Amiga, +5V, GND, aargh?

[joe34]

so a local friend of mine was kind enough to give me his old amiga 500 so im just in the process of servicing it, so its all in pieces (pcb out of casing, disk drive removed, keyboard dismantled, etc)

and currently at the moment im making up an amiga rgb scart cable

here is the amiga 23pin video connector, and the rest of the wiring diagram here:

http://eab.abime.net/showthread.php?t=47281

the problem im having is, in the diagram he says that pin 23 is a +5V pin, but, i notice that when i do a continuity test of the 23pin amiga video port with my multimeter, that pin 23 actually seems to be connected to ground instead?

so im confused.. does the pin 'become' +5V when the amiga is powered on (and otherwise when the amiga is powered off, pin 23 is 'grounded'?) .. well im pretty confused haha (note, i cant do any tests about what happens to pin 23 when the amiga is powered on, as my amiga is all in bits being serviced)...

any thoughts anyone on this?

joe342021-01-06 23:06:47

 

[John Bennett]

I know nothing about Amigas, but I suspect your worry is unjustified and it's flagging up a low-ish resistance as a continuity 'bleep'. If it read close to 0.00 ohms using the resistance setting I'd be more concerned.

Blanking is just a DC voltage signal anyway, so you could easily bodge it into something else that was 5V if that pin was knackered (unlikely).

 

[joe34]

thanks John, i just tested it again and its funny, i notice actually there is the 'beep' from the pin 23 to ground for like a second or two, and then it stops and i can see on the multimeter screen there is some resistance going on

yea i think im just worrying about nothing and will just get on with wiring the cable i think

 

[The Last Bandit]

That's perfectly normal. Pin 23 is the 5V line alright, the multimeter is just reporting continuity while it charges up some capacitors and then stops beeping when there done.

 

[joe34]

cheers, thats good to know whats happening there

by the way is using 1/4 watt resistors adequate for this wiring diagram? ... but for the +12v should i use a higher rated resistor? (if so is 1/2 watt enough for the +12v?)

joe342021-01-07 12:20:52

 

[John Bennett]

1/4 should be more than fine. The power is minimal. Even if the 12V was connected into a dead short in the TV (which is certainly isn't), the maximum power would only be 12*12/1000 = 0.144w.

 

[joe34]

ace, thanks John!