Mac Classic - will it ever run again!!

[Tomswork]

@rubber_jonnie

Any signs of happy mac beep



Tom

[rubber_jonnie]

@rubber_jonnie

Any signs of happy mac beep



Tom

Sadly not I'm afraid I've just not had the time.

The CPU has power, and using a spare one, it no longer gets ridiculously hot, and neither does the ROM. I have good power at all pins on the power plug. Continuity on the new CPU socket looks fine too, and no shorts between pins. Also, the /halt and /reset lines are high, which is a good sign.

No beep though and actually no display either, so next up I will check the solder connections for the wiring harness on the analog board, make sure all the moving around hasn't done anything to them. I need to find out more about how it should work, at the moment I don't even see the tube light up

[Tomswork]

Hmmm no tube glow is almost always a solder problem on the connector of the analog board. And again the capacitors could have leaked. No beep could be anything.

Tom
@rubber_jonnie

[stephen_usher]

You did plug the speaker in, didn't you? I often forget on the SE/30.

[rubber_jonnie]

You did plug the speaker in, didn't you? I often forget on the SE/30.

Yep, I nearly forgot at first, but did plug it in.

[rubber_jonnie]

Ok, so I seem to be back here again, and thinking of the Mac and seeing if I can get it going again.

The problems I have at the moment are:

1. I don't know if the logic board is good.
2. I don't know if the analog board is good.

Since I have no good way of testing the analog board, other than seeing if it is delivering the correct voltages, I'd like to rule it out.

The line of reasoning goes like this:

1. I have a PSU that I can use to deliver all of the voltages (5, 12, -12) though I suspect only 5v is needed to run the show, in a similar manner to the Atari ST.
2. The video output seems to be composite going by the main connection on the logic board.

Connector.JPG (19.19 KiB) Viewed 3009 times

Is it therefore possible to connect the logic board to a composite display, powered by my external PSU?

Or is the composite signal not a true one, but an Apple composite that doesn't work as expected? Or would I also need to combine the hsync/vsync signals from the plug with the composite out to make it work?

@Tomswork you seem to be quite clued up on these machines, do you have any thoughts?

[olivier.jan]

Hi,
Do you have access to a scope to look at this composite signal ? If it looks ok, you’ll be able to rule out a logic board failure.
Even with a cheap logic analyzer, plugged on composite, Hsync and Vsync you should be able to recognize a video signal (need to change the thresholds though as you might get something between 0v and 1v with a sync around 0.3v).
I have two Classic sand managed to get one to work properly. Fixed both logic boards, but still fighting with the second analog board and it’s always a bit scary with all the high voltage. Need to replace the MOSFET, it’s an usual issue with the analog boards.

[rubber_jonnie]

Hi,
Do you have access to a scope to look at this composite signal ? If it looks ok, you’ll be able to rule out a logic board failure.
Even with a cheap logic analyzer, plugged on composite, Hsync and Vsync you should be able to recognize a video signal (need to change the thresholds though as you might get something between 0v and 1v with a sync around 0.3v).
I have two Classic sand managed to get one to work properly. Fixed both logic boards, but still fighting with the second analog board and it’s always a bit scary with all the high voltage. Need to replace the MOSFET, it’s an usual issue with the analog boards.

As it happens, I am just going through the Bomarc schematics and wiring up the edge connector at the front of the board for power and was then going to get the scope going on it with out having to hook up the display, then see if I get what looks like a composite signal on my scope. I'll also have a look at the address, data, /HALT and /RESET lines, see what they're doing.

I did recap the analog board (Caps were ALL leaking), and it seems to be generating the correct voltages, but clearly it has a part to play from a video generation aspect.

I'd heard the AB opto-couplers could be problematic, so I was intending to replace that, but not heard about the MOSFETs. I'll have to look into that, though I guess if I'm getting correct voltages, then I suspect it's ok.

Do you have parts you can specifically recommend for the opto-coupler and MOSFET? Or do you just replace like for like? I'm pretty new to Mac Classic repairs, so still learning.

Appreciate your help.

[olivier.jan]

Didn’t change the opto-coupler on the analog board, but I should have a look on the defective one. The MOSFET is an IRFBC40 which you can find easily at Mouser or other.
The caps are totally crap on these Analog boards, they leak a lot. Mine was put on storage in 2004 fully working, got it back 2 years ago, it blows fuse each time I turn power on.
Did a full recap, but still blows fuse
The other one didn’t turn on, but after full recap it works (nearly) perfectly. Only small issue, when turned on the display is stretch horizontally but after 30s-1min it finally gives a proper picture. I know which part to change, but not urgent.

[rubber_jonnie]

Didn’t change the opto-coupler on the analog board, but I should have a look on the defective one. The MOSFET is an IRFBC40 which you can find easily at Mouser or other.
The caps are totally crap on these Analog boards, they leak a lot. Mine was put on storage in 2004 fully working, got it back 2 years ago, it blows fuse each time I turn power on.
Did a full recap, but still blows fuse
The other one didn’t turn on, but after full recap it works (nearly) perfectly. Only small issue, when turned on the display is stretch horizontally but after 30s-1min it finally gives a proper picture. I know which part to change, but not urgent.

Thanks, I will take a look for that Mosfet, you never know.

Out of interest, I connected the edge connector at the front of the board to my bench PSU, so 5v, 12v & -12V.

I can at least power on and probe the thing without having to fight with the stupidly short AB cable and a lingering fear of death by electrocution!!!

So it seems my theory is at least in part correct.

I am seeing a signal on the composite out pin, but need to get some pics (I stupidly forgot), but there is definitely something there.

In addition, I'm seeing the correct 22.3Khz hsync, and the correct 60Hz vsync values at the main connector. I believe it is possible to combine these with the composite signal to work with VGA in monochrome, so I'm having a look into that too.

I also had a probe around the CPU and found the /RESET & /HALT lines to be 5v, and drop on reset, returning to 5v as expected, and /BERR was also looking fine staying high as expected.

What was a bit concerning was that there didn't seem to be any activity on the address and data lines. I need to get pinouts for everything and check what's going on at the other chips, though the RAM did seem to have activity on the address pins which is potentially a good sign.

It is doing something, I just need a display other than the original CRT so I can see what's going on.

I found the attached notes too, which I thought might be useful for people in the same predicament as me!