exxos's Atari, Amiga & retro forum

Ok, I seem to have good news regarding the Mac this morning :)

Having been left overnight, it powered up immediately, so it's looking like the IRF-BC40 MOSFET, QP2 on the schematics, was the reason for the slow start.

In retrospect, as it seems to be the main switching transistor, I probably should have replaced that first, but TBH the electrolytics on these boards are known for leaking, and in fact the first batch I did last year, about 10 in the main PSU section, were all sitting in a puddle of brown goo!

So overall, the PSU has had a pretty comprehensive refurb!!

1. All electrolytics (PSU and Video circuits)
2. TDA4605 (IP1, switching controller)
3. CNY17 (QP1 Optocoupler)
4. IRF-BC40 (Main switching MOSFET)
5. MBR1045 (5v circuit main diode)

Items 2 to 5 were replaced based on research on the web, so sometimes you follow the advice you find, and it isn't always right, however it has helped me learn a lot about working with CRTs and a lot about switching PSUs, so it has definitely been worth the effort.

Also, changing the caps in the video circuit showed me that such a simple act can require a degree of re-tuning via the many pots, to get the picture stable on screen, plus how easy it is to upset a CRT by accidentally knocking the tuning magnets around the screen.

So, this machine is going to be temporarily parked whilst I look at Brett's STE, the final problem being that of the non working RAM expansion.

It's been quite the journey!!

rubber_jonnie wrote: 13 Jan 2022 09:33 Ok, I seem to have good news regarding the Mac this morning :)

Having been left overnight, it powered up immediately, so it's looking like the IRF-BC40 MOSFET, QP2 on the schematics, was the reason for the slow start.

Hurrah! Good news indeed!

So, this machine is going to be temporarily parked whilst I look at Brett's STE, the final problem being that of the non working RAM expansion.

Looking forward to those pictures & the diagnostics on that STE

It's been quite the journey!!

Yes, though I'm still not sure it quite equals your battle with that Mega ST!

JezC wrote: 13 Jan 2022 11:33
Yes, though I'm still not sure it quite equals your battle with that Mega ST!

Yeah, the Mega had a LOT more posts at the end!!

rubber_jonnie wrote: 13 Jan 2022 11:42 Yeah, the Mega had a LOT more posts at the end!!

I need to check my two Mega ST's before I get round to fitting the Dual TOS boards in them... :roll:

JezC wrote: 13 Jan 2022 12:07

rubber_jonnie wrote: 13 Jan 2022 11:42 Yeah, the Mega had a LOT more posts at the end!!

I need to check my two Mega ST's before I get round to fitting the Dual TOS boards in them... :roll:

I would!!

rubber_jonnie wrote: 11 Jan 2022 22:40

Tomswork wrote: 11 Jan 2022 16:56 Good news on the classic.

@Kai Robinson I believe he is working on a clasic ram board replacement

Tom

Indeed I'm rather pleased :)

Cool, I do have a Simm board that used to work, but has decided not to :(

@Kai Robinson are you building a replacement by any chance?

https://tinkerdifferent.com/threads/log ... nsion.106/

I did finish it - but not tested it, if you want a free PCB, and some simm sockets, you're welcome to assemble and try it :D

Kai Robinson wrote: 16 Jan 2022 01:32
https://tinkerdifferent.com/threads/log ... nsion.106/

I did finish it - but not tested it, if you want a free PCB, and some simm sockets, you're welcome to assemble and try it :D

Wow, that would be fantastic, thanks so much for the offer!! Are you sure I can't offer you something?

Let me PM you.

:)

Ok, so this little machine is on the bench again to try and finish off the RAM expansion problems.

I have a replacement expansion card on the way, as my current one doesn't work despite replacing the RAM with spare ICs from one of my dead Mac Classic mainboards. We'll see where we get to once it arrives, but currently booting fine from the ROM to System 6.0.3, so that's something.

Just want to get that extra RAM so I can boot to System 7 that's on the HDD.

Well, the replacement RAM card arrived, I plugged it in and it didn't work. Ok, do we have a problem with the SIMMs?

SIMMS out, jumper set to none installed and we bong, and boot fine to system 7 with 2MB RAM. Cool, I'll try the SIMMs from my old card, doesn't work...

Pull the SIMMs out again and swap the jumper, and now nothing works, just a sad Mac that suggests a memory addressing problem. Waaaahhhhh!!!

:dizzy:

Will have to look at the schematics, see if I can figure out what the hell is going on here, but it's bloody annoying.

Quite a few Macs are very fussy about the type of SIMM, even manufacturer and sub-model.