Commodore 128 - No Power. Diode on CR15 question

[byteback nostalgia]

Hello,

So I just bought a C128, and it appears that the accompanying power supply wasn't providing the right voltages (tested after the fact). Following a purchase of a new power supply from Amazon, it appears that I have all the symptoms of a dead C128.

My troubleshooting steps are as followed:
- The power supply is properly reading 9v AC and 5v DC
- The power switch is working as intended, reading 5v DC and 9v ac, respectively. Continuity is working as expected when the power switches flipped both on and off.
- tested CR 15, and I think there was a problem here. I'm reading values in the mV. Attached is a picture of its reading on my oscope:
- Voltage regulator at u59 is properly displaying 12v
As shown, the vpp is 30mV and the max is 10mV. The diode tests at ~99MOhms


- Cartridge port is only showing 4.62v across pin 1 and 2
- The Z80 5v rail is showing 1.75v on the 5v pin to GND


This is my first time attempting to fix broken hardware, so my experience on this is very low. Commodore is also a machine I haven't really touched either. Does anyone have any suggestions for me so I can move forward on troubleshooting? Thank you.

 

[byteback nostalgia]

I tested voltage on the voltage regulator (U59) and I am getting the following readings:
Pin 1 to Pin 2: 27v
Pin 2 to 3: 12v

Is pin 1 to pin 2 correct here?

I also checked the clock signal on the Z80 and the 8502R0, and I do not see a clock. Any help would be appreciated. Thank you.

 

[byteback nostalgia]

Testing CR11 gives me an opened diode, flapping between 100 to 0 ohm. This is the reading from my scope, which appears fine?


Can anyone please offer any insight?

 

[daver2]

How have you tested CR11? With a multimeter? Is it possible you are confusing 0 Ohms (a short circuit) with the multimeter trying to indicate 'infinity' Ohms (an open circuit)? A diode should exhibit a very high resistance in one direction, and a lowish resistance in the other direction. However, if tested in circuit, the circuit itself can give lower resistance readings than the diode in isolation.

27V input from 9V unregulated does seem high. Again, how are you reading this?

Ditto with CR15? Is this the voltage across CR15, or the voltage from both 'wires' of CR15 to 0V?

If the 5V supply is truly low on the Z80, then you need to find the reason for this FIRST. You can only check the logic circuits when all of them has the correct supply voltages.

Dave