Mac Se/30 black screen + simasimac + popping sound

[CosmoKramer]

Hello guys, A couple of weeks ago I received a broken Mac se 30 from a relative. Some of the symptoms were that the screen was black and there was no chime, however the fan spun. So I popped it open and found that the battery exploded and the capacitors leaked on the main logic board.
After cleaning the board extensively with isopropyl alcohol and a tooth Brush, I recapped and tried turning it on, but that did not help the issue. The next thing I did was I replaced the horizontal drive transistor (bu406) and the barrel rectifier located at cr3 on the analog board.  This fixed the black screen issue for a moment, the screen now displayed the mosaic pattern of simasimac. I tried to resolve this by Reseating the Rom and ram and by cleaning the board again but to no avail.

Then out of nowhere(a minute or so after turning it on) the mac started making a popping noise and the screen started to flash on and off. But not completely, the screen looked like it was still trying to scan across in between it turning on and off. Now it has stoped cycling between on and off and the screen is just black. Needless to say I am at a loss, i think the problem is on the analog board Because that’s where I heard the popping come from. I tried replacing the diode I mentioned, again with a heavier duty one but that did not fix it. I am having trouble figuring where the popping noise was coming from (no burn marks or hot components). Any input would be helpful, thanks.

ps. I have access to an oscilloscope and LCR meter.
pps. Thanks again for any help

 

[tdr]

Your problem sounds like something I also dealt with. It started with flashing and popping and changed into a completely black screen after a while. I also suspected a problem with the analog board. However it turns out that the analog board relies on the main board for a hsync/vsync signal, and if that signal corrupted, you may get the flashing symptoms. One reason this might happen is that the board is constantly resetting itself. In my case the problem disappeared after repeated cleaning. The SE/30 then worked for a while but is now back to its initial simasimac state, however I haven't seen the flasing/popping anymore.

 

[dochilli]

First I would recap the analog board and the psu. 

Did you measure the voltages at the floppy port? Should be 5 and 12 volt. Low voltages can cause a simasi.

When you did not hear a chime, then there may be a problem with the ROM. May be some corroded/ broken traces. You can check these with the schematics.

I had a simasi SE/30 that was caused by faulty F258 chips (logic board).

 

[CosmoKramer]

First of all thanks To you both for replying, I thought I might  be alone on this one. In regards to what dochilli wrote, The voltages that I measure hovered around 4.85 (5v) And 11.89(12v), so it might not be an under voltage issue. You pointed out that the Some of 258 chips might be bad and I think your right since they one or two probably got fried when the battery exploded so I will probe around. The hypothesis about the resetting that tdr said also makes sense. I am going to replace those a couple of those chips and and probe for continuity on the rom and video lines after a Another good cleaning. And of course thanks for all your guys help it is nice to know that someone has a similar issue.

 

[dochilli]

4.85 and 11.89 should work, but are a little low. Time to recap the AB and PSU.