Plus chirping sound (i think)

[ßœœ]

I just recapped the analog board and while the mac was operating it had some weird screen issues, so I took it back apart, poked around and turned it on to this sound. Pretty sure this is bad, how fix?

 

[Iesca]

Chirping is hard to diagnose exactly, but it and the common "flupping" sound are both a sign that the power supply is unable to get going, and is restarting at regular intervals.

Larry Pina wrote two books that will be helpful: Macintosh Repair & Upgrade Secrets, a repair manual for the Plus and earlier (and a little bit of the SE and Lisa), and Dead Mac Scrolls, a symptom and diagnosis manual, the "DSM" of early Macs....

I've attached them here. Possible problems could be: diodes, transistors, rectifiers, resistors... The list goes on, but Larry is very thorough, and has many checks for ruling various things out.

 

[desertrout]

If it's immediately after a recap, my first thought is that voltage output is now too high and the circuit is crowbarring... refer to the first book @Iesca linked to and see how to check voltages from the external floppy port (page 38) to see what's what. If +5V is too high, adjust the voltage regulator down (https://manualzz.com/doc/o/8qvv2/macintosh-plus-adjustments).

 

[ßœœ]

Thanks for the links! I'll try all this stuff when I get home. I had actually messed with the voltage pot to get the picture to look more square thinking it would make the picture more sharp. Any known way to make the image less blurry besides obvious ways, or are their screens just like that?

 

[LaPorta]

Sometimes that is simply due to the convergence issues: that can be due to the magnetic rings around the neck of the CRT tube coming out of alignment. There also the focus trim pot on the analog board.

 

[ßœœ]

Weirdly the focus pot did close to nothing

 

[bibilit]

Optosilator at U3

 

[ßœœ]

Optosilator at U3

I assume I need a new one?

 

[Iesca]

With one of my own Plus's analogue boards, replacing all 3 of the ICs did nothing to alleviate it's own startup symptoms, but maybe it will help with yours! It's always possible I didn't source correct replacements.

 

[desertrout]

Optosilator at U3

Evergreen advice. Inexpensive and readily available part (4N35), common failure point.

 

[ßœœ]

Is 30v good for these?

 

[bibilit]

4N35 Optocoupler Pinout, Datasheet, Equivalent & Other information - Components Info

4N35 is a widely used optocoupler device. The post below covers 4N35 optocoupler pinout, datasheet, equivalent & other information about this device.

www.componentsinfo.com

 

[ßœœ]

I've ordered some 4n35s, ill update when I install them!

 

[ßœœ]

Just replaced U3, no change, still got the chirping coming out of the speaker. Looks like I gotta read two books

 

[desertrout]

Did you check the voltages?