Stuff and Things.

[rubber_jonnie]

When I used to have to design circuits with ECL logic (especially when interfacing with TTL/CMOS logic) it was a nightmare working out what signals were active (and what polarity that was)...no chance of the logic analyser sorting it out, so we had to rely on a 'scope!

At least the majority of modern electronics (and I'm generously including the ST and Amigas in that category!) don't have that pesky-5.2v supply to confuse things further!
Even just using PECL made a world of difference...

Yeah, it's a bit of a bugger, I thought I'd found a short, then find out the way it's connected is electronically correct. Bah humbug. So annoying, this machine powered on 6 months ago.

One thing I did notice today was that with the logic board plugged in, the 5v rail drops to 4.75v, so I think maybe I need to tweak it again with the logic board connected. Supposedly there is also a CNY75GB OPTOCOUPLER on the PSU side which can cause problems, so I think I will also check that out.

Time to look at the schematics again.

[JezC]

Sounds like a lot of hassle however you look at it...

Good luck with it anyway - always wanted a Mac back in the day but never could afford one...and then found that I actually did better with the ST for the majority of tasks as well.

[rubber_jonnie]

Sounds like a lot of hassle however you look at it...

Good luck with it anyway - always wanted a Mac back in the day but never could afford one...and then found that I actually did better with the ST for the majority of tasks as well.

Yeah, thanks. I want to get it going again, because I know it was working, plus there is the heritage of 68K machines here. I do have another Mac that works fine, a Performa 6200, so all the good stuff, but I really want this machine to run again.

[Tomswork]

The most likely place for a short is corrosion bridges around the surface mount ic around the battery holder and down from the metal can capacitors. I use mild bakesode water mix followed by ipa or water rinse . The bridge is a mix of electrolytes trace and the metal off the capacitor. And it's a good conductor

@rubber_jonnie

Tom

[rubber_jonnie]

The most likely place for a short is corrosion bridges around the surface mount ic around the battery holder and down from the metal can capacitors. I use mild bakesode water mix followed by ipa or water rinse . The bridge is a mix of electrolytes trace and the metal off the capacitor. And it's a good conductor

@rubber_jonnie

Tom

The board has been thoroughly cleaned a couple of times now. There were no obvious signs of corrosion beforehand around the caps, and the battery hadn't leaked thankfully. What I thought was a short was actually correct design for a circuit using -5v, so that's off the cards and I'll put the caps back now I've done the tracing there.

Everywhere I look it says clean the board and replace the caps, which I've done. I've had another close look under the microscope and don't see any corrosion around the surface mount chips. The symptom is that when powered on without the extra ramboard I get wobbly fine horizontal stripes,and with the ramboard I get a checkerboard pattern.

What is concerning that I recapped the power section of the analog board and without the logic board connected I get 5v and 12.2v and the HDD spins up. If I connect the logic board they drop to 4.7v and 11.6v, despite having been previously adjusted under load, so I nee to look at that again.

I'v been told the CNY75GB OPTOCOUPLER can cause problems with power so will look at that too.

[stephen_usher]

It may be an idea to pop over to 68kMLA forums. They have some very knowledgable people on there.

https://68kmla.org/forums/index.php

[rubber_jonnie]

Already registered there

It may be an idea to pop over to 68kMLA forums. They have some very knowledgable people on there.

https://68kmla.org/forums/index.php

Already registered there

[Tomswork]

@rubber_jonnie

Up in the middle of the night. Do you have high voltage from the horizontal output on the analog board. Remember I asked if the board had the extra coil on the side of the flyback. The extra coil is there to supply voltage for safety xray reasons. Before the flyback can be driven. If you have good hearing it's worth listening to hear the hi frequency buzz is there. The power supply would struggle try to drive it. And the opto coupler could also not be doing its job. Have you check the voltages at the tube

Ps I was dreaming about the inside of mac's

Tom

[rubber_jonnie]

Up in the middle of the knight. Do you have high voltage from the horizontal output on the analog board. Remember I asked if the board had the extra coil on the side of the flyback. The extra coil is there to supply voltage for safety xray reasons. Before the flyback can be driven. If you have good hearing it's worth listening to hear the hi frequency buzz is there. The power supply would struggle try to drive it. And the opto coupler could also not be doing its job. Have you check the voltages at the tube

Tom

I haven't looked for that, as I'm not that familiar with Macs. What specifically am I looking for? Do you have a picture?

As for voltages at the tube I'm struggling to find things like power plug pinouts etc though I did just get a whole bunch of schematics to go through.

What I can say is that I have a picture (Horizontal lines or if the ram daughterboard is in, a checkerboard) so I think that the display side is ok, but I don't ever get to a startup chime like I used to.

I think this topic is getting big enough that I start a proper one now. I mean it is a retro forum and it is a 68K machine, so I think it best to give it it's own place.

[Tomswork]

I was thinking with the voltages you described there must be a load other than the logic board. The mac using the psu as the part of the monitor is a pain. Maybe find out if you can power the logic board separately to isolate the fault

Tom