General Stuff(tm)

[rubber_jonnie]

Yeah, though with the OPD being unreliable and the crowd pleaser (oh, wow, that's today's news!) it was a little stressful.

I've got the OPD on test this morning and I can't get it to fail. The only difference is that I'm using the smaller mono monitor. Maybe I need to do a full recap of the computer PSU in the monitor.

There are Rifa's in those PSU's, I know my own Colour OPD released the magic Rifa smoke

I expect a general PSU recap would help.

[stephen_usher]

Yeah. I replaced the smoke generators before I turned the machine on.

IMG_3704.jpeg (340.17 KiB) Viewed 529 times

[rubber_jonnie]

Yeah. I replaced the smoke generators before I turned the machine on.


IMG_3704.jpeg

Oooh, that looks ready to go!

[stephen_usher]

Well, the machine rans for hours and hours on the mono monitor's PSU.

Opened up the colour monitors and removed the CPU PSU and... one bulgy cap, so I fully recapped the low voltage side.

Put the monitor back together... swapped monitors, turned on and... Machine still doesn't boot. Checkered memory on the screen. Almost as if the CPU is being held in halt state.

Swap back to the mono monitor and it's fine.

I've run out of time for investigation for this week now.

[rubber_jonnie]

Well, the machine rans for hours and hours on the mono monitor's PSU.

Opened up the colour monitors and removed the CPU PSU and... one bulgy cap, so I fully recapped the low voltage side.

Put the monitor back together... swapped monitors, turned on and... Machine still doesn't boot. Checkered memory on the screen. Almost as if the CPU is being held in halt state.

Swap back to the mono monitor and it's fine.

I've run out of time for investigation for this week now.

Something else wrong with the PSU in the colour display perhaps given it works with the mono display?

[stephen_usher]

Yep.

I did one last test. Took out the Microdrive unit so I could get to the motherboard and checked the voltages with the oscilloscope.

+5V was up at 5.4V! +12 was slightly low at 11.6V. The +5V rail also had dips every 20ms or so.

I'm guessing that the switching isn't working quite correctly. I need to change the start-up cap and test again. I didn't have the correct value for that in stock.

[rubber_jonnie]

Yep.

I did one last test. Took out the Microdrive unit so I could get to the motherboard and checked the voltages with the oscilloscope.

+5V was up at 5.4V! +12 was slightly low at 11.6V. The +5V rail also had dips every 20ms or so.

I'm guessing that the switching isn't working quite correctly. I need to change the start-up cap and test again. I didn't have the correct value for that in stock.

Oh yeah, that's a bit high for 5v, hope you can sort that.

[JezC]

Oh yeah, that's a bit high for 5v, hope you can sort that.

Ouch!

[stephen_usher]

@exxos , you're the PSU wizard around here...what are your thoughts on what the fault may be?

[stephen_usher]

Well, I found the fault... and it's not the +5V, though that is high, but it's about the same on the mono monitor PSU and it drops to 5.2V once fully loaded. Still a bit uncomfortably high but just within TTL specs.

I found no -5V on pin 8 of the monitor/power socket. Traced it to cracked joints on all three of the pins in that area. Obviously the system hangs early in the boot process if there's no -5V somewhere.

Reflowed all the joints on the PSU, re-fitted and the system boots normally. Phew!