Macintosh plus anode cap => dead screen?

[didius]

I have a Macintosh plus which had some display problems.

After a while the display went black, squeezing the sides would temporarly solve the problem.

I've been reading a bit and found that weak soldering on the J1-connector often would be part of the problem.

When I opened it up, I found very weak soldering points, at some points there didn't seem to be any contact at all. So I redid the soldering them.

But then, as I wanted to put the anode cap back into place, I squeezed it at the wrong place and it fell apart.

Now I've got three pieces, but, how do they fit? How can I reassemble this cap. I tried putting the screw in the metal "U" and then pushing it back into the cap, but there's a wire coming out of the cap, how does this fit in?

I gues the metal "U" should be enough if it makes contact with the wire, so I've tried to leave the screw behind.. But then after booting I get a happy mac sound, and disk activity but no screen whatsoever.

I checked the C1 cap with a Peak Atlas ESR: 4.01uF and 0.07Ohm, that looks okay?

 

[James1095]

The metal clip is what clips into the ultor terminal on the tube to make the anode connection. The wire coming out of the cup is what attaches to the metal clip. Usually it's twisted around and soldered but that one may use a screw terminal. As long as the wire is securely attached to the clip and wedged into the rubber cup it will work fine.

 

[James1095]

Looking closer, I think that is definitely a screw terminal. Poke the closed end of the clip up into the anode cap so that it's under the wire poking out, then form the wire into a J shape and use the screw to attach it to the clip.

 

[nvdeynde]

I checked the C1 cap with a Peak Atlas ESR: 4.01uF and 0.07Ohm, that looks okay?

Have you tested for continuity as 0.07 Ohm is very low for such a low capacity capacitor ?

Given the age, one would expect much higher readings, so the capacitor can be shorted.

Given the age of the original C1 capacitor and because it's an electrolyte one, it's better to replace it just to make sure.

Actually recapping the complete analog board is always the best solution in the long term.

As for the loose clip with the screw. Make sure the red wire going to the anode cap is stripped for about 10mm. Then screw the clip back in the holder with the red wire firmly below the clip. When done, pull the red wire while holding the anaode cap just to make sure it's not loose as this is a high voltage lead from the flyback: you don't want that loose.

 

[didius]

Okay, the anode caps looks okay again. Thanks

I've checked the C1 cap again, today it's 4.02uF and 0.08 Ohm

I've ordered all replacement caps, can't wait to plug them in. |)

(PS: nvdeynde, I found your excell list on the forums, it was very helpfull)

 

[James1095]

That's a difference of 10nF and 10mOhm, that will be well within the tolerance of most meters. That does strike me as very low impedance though, for an electrolytic capacitor I would expect an ESR that is a sizable fraction of an Ohm. Have you tested that one with an Ohm meter rather than an ESR meter? The DC resistance should briefly show low ohms, then quickly rise to a nearly open circuit. One fault of an ESR meter is that from its perspective, a capacitor with a very low ESR looks the same as one that is shorted.

 

[nvdeynde]

The DC resistance should briefly show low ohms, then quickly rise to a nearly open circuit. One fault of an ESR meter is that from its perspective, a capacitor with a very low ESR looks the same as one that is shorted

That was my point: the capacitor looks shorted. Given it's age, low capacitance and being electrolytic, it should at least read several Ohms.

 

[didius]

Okay, I've replaced all of the caps.

But I still have a black screen. No screen activity whatsoever. When I plug in my external HD I can hear disk activity.

Small overview of the actions I've taken:

1) Problem: when i used the macintosh plus for a while, the screen went black. Pushin the sides would resolve the problem temporarly.

2) I've resoldered the J1 connector. After that still no screen.

3) Replaced all the caps and resoldered the joints on the flyback transformer.

No screen... I hear a buzzing sound about 30 seconds after turning the Plus on, the back of the CRT lights (faintly) up.

http://tinyurl.com/oewtdcc

http://tinyurl.com/pnlfh7s (buzzing)

The flyback:



J1:

 

[techknight]

Well if the filiments are lit the you have HV and horiz drive. So your missing the video drive signal somewhere

 

[techknight]

I cant watch the links because I cant open a qt file

 

[didius]

I don't know if the filaments light up, since i just get a very small oranje dot at the back:

The wires in this glass tube don't lit up:

 

[James1095]

I'm betting you still have a bad connection somewhere. Look closely at the cable between the motherboard and the analog board and check the solder joints on both ends. Pushing in on the sides causing the problem to go away is a dead giveaway that there is a bad connection. Always best to take care of problems like this quickly as you can damage other parts by continuing to use it. Also don't overlook the obvious stuff, did the brightness knob get turned down or the pot broken? It sounds funny but I have "repaired" equipment on numerous occasions that have had something switched off/turned down, etc and no actual fault.

 

[didius]

I've resoldered the flyback once more, the J1 and J2 connectors and now I'm stuck in some continous boot cycle (without a screen).

http://tinyurl.com/pbgw55a

The plus boots and after about 10 secondes it enters boots again and so on...

I"ve tried connecting a hard disk but didn't help.

Any ideas?

J1:



Flyback:



J2:

 

[nvdeynde]

Do you have a spare analog or logic board for a Mac Plus ? This will be easier to throubleshoot the issue.

As for now it can be a lot of things going from the wire harnass between logic and analog board, faulty flyback, voltages out of specs, bad deflection yoke, faulty resistor, diode, transistor somewhere...

Constantly rebooting often means the voltages are out of specs: what are your readings on the +5v, +12v and +12v line ?

Have a look at this:

http://home.earthlink.net/~gamba2/images/plus_analog.PDF

 

[didius]

Okay, resoldered the J4 connections.

The rebooting problem remains, but i got something on the screen. A vertical line moving up.

Readings on the lines are crazy. They start out fine at 5.02 V and 12.02 V, but after about 10 seconds they start going up really fast. Untill the reboot. I've measured like 15V on the +12V line.

@nvdeynde: no, i haven't got a spare board...

*edit: here's a movie of the +12V line and the screen:

http://tinyurl.com/ovthkwf

 

[techknight]

the 12v line is going WAY out of regulation. Something is still bad in that analog board.

Your going to have to test every component in the feedback circuit, starting with the voltages on the LM324, plus the 6.2V zener to make sure its not open.

 

[James1095]

I think what is happening here is the power supply voltages are climbing up until it triggers the over-voltage crowbar which shuts down the supply and it repeats. Are there any more questionable solder joints on the analog board? I don't have one of these to look at, but I would look at the feedback loop that regulates the supply. Usually there is a voltage divider formed by a couple of resistors hanging off one of the outputs, usually 5V. The output from that is fed back to the control side through an opto-isolator. Something is breaking that feedback loop and making the regulator think the output is low.

 

[techknight]

that power supply is TL431/optoisolated shunt regulated. any 1 of those parts in that feedback network can cause this. It can also cause the tick-tick-tick with no full startup too. lol.

 

[didius]

I've ordered replacement recitifiers for CR20 and CR21. When they arrive i'll install them and check the complete board again...