Another "Ticking" Macintosh Plus ED

[Ferrix97]

Hello!

I recently scored three ED Macs, two Macintosh Plus EDs and a Macintosh ED (which is a 512ke).

The Mac ED only needed a quick reflow of the J1 connector.

The first Plus ED produced a very quick ticking noise no matter if the logic board was connected or not. I quickly noticed that the flyback diode was shorted. I replaced the flyback, the diode and the transistor and now it is working perfectly as well!

The second Plus is driving me crazy: it has a slower ticking noise,  I don't think it is coming from the speaker.

The flyback seems in good shape, I swapped it with a NOS one and it still has the same issue. Did some other measurements on various diodes (according to Pina's books) and they all look fine. I also replaced two capacitors and the optocoupler.

If I disconnect the logic board the ticking gets a bit faster. After heating up the board, I was able to briefly stop the ticking, but still no chine and no video. I measured the +12V line and it was about 15V, the trimmer on the analog board does absolutely nothing. +5V is also way too high, about 6.5V. After taking the measurements I removed the logic board and tested it in the other Plus and it survived he high voltages. I also noticed that somebody clipped the third lead (output) of the 79045 regulator. maybe the power supply killed it (but it is used only for the serial ports, which I don't need).

There is very little dust inside, the HV area looks very clean, so it's possible that it broke when it was still in use (I got it from an Apple Dealer)

The analog board is the 220V INTERNATIONAL model with the W12 jumper removed.

I think there is an open/shorted component in the power supply area, but I cannot find it!

 

[uniserver]

Wow man, are you making good $$ from all these score's + repair work you have been doing?

 

[Ferrix97]

Well, most of the times I end up keeping the machine rather than selling it. So far I have 173 Apple devices scattered around my house and my grandparents garage.

As for repairs, I usually invest the money I earn in more (mostly broken) Apple Computers or Arduino/Raspberry Pi stuff.

Is the ED hard to find outside Europe?

 

[tanuki65]

Yes.

 

[pancakewagon]

I actually have a Plus I just picked up that's making a slow ticking noise too. Is that something that I should be concerned about?

 

[uniserver]

 

[pancakewagon]

That's my kind of solution.

 

[uniserver]

ferrix probably needs to turn the 5v cut out down.  

 

[Ferrix97]

The trimmer on the analog board does absolutely nothing. I desoldered it and it checks fine.

How do you control the +5V cut out? all of the feedback/control is done on the +12V rail.

Soon I'll check some diodes and the SCR, maybe one of them is open/leaky

 

[uniserver]

 

[Ferrix97]

In my case, it is R59, but it does absolutely nothing, no matter if it is all the way to one side or to the other, the voltages still remains the same

 

[uniserver]

then that is a problem...

 

[Ferrix97]

Sure is!

checked most of the diodes and transistors, none of them are shorted or very leaky.

at this point, I'm quite sure there is an open circuit somewhere

 

[uniserver]

Sure you changed all the caps, right?   please say yes.

 

[Ferrix97]

I've changed only a couple of capacitors (the ones after the two diodes), I don't have the rest of them in stock. I'll place an order soon.

out of the 5 or 6 ABs I've repaired, none of them had capacitor-related problems. They all had a mix of: broken joints, bad optocoupler , shorted diodes or flybacks. usually I recap them when I see wobbly video or random resets. I've never fixed a board with a recap only

 

[uniserver]

yeah change the caps.  they are a huge problem with these A/B.

 

[Ferrix97]

But usually when a capacitor is shorted/leaky, the DC voltage is lower than it should be.

If a capacitor is open I will get loads of ripple on the rail, but I've never seen an electrolytic capacitor go open.

Also, I noticed that the Boards in the 128K/512K don't have as many problems as the ones inside the Plus, they are the same exact board (as far as I can tell) but with older components. Maybe Apple decided to use lower quality parts for mass producing the Plus.

 

[techknight]

Optoisolator, or its loop filter network. 

 

[Ferrix97]

I did replace the Optoisolator with a brand new CNY17-3, which worked perfectly in the other ABs I've repaired.

Where is the filter network?