Yup. I picked up a stray capacitor (rather than stray capacitance) ![#-o]( "d'oh!")
I had changed all the ICs three times - the resistors, each removed and tested. I measured the traces from all points to all other points and they were all good. All that was left were the capacitors and that's when I noticed - I thought - "hmmmm, 33pF is going to be tough to measure" and then ah! I don't remember 33pF being in circuit... Now with your description of the way the circuit works (thank you) I will replace that capacitor with one approximately 300x more capacitance and hopefully the problem will be gone. These BEPI boards are a royal PITA to rework, I have to be super delicate - what's normally orthopaedics has turned into neurosurgery!
Regarding the grounding issue, when I have two Beebs connected at the same time and disconnect the one I am testing, everything is good. However, when I disconnect the other Beeb (and none of the others are connected to ground) the "ground" floats up to 120V - very annoying when I accidentally touch a test point with my finger![:shock:]( "Shocked")
so I'll definitely be looking to replace the Raspberry Pi supplied PSU - my Macbook Pro has the same problem, the "ground" is actually 120V so there's a surprise waiting for those wearing shorts and using it on their lap. Damn Apple deny this is a problem and then firstly blame my house wiring (duh!) then say I shouldn't use it on my lap whilst plugged in... Obviously the solution would be to ground the case to earth... Hahahaha, the electricity in California is only 120V (so 60V on the chassis) barely noticeable unless I licked it ![:lol:]( "Laughing")
I had changed all the ICs three times - the resistors, each removed and tested. I measured the traces from all points to all other points and they were all good. All that was left were the capacitors and that's when I noticed - I thought - "hmmmm, 33pF is going to be tough to measure" and then ah! I don't remember 33pF being in circuit... Now with your description of the way the circuit works (thank you) I will replace that capacitor with one approximately 300x more capacitance and hopefully the problem will be gone. These BEPI boards are a royal PITA to rework, I have to be super delicate - what's normally orthopaedics has turned into neurosurgery!Regarding the grounding issue, when I have two Beebs connected at the same time and disconnect the one I am testing, everything is good. However, when I disconnect the other Beeb (and none of the others are connected to ground) the "ground" floats up to 120V - very annoying when I accidentally touch a test point with my finger
so I'll definitely be looking to replace the Raspberry Pi supplied PSU - my Macbook Pro has the same problem, the "ground" is actually 120V so there's a surprise waiting for those wearing shorts and using it on their lap. Damn Apple deny this is a problem and then firstly blame my house wiring (duh!) then say I shouldn't use it on my lap whilst plugged in... Obviously the solution would be to ground the case to earth... Hahahaha, the electricity in California is only 120V (so 60V on the chassis) barely noticeable unless I licked it 
Statistics: Posted by maniacminer — Wed Feb 21, 2024 11:31 am