Voltages on 464 PCB

[llopis]

I'm currently working on (another!) 464. This one has no picture when I turn it on, so the first thing I did was check voltages.


It looks like I'm getting 4.74V around the board when it's on. It's full 5.15V when off, so the DC power is fine. The 4.74V seemed on the low side, so I checked other CPCs I have around me. The 6128 also has around 4.80V, so that's probably close enough.


But then I tried this other working CPC 464 and voltages are only 4.1V. How's that possible??? I thought I was hallucinating, so I stepped away from the machine and put it all away. I came back today and same thing. I measured it all over the board: RAM ICs, AY chip, Z80... Does that make any sense?




Another (perhaps related) question: I noticed that on both CPC 464s with the version 2 medium PCB, if I don't plug in the video connector, the power LED doesn't even come on (that would be on the working one with 4.1V and the one I'm trying to repair). On my other CPCs (6128 or 464 with large PCB), they come on even without the video connector plugged in. Is that normal, or is it a symptom of something deeper?

[Audronic]

I noticed that on both CPC 464s with the version 2 medium PCB, if I don't plug in the video connector, the power LED doesn't even come on  ??

No earth on the Power Plug (+5 Volts)
Check the volts on the Power lead to the 464 (From the Monitor ??)

Good luck
Ray

[llopis]

No earth on the Power Plug (+5 Volts)
Check the volts on the Power lead to the 464 (From the Monitor ??)

I just tested the power lead and it reads a perfect +5.15V unplugged. Is that what you meant? Or test something on the CPC itself?


The power source is one that I modified from a Cisco router that puts out both +5V and +12V so I can use it with the CPC 6128. But all the other CPCs seem to work fine with it, so I'd be surprised if there's something wrong with it.

[Audronic]

OK If the supply is OK then The 0 Volt connection on the socket is ?? Faulty ??
As when you plug in the Video plug you provide a return path for the +5 volt line ??


Ray