Macintosh SE blows transistor

[babelhoo]

Hi all! I'm new here, always wanted an old compact Mac, but in my country (argentina), these are difficult to find, not to mention quite expensive.

A few days ago, talking old tech stuff with a co-worker, he told me he had one, probably non-working, as it was about to be tossed in a former job.  He had it stored for 10 years.  Of course I accepted!

It was a very (dark) yellow SE, originally 2x800K floppies, upgraded to a SCSI drive.  I checked everything for obvious damage before plugging in for the 1st time, no apparent leaky or bulged capacitors (I know these still can be dry), and even the battery, a Tadiran with (I guess an expiration) date of 1989 didn't leak! (it even holds a very little charge).

This is a rev B board, with a socket for the battery.  I cleaned everything, put it back together and fired it up:  just a ticking noise from the PSU. The fan barely spin 1/2 a turn with each tick (the fan itself spins freely by hand).

I tested the PSU in isolation, no ticking and all the correct voltages are present.

As per Pina's book for similar symptoms, I tried replacing capacitors CR2 and CR3 and transistor Q2 (a BU406) in the analog board.

I also re-soldered any suspicious solder point (even though none seemed broken).

I just tested and... I was happy for a few seconds: no more ticking! the fan was spinning and a clear and loud chime was heard.  But, no video, nothing.

Obviously checked the brightness. not that easy unfortunately.
I tried this a couple of times, the last one, without the backcase, I was trying to see if the neck was glowing: I didn't notice anything, but I also didn't have much time: Q2, the one I replaced, burst undramatically in a very faint puf of smoke... immediately it was back to square 1:  ticking noise from the PSU! (No fan, no chime).

I understand that this ticking noise is the PSU's protection.

Any ideas what could be blowing this transistor???  it happened at least twice... 

I know that ideally I should recap the thing, but wanted to see if it worked at all.  Also, asked locally in a lot of places for a few caps, like the 3.9uf one, and can't find it! 

thanks in advance! 

 

[Jinnai]

In an SE that I have, the same symptom was caused by a short on the CRT neck board, capacitor C5. It's worth testing with a multimeter.

Does the PSU short if the analog board is plugged in, but not the motherboard? Could the short be electrolytic fluid on the motherboard?

 

[AwkwardPotato]

Pretty unlikely there's electrolyte on the mobo given that the SEs are all axials (never heard of these leaking).

 

[babelhoo]

Thanks for the suggestions!   As AwkwardPotato says, the SE's logic board seems pristine, I know that doesn't necessarily mean there's no problem with it.

I'll check the capacitor on the neck board.  It does seem as it never gets high voltage.  I understand that the way the analog board is done, any of these failures will prevent high voltage from building.

Reading the suggestions on Dead Mac Scrolls, it seems to point to a faulty flyback transformer, does it seem plausible (based on the symptoms)?  really hope not, as it would be impossible to get one locally, and I see there's only ONE guy selling these on Ebay, based on old posts, that same guy used to sell these for $17 to $20, but now, as he's the only one which seems to have stock, he "updated" the price to $100! (in my case, plus shipping and local taxes, it will be close to $200).

thanks again!

 

[babelhoo]

Another question: I might be able to get a Classic with a "jailed" sad mac, so at least it seems the screen is OK.

I know the analog board is quite different compared to the SE's, anyone knows if the flyback and CRT could be of use in the SE?

 

[Johnnya101]

Yes, both can be used. Flyback just solder in, for the CRT you have to swap the yoke. Easier to just find an SE CRT I would think? Less adjustments.

 

[babelhoo]

Thanks for confirming Johnnya101.  I (think) I need the flyback transformer, just wanted to check about the CRT, as the one in the SE has some burn-in (the menu bar is visible under light).

 

[babelhoo]

Just got the Classic today.  it seems to have a bad logic board, but there's video, so I know the flyback transformer is good!   I'll the required capacitors as well in order to do a complete overhaul of the analog board, hopefully it will work.