Mac Classic - will it ever run again!!

[Tomswork]

There not a large difference between them a jumper and one less capacitor on the 220v version

Tom

[stephen_usher]

In which case, on the 5V rail there are only three capacitors, an inductor and a diode beyond the transformer, from what I can see:

wibble.jpg

[rubber_jonnie]

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.

As far as I can tell the schematic is good for 240V dependent on the position of JP1, so I believe it to be correct. Full schematic is as follows:

Screenshot 2021-12-21 at 22.41.53.png

The jumpers are visible up by the rectifier, KBL08. It seems to match what I have very closely, but as I'm sure you know, the Classic schematics are 3rd party & not great. All the cap & other component values I've checked so far match the schematic.

Having printed out the schematics and stuck the two halves of the CRT section together, I'm a bit clearer.

Absolutely everything power related is does have a P, so RP, CP, DP, QP etc and the CRT circuit is labelled differently. Everything on the schematic above is power, so IP1 and QP1 are involved in power generation, so could be faulty.

As @stephen_usher correctly noted, there is a diode, and inductor and 3 caps between the transformer and the 5v rail. The only thing there I haven't replaced is the inductor. IIRC the inductor is a solid wire wound job, but I'll check that too, though it seems unlikely it's failed.

Given the 12V line 'seems' to be stable (I did measure both on power up after leaving it for 2 hrs, but it came on immediately, so I will try again tomorrow), and 5v takes a while to warm up, it feels like something to the left of the transformer (ZP1) is the culprit, as this is the side that works to generate 5v, 12v and feeds a 30-38v circuit that appears to head off towards the video circuitry, specifically HV2 and what appears to be HV7 (Jury is still deliberating.

Anyway, it's late and my brain is starting to ache as been at it a few hours, though I feel I have a bit of a better understanding of the logic board sections. I may try to read pin 5 of IP1, see if it really is showing 100KCS.

The schematics are using a number of symbols and concepts I have not seen before, which has been fogging the view of things to say the least.

Back at it tomorrow and I at least have capacitors coming!! Time for sleep.

[rubber_jonnie]

As promised here is the vid of power up.

During power up, there is no immediate 'bong''. Main sequence starts at 8 seconds in after a false start by me. 5v slowly ticks up until at 25 seconds, the machine 'bongs' when the 5v rail reaches 4.88v. At this point the 12v Rail drops to 11.77v from 12.39v then back up again and the machine boots.



I think the next step for me is to get the replacement caps in. Talking to various Mac people, the caps are notorious so I will get the rest of the electrolytics done so that I know I'm in a good position there, then if the problem persists after that, I'll look at getting the voltage test points marked on the bottom of the PCB do that I can measure them and compare to the test points on the schematic.

I'm going to have a look at the 2 power ICs too and see how easily available they are, it probably won't hurt to

What has really 'done my head in' with these schematics is that making the connection to the video section from the power section. Now that's clearer I can focus on the power section.

So next steps:

1. Finish replacing electrolytic caps as they are known to go bad.
2. Once that is done, take measurements and compare to schematics to try and isolate the problem if completing Item 1 makes no difference.
3. A a side quest, get replacement power ICs just in case.

As ever, everybody's input is gratefully received, especially since PSUs are my kryptonite.

EDIT: CNY17 and TDA4605 on order :)

[stephen_usher]

OK, seeing as both the 5V and 12V rails are changing it has to be on the mains side. As you say, it's probably one of the capacitors "leaking" current and slowly coming closer to spec.

It would be interesting to see what the shape of the voltages on the rails are though during the warm-up phase.

[rubber_jonnie]

OK, seeing as both the 5V and 12V rails are changing it has to be on the mains side. As you say, it's probably one of the capacitors "leaking" current and slowly coming closer to spec.

Trouble is, my parts are going to be mangled up in the Christmas post, but hey ho, what can you do? At least they are on the way.

I can at least in the meantime mark up the test points and see what they tell me.

I hadn't noticed the 12v rail changing before I videoed it as it was too quick, so that was worthwhile doing as it gave me insight I didn't have before.

The TDA4605 has a soft start cap on it, so it could be that, and though it isn't an electrolytic I have got replacements for that and some other non electrolytics coming.

Like you say, it does suggest something slowly coming back into spec, and given the effect on both rails, something on the mains side of the transformer.

[rubber_jonnie]

Shockingly my final replacement electrolytics turned up today, I thought they'd arrive after Christmas, but sadly they made no difference to the performance, which means it's something I haven't yet replaced.

So, whilst I wait for the replacement polyester caps and the CNY17 and TDA4605 I'll get testing as far as I can with the test points marked on the schematic, see if they are as expected.

It's slightly frustrating that the PSU needs a wait time of hours between power on fails though, as it'll make things go very slowly trying to check various points in a failed state.

Time to mark up the test points I think.

[stephen_usher]

Sod's states that the last component left to replace on the board after you've replaced all the others will be the one with the fault. ;-)

[rubber_jonnie]

Sod's states that the last component left to replace on the board after you've replaced all the others will be the one with the fault. ;-)

Much the same as when you find something you've lost, it's in the last place you look.

It's got to be something on that left hand side of the transformer, the stuff to the right is really simple stuff, and all the electrolytics in the PSU section have now been replaced.

[rubber_jonnie]

Ok, so I'm trying to ID the voltage of this capacitor, but it's avoiding all my attempts to do so!!

Value is 220nF, and I know what type, so all cool with that, and that ties in with the schematic, but where all the other large caps on the board are pretty clearly marked with voltage, this one isn't.

IMG_20211227_161013.jpg

My options would seem to be either 63v or 630v, but I'm kinda struggling to find any way to ID the working voltage. It simply seems to say 63|.

My gut feel is 630v, and I suppose if they were 63v and I used 630v, then they'd be very much inside their specs. I'm just kind of curious if anybody has seen that marking before.

I have some of the right type/value at 100v, but obviously they are no good if it really does need 630v.