Mac Classic - will it ever run again!!

[Tomswork]

@rubber_jonnie

Yup if you unplug the connector from the circuit board on the back of the tube and follow it the the analog board the connector burns and cause problems

Tom

[Tomswork]

@rubber_jonnie

20210106_115109.jpg (555.69 KiB) Viewed 3255 times

The board I have de capped and ultrasonic cleaning just need the caps will find a box to get a shipping cost

Tom

[rubber_jonnie]

Cool, thanks @Tomswork

[Kai Robinson]

Oh hi! What you're experiencing is exactly what i'm dealing with on the Macintosh SE Reloaded project. What you're seeing on the screen is the checkerboard, but with the contents of RAM being directly displayed - hence the distortion/odd dots all over the screen.

Thing is, the Classic, is just a cost reduced version of the Classic - with some simplified logic to remove the need for a 'GLU' HAL16L8. Everything else is almost identical, same BBU, same F257/LS245 RAM control setup etc.

I stumbled across the VCFED thread about a classic exhibiting the same symptoms - turned out to be the SND IC...which oddly, also includes the Reset circuitry. It can be bypassed by lifting pin 5 on the SE's SND Chip - on the Classic - the SND Chip is a PLCC - and Pin 7 is what's responsible. Look around C71 on the underside, too, to see if there's any broken traces, or if theres indeed continuity.

[rubber_jonnie]

Oh hi! What you're experiencing is exactly what i'm dealing with on the Macintosh SE Reloaded project. What you're seeing on the screen is the checkerboard, but with the contents of RAM being directly displayed - hence the distortion/odd dots all over the screen.

Thing is, the Classic, is just a cost reduced version of the Classic - with some simplified logic to remove the need for a 'GLU' HAL16L8. Everything else is almost identical, same BBU, same F257/LS245 RAM control setup etc.

I stumbled across the VCFED thread about a classic exhibiting the same symptoms - turned out to be the SND IC...which oddly, also includes the Reset circuitry. It can be bypassed by lifting pin 5 on the SE's SND Chip - on the Classic - the SND Chip is a PLCC - and Pin 7 is what's responsible. Look around C71 on the underside, too, to see if there's any broken traces, or if theres indeed continuity.

Ahh interesting, I will take a look, thanks.

I do also seem to have a problem now with the analog board too, I get no picture any more on the CRT, and in fact it looks like there is no glow from the neck of the tube. I'm working my way through the Dead Mac Scrolls to see what I can repair there, but it has been recapped and has worked before. Of concern are the voltages coming from the board, which are quite low, so 4.5V, 11.2V.

I know they can be adjusted, but they don't seem to be stable at all, even when adjusted.

Looks like I have a two pronged problem now, and need to get the AB working before I'll be able to do anything with the LB.

Are you by any chance aware of any circuitry that will allow the LB to display on a VGA display? I did find a link to some, but sadly it was dead. If I could work on the LB without needing the CRT it would make life a lot easier.

[Kai Robinson]

VGA? No. TTL? Yes - it's quite simple. The thing is, the internal CRT is a custom resolution - there's a 640x480 mod for the classic's but its very very awkward. You might have better luck running the logic board off a separate PSU for the meantime - i just built one out of a meanwell RT65 - has all the required voltage rails - just get a 14 way molex mini-fit-jr series plug and terminals, the crimp tool, the RT65 itself, some 16 gauge wire, a FPST switch, plastic box (incoming) and some mains cable.

I'm using this to fault find my Mac SE Reloaded board - can be sure i don't have to worry about a 34 year old PSU going pop, and i don't have to use the CRT, you can use the 'scope to probe the composite video signal being generated.

DSC_0423.jpg (92.5 KiB) Viewed 3174 times

[rubber_jonnie]

VGA? No. TTL? Yes - it's quite simple. The thing is, the internal CRT is a custom resolution - there's a 640x480 mod for the classic's but its very very awkward. You might have better luck running the logic board off a separate PSU for the meantime - i just built one out of a meanwell RT65 - has all the required voltage rails - just get a 14 way molex mini-fit-jr series plug and terminals, the crimp tool, the RT65 itself, some 16 gauge wire, a FPST switch, plastic box (incoming) and some mains cable.

I'm using this to fault find my Mac SE Reloaded board - can be sure i don't have to worry about a 34 year old PSU going pop, and i don't have to use the CRT, you can use the 'scope to probe the composite video signal being generated.

I found the connections on the edge connector that matched the power pins, so soldered on connectors there, but I think building a harness will be better. I have a Pico PSU and ATX power breakout board, which is perfect.

I have just received a logic board from @Tomswork which I'll need to take a look at now as well, but since it is known working I think that getting a proper power test harness will be the charm, and I can power it up and see what the signals look like on a working one.

Also, I think my Analog Board has died, so that's questionable as far as power goes.

[rubber_jonnie]

@Tomswork The board arrived today, thanks so much for that, together we will get this thing running again!!

[Tomswork]

@rubber_jonnie

Good to hear the board arrived and the post still works.

Have you looked at this site for mac recapping guide he has some you tube video on the analog boards repairs to

https://recapamac.com.au/

Tom

[rubber_jonnie]

@rubber_jonnie

Good to hear the board arrived and the post still works.

Have you looked at this site for mac recapping guide he has some you tube video on the analog boards repairs to

https://recapamac.com.au/

Tom

Yes thanks, I've already been there, I have actually recapped the AB, but you know you get the feeling something is off? I just can't shake it as I don't see any heating going on in the CRT.

I have got the parts suggested by Dead Mac Scrolls for this particular issue, so I will get that done and see what it throws up.