Mac Plus Power Supply Modifications

[theminidriver]

Trying to boot up my old Mac Plus after 25 years. It makes a strange sound from the speaker.
No display on screen either, not even the sad Mac

I got my hands on a copy of "Macintosh repair and upgrade secrets". According to that, it seems like the power is automatically cutting out and restarting through the over voltage protection circuit. I'm going to try get all the recommended replacement parts for that.

But when I took off the protection jacket on the analog board, I saw what looks like a massive modification. Anyone know what the purpose of this might have been? There are also some strange mods the other side too.

 

[MacTech68]

Would need some pics of the whole board (both sides) and of the other mods to be certain.

As I understand it, you had this working 25 years ago with those modifications as they appear today, correct ?

There can also be several reasons for a switch-mode power supply to behave like that.

Can you confirm your country's AC mains voltage ?

 

[theminidriver]

Hi, thanks for your reply MacTech68!

Yes, it was working with these mods back then (although I had no idea it had been modified as this is the first time I've opened the Mac).

I've attached some more photos of the modifications on the component side of the board. A big thing I've noticed is the 2 extra capacitors for this board.

My country's main power is 240V. This board is marked as a 120V board (and as far as I can see from pictures is a 120V board). As I remember, I used to power this Mac using a Variac (variable power supply) with 110V. I've also got a step down transformer now which I am using.

I suspect the changes might have been to make it compatible with the 240V supply here, although I'm not brave enough to plug it in directly!

 

[MacTech68]

I had a sneaking suspicion that it was a 120VAC modded to 240VAC. I've not seen it done before, so hopefully it's not caused any unique problems.

 

[theminidriver]

Would it not need the main transformer swopped out if it was for a 240V conversion? It still uses the 120V part number. How would this mod work?

Is it possible that the mods are for a more stable power supply? It has 4MB of RAM, so I think it probably was a power hungry machine.

 

[MacTech68]

Indeed. The 120V only board shows the part number 157-0025 whereas the International 240v board shows 157-0047

EDIT: I'd be intrigued to document the modification. And whilst it might be good to return it to original, i'd be somewhat concerned of missing any changes in the reversal.

 

[theminidriver]

Haha yeah, it's a bit of a puzzle as to what it was actually for.
Happy to document it further - what else would you like to see?

There are a few components that were removed as well, like R41, R49, CR18 (positive side jumped to W9), R32 (replaced with a transistor with base to R33), CR27 replaced with 2 diodes in series. A big 1k 5W resistor is also added under the main transformer, between C34 and the bottom right pin of the transformer.

An aside - it is possible to power the analog board without having the logic board plugged in? I'm guessing the CRT flyback needs to be plugged in, to avoid sparks everywhere. I'm just concerned switching it on and off to test, as well as the voltage spikes damaging the logic board.

Another thing I'm pondering - if this indeed is for a 240VAC conversion, would it still work on 120VAC power? Has my memory failed me, and I didn't run it through Variac? (I do remember it being next to my desk).

 

[MacTech68]

What are the markings on the TO-92 transistor mod near R33, and can you make out the marking of the yellow diode there. This is part of the crowbar circuit, so I'm wondering what is going on.

If you measure the resistance across each of the two rectifiers on heatsinks just above the 157-0025 transformer in both polarities, (CR20 & CR21) what are the resistances? They should measure 30 to 50 Ohms in either polarity IIRC.

Regarding turning on & off, you'll need the logic board connected as well as all grounds and flyback anode cap
Which leads me to say you'll want to replace the Class X and Class Y capacitors on the mains input. Don't get these from eBay or non-reputable sellers, since these are safety devices. Most are blowing apart with very loud pops at this age.

C38 Class X2 0.1uF = PME271M610MR30

C33 & C37 Class Y = PME271Y447MR30

If you need assistance in sourcing them, let me know your approximate locality.


EDIT: - BTW, looking closely at the modification, it looks as though the bottom lead of R46 (large green resistor normally 22ohm) is mostly un-soldered.

 

[theminidriver]

The transistor is a 2N3906.
The diode is a BZX79C5V6.

The resistance over those 2 rectifiers is very weird - it fluctuates between about 0.7k - 2k Ohms as it's being measured. I've ordered replacements for them tho, as well as many other of the parts in the "likely to fail" components in the Dead Mac Scrolls and Mac Repair Secrets books.

I'm in South Africa - yeah I've heard those RIFA caps are being visited by the reaper!

 

[MacTech68]

You could try ElectrocompExpress which source from UK stock.

Search - Electrocomp Express - Electrocomp Express

Electrocomp Express - Licencenced distributer of Farnel products in South Africa

www.eexpress.co.za

Or direct from UK Farnell Export:

PME271M610MR30 - Safety Capacitor, Metallized Paper, Radial Box - 2 Pin, 0.1 µF, ± 20%, X2, Through Hole

The PME271M Series is constructed of multilayer metallized paper encapsulated and impregnated in self-extinguishing material meeting the requirements of UL 94 V–0.Typical applications include worldwide use in electromagnetic interference suppression in all X2 and across-the-line applications.

export.farnell.com

 

[theminidriver]

Great! Thanks, had no idea about that supplier.
And well spotted on the solder joint - That was actually me. I was removing and re-applying solder wherever it looked a bit off. I missed that one when going back. So not part of the modifications!

 

[theminidriver]

A quick update: I've replaced the main suspects from Mac Secrets book - CR20 + CR21 and a bunch of caps and a few resistors. Still the same problem. I'm going to order more parts to revert back the modification, and replace as much of the switching circuit that I can (op-amp, smaller transistors etc)

Before I go totally crazy tho, is it possible to power the logic board from a regular AT/ATX style PSU, to ensure that there isn't a problem there as well? I know there wouldn't be a monitor output, as it would be disconnected, but hoping there would be a 'bong' through the headphone jack. Anyone done this? From what I can see, it doesn't seem to have much feedback to the analog board, other than speaker and video out. I don't really have any other way of testing the logic board independently of the analog board.

 

[MacTech68]

You should be able to use a small AT power supply (provided you trigger it's soft-power-on).

You need +5 volt, +12 volt & -12 volt. Sadly, it would appear that the only requirement for -12v on the logic board is for the speaker/headphone amplifier.

Note pin 2 is the un-installed pin.

 

[theminidriver]

Great, thanks, looks simple enough. An AT power supply is going to be a lot easy to get hold of than another analog board to test!

 

[theminidriver]

After much recapping, reversing modifications, replacing all diodes and rectifiers, and basically refurbing the whole power supply and floppy drive, we finally have victory! Just need to get a small gear 3D printed for the floppy auto-eject, and she will be as good as new! Thanks for all the info and tips

 

[MacTech68]

That's great news. Congratulations on a job well done ! 👍

 

[theminidriver]

Thanks!

 

[theminidriver]

The saga continues!

After a few weeks of running the plus, I finally got around to putting the case back on and final cleaning.
Then after about 2 days, I was using it, and it abruptly shut off.

Now it doesn't turn on at all - just a black screen. No strange noises etc.

Reading the books and forums, it sounds unfortunately flyback related.

Is there a way to definitively test the flyback transformer, if I remove it? I can't see anything visibly wrong with it - in some horror pictures there are cracks and arcing, but nothing like that on this one.

Should I also be looking at something else as the issue?

I'm going to try replace CR5, the rectifier for the flyback as an option as well, because at least that is simple and I have a spare one.

Any thoughts appreciated!
Thanks

 

[MacTech68]

If it's suddenly gone completely dead without noises and no longer makes a sound, check R52 (out of circuit) - original is half watt 10K.

Also check R42, 1K quarter watt, which should be next to the fuse.

 

[theminidriver]

Great, thanks, I'll check those out first.

 

[theminidriver]

Hi, I checked out those resistors, they are fine.
I'm thinking checking the crowbar SCR next.

If I desolder and remove the flyback, and power up the Mac, I should get still be able to get bong right? Would it damage any circuitry to remove the flyback and power up the Mac? And then I should be able to measure the 12V and 5V lines from the power supply (at the moment they measure 7.9V and 3.3V, so sounding like a short to me?), to narrow the cause down.

 

[MacTech68]

Have a look at page 15 thru 17 of this PDF

https://cdn.hackaday.io/files/1681007191990400/classicmac2.pdf

Page 16 has notes of the resistances and pins of the flyback. Also take note of the windings that are NOT connected and check those separate windings are not shorted.

You might also re-seat and/or clean the J4 connector (the cable from the motherboard to the analog board). This may cause voltage problems, as noted on page 5 and 6 of the above PDF.

As for removing the flyback to test the PSU, I do not recommend that action. You may damage other components within the Horizontal circuit.

If you are getting a steady voltage (even if it's incorrect) without a "fupp-fupp" noise, I wouldn't suspect the crowbar circuit.

The Switch Mode Power Supply's control circuit could be a cause.

Check CR19 (500mW 6.2v Zener) and C39 (470uF 10V)

 

[theminidriver]

Hi, update time.

I replaced CR19 as suggested above, and U3, as I saw a few other people had issues with that. After I powered it on, there was a really high pitched sound, but still nothing.

I did notice then that the horizontal output transistor Q3 was extremely hot. So I replace that, along with C1 and C2.
Although for C1 I couldn't find the exact replacement, but I read in that pdf above that 4x 1uF ceramics would do the trick, so I tried that (see pic).

Now I get a bong, and the 5V and 12V lines are fine, but the output is completely garbled. Q3 still seems to be getting hot (not sure if that is normal hot or not tho).

Any clues?

 

[MacTech68]

Sad to see that. It is indicative of a failed Flyback transformer

They are horrendously expensive today. Links below are NOT a recommendation or endorsement:

157-026C Flyback 157026C / Replaces Apple / HR 42031 | eBay

All credit cards are processed with Authorize.net. © 1982-2015 Dalbani. Serving the Electronic Industry Since 1982. Miami, FL 33166. 4225 NW 72nd Ave. Dalbani is a national and international distributor of high quality electronic components and parts.

www.ebay.com

Line Output Transformer / Flyback: HR42031 (HR 42031) - 1570026B APPLE 157026C MONITRON...

hr 42031, 1570026b apple 157026c monitron 157126v macintosh

www.donberg.ie

The only other option would be another used machine, and hope it's Flyback is longer lasting.

 

[theminidriver]

Thanks, yes I guess it was inevitable

The part price is not too bad, since another compact Mac would cost way more than that (and there are pretty much none left in South Africa). The only problem is getting the flyback shipped here - to do it without using the local (collapsing) Post Office would cost more than the flyback.

I guess this Mac will need to sit on the shelf until I travel overseas / someone I know does. Maybe I'll look into a video out option in the meantime.