Macintosh plus anode cap => dead screen?

[didius]

Okay, today i've replaced the rectifiers (because I've read in the classic mac repair document that those are weak spots)

However it didn't solve my problem. I started checking the feedback control circuit. I desoldered CR19 (the 6.2V zener diode)

I connected it to a 9.5V battery and included a 2.2kOhm resistor in series. I've checked the voltage over the diode, and the readout was 0.62V.

Is this the right diode? I don't think this is a good readout? The partnumber is IN4001GI (internet research shows this is a 50V diode?)

(I've encountered an error in the mac repair document, in the text of the feedback control circuit they talk about the R56 potentiometer, but it should be the R59 pot, at least on my mac plus)

 

[didius]

I took away about every part in the feedback control circuit, and tested them out.

However, my findings are different

Resistors:
R34: OKAY, reading of 1kOhm
R35: OKAY, reading of 1kOhm
R36: i have a reading of 12Ohm, schematics talk about 470? (resistor is also very different, more like disk shaped)
R37: instead of the 10kOhm i have a resistor of 390 Ohm +-5% (reading is ok)
R38: instead of the 1.2kOhm i have a resistor of 10k Ohm (reading is 9.8kOhm)
R56: on my mac plus this should be R59?

Capacitors
C27 and C28 have been replaced in the recapping proces
C21: instead of 0.1uF mine reads 103k => 0.01uF
(there is a 0.1uF in the area but that's C22

I've tested the zener, and there i get a reading of 0.62V (part number IN4001GI)

 

[techknight]

1N4001 is a general purpose rectifier diode. No zener properties at all.

Did the diode look like its been replaced before? i.e. have rickity solder joints from a pull-out job?

 

[nvdeynde]

I've encountered an error in the mac repair document, in the text of the feedback control circuit they talk about the R56 potentiometer, but it should be the R59 pot, at least on my mac plus)

The repair document is probably right as it's made for USA versions of the Analog board working on 110 volts.

We in Europe have the International version at 240 volts that can work at 110 volts as well. That's why the numbers of the resistors and other components don't match on the schematics but also the values are different. So don't use the values try to repair your board.

In the American version it's indeed common for CR20 and CR21 fail, however I haven't seen one that goes bad on an International 240 volts board. So upgrading them to higher specs is also not needed in my opinion.

Go through the whole schematics of the board and test every component. In Larry Pina's book is a diagram with all the voltages you can check on over 10 spots. Have you done this test ?

Otherwise it's probably best to cut you losses and give up the board.

 

[didius]

Hi nvdeynde, I haven't heard of Larry Pina's book?

 

[James1095]

Don't just throw parts at it. Test the parts and only replace them if they are bad, or if they are something you are unable to test and you have tried everything else. Each time you replace a part, you create the opportunity to error. Wrong part installed, part installed the wrong way around, part that needs to be insulated from a heatsink installed without insulation, part that uses the heatsink as a connection installed with insulation, solder bridge created, damaged traces. These are all things I've seen when cleaning up after previous repair attempts, and in most of those cases I spent longer correcting errors that were introduced by the previous repair attempt than it took to fix the original problem.

Even if you want to replace all the electrolytic capacitors which is not a bad idea, get it working first, and then go ahead and replace old but functional parts as preventative maintenance.

Is there a full schematic of the analog board online somewhere? I sold my last pre-SE compact a while back so I don't have a board to look at.

 

[didius]

New weekend, new hope

I've started comparing the board with that of my working mac 128.

(the 128)



(the plus)



I'm measuring the values inline. I have already replaced the caps on the macintosh plus board, I guess some variations in readings would be possible, but they can't be excessive?

Could anyone mark the feedback control circuit? Since the schematics in the 'classic mac repair'-document only seem to cover non-240V models...

Also, does anyone know the values of those small blue axial capacitors? (like C19,C20, C23, C26, C46)

 

[nvdeynde]

These are both boards from a 128K/512K, not an original Plus board.

I see you have not replaced the brown film capacitors in the high voltage circuit, the filter caps and the axials.

C5, C9 and C18 are not original and have been replaced earlier. Are these values right ?

As for variations for inline measurements when you compare with your other board: difference should be less than 15%, otherwise something isn't right.

Do an ESR test on all your capacitors as even new one's are occasionally faulty, eventhough very rare but I have seen it.