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arcademan0

macrumors newbie
Original poster
Nov 17, 2021
15
7
Colorado
So, I have an old Mac Plus I'm trying to fix. I have an idea on what the problem might be but I want the opinion of someone who knows what they're doing. I'm pretty sure the problem is the flyback transformer but I think there might be more to it than that. When I turn on the computer there is no startup bong, the screen is blank, there is a high pitched whirring, after 20 seconds there is an ozone smell, and the top left part of the computer (where the analog board is) gets super hot. Ideas, thoughts, advice?
 

MacTech68

macrumors 68020
Mar 16, 2008
2,393
211
Australia, Perth
It does sound like the flyback, and it's cooking the HOT (Horizontal Output Transistor).

I expect when you get it apart, the heatsink for the HOT will be a lovely, purple/blue.brown color.

FBT & HOT will most likely require replacement.

FBTs are horrendously expensive these days.

For example (NOT an endorsement) :


It may work out cheaper to buy another 128, 512, or Plus and scrap it for parts.
 

arcademan0

macrumors newbie
Original poster
Nov 17, 2021
15
7
Colorado

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arcademan0

macrumors newbie
Original poster
Nov 17, 2021
15
7
Colorado
It does sound like the flyback, and it's cooking the HOT (Horizontal Output Transistor).

I expect when you get it apart, the heatsink for the HOT will be a lovely, purple/blue.brown color.

FBT & HOT will most likely require replacement.

FBTs are horrendously expensive these days.

For example (NOT an endorsement) :


It may work out cheaper to buy another 128, 512, or Plus and scrap it for parts.
I noticed the heatsink was coated a brownish color like you said. And I found a surprisingly good deal on a flyback transformer so that's coming in on Saturday.
 

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dmr727

macrumors G4
Dec 29, 2007
10,703
6,016
NYC
Stupid question, but I don't see the brownish color on the heat sink. Is that a picture thing or am I just blind?
 

MacTech68

macrumors 68020
Mar 16, 2008
2,393
211
Australia, Perth
Stupid question, but I don't see the brownish color on the heat sink. Is that a picture thing or am I just blind?
If you compare it to the other heatsink next to it, it should be more obvious, but I'll agree it is very slight compared to some I've seen.

The very first photo also shows a dulling of the surface for about 2/3rds of the lower portion.

EDIT: as an example, this page shows the range of colours expected at different temperatures:


Tempering_standards_used_in_blacksmithing-1.jpg
 
Last edited:

arcademan0

macrumors newbie
Original poster
Nov 17, 2021
15
7
Colorado
At the very least, yes. You'll need a small nut & bolt to attach the transistor to the heatsink (or a rivet) and some heat transfer compound).
Would it be acceptable to reuse the current heatsink, nut, and bolt for when I replace the transistor?
 

arcademan0

macrumors newbie
Original poster
Nov 17, 2021
15
7
Colorado
The flyback transformer came in yesterday and I soldered it in today. I turned it on and it works! I'll have to test it long term to see if the screen maintains its normal functions but so far everything looks promising. I didn't even need to replace the Horizontal Output Transistor but may need to if the screen gets wonkey.
 
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arcademan0

macrumors newbie
Original poster
Nov 17, 2021
15
7
Colorado
All of the hardware issues are taken care of and now that just leaves the software. I've done some research and it appears the latest software version it can run is System 7.5.5. Further research has led me to believe that in order to run this software I would need an external hard disk as the mac doesn't have enough RAM to run the software by itself. It also seems I would have to install the software onto the hard disk from a different computer as the Mac Plus only has an 800kb disk drive and the installer disks are 1.4mb. Would anybody happen to know where I can get all the necessary installer files for 1.4mb floppy disk?
 

MacTech68

macrumors 68020
Mar 16, 2008
2,393
211
Australia, Perth
You may want to reconsider running System 7.5.

Whilst it's possible, even with a full 4MB of RAM, it doesn't leave much free for programs to run.

Yes, each owner will have their own opinion, and they vary.

In any case, to perform an installation of say System 7.1 or 7.5 would (I'd recommend) require a SCSI hard drive or even a FloppyEMU or SCSI2SD.

If you don't have the $ for the latter two, what other older Macs do you have, if any ?
 

arcademan0

macrumors newbie
Original poster
Nov 17, 2021
15
7
Colorado
You may want to reconsider running System 7.5.

Whilst it's possible, even with a full 4MB of RAM, it doesn't leave much free for programs to run.

Yes, each owner will have their own opinion, and they vary.

In any case, to perform an installation of say System 7.1 or 7.5 would (I'd recommend) require a SCSI hard drive or even a FloppyEMU or SCSI2SD.

If you don't have the $ for the latter two, what other older Macs do you have, if any ?
As of right now, the Mac Plus is the only mac I have.
 

arcademan0

macrumors newbie
Original poster
Nov 17, 2021
15
7
Colorado
So, I've noticed the screen is blinking out every couple of minutes but after I tap the computer, the screen turns back on. Is this a result of the Horizontal Output Transistor failing?
 

MacTech68

macrumors 68020
Mar 16, 2008
2,393
211
Australia, Perth
No. Most likely a dry/cracked solder joint on the video pin of the analog board to logic board connector.

Worth checking the soldering of all the circled connector pins.

full
 
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