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Bolacore

Macintosh Classic - low beep with garbled screen

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Hello, I'm repairing a Macintosh Classic I picked up with battery damage. When I received it, it was totally dead (0V from the power supply on all rails), and it's battery had leaked on it's board. It has it's original SCSI drive and RAM expansion card with dual SIMMs. 

I've since repaired it's power supply (4.95v for 5V supply, 11.82 for 12V supply), cleaned up the board, replaced all caps, and have been trying to get it to boot. Despite the battery leakage, the board seems in good shape, and the only loss was the RTC IC (UA2 on my board) since it's leads fell off, and what I presume are the two FETs to charge the lithium battery. Both have been removed from the board. The entire area around the 68000 and VIA check out just fine if I follow them according to the schematics. 

 

However, when powering it up I get a screen with random artifacts on it, and a low pitch loud beep from it's speaker after a few seconds. It then sounds like it attempts to read the floppy, then stops after a few seconds. 10 seconds later this repeats with another beep, and more floppy noise.

I can't really find anything describing what this low pitch chime might be. Not sure if it's actually intentional or some fault on the board still. Does anyone know what it means? Is it an error code?

Video of the bootup + sounds: https://photos.app.goo.gl/kyN9d6Rxwcb1Vb5V7

As well, had a few questions on replacement parts as well -

  1. Can the Classic boot without it's RTC IC (UA2 on the board)? As mentioned, I pulled it from the board due to corrosion. 

  2. Can the Classic boot without it's ROM (UE1 on the board)? I have a 40-pin ROM and the board has a 42-pin socket. During repairs I had the ROM off by one pin and it got quite toasty, so I presume it's dead. If I pull it, the screen produces rectangles on the screen with no beeps, with it the screen is totally garbled but it does chime. 

  3. What is the PCB finger connector at the bottom of the board for? Not too relevant, just curious. My schematic doesn't mention what it's for.

Thanks in advance!

Edited by Bolacore

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For sure the board cannot boot without the Rom, as most of the boot procedure is connected to. 

 

Probably not  without the rtc, but never tested myself. 

 

Not sure what you are talking about in your last question, but i think you are talking about the connector used for testing purposes at the factory. 

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19 minutes ago, bibilit said:

For sure the board cannot boot without the Rom, as most of the boot procedure is connected to. 

 

Probably not  without the rtc, but never tested myself. 

 

Not sure what you are talking about in your last question, but i think you are talking about the connector used for testing purposes at the factory. 

Ah that's good to know then, I wasn't sure if that was an extra ROM for the built in System 6. I'm wondering if I've just killed the ROM and it's resulting in garbage on the bus, hence the garbled screen with it in. I can't seem to find replacements, so will try an find an EPROM that's pin compatible. 

 

Can't find any replacement for the RTC. Will just keep going under the assumption it isn't needed. I believe it's I2C, so, would be surprised if it didn't work at all without it, but I could be wrong.

 

And that's the one, I hadn't heard anything about what it's for. Glad to know it's for testing. 

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The RTC also stores the PRAM.

Some Macs will not work properly without it. You should reach the floppy icon/question mark but it might not boot from SCSI without the RTC chip.

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Just now, Bolle said:

The RTC also stores the PRAM.

Some Macs will not work properly without it. You should reach the floppy icon/question mark but it might not boot from SCSI without the RTC chip.

Is there a compatible chip? I see it's Motorola branded, and I've found other ICs from them that were RTCs + SRAM. If not, I'll try the trading post here seeing if someone has a spare. 

 

As well, do you know what the screen should look like without the ROM installed? Should it be blank, or actually display lines/rectangles like mine does? 

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