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Mac Classic - Assistance needed
- Thread starter Roninfourseven
- Start date
Roninfourseven
Member
I'm not that dumb!
They aren't missing, just not fitted in that picture as I was testing for shorts. It looked like C5 and C9 positive sides were connected to GND, however looking at the circuit in that section, that is correct. They are connected to a -5v regulator, so the negative side of the cap is connected to -5v and the positive side is actually connected to GND. Electronically correct, but when you're testing for shorts it does give what looks like a short.
Getting schematics helped massively in understanding why it wasn't actually a short.
Both caps are reinstalled and I'm in the process of fitting a PLCC socket for the CPU.
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Roninfourseven
Member
Roninfourseven
Member
@dochilli Thanks so much, your 'care package' arrived today, it shows how bad the Maxell bomb can be!! My board luckily didn't have that, but clearly there is other stuff wrong.
I have the old CPU off my board and a socket to install in it's place, plus a spare CPU (One from an Atari STE, one on the board you sent) to work with, plus I have my microscope properly set up now, so I can get to work on the next phase. The ROM on board will be a great help to as I'm pretty sure mine is toast.
Fingers crossed I can get it back up to speed and then onto the next thing, my beeping Amstrad ALT386!!
I have the old CPU off my board and a socket to install in it's place, plus a spare CPU (One from an Atari STE, one on the board you sent) to work with, plus I have my microscope properly set up now, so I can get to work on the next phase. The ROM on board will be a great help to as I'm pretty sure mine is toast.
Fingers crossed I can get it back up to speed and then onto the next thing, my beeping Amstrad ALT386!!
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techknight
Well-known member
Hopefully it all works out!
Roninfourseven
Member
I'll let you know once I get time to work on it! Maybe this weekend all being well.
Roninfourseven
Member
So I have a socket fitted for the68K now (Checked for shorts and continuity BTW), and the CPU doesn't overheat like an air cooled car in death valley anymore! Voltages at the power connector seem fine and I have -5v where I expect it too.
But right now I have nothing on screen at all, no bong or checkerboard so it seems deader than ever
On a positive note I am seeing 5v at the /halt and /reset pins on the CPU, which means it isn't stuck in reset and it hasn't halted. I guess I need to break out the scope and look at the address and data lines over the board and see what's going on.
I have the schematics, but was wondering if anybody could point me at a description of all the chips and their functions.
thanks again to @dochilli for the spare if slightly melted logic board, I at least have a supply of spare parts now.
TIA
Roninfourseven
Member
Sadly no luck as yet, I will keep at it though.
Roninfourseven
Member
Ok, so it's been a while since I looked at this machine and it occurred to me that it might be worth trying to bypass the AB because I don't have a lot of space available to put the whole machine out to work on.
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 LB.
Is it possible to connect the LB 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?
I don't intend for this to be any kind of permanent solution, just something to make my life easier whilst troubleshooting, so if anyone has any thoughts or experience around this, I'd love to hear it.
TIA
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 LB.
Is it possible to connect the LB 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?
I don't intend for this to be any kind of permanent solution, just something to make my life easier whilst troubleshooting, so if anyone has any thoughts or experience around this, I'd love to hear it.
TIA
Roninfourseven
Member
I did, and am getting about 8Mhz as expected. /HALT & /RESET lines are behaving as expected, so 5v when running, and 0v when reset is pressed.
Roninfourseven
Member
Wow, I just realised how long it's been since I last posted about my Classic on here!! What with covid and moving house this year, I've not had much time to look at this.
However, it now lives again!!
So the logic board was dead, and I have been able to secure a replacement, which is great. At some point it had no display which wasn't helping and I found a damaged track on the analog board, near the middle on this image:
Once repaired I started to get a picture again, and in fact the CRT image is very nice once it's running.
However, we're not out of the woods yet.
The machine still has a power problem. If I switch it on, I either get a blank screen or unhappy Mac. I then leave it powered on and after about 5 minutes it bongs and there is a sound like a 'flup'. The dead Mac scrolls describes this sound, and now I understand what it is!!
It then continues to bong/flup for a minute or two before finally booting, so I think the PSU is failing to start and eventually does so once it warms up.
In terms of what I have already replaced so far this is it:
CP8 & CP12 in the 12V output circuit
CP6, CP7 & CP36 in the 5V circuit
In addition I have replaced CP2, CP9 & CP10 as they're in the same general area, and as it happens every last one of these were identified as leaking with brown goo at the bottom of each one.
I'm going to change the Schottky rectifier diode at DP6 which feeds the 5v supply (The 5v supply seems unstable when measured, plus these components are pretty cheap), and am intending to replace CP3 (This 'might' be the starter cap, I am not 100% sure due to lack of PSU knowledge) and CP11 and CP37 as they seem to be part of that circuit too.
If it's still doing this after replacing the components then I guess it could be another logic board problem, though everything I've found thus far does say logic board, and there are 2 ICs that could be playing up. One step at a time.
Overall I'm getting there, and hopefully pretty soon this little gem will be back to full working order
However, it now lives again!!
So the logic board was dead, and I have been able to secure a replacement, which is great. At some point it had no display which wasn't helping and I found a damaged track on the analog board, near the middle on this image:
Once repaired I started to get a picture again, and in fact the CRT image is very nice once it's running.
However, we're not out of the woods yet.
The machine still has a power problem. If I switch it on, I either get a blank screen or unhappy Mac. I then leave it powered on and after about 5 minutes it bongs and there is a sound like a 'flup'. The dead Mac scrolls describes this sound, and now I understand what it is!!
It then continues to bong/flup for a minute or two before finally booting, so I think the PSU is failing to start and eventually does so once it warms up.
In terms of what I have already replaced so far this is it:
CP8 & CP12 in the 12V output circuit
CP6, CP7 & CP36 in the 5V circuit
In addition I have replaced CP2, CP9 & CP10 as they're in the same general area, and as it happens every last one of these were identified as leaking with brown goo at the bottom of each one.
I'm going to change the Schottky rectifier diode at DP6 which feeds the 5v supply (The 5v supply seems unstable when measured, plus these components are pretty cheap), and am intending to replace CP3 (This 'might' be the starter cap, I am not 100% sure due to lack of PSU knowledge) and CP11 and CP37 as they seem to be part of that circuit too.
If it's still doing this after replacing the components then I guess it could be another logic board problem, though everything I've found thus far does say logic board, and there are 2 ICs that could be playing up. One step at a time.
Overall I'm getting there, and hopefully pretty soon this little gem will be back to full working order
Johnnya101
Well-known member
Good to hear it's running. Make sure you replace ALL CAPS. That could very well be your problem. What youre hearing is not a logic board issue.
Roninfourseven
Member
I will, working on a list of caps and replacing what I can from stock in the meantime
Roninfourseven
Member
So, I have a final update on this little guy, and it's all running just fine!!
I sorted out the bong/flup issue, which was QP2, an IRF-BC40 MOSFET, which after replacing left me with a fully working power supply that powered up everything just fine.
I then hit another issue whereby with the RAM expansion card installed, the machine would fail with a black screen and error code.
It would work fine with the card removed, including booting to System 6 from ROM. So I did some circuit tracing and found that a resistor in a line to the CASPAL IC was reading 330R rather than the 22R it should have been. As can be seen in the image, it was cracked and damaged, but after repairing and restoring the 22R value, it all now works just fine and fully recognises the expansion card.
It's been a long journey, but worth it.
I sorted out the bong/flup issue, which was QP2, an IRF-BC40 MOSFET, which after replacing left me with a fully working power supply that powered up everything just fine.
I then hit another issue whereby with the RAM expansion card installed, the machine would fail with a black screen and error code.
It would work fine with the card removed, including booting to System 6 from ROM. So I did some circuit tracing and found that a resistor in a line to the CASPAL IC was reading 330R rather than the 22R it should have been. As can be seen in the image, it was cracked and damaged, but after repairing and restoring the 22R value, it all now works just fine and fully recognises the expansion card.
It's been a long journey, but worth it.
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Phipli
Well-known member
Glad you got it running. Luckily the one I recapped for my dad seems to have been in reasonable condition.
I thought for a second you were going to say it was labelled 330 when it should have been 33R and I was about to politely interject
