Seeking advice with my Macintosh Plus

[haemogoblin]

Hi All

New to the forum, but been on IRC over the past couple of days talking to some of you, also a big thank you to the mod who authorized my registration, dont remember your name, but thank you for doing it!

So last year i had my Plus set up at my GF's for gaming and writing entries for my vintage computer blog. Only two months in to using it, the more computer began to show signs that something wasn't quite right. Finally one afternoon the screen went out and it didn't come back on. I assumed this might be the capacitor located at C1 on the power board. I followed a Plus repair guide i found online, Jags House i think and sourced a 50v non polar cap and replaced the existing one, which had no signs of bulging. Turned the system on and nothing, the screen did not fire up, i dont even recall whether it produced a 'Bong', I dont think it did.

Not knowing where to go next, i bagged the Plus up and put it in the attic until i could find a replacement analog board or an answer to what the heck was wrong with the machine. Then recently i read something that gave me hope. After the repair, I had neglected to check the voltage coming from the external floppy port, if it wasn't reading 5v and 12v, the machine probably wouldn't boot up. So I've taken it from the attic and hooked it up once more. This time the machine chimes with a clean 'Bong' however the screen is still very much dead. I checked the external floppy port and it reads 4.97v and 12.17v, I ran a line test on the yoke connectors on the power board and they appeared fine. I was wondering if the screen wasn't receiving power, however if i cut the power at the wall and not the switch on the rear of the mac. I can hear the fizz of the CRT as it discharges.

I'm really hoping someone might have some advice for me, so i can revive this system.

Cheers

James

 

[haemogoblin]

30 views and not a single suggestion?..this can't bode well

 

[Byrd]

Come on mate, it can take a while for others to respond

Dry solder joints - have you checked/resoldered the analog board? This is one of the more common points of failure before pointing the finger at components.

 

[haemogoblin]

Sorry, the old lad is sitting on my kitchen floor looking sad and pathetic every time i go for a drink

I've gone around the yoke joints and i ran a line test on them, all seemed ok. It's been suggested i check J1, apparently it's one of the yoke pins which sits away from the others. Which could have led to me missing it. Going to take a look today. Really hope i can get this Plus working, done some miles on this machines since i was given it back in 2006, I need to finished Space Quest too, i was getting really far darn it! :lol:

 

[beachycove]

Well, I read it yesterday, dithered, and did not respond. Here's what would have been.

I was given a Plus that had no video a couple of years ago. It turned out that the brightness knob had been turned way down. I know it sounds trivial, but the guy who did the giving actually knew his way around technology, yet missed the blindingly obvious.

It's most likely the components in your case, given how it started, but before you go gutting the thing, make sure you've twiddled your knob.

 

[James1095]

The flyback/LOPT transformer is known for failing on these and can result in a dark screen. I had to replace the one in my own Plus around 15 years ago.

You could also check to make sure the filament in the neck of the tube is glowing, it's not very bright but should be clearly visible in a dark room. You could scope the video signal pin on the connector to the analog board, a logic probe or multimeter will work in a pinch to tell you if there's something there.

 

[haemogoblin]

Got back to looking at the plus this evening, I checked to see if the tube glowed. I could see a very faint glow from the glass tube at the rear of the CRT. I have also checked to see if the machine would boot from a floppy and it does. I've checked both the yoke connectors on the analog board and they appear good. No bad connections, unless the actual connectors themselfs are dirty.

Could someone also confirm with me if I've replaced the cap on C1 with the correct component. I've replaced the old cap with a 10uf 50v none polar capacitor. The Mac repair PDF i had, did not specify a uf rating, so it struck me it might be wise to ask. Incase i've chosen the wrong component.

 

[onlyonemac]

Check the analog board connection. I'm not sure if the filamint glows with poor analog board connection, but just check it anyway.

 

[trag]

The connector on the analog board to check is not a yoke connector. It is the connector for the cable which runs between the analog board and logic board. There is only one cable which connects the two, so it is easy to identify. Pin 1, which is the pin by itself (position 2 is blank) should be desoldered and resoldered. I can't guarantee that this is the problem but it usually is in this type of case.

A simple but not perfect test is to slap the machine on the left side while it is turned on. If the screen comes back for a moment, then it's definitely that pin 1.

The failure will not be visible to the naked eye -- at least, not unless you're very skilled at judging solder joints.

 

[haemogoblin]

Hey everyone

I come with good news! After 12 months of trying to fix this machine and almost giving up. I have finally brought it back to life. I brought it out one more time yesterday, determined that this was it, defeat was not an option. I tested each and every solder joint I thought could cause a lack of picture. Everything checked out, no bad joints as far as I could detect.

It was at this point with the machine sitting in front of me, defiantly refusing to work. That I felt like hitting it out of frustration, which caused me to remember what trag has said. So using the plastic handle of my screw driver, I struck the back of the analog board, which caused a suddenly flicker on the screen. Through process of elimination, i found the flickering happened most in the center of the board. Which just happened to be where the cable linking the two boards is located. I was wearing protective gloves, so gentle wiggled the cable and sure enough the Plus rewarded me with a clean picture

I re-flowed the connector and swapped the cable around, also checking for any breaks. With everything back in place, the machine is now once more working. I had to follow the PDF repair guide to realign the geometry of the screen as it was slightly off. I have it about as square as I think i'm able and in the next few days intend to bring down the hdd, keyboard etc from the attic and set the machine up for a few days on my desk.

Will post some piccies later!

Thanks to everyone who offered advice and encouragement!

James