CPC464 voltage drop

[neo2154]

Hello everybody.

I recently bought a CPC464 with the Board MC0044D (medium sized), and unfortunately it does not work.

I am supplying the board through the barrel connector and a lab power supply, and when i switch on the computer with the computer switch, the voltage drops suddenly to 3,7volts an flow a current of 4Amperes!!! I dont have monitor so i cant check video output for the moment, but the red led is not lighting.

In order to discard fails, i have disconnected the cassette connector, and i switched on only the board shorting pins 1 and 3 of the board connector. The result is the same, voltage drop and excessive current. So the problem is on the board.

I retired the 47uF capacitor and i tested ceramic caps so as to identify any shortcircuit, but i have not found any fail...

Looking the ic's temperatures, when i leave only the board switched on for a minute, i noticed that the 74HCU04AP and 40007 IC are hotter than the other ICs.

I measured voltage on the ics and is in general roundind 1,2Volts, very far of 5 volts...

I dont know exactly what more to check. The general status of the board is good without signals of oxide.

I am almost certain that there is a short circuit on the board, but i am a bit lost right now... Any help or orientation would be appreciated...

Thanks in advance

[neo2154]

I am thinking if the board can be switched on without monitor and without cassette unit, supplying it through the barrel connector and shorting pins 1 and 3...

I suppose yes, specially for check voltages, but i ask here just in case the answer would be NO ...

Thanks!!

[Rabs]

I am thinking if the board can be switched on without monitor and without cassette unit, supplying it through the barrel connector and shorting pins 1 and 3...

I suppose yes, specially for check voltages, but i ask here just in case the answer would be NO ...

Thanks!!

Hi, sounds like a short, and looks like you are doing all the right things. Unfortunately you do not know what has happened to it before you got it. Possibly an overvoltage but a guess. If it is follow @eto recent thread.

[eto]

If it is follow eto recent thread.

In my case, overvoltage fried several RAM ICs where at least one of them then created a short between VCC and GND:

https://www.cpcwiki.eu/forum/hardware-related/fried-my-6128-3-ohms-between-gnd-and-vcc/
 

I am almost certain that there is a short circuit on the board, but i am a bit lost right now... Any help or orientation would be appreciated...

You can measure the resistance between GND and VCC on the board to check if there is a short (or almost a short with low resistance). Then your CPC will draw whatever it gets from the power supply. It definitely makes sense to limit the current to less than 4A. 

Btw: The 40007 is always hotter than the rest, that's normal. 

[neo2154]

If it is follow eto recent thread.

In my case, overvoltage fried several RAM ICs where at least one of them then created a short between VCC and GND:

https://www.cpcwiki.eu/forum/hardware-related/fried-my-6128-3-ohms-between-gnd-and-vcc/
 

I am almost certain that there is a short circuit on the board, but i am a bit lost right now... Any help or orientation would be appreciated...

You can measure the resistance between GND and VCC on the board to check if there is a short (or almost a short with low resistance). Then your CPC will draw whatever it gets from the power supply. It definitely makes sense to limit the current to less than 4A.

Btw: The 40007 is always hotter than the rest, that's normal.

Good Morning.Thanks for the answers.Yesterday night, i was checking voltage and resistance in the input, and i noticed when i switch on the board a negative voltage 

So checking the barrel input cable i was using, surprise, i was reversing the polarity  .

After set the correct polarity, the board started normally. For troubleshooting checklists it could be a classic fail 

I felt like Homer Simpson with monkeys in my head 

10 hours lost in my research. The only thing that comforts me is that the board has not apparently died in the process because of the 4Amps of current i have been injecting to the board.

I will follow reporting my results.

Thanks again...