Macintosh Classic Analog Board whistling

[MrArthegor]

Hello everyone

I have a Macintosh Classic (I have made a post for this computer several years ago), the initial issue has been a checkerboard screen, and the attempt to repair was ended in a blown fuse.

Back to this week, I have resumed the work on this computer and I have made some progress, the culprit of blowing fuse was the primary MOSFET, I have also replaced all secondary caps and checked the bridge rectifier and the octocoupler, which is good.

The Macintosh with the new MOSFET have started briefly (showing the floppy icon), confirming it’s was the original culprit for checkerboard, but now I have this strange whistling and it reset himself, the issue seem to go worse and worse with the rest frequency shorter and shorter

Has anyone experienced thing like that before or has any idea of what can cause this thing to happen

Thank you for reading me and sorry for my English is not my native language

 

[Builder68]

That whistling noise and the resetting behavior could still be power supply-related. Even though you’ve replaced the MOSFET and some caps, there might be something else going on.

It sounds like the power supply is struggling under load.

Take a close look at your solder joints on the new components and the connectors (J).

Have you checked the power supply outputs (+5V, +12V, -12V) to make sure they’re stable and within spec?

Also, check the overall resistance of the logic board separately between +5V and GND. The reading should stabilize around 200 ohms, give or take.

The caps on that logic board are crap—it’s worth re-capping the entire LB. However, it’s a delicate task, and the pads can easily be lifted or broken, so proceed with caution.

 

[bibilit]

I agree a 100% with builder68, I will replace caps first, not so delicate though, and will probably solve your issue

 

[MrArthegor]

Tank you for your response

The LB caps have been recapped (with electrolytic, not so great idea but it’s the only thing I have on hand), and the 5V rail is at 295Ohm

I have managed to make the situation worse, I have desoldered DP3, DP4, DP5 and DP7 for check (all pass) and now the board doesn’t give more than 800mV on both 12 and 5 rail. I also changed the primary MOSFET for a K3569 who seem more appropriate than previous MOSFET in place (the previous only go up to 5A)

The issue with my previous jobs is DP5 and DP7 have their mark completely gone, so if I have check the polarity I don’t know witch is one, and it seem one is 200V and other is 400V

 

[MrArthegor]

Some progress, the Analog Board issue is partially fixed. I had just reinstalled the diode is reverse.

Now with the MOSFET previously installed, the Macintosh Classic boot properly (it was the amperage of previous MOSFET witch was too low) , but I continue to have this whistling noise, but I am not sure if it’s from the logic board or the analog board.

 

[Builder68]

Some progress, the Analog Board issue is partially fixed. I had just reinstalled the diode is reverse.

Now with the MOSFET previously installed, the Macintosh Classic boot properly (it was the amperage of previous MOSFET witch was too low) , but I continue to have this whistling noise, but I am not sure if it’s from the logic board or the analog board.

The common culprit of a high-pitched whine or squeal is the flyback transformer. As the varnish breaks down over the years, the noise will become louder and louder. And get this, I'm in my late 50s, and honestly, it's not that bad to me. I read somewhere that older folks like me don't hear those high-pitched sounds as well. Go figure! Guess getting older has some perks, eh?

 

[MrArthegor]

I was able to make more test

The noise comes from Analog Board, and when I apply power to the Analog Board without any load connected (no CRT or logic board) it make no noise.

 

[bibilit]

And get this, I'm in my late 50s, and honestly, it's not that bad to me. I read somewhere that older folks like me don't hear those high-pitched sounds as well. Go figure! Guess getting older has some perks, eh?

Same here, but having also troubles hearings some other sounds as well

 

[MrArthegor]

Some progress, I was able to correctly measure the ripple currents and the voltage under load (logic board connected) in my Analog Board and the result is not that good, I have 4.72V in the 5V rail, witch seem pretty low, and 11.95V in the 12V rail, witch seem correct.
The ripple current for the 2 rail is approximaly 200mV, witch also seem not so good, I was able to screenshot the two rail in my oscilloscope (ripple is "oscillation" in my scope, set to french), again no look very good.
If anyone has an idea, I think this issue can be linked to my whistling issue (I cannot be sure to 100%, but the noise seem to be coming from the secondary of the power supply)

 

[Builder68]

So, if I were in your shoes, I'd jump right into recapping that analog board, and then get those voltages set with the knobs. You can grab the cap list from a bunch of threads here.

 

[MrArthegor]

So, if I were in your shoes, I'd jump right into recapping that analog board, and then get those voltages set with the knobs. You can grab the cap list from a bunch of threads here.

Yes, it’s seem to be the best idea, it’s wild because I have previously recap this analog board (the secondary only but most of caps is here) with new caps (and good cap, some Panasonic) but maybe I have omitted one or two or a ESR thing (when I have made some search I have seen some people who too low-ESR caps seem to cause issues).

But clearly for the price, and the fact I have to also replace the temporary electolytic caps on the Logic Board, I’m going to recap all the Analog board and change 2-3 other things, mainly the QP5 and QP7 because the legs was corroded, and if it’s work I’m little suspicious

 

[bibilit]

I will replace DP3 and DP4, those get leaky with the years, they are ok when tested but they are cheap enough not to change as a matter of fact.

The 5 volt rail is a reference for other voltages, so if you are trying to tweak the 5 volt line, the 12 volt will increase heavily.

Keep in mind that cap goo is conductive, so the board should be cleaned to avoid any issue.

The perfect voltage is around 4,9 volts, and 4,85 is a minimum… 4,72 is far too low.

Last but not least, as the 5 volt rail is a reference, the feed-back is done through that rail, and another usual culprit is the optoisolator.