Mac Plus.. ticking analog board

[Michelangelo]

Hello : )
Glad to be a new member of this community! I hope that someone can help me a bit : )

I recovered a Macintosh Plus, almost completely dead. Battery leaked on the analog board but no big damages on the traces or other components.. still. Once plugged in and turn on the only thing that happen is a repetitive "ticking" or chirping sound. The area is definitely around the T3 transformer, but cannot pinpoint exactly the location. Here's some data

INTERNATIONAL BOARD 240v

What I have done so far.
- reflow all the cracked solder joints and beyond
- replaced the C33 and C36 rifa caps.
- made sure that R55 is still good, reading 33k o as per colour coding.
- tested diodes, some allowed current both ways, but when out of the board were perfectly fine

Got some cigarette marks on the vinyl cover in proximity of R 52 (the big cemented resistor below T3) but again, tested just fine at 1Ko.

Video of the clicking attached, note that when close to the main transformer volume goes higher.

Once On voltages reading from the floppy connector are very weak and oscillating randomly between 0.5 and 1 V on both the 5 and 12V lines..
I will investigate further but any help will be much appreciated.

Thanks!

 

[robin-fo]

My Mac Plus sounds similar when no logic board is connected…

Check Larry Pina‘s Dead Mac Scrolls which is a vital source of information when repairing early Macs

 

[retr01]

The Dead Mac Scrolls can be accessed here:

https://tinkerdifferent.com/resources/the-dead-mac-scrolls.20/

 

[JDW]

@robin-fo & @retr01
I have been conversing privately via email with @Michelangelo for a while now about this problem and the Pina books have been discussed. Unfortunately, there is not a case mentioned in either of those books which perfectly parallels the problem he is seeing. I don't have an international version board myself, and there are some subtle differences with US edition boards.

I suggested he post here on TD to see if other international board owners might have heard a similar ticking sound and could perhaps offer some insights.

 

[Michelangelo]

Thanks to all. I will keep going and testing a few things... when my job for Netflix will allow haha. I will keep you posted and share any discovery!

 

[68kPlus]

Something worth mentioning:

My Plus makes a similar tick sound when it's turned on, but only one tick is heard. It's almost immediately cut off by the boot chime, however. I'm pretty certain it's a normal thing for a Mac Plus to do. It might be that your Plus is being shorted out and it's infinitely restarting only to get cut off before it can chime.
Just my two cents.
Also, my Plus is an international one, from Australia.

 

[JDW]

It might be that your Plus is being shorted out and it's infinitely restarting only to get cut off before it can chime.
Just my two cents.

I believe so because he said the voltages are "oscillating randomly between 0.5 and 1 V on both the 5 and 12V line." But the question is, what specific component on the analog board is causing the voltages to become like that. I told @Michelangelo that I suspect it's a diode and/or transistor, but I don't know which specific components.

 

[LeadedSolder]

I would guess it’s a short and the multimeter is just reading those lines as they collapse from the power supply protection circuit kicking?

What is the resistance like between 5V/12V and ground? If it’s really really low at least we know which rail may be the one that has a bad part on it.

 

[retr01]

Another thing to consider is cracked solder joints that are common on those analog boards, which need reflowing.

 

[JDW]

Another thing to consider is cracked solder joints that are common on those analog boards, which need reflowing.

He painstakingly did a lot of that work as mentioned in his opening post...

 

[retr01]

He painstakingly did a lot of that work as mentioned in his opening post...

View attachment 9453

Sorry, my bad. Thank you, @JDW.

I wonder where the clicking sound is coming from if it is not a sound output from the speaker. Is the Mac Plus rebooting again and again?

In this discussion on Google Groups some years back, two persons have stated about the power connector. One person stated about adjusting the PSU:

Mac Plus is constantly rebooting!

groups.google.com

 

[JDW]

Sorry, my bad. Thank you, @JDW.

I wonder where the clicking sound is coming from if it is not a sound output from the speaker. Is the Mac Plus rebooting again and again?

In this discussion on Google Groups some years back, two persons have stated about the power connector. One person stated about adjusting the PSU:

Mac Plus is constantly rebooting!

groups.google.com

My guess is that ticking is coming from the speaker, and desoldering one of the speaker wires would be a quick way to prove that. But even if you did prove that, the fundamental problem remains. What specific components on the board are resulting in the voltages going up and down like that?

 

[Michelangelo]

Thanks a lot for your messages.. been new in this community it's great to see all this support.
I will replace a few resistors and diodes around the T3 area, got some odd readings on board, but so far when resistors and diodes are desoldered.. they just read fine. Will replace a suspicious capacitor and get back to you with any discovery.

Dumb question. Is it strictly necessary to assemble ALL the main components to test the analog board ?
In other words, if I connect the logic board, but NOT the floppy for example... am I messing up with the power up ? Just asking to completely exclude the logic board from the equation.
And I presume that even if I don't connect anything else then the analog board (and relative grounds clearly), at lest the screen should power up or give some sign of life.

In the meantime a little victory: I manage to completely bring back to life a MacSE 30 with a dodgy analog board and a dead logic one... felt like an easy task compared to this tricky Mac Plus : P

 

[Patrick]

Dumb question. Is it strictly necessary to assemble ALL the main components to test the analog board ?

1. not a dumb question
2. nope you don't.

no need to connect the floppy up.

Some of us even make really long cables so we can easily connect the logic board and the analog board without having to put the logic board back into the case. but with the 128k/512/plus macs thats a bit harder to doo....

 

[JDW]

No need to connect floppy drives or hard drives in order to get the Mac Plus power supply to work. However, I have never powered mine without ANYTHING connected (which is called a "no load" condition). I know that the SE & SE/30 PSU works fine with no load. Voltages are a tad different than when loaded, but the SE PSU works as expected. Some power supplies requires a minimum load. Not sure of the Mac Plus power supply analog board requires at least the CRT to be connected or not. I would need to test that to know for sure, but if anyone else has tested that, then please chime in.

 

[Daniel Hansen]

Two possibilities pop to mind here, given the location of the noise:
1. The optoisolator at U3 is bad, which is a common failure point. Replace with a 4N35 or CNY17 - inexpensive and plentiful
2. The SCR at Q12 is bad, a less common failure point but does result in these symptoms (though ticking is faster in my experience, fwiw)... a replacement can be difficult to find as the SCR needs to be a 'fast' type, with a tq max of 10μs and those aren't made any more... however, the BR103 is a suitable replacement and there seems to some available on eBay right now: https://www.ebay.com/sch/i.html?_fr..._sacat=0&LH_TitleDesc=0&_odkw=br103&_osacat=0

PS - if you try the BR103, note the CGA is reverse from the original part



As for assembly for testing, my recollection is that the CRT, analog board, and logic board all need to be connected.

 

[Michelangelo]

Two possibilities pop to mind here, given the location of the noise:
1. The optoisolator at U3 is bad, which is a common failure point. Replace with a 4N35 or CNY17 - inexpensive and plentiful
2. The SCR at Q12 is bad, a less common failure point but does result in these symptoms (though ticking is faster in my experience, fwiw)... a replacement can be difficult to find as the SCR needs to be a 'fast' type, with a tq max of 10μs and those aren't made any more... however, the BR103 is a suitable replacement and there seems to some available on eBay right now: https://www.ebay.com/sch/i.html?_fr..._sacat=0&LH_TitleDesc=0&_odkw=br103&_osacat=0

PS - if you try the BR103, note the CGA is reverse from the original part



As for assembly for testing, my recollection is that the CRT, analog board, and logic board all need to be connected.

Thanks a lot. Today I was indeed looking closely at Q12.. had some sort of patina on the legs .. but no burning marks. I was going mad to find a modern replacement, thanks for that link! Will definitely replace the components.

 

[Michelangelo]

Quick update.. Replaced the Q12 and now from an irregular "tick tick" I have a very loud, regular " chirp chirp". Improved.. sort of haha. Will go on and replace U3 as well and see what happen.

 

[68kPlus]

Good to see some improvement!

 

[Michelangelo]

Ok.. after replacing U3 the problem persisted. I reassembled the Mac and I was ready to put the project aside... and I saw this (image attached)
That transistor was staring at me with a huge smile... this is just to state how important is visual inspection.. and how something so obvious could pass unseen.
I am quite sure I replaced a bunch of functioning parts, and I was proud to have swapped R55 with a beefed up resistor : )
I order the part and will keep you posted...
Out of curiosity.. what could cause a transistor to almost explode like this ?