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Old Dec 3, 2012, 02:04 PM   #1
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Join Date: Oct 2010
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A 128k that no search engine can seem to help.

The 128k I got on September 10th, 2010 is having a rough time. I was using it with a System Saver and everything was fine until October 22nd, 2012-when I booted it up and the CRT shut off like you turned the dial off on a 1970's vintage TV, first losing horizontal deflection, then having vertical deflection going out. It automatically restarted and the startup bong sounded like a screech instead of a bong. A Plus with a C1 and C3 capacitor near failure pulls up the 128k's logic board perfectly despite having a far more suspect analog board, but supposedly the flyback transformer doesn't affect audio.

In the original 128k case witth it's original analog board, and CRT, the original 128k has severe audio distortion-but not lack of sound-regardless of internal or external speaker, while in the Plus case, CRT, and analog board the 128k logic board boots up with perfect sound regardless of speaker choice.

The 128k's digital board blew all three axial digital board capacitors at the same time the analog board had the issue where it powered off the CRT and rebooted with screwed up sound. But the capacitors have been replaced, and an extensive inspection of the analog board on the 128k only reveals one small issue with SW1, where the on-position solder pad is located; the leg of the switch is poking 90% through the solder pad.

I would do multimeter testing, but we don't have the money for a tester that can test all the different voltages-Daddy says it's a collector's item and can't afford to keep it going.

Comparing the visual condition of the analog boards, the 128k's flyback transformer is not disturbed in any way-while the Plus has a failing seal and goop is poking through the contacts on the RFI shroud side, which does not happen on the 128k-and the analog board capacitors on the 128k are not leaking goop the way the Plus testbed is.

I have looked through every resource all the search engines, including Dogpile, to find someone who could manufacture a new replacement or find a new old stock version of the flyback transformer the various compact Macs used and have had no luck.

I have a feeling the flyback transformer might still work if I swapped it into the Plus, as there is no permanent burning anywhere on the flyback transformer (just normal oxidation due to increased heat as the CRT stays on, unless it isn't really normal.)

I need to know that since there is no arcing, no smoking, no ozone smell, but no picture and distorted sound, that the impossible to repair or replace flyback transformer isn't bad.

Resistors and low-voltage components can be replaced, but the flyback transformer, surely it must work.
In addition to the items listed on my profile, this morning I just acquired a Macintosh 128k! Needs a new CRT, but that can be worked around. Boy does it feel good to be me...
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Old Dec 7, 2012, 10:55 PM   #2
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Location: Australia, Perth
Really hard to say, but looking at the circuit, both the +12 and -12v rails are used in the sound amplifier circuit. So if either rail is down or being held low by a failure elsewhere, it would be possible for the sound to be clipped somewhat.


Given that horizontal went out first, I'm still going to point at the horizontal drive circuit, but would advise checking the usual suspects like C1, the yoke connector itself including dry or cracked solder joints and the Horizontal Width Coil, just for starters.

It is certainly unusual for the machine to power up (albeit behaving badly) if the FBT is shorted. My experience was that a shorted FBT resulted in a crow-bar effect across the power supply, making it tick rapidly and failing to power on. Of course, anything is possible.

You could try swapping the FBT from the MacPlus to the 128, Simply to see if that alleviates the problem. Donberg used to supply the FBT, but I don't think they do any more. I've seen some vendors around that list them, but are then out of stock. I shudder to think how many perfectly working FBTs (and whole machines for that matter) are sitting buried in landfill.
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Last edited by MacTech68; Dec 7, 2012 at 11:11 PM.
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