Short when CRT board connected

[benanderson89]

I'm now faced with a new problem trying to repair my Macintosh Classic. In a previous thread I thought I had worked out that DP7 had failed since there was a short over the diode. I removed the diode and tested it completely out of circuit. It's fine and works perfectly. There was still continuity between the two now empty pads on the board where the diode used to be, as well as across almost the entire low voltage side of the transformer including every pin on that side of the transformer itself and one of the power rails of the molex connector for the hard disk.

I disconnected everything to remove the board from the chassis and the problem suddenly went away and I was no longer getting shorts to ground across the low voltage side of the analogue board.

Reconnecting everything one by one, I got a short again once I connected the small board to the back of the tube.

I tested the components on the small board with my multimeter and I thought it was CC2 that had failed (a 0.1uF 100v J cap), so I snipped it off. Turns out it was fine and I still get a short to ground. The value was just so low that my cheap meter didn't register it properly.

Does anyone have any idea what on earth (no pun intended) is going on with this?

Cheers!

 

[mraroid]

I wish I could help, but this is out of my league. 

But I do appreciate the very good detailed photo, plus the additional information on the photo.  If everyone were this good at explaining issues they are having, plus provide good photos as you have, troubleshooting will go much faster. 

I am sure someone can come up with something.  I look forward to learning more about this problem.

mraroid

 

[benanderson89]

Some more info:

I've traced the brown cable from the CRT to ground. It looks like it's part of the +12v rail that also supplies pin 1 (yellow) of the motherboard. I've found a grand total of one schematic for the Classic Analogue board (both rev A and B ).

Looking at the circuit diagram, there's nothing really linking +12v to ground. Just crossing my fingers that it's not sent +12v over the +5v rail when it shorted, else the motherboard will be toast. Given you can clearly see ground connected and able to rush through to the orange motherboard cables without restriction, it may very well be the case that it's toast!

The attached image showing the line I've drawn is located just off pin 9 on the transformer in the schematic.

It's 22:38 at night and I really shouldn't be messing with this when tired! :lol:

 

[techknight]

You say the short is only there when you plug it into the CRT? Your CRT itself could possibly be shorted. it happens. 

It also depends on where in the circuit it is. Could be an H-K short. 

Its equally as possible your reading through the CRT filament and this is normal. I am not too familiar with the classic, so i dont recall exactly how the CRT filament gets its power. but it is a very low resistance filament winding. 

 

[benanderson89]

I'm not the best at electronics: H-K would be where on the board? I could pull the Classic out of storage and give it a quick test.

Hopefully the monitor isn't shorted, and I'm 50% sure that it may not be the problem since on first power up it did actually come to life (would draw one quarter of the screen then centre the beam to a single dot very rapidly as the computer struggled to boot, the speaker clicking each time it tried to scan -- after that first attempt the computer doesn't anything except crackle the speaker and spin up the fan).

 

[techknight]

H-K short is internal to the CRT Tube itself. I was simply trying to follow your narrative in the first post, assuming I am following it properly. 

 

[benanderson89]

Yes you read correctly.

Do you know what pins on the tube neck I could test to double check that it is the tube itself?

 

[techknight]

 

Pins 3 and 4 are the only two that should ever have continuitiy. 3 and 4 should not have any continuity to any other pins, and none of the other pins should have continuity between any other pin. 

 

[benanderson89]

Just did a continuity test across the pins and Pin 1 is shorting to Pin 5 and, oh, look at that, Pin 5 is to ground! Thank you so much @techknight, you helped a lot. I've got a replacement CRT on the way (hopefully it doesn't get damaged in transit). Would there be a way to rescue this CRT when there is a short between pins 1 and 5? A quick google shows that it's way above my skill level regardless but I know a local electronics repair shop that mostly does vintage synthesizers but he'll happily do old computers and CRTs, too.

P.S. Because I originally misdiagnosed the problem I ended up snipping CC2 off the CRT neck board. I want to double check that it is in fact a 100V cap in this location. All the other caps around it are labelled with their voltage on the diagram  (0.1uF 2KV) except CC2.

 

[techknight]

You need a CRT tester/rejuvinator that has shorts restore capability to do so. It tries to blow the debris away from the grids to restore operation, but its not perfect. sometimes it can blow the grid completely open and thats it. 

 

[benanderson89]

Given its not working regardless it'll be no loss if it blows the grid, really. I'll see when the electrician is next in at the repair shop and I'll take it down.

 

[benanderson89]

Replacement tube arrived and there's a short between 1 and 5 on this one, too.

Looking at the pin out again I don't think this is a H-K short since pin 1 is G1 (it's linked to the software brightness control and is the top-most pin). I'm thinking it might be some debris lodged in there as when I unboxed the tube, it was showing a dead short across the pins, but after jostling the tube around, dropping it about a centimetre onto a folded towel and testing the tube tilted at different angles, it shot up to about 35 ohm. Tilting to the left it fell back to a dead short.

Someone suggested I blast the neck with a hair dryer whilst tapping on it gently but that just seems counter intuitive :lol:

 

[techknight]

Thats just pot luck right there.... I mean, two tubes in a row with the same short? 

 

[benanderson89]

It really is! It's infuriating because this is the last thing I need to do for my Macintosh Classic for it to work. ARRGH!

 

[techknight]

It really is! It's infuriating because this is the last thing I need to do for my Macintosh Classic for it to work. ARRGH!

Might be worth picking up a CRT tester/rejuvinator that has a restore function for shorts. blasting it with a capacitor charged with high voltage while tapping on the neck may blow out that debris. 

 

[cheesestraws]

It really is! It's infuriating because this is the last thing I need to do for my Macintosh Classic for it to work. ARRGH!

There's a Classic CRT + analogue board combination on eBay UK at the moment for 30 quid.  Was tempted to pick it up myself but if you need it I won't.

 

[benanderson89]

Let me quickly call some local repair guys first. One is an old TV repair specialist that's been in the business for decades and he might still have his old CRT equipment. The other is more for vintage synthesizers but also has all the old equipment since I've had several vintage things repaired by them in the past (albeit mostly synthesizers but they've done TVs, too).

 

[benanderson89]

P.S.

Just checked, that's the wrong monitor revision, sadly. I have a Rev B with the slim connector on the yoke.

 

[benanderson89]

Asked ye-olde reddit electronics sub for help. Old TV repair guy says he's confident it's a GK short. From what I can tell you basically just discharge a whacking huge capacitor between the two pins and it vaporises the debris off of the gun. Wondering how I could do that with what I have.