power up reset circuit, but that's worth looking at too seeing as things work after a warm boot. Maybe also check the connections between the mouse port and the PCB (I'd do that first actually as it's probably the easiest!).

April 15th, 2009, 01:09 PM
Unfortunately my electronics skills aren't quite that good :(
I'll look for leaking capacitors but I think that's about as much as I'll be able to do. I'll just have to live with it I think, it's not like I'm going to be turning it on and off several times an hour anyway, altho I know it's going to annoy me eventually.