Unusual Analog board problem - Mac 128k, 512k, Plus

[BadGoldEagle]

Hi guys and gals

My 128k AB doesn't work correctly. Screen is completely dark (crt tested ok), but it bongs and it apparently boots ok from the floppy drive.

Since bibilit and I are practically neighbours (we live 14 miles apart), he's usually the one that fixes most of my stuff. Thanks again btw for fixing my SE/30's LB, AB, FDD as well as the Plus AB!

Last week end he tried to fix that board. He reflowed all the known-to-break solder joints and replaced all three filter caps that were about to burst. But it didn't work. 

I suspected the flyback, but then he told me he had a Plus with the same problem and that he tried replacing its flyback with a know good one among with other parts with no luck. 

C1 & C2 get abnormally hot 10 seconds or so after switch on.

So, do you have any idea what might be causing that problem? 

 

[apm]

Just because the flyback didn't fix the problem with the Plus doesn't mean it's not the problem here, especially if (as a 128k) it has one of the early AC-058 flybacks, which were notoriously prone to failure.

If C1 (the horizontal yoke DC-blocking cap) is getting hot, that would seem to indicate that there is some AC current through the horizontal coil, which would mean that the earlier parts of the horizontal circuit are working. I'm not sure why C2 (4700uF 16V filter cap?) would get hot.

 

[BadGoldEagle]

Hi apm

Yeah it's got one of those early flyback transformers. I'm going to order a new one. As much as I wanted to keep things original, AC-058s don't seem to be available anymore. That's good I guess, since they were really unreliable. 157-0026B is more or less appropriate for a 128k, I'll go with one of those instead.

I'm worried about the BU406 transistor though, which often dies when the flyback fails and those aren't available anymore...

 

[apm]

Well as nice as it is to keep things original, having things work is usually better. It happens with the RAM chips in these machines too, amongst a few other parts.

The BU406 is definitely still available, e.g. at Farnell/Element14 and probably most other places.

 

[BadGoldEagle]

Yeah I had to replace the RAM when I got this machine. So it's not completely original anyway. 

I sear I read somewhere that BU406 was impossible to find nowadays. But you're right. 

Will update once I swap the flyback for the new one.

 

[aplmak]

Ok this is just a shot in the dark... I had a 512k that the screen flickered in and out... I would get a bong and it would boot!!  I found fiddling with the main wire that goes from the logic board to the analog/power supply board would bring an image by tapping it... I reflowed the solder in the 11 or so connecting pins on the back fixed the issue!!! It's worth a shot.. I also re-inforced each pin in the connector by removing them and bending them a little so it was a stronger connection... It's a bit of work but I'm sure it helps...

 

[unity]

Excluding the dimmer dial simply being all the way down, I agree with aplmak. Not only that, the ones on the logicbaord must be done also. So if you have not done these, do so for sure.

 

[BadGoldEagle]

The brightness control knob was the first thing I checked. Unfortunately it had nothing to do with it.

bibilit reflowed the A/B connector too and it didn't help. Also, the screen doesn't flicker at all, not even if I tap (gently) the side of the cabinet...

The screen went boom: it was on then 2 seconds later it wasn't. The Mac was running really hot when it happened.

It did smoke too...  But the rest of the mac work fine (tested with a different analog board).

The flyback is still on its way. I didn't wanna pay 40 bucks to have it delivered using the usual carriers. Dammit it's only a $23 flyback! So I opted for Shipito's cheapest handler. I think it's called AirMail. Cost me $9 insured. I know they deliver. It takes time, but they'll do it. 

Also the next thing on my list is a soldering iron. I will need one of those.

 

[trag]

I'm worried about the BU406 transistor though, which often dies when the flyback fails and those aren't available anymore...

That Dalbani Corp. seller has five or so different BU406 transistors available on their real (non-Ebay) web site. Or did lsst time I checked. I bought an assortment intending to compare them, but haven't.