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128k/512k boot sequence


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I am trying to diagnose a problem with vertical bars on a 512k and want to know if anyone has seen a documented boot sequence for a 128k/512k Mac so I can work through the sequence. I don't have any visual issues on the logicboard so I am down to trying to work out which chips may have problems.  The fuzzy schematics I have been able to find aren't that helpful - or readable.  What I am after is the triggering mechanism from when the power is turned on to get to a "bong",  somewhere in there I have a problem.

 

Details can be found over here - unfortunately it seems to have been taken over by discussions re image formats on modern Apple's. :-(

 

 

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Try this article: Macintosh Startup Revealed: From Boot Blocks to Finder

 


See also, Inside Macintosh, volume III, page 42

 

As to your particular problem, the ROM clears the screen to black very early in the boot process, even before the boot chime. So if you're seeing those bars, it means the video buffer in RAM is not being cleared. Likely it's not being cleared because the code in ROM is not being executed.

 

Therefore, I would suspect the problem most likely lays between the Motorola CPU and the two ROM chips. Since you indicated in your earlier thread that you believe that the two ROM chips are OK, I would look into the CPU.

Edited by Dog Cow
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So some progress but only accidental.

 

I have checked all the connections coming off the CPU and traced them to other chips and performed some general continuity testing across the Logicboard today from what I can read on the very fuzzy Schematic of the 128/512k board.  (Note I actually found an error in the schematic on pins 45 & 46 on the 8530 chip,  they read D14 & D12 but should be D15 & D13 !)

 

I had all but given up and thought I would try the interrupt switch at the vertical bars,  nothing.  Then I tried the reset switch and I got a bong and a Mac screen with flashing question mark disk,  what ?  This is able to be repeated,  but,  when I insert a disk I get wurring but a disk with a cross,  if I put an external drive on I get wurring with a disk and then X disk and then a sad Mac.  If I use a FloppyEMU in the external drive port it doesn't initiate and can't be used.

 

Voltages are good 4.99v on the 5v rail.

 

I have posted a video of the sequence.

 

This leave me to think that the CPU isn't initiating properly with the power on switch and needs a reset "Kick" to get it going.  But then it doesn't receive a good signal or can't interpret the signal from the IWM ?  Does this lead me to think I have a dodgy 68000 chip that I need to replace ?

 

 

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2 hours ago, Cinan said:

This leave me to think that the CPU isn't initiating properly with the power on switch and needs a reset "Kick" to get it going

This much is normal on a lot of CPUs—it's certainly true of the 68000—but there's normally a circuit on the board that generates that kick automatically.  (see http://www.users.cloud9.net/~stark/txhmt4.pdf)

 

So it may either be that the reset circuit that automatically generates this has gone kaput, or the power isn't sufficiently stable when it happens.  You might be able to test the former with a decent multimeter with min hold, since to reset the system, both the RESET and HALT lines are pulled low, but I'm not sure I'd want to rely on that method myself.

 

(edit: that chapter I linked to tests their reset circuit just using an LED, which might be feasible; the human eye is pretty good at picking up brief pulses of light.  But I don't know how long the pulse on the Mac is, so take this with a pinch of salt).

 

I don't know exactly how this is implemented on the Mac, nor how this interacts with your other disc-related problems.

Edited by cheesestraws
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2 hours ago, cheesestraws said:

OK, so both the Plus schematic and the Guide to the Macintosh Family Hardware agree that the initial RESET is generated by, of all things, the sound chip

I don't believe that the original Mac 128K and 512K motherboard has a sound chip. Inside Macintosh volume III shows how sound is generated on these models.

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On 11/7/2020 at 10:44 AM, cheesestraws said:
On 11/7/2020 at 10:44 AM, cheesestraws said:

I don't believe that the original Mac 128K and 512K motherboard has a sound chip. Inside Macintosh volume III shows how sound is generated on these models.

Yup.  But I don't have a 512k schematic, so that should have a 'for what it is worth' rider.

 

Here's the followup from some research I did, because I'm not an expert on all things, and I am still learning. The Sony Sound chip was new with the Macintosh Plus and Macintosh SE. The original Macintosh logic board has an Analog Sound Generator chip. From its pinouts found in this schematic, it does not appear to interface with the 68000's reset line.

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Agree.  I have ordered a new LM311 chip so lets see what happens.  I did some more testing last night and it seems that after resetting once and then using a multimeter on pin 7 to see what was being sent,  the machine then started working normally after a few times of on/off's but then resorted to not starting.  I also got the FloppyEMU working when using System 2.0 as the system disk.

 

Some strange behaviour but at least it is showing signs of life.

 

Does anyone have a spec sheet of the smaller components - Especially the Diodes on the 512k board as I may need to replace some of these ?

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10 hours ago, Dog Cow said:

Here's the followup from some research I did, because I'm not an expert on all things, and I am still learning. The Sony Sound chip was new with the Macintosh Plus and Macintosh SE. The original Macintosh logic board has an Analog Sound Generator chip. From its pinouts found in this schematic, it does not appear to interface with the 68000's reset line.

 

You're right, I was mislead by some wording in the hardware reference book (or, rather, I misread it).  Sorry for the dodgy information.  From the schematic linked above, it's pretty clear that the LM311 is generating the reset.

 

 

Screen Shot 2020-11-09 at 09.23.14.png

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So I replaced LM311 without any change.  I am now thinking it might be C9 as per the image above.  In my understanding if C9 isn't working properly then the -ve won't be presented to pin 3 and therefore LM311 won't create the reset signal.  But if the reset switch is pressed (SW2) then it will complete the circuit and the negative presented to P3 and hence LM311 creates the reset signal when I press SW2 and the CPU reboots ??  is that logic right ?

 

My eyes aren't the best but C9 looks to be a non-polarised 4.7uf 35v on the schematic but on my board C9 looks more like a diode than a cap (image attached).  If it's a cap then what type is it ?  If it is non polarised (two parallel bars) then why does it also indicate a + symbol on the top bar ?

 

 

IMG_7390.jpeg

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I have taken C9 out of the board to test it.  it is just showing an open circuit on the multimeter when I test for capacitance.  I have a spare non-working Mac Plus board,  is there a glass bead cap on here that I can transplant,  too hard to read so I need some experience to tell me - please.

 

If I can't transplant can I replace C9 with an electrolytic or a ceramic cap ?  On the schematic it looks like it is polarised but on the board there is no +/- marking,  help !!!

 

 

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On 11/13/2020 at 9:15 AM, Cinan said:

So I replaced LM311 without any change.  I am now thinking it might be C9 as per the image above.  In my understanding if C9 isn't working properly then the -ve won't be presented to pin 3 and therefore LM311 won't create the reset signal.  But if the reset switch is pressed (SW2) then it will complete the circuit and the negative presented to P3 and hence LM311 creates the reset signal when I press SW2 and the CPU reboots ??  is that logic right ?

 

Sorry for slow reply.  That's pretty much what I thought was going on in that circuit, too.  If I'd been thinking, the reset switch working suggests that the chip was fine.  Oh well. 

 

On 11/13/2020 at 9:50 AM, Cinan said:

I assume I can replace this with a ceramic cap of 4.7uf 35v ?

 

This is probably what I would try in your position.  If I'm reading the circuit right, C9 is mostly there to set the time delay before the reset signal is being generated, so you probably have a certain amount of latitude in precisely what you replace it with.  With that in mind, I'd start with whatever I had in the cupboard that had about the right specs, which probably would be a ceramic of some description.

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No change unfortunately,  I used a tantalum and then an electrolytic. the sequence is as follows :

 

1. power on - vertical bars

2. reset switch - boot ok with bong.

3. power off

4. power on - boots ok with bong

5. power off

6. power on - boots ok with bong

 

point 4& 6 only work if they are within a few seconds of 3&5 leading me to believe that there is a capacitance issue somewhere as once the charge has been lost it goes back to point 1.

 

I can't see other capacitors on the schematic that would be causing that.  So unless there are any suggestions looks like I am using the reset switch as the 2nd power on switch.

 

 

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I wonder if the power supply is taking a while to become stable when the machine is first "switched on", either for thermal or capacitance reasons.  The reset signal to the CPU will only work if the power supply lines are nice and solid at that time.  On later machines, I believe, the reset signal is actually not generated until the power rails are stable, but on this one it just looks like a timer.  I don't suppose you happen to have another one of these machines or a Plus do you?  It would be useful to try this board out with another analogue board/PSU to rule this out

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