M5126 Little Signs Of Life, Help needed

[MacPlus2300]

I recently inherited an M5126 Macintosh Portable with the modem and ram expansion card, hard disk and floppy drive. I tore down the device before powering on and of course had to clean up the capacitor electrolyte and replace them. During the recap some pads became fragile so I made a few bodges that came out successfully. After I did a board wash, let it dry and upon powering on with a new battery and working recapped power supply all I got was a speaker crackle. The voltage regulator is working and spitting out 5.05-5.26 volts when I have everything connected to benchtop power supply. I also tried reset methods with the reset and interrupt buttons and I was able to get a short lived chime once, but haven’t since. Any ideas or help?

 

[SuperSVGA]

You may want to check the powered on +5V which is at a few points on the board but the main source is at Q11, which has the always on +5V and /SYS_PWR as inputs and the powered on +5V as an output. The powered on +5V is the large tab. If /SYS_PWR is stuck high then the Power Manager probably isn't turning on the system.
If that is working properly and you get a stable +5V output out of there, then the system is at least trying to turn on and you may need to look at what the CPU is trying to do.

 

[MacPlus2300]

You may want to check the powered on +5V which is at a few points on the board but the main source is at Q11, which has the always on +5V and /SYS_PWR as inputs and the powered on +5V as an output. The powered on +5V is the large tab. If /SYS_PWR is stuck high then the Power Manager probably isn't turning on the system.
If that is working properly and you get a stable +5V output out of there, then the system is at least trying to turn on and you may need to look at what the CPU is trying to do.

I am getting stable +5V readings across the board. However, Q15 is not opening, and I am not getting a 12V signal sent to the regulator. No 12V signal is present. Additionally, i am not hearing a -5V signal either. I have replaced all the MOSFETs on the board and the power regulation IC. The 1.2V reference is outputting as expected. Neighboring resistors and capacitors were checked, but my next point of replacement may be the operational amplifier on the board. I'll have to check a few more points when I get around to it. I scanned the board for any cut or broken traces, and I found none.

 

[SuperSVGA]

I don't think you'll get -5V until the Power Manager turns it on.

If you're seeing battery voltage on pin 3 of Q15 but not pin 2 while the system is on (/SYS_PWR low), then check and see if pin 1 is low. If it's not low, trace pin 1 of Q15 back to pin 3 of Q204, and if that looks good trace pin 1 of Q204 back to pin 3 of U13C, and also verify that pin is high.

 

[MacPlus2300]

Q15 was high and that lead to everything else not working out right. I reflowed U1M and I get a chime and I can NMI. For some reason it works fine now, but I got a lot of new components just in case. I find this very strange.

 

[MacPlus2300]

I don't think you'll get -5V until the Power Manager turns it on.

If you're seeing battery voltage on pin 3 of Q15 but not pin 2 while the system is on (/SYS_PWR low), then check and see if pin 1 is low. If it's not low, trace pin 1 of Q15 back to pin 3 of Q204, and if that looks good trace pin 1 of Q204 back to pin 3 of U13C, and also verify that pin is high.

Okay, nevermind, after a few minutes of being good and happy it went into a dead state again where Q15 is high. It seems to be random and unhappy.

 

[MacPlus2300]

After reflowing a bunch of components and checking again, no dice. I have a new opamp to install soon but Q15 stays high on pin one. No idea what that lapse was where things worked suddenly.

 

[SuperSVGA]

Were you able to check the path back through Q204 and U13C? Because the gate of Q15 will be high until Q204 sinks it low, and Q204 won't sink it low until U13C outputs high.

 

[MacPlus2300]

Were you able to check the path back through Q204 and U13C? Because the gate of Q15 will be high until Q204 sinks it low, and Q204 won't sink it low until U13C outputs high.

u13c... outputting absolutely nothing.

 

[MacPlus2300]

U13 will be replaced tomorrow as well as many other cheap IC's i found. I'll replace it first to see if it was the issue. I'll let you know by tomorrow if it is the problem. I have a lot of parts for various projects and threw in a bunch of portable replacement ICs in there too.

 

[SuperSVGA]

You can also verify the power and inputs on U13C if pin 3 output is low. Pin 20 should be the 5V power for U13C, pin 19 is the output enable connected to /SYS_PWR which should be low, and the input on pin 17 should also be connected to the same 5V power.