Mac plus, blank screen after recap

[luckybob]

Title tells 99% of the story.  Got a mac plus off craigslist for $40, no worky.  recapped it, still no worky for the same reason.  popped in a disk, it reads it and acts like it boots.  Can even apple-E to get the disk back.

Looking around, I come across the "Macintosh repair and upgrade secrets" book and long story short it says the LAG chip and a quad nand chip are toast.

The nand chip is easy enough to get ahold of.  the question becomes the LAG chip.  I would imagine I would need to cannibalize another 128/512/plus board for it.

So before I start down this path, how exactly can I be relatively sure this is the proper way to go?  Keep in mind I don't have a scope or any boards I can swap in.  I have two SE's, but it was very clear I couldn't just swap boards.

 

[bibilit]

Plus is well known for having bad solder joints on the analog board, 

Most of the times, is just that, hit the side casing for a test if the screen gets back...also have you turned the knob under the screen ??

 

[BadGoldEagle]

It could be a bad flyback transformer... But try what bibilit said first, just in case.

 

[luckybob]

Yes. Cold joints was the first thing I checked. I resoldered both connectors. Sorry I didn't mention it first.

 

[Juror22]

Did you already check the harness for continuity?  Page 104 of the same book. I had a working plus that I put away for a few months and it had no video.  Since I had already replaced C1, I started through the list by re-soldering the J1 connector, then unplugging and re-plugging in the motherboard cable from BOTH ends.  That step fixed it  - I really would not have thought that would work.

 

[unity]

Both connectors? There are three. Four if you count the logic board which also requires a reflow. Never forget the stand-alone pin on those connectors, the odd pin on the end. Also I have seen cases of the brightness control needing a reflow as well.

 

[luckybob]

@juror22

I'll check the harness.  There is no obvious signs to it NOT working, but I guess there isn't any harm in trying now.  

@unity

I haven't messed with the connections on the logic board.  That board is in pristine condition.  I will go ahead and re-flow and recap the logic board as well.  Not much point in not doing it now.

I just ordered a replacement quad nand chip mentioned in the book.  2 in fact.  I figure I'll replace that if the above doesn't work and hopefully it will shed some light on whats going on.

 

[unity]

Condition does not matter. The solder cracks.

 

[luckybob]

So I had some time today.  Completely re soldered the connectors.  Made no difference.  I measured the resistance of the connector, all came up at zero ohms.  (at least less than .01 ohms)  went ahead and recapped the logic board.  SO much easier than the socket A board I also did today.  

next step is the nand gate and I'll go from there.

 

[luckybob]

Got the replacement for U2 today.  replaced it.  made no difference.  Still 100% black screen.  Am I at the point where I need to replace the flyback?  I've resoldered it and all the connectors.  I tested the voltage at the logic board.  the voltages are BANG ON PERFECT.  That was as far as I wanted to poke around a live TV without some guidance.

I'm looking at this guy on ebay:  http://www.ebay.com/itm/332196315606

It's going to be a week or two before this poor fat bastard has the money.  

332196315606

 

[unity]

Does the cathode illuminate at all? It would be a weee faint, but you should see an orange glow. If it is, it rules out much. Also keep in mind, the logic board itself could be at fault and produce no video if the RAM circuitry is bad. Or even if the RAM is bad. Not sure if that was mentioned yet.

 

[luckybob]

Ok so, this is going to sound crazy, but it started working.

I set it up again, turned off the lights to see the area behind the yoke and flipped the switch.  I saw a slight warm glow and then a bright one from the front.  The damn mother f-er came to life and it looks damn near perfect.  I had a WTF moment so hard it woke up my dogs.  I shit you not, I had this machine on 2 hours ago for like 10 minutes measuring voltages, and got NOTHING.  power cycled it 3 times, all 3 times it worked.  dropped in a system 6 disk and it booted.  (got it back)

I know what you people are thinking, he never checked the connections, but no I did.  I even completely removed the connectors and resoldered each and every one.  I was so certain I popped U2/lag because I TWICE discharged the crt and got a very decent pop.

Also, for the record, I have NOT been drinking.  I'll post a video of what it looks like tomorrow.  

 

[unity]

I still put money on cold solder joint. When you have the rear bucket back on, slap it up a little and see if the display flickers at all.

 

[luckybob]

I've smacked it as hard as I dare, no flickers.  

 

[unity]

Ok. Drop it from 30 feet and see if it does anything.

Well I guess thats good news! And weird. I have a distrust for electronics that pull stuff like that though.

 

[luckybob]

well, at least I now know the flyback and actual screen if fine.  I'm not going to look a gift horse in the mouth at this point and just roll with it for now.  

 

[luckybob]

I fired it up again this afternoon.  worked fine.  Go to install an external HDD and I come back to a black screen.  Hour later I try again, it works, and then dies after 5 minutes.  So somewhere is a bad joint.  I'm going to resolder EVERY component on that analog board.

There is no way this could be a faulty flyback at this point?

edit:  there is also a very very slight hiccup sound.  kinda sounds like when you disconnect your headphones while music is playing.  This happens after the screen is grey, but just before it rounds the corners and flashes the disk icon.  only happens once every time it is turned on.  my inital thought is just the INIT of sound, but I'm not taking anything for granted on this system at this point.

 

[luckybob]

I just tossed this on youtube if it helps:  https://youtu.be/xD2a4hbcT4Y

 

[bibilit]

There is no way this could be a faulty flyback at this point?

Well the Flyback IMHO won't work half of the time, one died on me and never worked again.

Can be a problem in the CRT side, where the connectors are connected (be careful here, can be dangerous)