Mac SE CRT issue

[clh333]

Working on a Mac SE (same as was referenced in PSU issue post earlier).  After substituting a known-good PSU I assembled the computer and powered on, with the back shell removed for observation.  Observed the following:  No chime, black screen and a hissing, crackling sound.  Immediately shut down.  Checked all connections tight.

Crackling seemed to have come from the rear of the chassis but needed a second brief power-up to locate the source: from inside the neck of the CRT; small arcs visible.  Not good:  I have no idea how serious this is or what can happen if it is allowed to recur but I assume this could fry something expensive.

Referring to Pina's Dead Mac Scrolls, this condition is described on P 147 as an analog board problem, with shorted barrel-rectifier diode CR2 ( GI854, 600V 3A) the likely fault. 

While the unit was apart I refreshed the solder on the board; it's possible that I caused the breakdown of the diode.  I can test this of course but other questions occur to me as well:

In order to work on the analog board I disconnected the HT lead from the flyback to the CRT.  When I reassembled I was careful to reseat it, but might I have missed something there; got it wrong?  Also: in the course of disassembly I did not remove the CRT board from the CRT or inspect its pins for tarnish or signs of a poor connection.  Should I have done that automatically?  Could that contribute to this problem?

I'm pretty sure the ground wire from the CRT board to the CRT chassis / frame is in the same position (upper left lug) and making good contact.  Beyond that I'm not sure of anything. 

Any suggestions or help with diagnosis would be greatly appreciated.  Thanks for your replies.

-CH-

 

[Johnnya101]

Could you have nicked it and unsealed it? Pretty sure those are the symptoms.

 

[clh333]

Thank you, Johnnya101, for your reply.

" Could you have nicked it and unsealed it? Pretty sure those are the symptoms. "  Meaning nicked the CRT while re-inserting the flyback lead?  It's possible, but I thought the CRT implodes if the tube is breached.

I checked both CR2 and CR3; each measured (in circuit) infinity in one polarity and about 4.3 K Ohms the other direction. 

I lifted the CRT board from the CRT pins and saw a small (neon?) bulb in the center of the socket.  Apparently that's an over-voltage protection.  The pins were shiny and untarnished.

I put everything back together and tried power one more time, with the same result.  I guess next I verify the components individually in other systems.

 

[AwkwardPotato]

The CRT doesn't always implode if you break it. Usually you'll just hear a faint hissing sound as air fills the tube.

 

[Crutch]

If the problem was that he’d nicked the CRT, shouldn’t he still get a chime? He said no chime which indeed makes me think analog board. 

 

[Gorgonops]

Between the pins on the neck of your CRT is a small glass nipple; it's there to allow the vacuum to be released intentionally by smashing it with pliers in order to dispose of the unit safely. Word has it that it's not that unusual to inadvertently break that by knocking on the CRT neck board when disassembling/assembling compact Macs because of the minimal clearance in the back of the case. If you don't see a crack elsewhere that could be it.

Re: implosions: not to discount the risk too much, but modern CRTs (IE, from the mid-1960s and later) are made out of impressively strong glass. A small CRT like the one in your Mac is pretty unlikely to shatter theatrically into glass confetti if it takes a crack on the neck. (It's pretty common for necks to get completely broken off in shipping disasters without an earth-shattering kaboom.)

But I agree, I'd think the machine would still POST if it was just the CRT?

 

[clh333]

Later this morning I consulted the other Pina reference, Mac Classic & SE Repair and Upgrade Secrets which covers the same symptom but in greater depth, pp 52-57.  Here he states that for the diodes CR2, CR3 and CR5 acceptable in circuit readings are in the range of 0 to 50 Ohms and bad readings are in the range of KOhms, which is what I got for CR2 and CR3.  There is a transistor, Q2 (7 A 60 W, TO-220 case) that often fails along with these components.  I will be ordering the parts and replacing all as a matter of course.  I'll report back when I know more.

Thanks to all who replied.

-CH-