Mac plus, blank screen after recap

[luckybob]

Well I pulled Q3 - BU406.  I'm 99% certain it's ok.  Diode testing on my dmm reveals the "correct" biases.  Also pulled the CR1 & 5, they came out the same way.  opened the gun connection, the individual wires looked good, and with a pair of tweezers I slightly closed the connectors.  

The pins themselves are clean but do have a tinge of blue.  Alcohol does nothing.  I'm guessing I need to buy a can of deoxit?

 

[techknight]

The fact that the machine works one minute, and not the next rules out component failure unless you have a horribly intermittent component which is really rare. I have never come across it, and when I have, its usually been thermal related, heat gun/freeze spray triggers it. 

Chances are you have a failed solder joint somewhere, or something going on in the sweep power supply itself. 

When the raster goes blank again, you need to monitor the voltages and make sure they are still present, and if you have a scope, probe the drive signal at the horizontal driver transistor. if its missing, you wont have a raster. 

 

[luckybob]

sadly I've never been able to justify the cost of getting a scope.  I'll measure the voltages I get on J2.  I'm trying to avoid directly soldering the connection together.  There is no light at all when the system is running.  I'll check what I can and get back.

 

[luckybob]

Ok, took a sharpie and marked lots of points on the board to test voltages.

Naturally the damn thing decides to work.  So while running all voltages are excellent.  +31.7V @ C14.  (sheet says 30v)  +12 everywhere it should get it, same with 5 and -12.  (U1) , +12 on the crt (pin 8 ).  I didn't check the voltages of the bias / focus / brightness.  I saw 750v on the sheet and my meter is only rated for 600 so I didn't touch those.  The 170V area clocked in at ~153v.  That seems low.  Considering how bang-on the secondary was, I don't know if this is necessarily bad.

Whole time, for a damned hour the thing just happily sat there and mocked me.  Running just fine. 

What if maybe the brightness is getting shorted all the way down?  that's my new theory.

 

[luckybob]

I think I found it.

The brightness knob.

Nothing looked bad, but I desoldered it, checked it with a DMM (was fine ~3 to 1M ohm) and reinstalled it.  Its been working just fine now.

Next step for me is the keyboard.  seems about 5 of the keys aren't responding.  YAY.