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ADB and SCSI issues with a recapped Mac LC
Posted by: ironborn65 on 2021-12-30 15:51:33
Dear mac lovers,
I got a badly recalled LC what could just chime, no screen, no boot, no sign of activities.
I removed the radial electrolytic capacitors, I washed the MB, added the SMD tantalum, except for the 100uF (non tantalum SMD) and the 1uF (radial).
It is back again "bong", screen, pointer ... but:
the ADB does not work, i.e. mouse and keyboard does not work.
the SCSI2SD shows some issues, the led flashes very faintly after the initial bright blink, after a few seconds I have the questionmark floppy icon. Internal of external SCSI connector does not change the behavior.
And it does not bother the floppy either.

The previous guy reconstructed a couple of traces because two pads were corroded, I checked the continuity and those were just fine.

I do not have the schema to check which capacitor are related to these issues.
Can someone put me in the right direction please? Can I test the ADB, the SCSI? Would a picture of the MB help?

thanks
Posted by: MacKilRoy on 2021-12-30 16:51:56
I don’t recall this logic board having any radial caps. Can you post any pictures ?
Posted by: ironborn65 on 2021-12-31 00:49:59
correct, the previous owner "nastly" replaced the original electrolytic SMD with radials
Posted by: ironborn65 on 2021-12-31 01:02:08
the C8 and C6 have been tested for continuity across the SMD pin and the trace
the C108, the hot glued one, is very fragile, I've not the ability to rebuild traces, so I had to solder the SMD on the tiny trace fragment and to add a bodge wire, it has been tested for continuity on the other side of the board.
What I'd like to know is where to inspect for issues close to SCSI and ADB IC

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Posted by: ironborn65 on 2021-12-31 01:03:24
ah, there is a thread about booting without the PRAM battery, mine can without it
Posted by: max1zzz on 2021-12-31 04:15:53
If you have access to a hot air station it might be worth removeing the EGRET and cleaning under it and seeing if that restores ADB, even on really well cleaned boards I often find there is still cap juice under the EGRET
Posted by: ironborn65 on 2021-12-31 07:10:58
thanks Max: nope, an hot air station would be the next investment, and I need to learn how to solder SMD ICs.
I could try to soak it in Isopropanol and use an air compressor under che EGRET
Posted by: ironborn65 on 2021-12-31 07:16:38
for someone who might be reading here for the same issue, I found this extremely useful link

www.leadedsolder.com

A “Practice” Mac LC

Repairing broken and corroded motherboard traces on a Macintosh LC computer that will not power on, and whose ADB keyboard and mouse don’t work.
www.leadedsolder.com www.leadedsolder.com
Posted by: ironborn65 on 2022-01-20 05:29:50
I bought a hot air de-solder, I removed, cleaned and re soldered the EGRET/UD8, the undervoltage sensing 34064/UD9 and the voltage regulator 78L08/UA10.
It chimes, but no video, no bootstrap, no video...
I tested for continuity the ICs from the pin to the pad, The bottom most right pin on the EGRET seems loose, but it's connected. Traces on the right side are not interrupted. The ADB signal goes to the EGRET.
I had to fix the C108 and UA10 on the left side because pads were missing, it all tests ok. The I don't know what the DFAC/UB10 does, I think it might be the sound chip. I do not know what this part is responsible for.
The bodge close to the C6 is for a missing pad, it tests ok.

I did not find Bomarc's schematics for the LC, do I'm a bit lost now. Is it the same of the LCII expect for the CPU?
I might try to remove and test the undervoltage sensing 34064/UD9 or assume the EGRET is broken

Any comments or suggestions?
thanks

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Posted by: LaPorta on 2022-01-21 07:20:52
One suggestion, and caution warning: I inadvertently fried an EGRET on an LCII board once - they are very sensitive to heat, I preheated it and used hot air and still it got messed up. Be careful with yours.
Posted by: ironborn65 on 2022-01-21 15:37:40
@LaPorta thanks. The ADB/EGRET was not working since I got this LC, after the resoldering with hot-air ... it's the same. At least I did not fried it.
Could the DFAC/UB10 be involved in the bootstrap?
I'm close to archive this MB as "forget it, it does not work, use it for parts"
Posted by: jajan547 on 2022-04-22 12:18:30
@LaPorta thanks. The ADB/EGRET was not working since I got this LC, after the resoldering with hot-air ... it's the same. At least I did not fried it.
Could the DFAC/UB10 be involved in the bootstrap?
I'm close to archive this MB as "forget it, it does not work, use it for parts"
Well if it chimes there’s hope, did you try PRAM reset, I might have spare chips for this board.
Posted by: max1zzz on 2022-04-24 04:21:05
I did not find Bomarc's schematics for the LC, do I'm a bit lost now. Is it the same of the LCII expect for the CPU?
This might be of some use to you: https://github.com/max234252/MacLC-Reloaded/blob/main/LC_Schematics/PDF/LC_Schematics-V1.1A.pdf

The only note i'll add about it is that on page 8 UJ10 has two pins labelled as pin 7, /cblank is the correct one (The VCC pin labelled as pin 7 should actually be pin 5)
Posted by: ironborn65 on 2022-04-24 14:52:03
I tried to reflow to EGRET but now it does not even chime. The problem I believe it all there.
@jajan547 I could try to replace the EGRET, have you a spare one?
@max1zzz thanks, thank for reporting the issue in the schematics.
Posted by: jajan547 on 2022-04-24 16:14:25
Which egret do you have?
Posted by: jajan547 on 2022-04-27 09:06:15
Which egret do you have?
I have a spare that should work.
Posted by: ironborn65 on 2022-05-01 12:48:01
this is it
is it? 😉
Posted by: jajan547 on 2022-05-01 13:49:14
this is it
is it? 😉
I have 341SO851 which is the same EGRET just a newer revision fully compatible.
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