LC III - No video, no sound, and weird schematic

[EffingController]

Hi everyone! I recently acquired an LC III which I’ve recapped (both the logic board and the power supply). After recapping, the machine will power on, but will not output any video, and only outputs sound via the speaker jack in the back. I get a boot chime which is immediately followed by a Sad Mac chime. This has been consistent despite an alcohol bath and gentle scrub, a new PRAM battery, reseating the ROM, FPU, RAM and VRAM chips in their slots, and trying with various combinations of powered components (HD, fan, etc) unplugged.

I decided to second-check my recap work and I discovered a possible fault that I need some guidance on. Based on the schematic below, it looks like the positive pad of C5 should have continuity to one side of R7. I don’t get a continuity beep, and when I switch to resistance, I’m getting 1.6 kOhms.


That just so happens to be the resistance rating of the nearby resistors, namely R8. Here’s a picture of the board I’ve been working on. I’ve removed R7 and C5 while I work through this. Based on the photo, I’m not sure the schematic is right (or at least my reading of it). It looks like a trace leads from the right pad of R7 to the left pad of R8. (Sorry I can’t get it any closer, I’m stuck with my phone’s dinky camera.)



Can anyone with a working LC III logic board confirm this? If this isn’t the issue, are there any other suggestions for what I should check next? I’ve checked the voltage under load and everything seems fine. The -5V rail is only at -2V, but as I understand it, that rail is primarily used for AppleTalk; please correct me if I’m wrong!

This is my first restoration project and I am so grateful this place exists. Thanks for any help!

 

[mmu_man]

I don't have time this week-end (I opened it but had to close it back to make room), I'll have a look next week.

 

[WombatPredator]

Here's what mine looks like. It looks indeed like R7 and R8 are connected.

Is there a specific test or measurement you'd like me to run?

 

[mmu_man]

Yes from the schematics R5, R6, R7 and R8 have a common trace between them.

 

[EffingController]

Here's what mine looks like. It looks indeed like R7 and R8 are connected.

Is there a specific test or measurement you'd like me to run?

Sorry, I think I did a crummy job of explaining what the problem was. If you could test whether the positive pad of C5 has direct continuity with one of the pads of R7, that would be great!

However, I think I uncovered a more serious issue. Both of the pads of R26 on my board have continuity with ground. I removed the resistor to see if there was a bridge, but the problem remains. I also don't see any obvious shorts elsewhere. So, with that in mind.... could you also test to see if both pads of R26 are connected to ground? I would be surprised if they were, but you never know!

 

[WombatPredator]

Sorry, I think I did a crummy job of explaining what the problem was. If you could test whether the positive pad of C5 has direct continuity with one of the pads of R7, that would be great!

However, I think I uncovered a more serious issue. Both of the pads of R26 on my board have continuity with ground. I removed the resistor to see if there was a bridge, but the problem remains. I also don't see any obvious shorts elsewhere. So, with that in mind.... could you also test to see if both pads of R26 are connected to ground? I would be surprised if they were, but you never know!

So:

C5 does not have any continuity with R7. Its positive pad has continuity with the right side of R8, the left side of C6 and the test point above and to its left. Its negative pad has continuity with the middle and rightmost pins of LP2, both on top and bottom.

As for R26... well, as weird as that is, both pads are indeed connected to ground! Waddayaknow. In the pic you'll see what that area looks like on mine.

I recognized the inimitable style of the Bomarc schematics in your capture above, so I went to check and it's indeed saying that R7 is connected to C5. Which is weird because it doesn't reflect what I see in my LC III either. R7 is not on the path of C5 at all.
And R26 is a little island of ground, unlike what it says in the diagram.

So, couple of things:
- Do we have a different model than the one documented by Bomarc? Mine is an LC III with the sticker label (see pic). Is yours?
- That voltage at -2v instead of -5v is weird. The power supplies on these babies are getting flaky and I wonder if yours is still pulling its weight. If it's not, it might explain why your Mac is being unhappy.
Do you have access to an oscilloscope by any chance? Or anything that could allow you to see if there are voltage changes over time (maybe your multimeter can do that)?

Let me know if there's more that I can test. Good luck!

 

[EffingController]

Thank you for checking all of that! Strange to have that resistor jumped, but I guess if it works, it works!

In the course of working on the board last night with some friends, they noticed that the Bomarc schematic is for 820-0650-B, but my board is 820-0650-A. I don't have the sticker label version of the LC III, mine has the stenciled "Macintosh LC III" mark.

I think I'm going to focus more of my attention on the power supply like you mentioned. Sadly, I don't have an oscilloscope. I have noticed that the -5V rail starts at around -2.3V and then gradually starts to decline to -1.9V or so before plateauing. I followed Recapamac/Branchus Creation's recap guide, so I didn't replace the big 385V cap. I've done some shopping for it but it's been tricky to find a replacement that'll fit inside.

That'll have to wait until I can replace one of the resistors I've been working on. It went flying out of my tweezers and I'm sure I'll never find it.

 

[WombatPredator]

Thank you for checking all of that! Strange to have that resistor jumped, but I guess if it works, it works!

In the course of working on the board last night with some friends, they noticed that the Bomarc schematic is for 820-0650-B, but my board is 820-0650-A. I don't have the sticker label version of the LC III, mine has the stenciled "Macintosh LC III" mark.

I think I'm going to focus more of my attention on the power supply like you mentioned. Sadly, I don't have an oscilloscope. I have noticed that the -5V rail starts at around -2.3V and then gradually starts to decline to -1.9V or so before plateauing. I followed Recapamac/Branchus Creation's recap guide, so I didn't replace the big 385V cap. I've done some shopping for it but it's been tricky to find a replacement that'll fit inside.

That'll have to wait until I can replace one of the resistors I've been working on. It went flying out of my tweezers and I'm sure I'll never find it.

Hey no problem. Glad I could help removing some doubts.

I haven't done much research so it's a wild guess but isn't reference 493-14773-ND on Digi-Key a good candidate? Too tall maybe?

And that's too bad about the resistor I hope you can locate it. A good way is to walk around barefoot and wait for it to jam itself in your foot when you least expect it. Speaking of experience here

 

[EffingController]

Hah! Unfortunately, my workbench is in my crummy old basement and I'm sure I'd get plenty of false positives using that method, and maybe a case of tetanus. I have replacement resistors coming from Digikey this week. Unfortunately, I didn't order a replacement for that big capacitor. I think the one you linked might be just a tiny bit too tall (I think the installed cap measured about 0.75" tall), and the voltage is too low (350V vs. the 385V rating on the existing cap). I'm not at all sure that it's related to the -5V problem anyways.

I've been spending the past few weekends on this poor old machine so I figure I should let it sit for a bit and I'll come back to it. I have a beige G3 Desktop that needs some attention too.

 

[WombatPredator]

Hah! Unfortunately, my workbench is in my crummy old basement and I'm sure I'd get plenty of false positives using that method, and maybe a case of tetanus. I have replacement resistors coming from Digikey this week. Unfortunately, I didn't order a replacement for that big capacitor. I think the one you linked might be just a tiny bit too tall (I think the installed cap measured about 0.75" tall), and the voltage is too low (350V vs. the 385V rating on the existing cap). I'm not at all sure that it's related to the -5V problem anyways.

I've been spending the past few weekends on this poor old machine so I figure I should let it sit for a bit and I'll come back to it. I have a beige G3 Desktop that needs some attention too.

That's what I do too: in the face of adversity, change your adversary

(and sorry, I misread the 385V)

Keep us posted when you come around to giving the LC III another shot!

 

[marcelv]

the -2V is probably because C22 has the wrong orientation, the silkscreen on he board is incorrect. if you rotate C22 you will have -5V

 

[MacKilRoy]

the -2V is probably because C22 has the wrong orientation, the silkscreen on he board is incorrect. if you rotate C22 you will have -5V

View attachment 39187

View attachment 39188

I’m confused by this. I checked my LC3 logic boards and they have the factory caps orientated all 3 the same direction. C22 is orientated with positive towards the floppy drive, same as c21. In your photo you show C22 with positive facing the positive of C21 saying the silkscreen is wrong.

 

[marcelv]

just recapped my LC III and noticed this, the production of the LC III has C22 reversed, see the bomarc schematic

when installed as factory, I get -2.8V, reversed as in the schematic (and as intended) -5V

see also:

LCIII Recap - Apple design fault -47uF reversed!

Hi, Am I the first person to discover this? C22 (the 47uF capacitor in the corner beside the power supply connector) on our LCIII has the + sign on the PCB on the negative capacitor terminal. This capacitor is between -5V and ground. The original C22 electrolytic on this board, and on...

68kmla.org

 

[marcelv]

maybe you are focussing on the wrong part of the board, the LC III will start without battery, the capacitors you replaced ( C2, C4, C5, C7, C8, C13, C18, C19, C20, C21, C22) , FPU, RAM simm and VRAM extension

could you test the board without battery, FPU, RAM simm, VRAM extension

could you post a picture of the entire board?

 

[jajan547]

Is this of any help?

 

[marcelv]

do you have some progress on this LCIII?

happy easter

 

[jajan547]

do you have some progress on this LCIII?

happy easter

Ok so add a new battery do PRAM reset off then on. Repeat this 3 times, follow this by holding PRAM for 10 boot up initializations. Make sure you have RAM and VRAM connected I has the exact same issue and this corrected it

 

[David Cook]

and only outputs sound via the speaker jack in the back

I recently received an LC III that also only outputted via the headphone jack. I replaced the jack, but without any improvement. I found that the vias around C13 suffered the most on this particular board -- and (spoiler alert) fixed the sound by cleaning and threading new wire through two of the vias.

In case it helps others, here is a quick guide of test points:


With a multimeter in continuity mode:

1. Without anything in the headphone jack, test 1 to 1A to confirm it is grounded.
2. Test 1 is NOT connected to 2, 3, 4, or 5. They should not be grounded.
3. Test 2 connects to 3.
4. Test 4 connects to 5.
5. Test 5 connects to one of the speaker pins (J15) but not the other.
6. For all the remaining tests, plug a disconnected headphone plug (cut from headphones, or a jack extension cord, or any headphone plug that is not electrically connected) all the way into the headphone jack. This physically separates pins 2 from 3, and 4 from 5.
7. Test 2 is no longer connected to 3.
8. Test 4 is no longer connected to 5.
9. Test 2 connects to 4. This is part of the board circuit. This Mac only has mono output and thus these pins are mixing both left and right channels.
10. Test 2 connects to 2A
11. Test 4 connects to 4A
12. FYI. 4 connects to vias 4B and 4C. Because those vias will be coated in soldermask, you probably can't test them. But, it is worth noting these are all connected to 4.
13. Test 3 connects to 3A (right pad of R37). FYI. Via 3B is also connected but likely covered by soldermask. FYI. Via 3CR is connected through resistor R37 which is 39 ohms.
14. I have circled C55 because the schematic has a typo which lists this as C75. The actual C75 (on the board and in another place in the schematic) is below and to the right.

Flip the board over. Capacitor C13 is removed from the photo below simply to show the traces underneath.

15. Test 4B connects to 4D. FYI. Via 4C also connects to these but is likely covered by soldermask on your board.
16. FYI. Via 3B and 3CR are the same vias listed on the previous photo.
17. Test SP1 (pin 8 of U6) connects to one of the speaker pins (J15) but not the other.
18. With one probe underneath the board, test 3 connects to 3D (pin 6 of U6)
19. Unplug the plug you inserted into the headphone jack in step 6 before powering on your Mac. A disembodied plug will prevent sound from being played.



When I performed testing on my LC III without speaker sound, I found no continuity in step 13 and 18. I also found no continuity in step 15. It was because vias 4C and 3B (in the above photo) had broken to capacitor leakage.

Take a look at the "before" image. Honestly, would you have guessed that these two vias were the problem? There are four or five other vias that look much more suspect. Yet, probing them with a dental pick, all the other vias were firm, but 4C and 3B were mushy.



Using a scalpel, I flaked off the soldermask coating on the pads around all the questionable vias. Bad soldermask flakes easily. Good soldermask tends to scratch as opposed to flake. I cleaned the area with flux and a copper solder wick, followed by cleaning with IPA.

I then drilled out the two bad vias with a tiny carbide 0.135 mm drill by using just my fingers. The top portion of each via was gummy and mushy. I could immediately tell the good portion of the via towards the back side of the board as it was dry and harder. I scraped the pads around the back side vias.

I then threaded extra long 30 AWG tinned buss wire (not enamel wire) through each via and soldered both sides to their respective pads. The remaining extra length can be wiggled back and forth until it snaps off.



Notice all exposed traces are coated with solder and the other questionable vias were cleaned and resoldered to be sure. The LC III sound has been restored!

Even your Mac doesn't have the exact same issue as mine, hopefully these tests will reveal issues your board may have.

 

[David Cook]

As for R26... well, as weird as that is, both pads are indeed connected to ground!

Not quite. One end is connected to ground and the other is connected to a capacitor and through an inductor (LP2) to a another capacitor and a microphone jack pin which physically connects to another pin (which connects to GND) only when a microphone is not plugged in.

Plug in a microphone and the island goes away.

This LCR circuit protects the input and quiets a pop as a microphone is added/removed.

 

[timmcross]

Hi there, I also have a LC III that was working but had leaky caps. I bought a tantalum kit from console5 and recapped. As far as I can tell everything looks good. Except now I have a system that powers on but no chime or video. No sound through the headphone jack either.

I saw -2.8v on the -5v rail so I recapped the TDK power supply too. -5v only came back after reversing C22. However, still nothing.

Anything else I can check? Is there a solution to this issue? Thanks!