Macintosh II SI capacitor question

[T-Man]

Cannot remember how I ended up with this board (one of 3 nonworking SI boards I've collected over the years) but it is nonworking (no chime, no video) and has been recapped. Whenever I think of replacement by tantalum capacitors, I am looking for a positive and a negative. Are these bipolar capacitors; do these capacitors need to be replaced?

 

[T-Man]

Working but not as designed. Big MAYBE someone might know how to interpret this behavior. Initially, when using just like my other IIsi, no chime, no video, LED on, internal HD spins up initially but then stops. Eventually got video adapter with switches and found a video output white or grey screen only with the settings on the adapter for a 14 inch multi sync monitor composite sync (the actual monitor is the 16 inch blue CRT apple display used with the first blue iMac G3 tower). Then put Big Mess of wires Macintosh ROM in the slot, with the jumper closed and it works. If I leave the BMOW in place but remove the jumper, I get no video but the crash startup chime. If I remove the BMOW ROM, I'm back to blank white/grey (no pattern) screen and no chime at all. I cannot find any broker traces around the onboard ROMs.Maybe just bad onboard ROMS, but the video still isn't working quiet correctly and hate to have this machine dependent on the BMOW ROM.


Addendum: The video output works fine with the BMOW ROM in place......so it's all pointing to bad onboard ROM.

 

[David Cook]

Are these bipolar capacitors; do these capacitors need to be replaced?

Those are ceramic capacitors, which are non-polar. That's why there aren't any +/- markings. However, the capacitors there are supposed to be 47 uF. There is no way those tiny capacitors have that much capacity. So, you should replace them.

Here are all the locations and expected values:

Macintosh IIsi - Recap-a-Mac

The Macintosh IIsi has it all, leaky caps on the logic board and the power supply. The logic board image shown at the...

recapamac.com.au

 

[speakers]

Weak or totally absent decoupling caps like C20/C22, may not stop the machine running. In fact, a logic board could work with no decoupling caps at all, albeit unreliably!

If those tiny ceramics have been elsewhere, in particular C10/C11, then that would explain no audio.

 

[T-Man]

That was disappointing. Removed and replaced caps with electrolytics. Same behavior: white/grey screen with internal ROM, works with BMOW ROM.

 

[RetroViator]

With my Mac2VGA, connected to a IIsi, I have pin 1,4, and 5 on and the rest off. This works with my VGA displays. This corresponds with 640x480 at 66Hz.

I’ve attached the cheat sheet for my adapter.

It seems doubtful the BMOW ROM gives the IIsi more video output options, but it’s worth trying boring old VGA and see what happens.