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| Diagnosing Macintosh Classic |
Posted by: SpocksBeer on 2026-06-13 00:17:25
It does that much without UA5 being present? Interesting I would not have expected that.
The variation in screen brightness may be down to electrolyte that has seeped into the PCB of the analogue board. It's quite porous and a real pain to deal with. Usually a deep soaking clean is required if this is the case - hot soapy water, or a ultrasonic cleaner if you have one |
Posted by: DJ68K on 2026-06-13 05:20:01
It looks like the "Screen" adjustment is overdriving the flyback. I suggest you adjust the brightness/contrast dials on the analog board. The Service Manual (page 89) has instructions on how to do this; ignore the light meter stuff (unless you have one) and adjust it so the non-lit areas around the sides of the screen (glass, but no image being projected) is as dark as possible. |
Posted by: musawir on 2026-06-13 07:30:33
will try the adjusting the dials and if that doesn't help i'll pull the boards out for another cleaning.
Any clues on the sad mac icon and the error code? |
Posted by: Byrd on 2026-06-13 23:16:32
Nicely done! I suspect cap goo is still present, have you checked the other culprits as noted TDA4605 @ IP1, CNY17G @ QP1, and diodes on the analog board, also clean the CRT adjustment pots and check voltages now showing life.
The Sad Mac: related to the motherboard likely caps or bad RAM; check again for cap goo, double check recap work and reseat the RAM, ROM chips (if socketed), electronic solvent cleaning spray on the sockets/slots helps. |
Posted by: musawir on 2026-06-16 09:21:56
Did a bit more work on the board. In particular, I found that UH6 had significant rust and rot. So I removed it from the board and sure enough it was a giant mess under the chip. I cleaned up the spot, a few pads were gone. Then I soldered it back on and jumperwired the pins without pads. Continuity tests showed that each of the pins is connected to where it should be. At the same time, I also put the sound chip back on in the same way.
And the result... was a black and white checkered screen 😢 It feels like I've regressed from the sad mac icon to this.
Searching around, the checkered screen seems to indicate issue on the logic board. Lot of people mention the ram. The ram on my board is soldered on, and the chips SEEM OK. I don't see the usual rust and rot I see on other chips pins. I went ahead and cleaned up all the chips anyway, but no go!
Any suggestions on what I should try next? Can the problem still be with the analog board? |
Posted by: LaPorta on 2026-06-16 10:19:48
This is usual: I've taken steps forward and then steps back before. There's got to be stuff going on elsewhere, possibly under other chips that you haven't looked at yet. Don't feel bad; it happens. Likely your issue is still on the logic board, but, if you want, try and run a voltage test on the floppy port with the machine on to make sure that the correct voltages are getting to the logic board. If that is good, for now I'd just focus on the logic board. |
Posted by: musawir on 2026-06-18 02:17:35
Found the issue causing the checkered screen! the cpu's reset and halt pins were being held low due to a solder bridge under one of the caps i changed. Fixing the problem got rid of the checkered screen but still no mac icon. Instead I get this stripe screen now 🤔 |
Posted by: SpocksBeer on 2026-06-18 15:51:14
Think that's indicative of CPU not executing code. Try reseating the ROMs? |
Posted by: musawir on 2026-06-21 09:26:06
further research points to UH6 still being the problem. I desoldered the chip again to inspect it more carefully from underneath. there was a good bit of rust still present, so i took an xacto knife to gently scrape off the gunk from the pins. In the process, a couple of pins broke off 😢 i have ordered a replacement flipflop in a dip package and will attempt to deadbug it to the vias around the slot. Kind of feeling this is turning into a losing battle. |
Posted by: 8bitbubsy on 2026-06-22 04:05:42
EDIT: Whoops, replied to something on the first page while thinking it was the last post. Ignore this, unless you still have problems with unstable CRT brightness.
My guess is unstable voltage regulation on the analog board PSU. Did you properly remove all of the electrolytic leakage when you recapped it? Especially the optocoupler IC (QP1) needs to be cleaned properly as any conductive electrolyte from nearby caps (which is common) can easily mess up the voltage regulation. |
Posted by: musawir on 2026-06-26 17:55:19
New 74LS174 patched in. Continuity at appropriate points checked. Scope shows chip is latching. But still the same vertical bars screen.
There is one thing i'm unsure about:
Pins 12 and 15 on UH6 seem to be not connected, and Pins 13 and 14 are tied to Pin 1. Is this correct or is the schematic i'm looking at wrong? I have soldered the chip according to it. |
Posted by: DJ68K on 2026-06-27 04:40:44
I found a fairly extensive walkthrough of a repair here that might have some extra hints/clues; the attached picture is borrowed from that site. It appears that pins 1, 13, & 14 are all tied together and connected to a VIA under the IC. The VIA should connect to one side of R69 on the opposite side of the board, and pin 17 of the big VLSI chip BBU (UI4) as well as pin 10 of the SCSI chip (UE8) are all connected to that trace (per the schematics). Pins 12 and 15 have no traces coming off of them, so you should be correct to not connect them to anything. |
Posted by: musawir on 2026-06-27 07:28:36
yep, that is exactly what i've done. well that sucks. I was hoping the vertical stripes would have something to do with it. |
Posted by: DJ68K on 2026-06-27 08:13:29
Does anything change if you plug in the 1MB RAM card? How about with the card plus two 30 pin SIMMs? |
Posted by: musawir on 2026-06-27 11:26:11
Does anything change if you plug in the 1MB RAM card? How about with the card plus two 30 pin SIMMs? the system didn't come with the ram card. only the soldered on ram chips. |
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