exxos's Atari, Amiga & retro forum

Tomswork wrote: 21 Dec 2021 18:41 Could some of the diodes or resistors be breaking down ?

Tom

It's possible, but I still think it's a capacitor problem. I have replaced the main diode in the 5v circuit BTW.

I sat and monitored the 5v rail on a cold boot, having sat for some time, so it didn't boot right away.

The 5v rail starts about 4.8v and slowly but surely climbs to 5.1-5.2v when it bongs and boots up. Prior to that it sits with garbage on the screen. As soon as I can test again, I will monitor 12v at the same time.

I'll use 2 multimeters and video what happens.

The other reason I think it's a capacitor somewhere it that it takes sometime to get back to the stage where it won't boot, suggesting something is discharging.

As exxos suggested, I think I need to have a look at the values at suggested test points on the schematic, I just need to mark the points on the bottom of the analog board as it's too difficult to measure on the component side with the machine assembled.

Back to the schematic too, see if I can figure out why the 5v circuit, which is pretty simple, would take time to deliver a working voltage.

It's tricky to diagnose when you can't simply have it "on the bench" :( sounds like you replaced most of the stuff in the secondary side already. The next logical step is to trace back the feedback line as mentioned earlier.

If that feedback voltages stable and does not change but the output voltage does.. There is not exactly much left in the circuit other than the Opto itself. Those things have bonkers specifications when new, never mind when they are years old.

exxos wrote: 21 Dec 2021 19:11 Is tricky to diagnose when you can't simply have it "on the bench" :( sounds like you replace most of the stuff in the secondary side already. The next logical step is to trace back the feedback line as mentioned earlier.

If that feedback voltages stable and does not change for the output voltage does. There is not exactly much left in the circuit other than the Opto itself. Those things have bonkers specifications when new, never mind when they are years old.

Yeah, it's not simple to work on, however I'm not going to let that stop me!! STs, BBCs, A8s etc are so much easier without the internal PSU that includes all the CRT stuff.

I have just been looking at the datasheets for the opto (CNY17) and the TDA4605. The latter seems to be for flyback control, and given I have a stable image during boot I'm thinking it's ok. I think the opto may also be part of that circuit too so not related, BUT I'm just printing out the full schematics as looking at half a page at a time on my laptop is driving me mad and I could be wrong!!

rubber_jonnie wrote: 21 Dec 2021 20:06 I have just been looking at the datasheets for the opto (CNY17) and the TDA4605. The latter seems to be for flyback control, and given I have a stable image during boot I'm thinking it's ok. I think the opto may also be part of that circuit too so not related, BUT I'm just printing out the full schematics as looking at half a page at a time on my laptop is driving me mad and I could be wrong!!

Just sanity checking this here, that transformer on the schematic is that actually a switch mode transformer or the actual flyback transformer ?

exxos wrote: 21 Dec 2021 20:14

rubber_jonnie wrote: 21 Dec 2021 20:06 I have just been looking at the datasheets for the opto (CNY17) and the TDA4605. The latter seems to be for flyback control, and given I have a stable image during boot I'm thinking it's ok. I think the opto may also be part of that circuit too so not related, BUT I'm just printing out the full schematics as looking at half a page at a time on my laptop is driving me mad and I could be wrong!!

Just sanity checking this here, that transformer on the schematic is that actually a switch mode transformer or the actual flyback transformer ?

That is a very good question, and one of the reasons I just printed out the schematics. I need to try and piece it all together and figure out if that is the flyback or not. The schematics are basically not great, so trying to untangle it, especially as I am not great with power supplies.

More head scratching to be done, but I'm used to that. I'll hopefully check both 5v and 12v rails on a bad power on shortly, see if the startup is different on each.

@rubber_jonnie

This is a good resource

https://recapamac.com.au/macintosh-classic-analog/

He has some videos on these board repairs

Tom

Instead of two multimeters you could use your oscilloscope's two channels. That would allow you to see any ripple or noise too.

Tomswork wrote: 21 Dec 2021 21:18 @rubber_jonnie

This is a good resource

https://recapamac.com.au/macintosh-classic-analog/

He has some videos on these board repairs

Tom

Cool thanks for that.

stephen_usher wrote: 21 Dec 2021 21:22 Instead of two multimeters you could use your oscilloscope's two channels. That would allow you to see any ripple or noise too.

Good thought.

Just a mo... Are those schematics for the international 220V version or the USA one, which is very different?

I have a feeling that they're the US version you know.