Mains transformer from Apple II monitor - open circuit on primary side [FIXED]

[synoptica]

Hey all,

I've recently found a full Apple IIe on hard rubbish - the machine itself works great, but the monitor is dead.  I found the service manual for the US model, started testing and eventually worked my way back to the transformer where I found no voltage on the secondary side!

I tested the 240v points on the primary side, which were live, so pulled the unit out and found it's an open circuit on the primary.  I've never found a transformer to fail but I'm only a casual tinkerer, so I'm sure it happens!

Here's my problems:

I have a model (APT-001) which I can find no information on.   In the service manual, it states there should be 22.5v on the secondary (edit: that's after the rectifier; it looks like 18.7v on the schematic), but that's for the US 60Hz model, so I'm not sure if the same will apply.

The primary side is an open circuit (no taps) so I'm not sure if I can take any measurements to determine a suitable replacement.

Does anybody have any hints or tips they can offer?

Service manual link: https://www.eserviceinfo.com/download.php?fileid=64031

Many thanks!

[BradC]

Something like this would probably do the job.

http://www.altronics.com.au/p/m4909a-powertran-240-9+9-30va-toroidal-transformer/

[synoptica]

Something like this would probably do the job.

http://www.altronics.com.au/p/m4909a-powertran-240-9+9-30va-toroidal-transformer/

Oh, I always forget about Altronics!  I've just moved and they're not far from me now

I'm guessing something like this is likely to be a bit under-rated? http://www.altronics.com.au/p/m2154l-powertran-12v-18v-240v-1a-ei-core-transformer/

[BradC]

I'm guessing something like this is likely to be a bit under-rated? http://www.altronics.com.au/p/m2154l-powertran-12v-18v-240v-1a-ei-core-transformer/

Look, it may be ok. The schematic clearly says ~9xxmA. If it came down to that being the only one that fitted you'd probably get away with it. I always err on a bit of extra headroom.

[james_s]

Cut off the outer wrapping on the primary winding, usually there's a thermal fuse buried near the surface and I've seen those blow due to nearby lighting strikes and such. If you can find that and bypass it that should get the transformer working well enough to determine the ratings at least. You might also find there's a broken connection where the lead wires connect.

[synoptica]

Cut off the outer wrapping on the primary winding, usually there's a thermal fuse buried near the surface and I've seen those blow due to nearby lighting strikes and such. If you can find that and bypass it that should get the transformer working well enough to determine the ratings at least. You might also find there's a broken connection where the lead wires connect.

This is a solid brick of a unit - fully enclosed and tack-welded!  I'll have to go at it with a grinder to get in there, but nothing to lose!

Thanks

[james_s]

Oh, well in that case it's probably not worth the effort. You might find that the shielding is necessary though to prevent the picture from being wiggly. I doubt the voltage is all that critical, something in the right ballpark ought to work electrically.

[synoptica]

Oh, well in that case it's probably not worth the effort. You might find that the shielding is necessary though to prevent the picture from being wiggly. I doubt the voltage is all that critical, something in the right ballpark ought to work electrically.

Hrm, now that's something I hadn't considered!  In that case I'm guessing the above suggested transformer may not be ideal...?

[BradC]

Hrm, now that's something I hadn't considered!  In that case I'm guessing the above suggested transformer may not be ideal...?

Potentially. The toroidal will be better in that regard.

[james_s]

I would at least try it, if you do have issues you can probably find a scrap of mu-metal or maybe even ordinary steel will work to shield it.

[synoptica]

UPDATE - FULLY REPAIRED!

So following james_s' comments, I decided to drill the welds and have a look inside the unit.  Good god, so much shielding! Fortunately what wasn't welded had some fairly brittle epoxy holding it all together so after drilling 6 or so welds I was down to the core.

Inside the core, was two steel wires going into the core.  One of these was soldered to the center tap, the other was attached to nothing... the winding just wasn't there!   Over the other side was the wire for the other end of the winding, but this wasn't where the wire had been attached to - it was on the other side.  So presumably this was a single unit for 120/240v configurations with the wires attached to center/outer or outer/outer taps depending on market? 

So I stripped back the other end of the winding and fed in 240v... and got ~20v on the output!    I soldered it all back together, put a thick layer of JB Weld over it all and left it overnight,  then wrapped all the shielding back around it with a couple of fat cable ties in lieu of the welds.  No other problems apparent now we've got power either (almost disappointed - was hoping for a bit more of a challenge lol)

Feels like I repaired something that probably isn't ever supposed to be repaired, but happy with the result!  Thanks to all who offered advice

Will post up pics if anyone's interested.

[james_s]

Nice job, reminds me of the times I've fixed epoxy potted HeNe laser power supplies, it's a nice feeling.