MacSE analog board issue – squished video, flyback overheating

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CamInTosh

New Tinkerer
Sep 20, 2025
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Hi everyone, lot of info I'll try to break down. This is a new acquisition I've had for about a month now.

Current Problem
  • Screen shows lightning/static, then suddenly compresses and darkens (“squish”) – see video and photos.
  • Now: no picture, burning smell around flyback.

Hardware
  • PSU: Astec AA13800
  • Analog Board: 820-0206-B (630-0147-C)
  • Flyback: 157-0042A
  • Logic Board: 820-0176-B

History / Timeline
1. Initial work
  • Reflowed analog board connectors (per Adrian Black - but I'm reading the reflow tequnique may not be the most effective).
  • Replaced logic board battery with CR2032.
  • No obvious signs of corrosion / broken traces on analog board, logic board, or PSU
  • Mac booted, showed 1 MB RAM (correct), then I shut down awaiting keyboard/mouse and memory upgrade.
  • Ordered 4 × 1 MB / 80 ns RAM from Apple Rescue of Denver.

2. Upgrade
  • Installed 4 MB upgrade, clipped R35.
  • Booted fine, via BlueSCSI, reported 4 MB, ran programs successfully.

3. Failure
  • The next or second boot: static/lighting effect -> squished/dark screen (see video/pic).
  • Attempted PSU reflow (per Larry Pina: T1, T2, L7, D4, Q2, Q3). Some joints wouldn’t cooperate, but otherwise "looked" okay.
  • No discoloration near T2.
  • Result: still squished/dark display.

4. Troubleshooting
  • Mac still chimes, mouse works.
  • Measured voltages at HDD connector: +5.05 V, +11.94 V (under load).
  • Reverted back to 1 MB RAM (re-soldered R35). Thought maybe extra RAM triggered video issue.
  • Same problem, though the squished image looked a bit clearer on 1 MB.
  • Left Mac running briefly to get my phone -> came back to hot burning smell and an auto-reboot (I heard the chime).
  • Flyback cage extremely hot, electronics burning smell in the air, air around flyback was hot.
Next Steps / Questions
  • Suspect failed flyback (157-0042A) maybe?
  • None of the caps on the analog board appear damaged or bulged.
  • I'm willing to desolder/resolder/test/replace parts as needed. I just didn't want to shotgun every component blindly without asking for advice!

Thank you!
-cam


after lightning.jpeg
after psu reflow.jpeg
 
JDW

JDW

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Kai Robinson said:
The pdf is in the resource section.
Click to expand...
In the Resource section, I cannot find the "Mac Classic & SE" book that I linked in my previous post, but I do see these "related" PDFs...




Here's a photo of the three Pina books I have physical copies of...

tempImageOyIkFI.png
 
C

CamInTosh

New Tinkerer
Sep 20, 2025
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I'm so embarrassed... I have all of the PDFs mentioned, but I was in "panic mode" and decided to reach out before consulting the books! But I also prefer the extra personal confidence from talking to people!

Having said that, I see what Larry is saying about Q2 and CR2. To me, the last paragraph reads like: "while you're in there, replace CR3 and CR5 as well" since they are both linked, in someway, to Q2. I'll source those 4 parts.

I hope to have an update by next week-ish.
Thank you!
 
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CamInTosh

New Tinkerer
Sep 20, 2025
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I have a followup I can't find / missed in the PDFs. I've got everything cleaned up and re-soldered with new parts. But I'm curious about BU406 (Q2). The Base connects to resistor R18. The Emitter to ground. On my board, both ends of R18 have continuity through that resistor. And one end is coupled with a ground connection; you can see this in the schematics and in PCB recreations. This is true for a few other resistors. But for R18, this means the base and the emitter are electrically connected to each other, and to ground, and I'm not sure that's correct. Three different chat bots are beyond-adamant that I have a bad R18.

I don't want to power on the machine without asking about this, or if R18 is bad. I can test R18. Just thought I would ask first. The transistor out of circuit seems okay (no short).

Hope this question makes sense. If not, another way I can ask is: do the 2 legs of Q2 closest to the flyback gage have continuity when attached to the PCB?
 
JDW

JDW

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Anything connected to that BU406 transistor is a candidate for possibly being bad if the transistor dies and fries, but that appears not to be the case in your machine.

It should be simple enough to desolder that resistor and test it though.

Don’t rely on your “beeper mode” (continuity mode) for accurate info. With a highly accurate resistivity-measurement meter that can accurately measure below one Ohm, you need to make actual resistance measurements to see if something is truly connected as a dead short. I did that when making my VGA Mod and 13” Hi-Rez mod videos for the Color Classic, and I found what appeared to be a dead short in beeper mode was in fact 140m-Ohm different in Resistivity mode, thereby proving it was not a dead short.

The problem though is that if your meter is a cheap one and doesn’t provide good accuracy below one Ohm, doing such resistivity measurements will largely be in vain. Your meter would need to reliably differentiate between about 8 milliOhms and 150 milliOhms, for example. And making those measurements with the transistor desoldered and removed is also important.

You also need to check the schematic to see what is connected to what, and I don’t have that in front of me right now.
 
C

CamInTosh

New Tinkerer
Sep 20, 2025
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3
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Thanks for the sanity check JDW. Looks like I have some homework ahead of me! I’ll test it out and also check out the rest of the connections via the schematics. Thanks for the reply!
 
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