Macintosh SE/30 Faint chime and no video, possible short on +5V

[techknight]

you have a broken address/data line between the video circuit/sound, and SCC/SCSI. 

Also, listen to your sound, if it sounds "funny" then thats what it is. 

 

[Ferrix97]

The sound seems to be ok, both the startup chime and the sad mac tone are loud and clear. Only once, the sad mac tome was "slower" than usual.

 

[techknight]

Do you have a screenshot of the issue? 

Edit: does this have a completely black screen? The only time I came across a unit that was totally black even though it chimed, was bad VRAM. I have had to change out VRAM at least 3 times that I can recall. 

 

[Ferrix97]

The screen remains all black, the analog board is good (tested with another SE and the half-video SE/30, the one in the other thread)

The "slower" tone only happened once, after rebooting the machine it came back as usual

I also replaced one of the Bourns filters, RP2 if I recall correctly (desoldering it was quite a pain) but it still doesn't boot

So, At this point I'm still stuck with black screen + no boot

 

[techknight]

Feel around on the chips. see if the SCSI, SWIM is getting excessively hot. 

the PALs and the SCC will get really warm. thats normal. 

Bourns are easy to test. The end pins are grounded, and the middle pins should be high resistance. If you get low ohms to nearly a short on any other pin besides the outer pins, the Bourns is bad. 

 

[techknight]

Also note, the video circuit in the SE/30 will run all by itself regardless of what the BUS is doing... period. It will draw on the screen, the default state of the VRAM. Different brands of VRAM draw different initial patterns. Simasimac is created by only 1 type of RAM, while zebra stripes are created by a different brand of RAM. NEC vs TI. I think. 

So, if you get black screen, the video circuit is bad and needs troubleshooting. in my EXPERIENCE, the #1 cause was the VRAM itself. Second to the PALs. if the PALs are bad, it will prevent the machine from booting as well, especially if its holding up the BUS which could also cause no video, as they are also used at refreshing the screen with whatever contents are in VRAM. 

there 5 PALs. Find the one that is common to the video generation, and also attaches to some control lines on the main data bus. That would be where I would start. 

 

[Ferrix97]

At this point, I think that the problem might be a bad PAL, maybe UE6 or UI6.

Unfortunately they are soldered, so they are not so easy to swap with the other board

 

[techknight]

desoldering station time. 

 

[PotatoFi]

@Ferrix97 Did you ever get this one fixed?