MacIntosh Plus strange startup behavior

[fit2page]

Hi,

Got some strange startup behavior on one of my Plus machines. I can start it but on the first screen there is video garble, the moment the second screen appears (with the floppy question) the video is OK. I already swapped boards and it is a logic board issue. Can this be some kind of screen buffering thing? Thanks in advance for any input/pointers!

Marc

 

[bibilit]

A stuck reset line ?

 

[arroz]

Hi!

The Plus (and many others like the PowerBook 145) always boot with video garbage on the screen for a second or two. You usually can’t see it because the CRT takes time to warm up, but on a PowerBook it’s visible since the LCD fires up immediately. I noticed that on the Plus as well while developing my MacDVI adapter and also when using RGBtoHDMI.

It shows garbage for about a second, then clears the screen and shows the diagonal pattern while the RAM is being tested (may take quite a while if you have 4MB, and the screen flashes black occasionally) and finally switches to the checkerboard pattern with the question mark floppy disk.

Regadds

Miguel Arroz

 

[fit2page]

Hi Miguel! Thanks for your reaction, this behavior is new to me, never experienced it before on a Plus. But OK we have to live with that then. Another problem on this machine is that there is no sound...a while I had a kggggg instead of the start chime but now the sound is completely dead. Speaker is tested and OK, I can hear the click on start-up. When I listen very carefully tyhere is a 1Hz ticking sound coming from the audio line. I also tested all traces/connections round the SND chip, everything OK there. Is this a known issue to you?

 

[arroz]

Hi!

Still regarding the random garbage at startup, for how long do you see it? It should be display only for a second or two at most (it’s displaying whatever is in RAM chips, which is unpredictable, before they are reset by the CPU).

Did you recap the board? Based on both symptoms I wonder if what you are seeing is related to bad caps. Audio definitely uses electrolytic caps (3 I think) and the other 3 stabilize the power rails. If you have a bad cap at the 5V rail it can be taking longer than usual for things to stabilize and that would explain a longer period before the CPU kicks in and resets the memory.

If not, I would not be surprised if recapping fixed both issues.

Be careful because this is a multilayer PCB so soldering (and especially unsoldering) is a bit of a pain. The only work I did on the Plus board so far was removing the RAM size resistor (required to upgrade from 1MB to 4MB). I didn’t want to cut it so I used the soldering iron to take it out and it was surprisingly difficult (I didn’t know the board was multi layered at the time and I was sleepy so that didn’t help, but still).

Regards

Miguel Arroz

 

[arroz]

Also, if you have a scope, check how long does it take for the 5V to stabilize at the positive end of the 5V cap (C6 I think) when you power on the board, and compare it to another board that works fine.

If you do this with the original analog board and CRT, obviously be careful with the high voltage. If you don't feel comfortable working near a charged CRT (or if you don't know why you should feel uncomfortable ) please don't, because that stuff can really hurt or kill you. Build a new cable and use an external power source that provides the 3 necessary voltages (5, 12 and -12), and a converter to modern displays (MacDVI, RGBtoHDMI, VGA convertors, etc).