I've finally decided it's time to get some of my collection to an operational state, and first up is an Osbourne 1.
After spending a few days treasure hunting - I'd forgotten where it was stored as it has been sitting in the same box for the last 20 years.
Once located, I unpacked it and checked it had survived the rodents - luckily they only ate large holes the cardboard box and paper packing around it.
Being a knowledgeable electronics bloke, I dismantled it and set it up to reform all the caps, especially the tantalum ones that usually explode on power up after a few years of storage.
So, with the schematic handy, I locate two suitable points to connect to - it has an optional DC supply input, so the ends of the isolating diodes were perfect for a couple of crocodile clip.
Applying voltage from my bench psu via a120 ohm 5 W resistor to both rails at once, I left it an about 0.2 volts for about an hour and slowly stepped it up to 0.5 volts.
I then ran the meter over the board to confirm things were as expected.
Problem - no voltage on some sections of the board. Check the schematic - R21 should be on the +12v rail, recheck - nothing!!
There are two connectors and leads from the power supply to a single plug on the board. Digging further I discover that these separate connections are indeed separate even on the main PCB.
Each of the two +5 and +12 pins feeds different board areas.
But this isn't what the schematic shows:
So maybe my older board is different, who knows, there is no indication this has ever changed in the manual. Never mind, lets move on.
So I simply plugged in the PSU and it's cables,which then connected them together as it would when running and started again.
The plan is to start with the main PCB and then do the PSU mains side separately, before wiring in the floppy drives, display unit and then re-doing the whole system from the start again when all together.
I'm looking to replace at least the PSU caps, since they are the most likely to give issues later on.
So once this is eventually up and running, it will be one down, and many more to go, with a DEC Rainbow next in the queue.
But I am not expecting it to be all as easy as step 1.
An interesting feature is the thermal protection - a thermal circuit breaker more commonly found in dryers and appliances, in series with the mains power.
This isn't mentioned in the technical manual other than as a spec, and poses a live mains hazard as the connectors aren't well insulated.
Mark.