Recapped my 128k analog board, now it turns on but the display won't show anything

[Huxley]

Hi all,

I'm at the stage of my self-taught-repair-skills journey where I'm just knowledgable enough to be dangerous. Case in point: I have two 128k Macs - one "standard" and one which was upgraded to "Fat Mac" status, and both had issues:

- The Fat Mac would power on but it would take several minutes before the display would come up, and even then it was extremely sensitive to any movement - even light bumps or movement of the desk would cause the display to flicker wildly or just turn off. I carefully reflowed all the solder joints on the analog board, paying extra attention to the soldering of the flyback and the cable sockets. Now it's working like a champ!

- The Standard 128k was dead as a doornail, no beep at power on, nothing on screen, no floppy sounds, etc.

Feeling confident after getting the Fat Mac running well, the other night I went through and carefully replaced each capacitor on the "Standard" 128k's analog board with new ones purchased in a kit from Console5. I'm not the most experienced or knowledgeable solderer but I have decent tools (including a Hakko desoldering pump) and I followed a really nice step-by-step video on YouTube as I progressed. I was super careful about ensuring that the polarized caps went in with the right orientation, I did my best to ensure that my solder was neat and not shorting anything. etc.

Unfortunately, when I finished and tested, it was only a partial success: now the machine gives a nice loud beep at power-on and I can see the "heater" at the back of the CRT glowing (and the flyback transformer gets warm), but there's never any indication of an image at all onscreen. Bearing in mind that I'm a beginner with this work, I've done the following:

• I've worked the brightness knob - it's definitely set to full brightness
• I've visually inspected my soldering work and redid a couple spots where it didn't look perfectly uniform
• I've carefully reflowed the solder joints on all basically everything, including all the plug connectors on the analog board
• Double-checked the polarity of the caps I installed
• Carefully re-seated the "suction cup" into the CRT glass

Bearing in mind that I'm still learning here, what would you suggest as a good direction to go in here? I'd love to get this machine working again, and I'm closer than before (it's beeping!) but not sure what to do now. I do have a multimeter, assuming it will be useful in troubleshooting...

Thanks for any advice!

Huxley

 

[robin-fo]

Do you get any voltage on the floppy connector?

 

[dv-]

I had similar problems and it turned out to be a short. Grab a continuity tester and check to make sure all the adjacent pins on the stuff you soldered don’t have continuity between them.

Good luck!

 

[JC8080]

Did you try the 128k logic board with your known-good Fat Mac analog board? This doesn't sound like a logic board issue, but this would be an easy step to rule it out.

 

[superjer2000]

Sounds like a video signal issue. Try reflowing the pins plugs on the logic board plug that goes to the analog board. Also worthwhile to try swapping logic to analog board cables with the 512k.

 

[cheesestraws]

Sounds like a video signal issue. Try reflowing the pins plugs on the logic board plug that goes to the analog board. Also worthwhile to try swapping logic to analog board cables with the 512k.

Agree with this. @Huxley, if you have an oscilloscope and feel safe using it in this situation, it's worth checking to see whether you have video signal and sync at the connector where the cable goes off to the LB. If you do, you can follow the signal through the signal path until it all gets too scary with the aid of the 'macintosh analogue board' PDF that's sloshing about. For what little it's worth, I had symptoms a lot like this and it turned out to be the 74 series inverter (? or perhaps a NAND gate being used as an inverter, I'm working off memory and can't remember) near the connector which had failed, which I only found out by tracing the video signal until it disappeared...

 

[Huxley]

I just spent a couple hours re-re-re-checking my work on this machine. My soldering won't win any prizes, but as best I can tell it was all done correctly - no continuity issues that I can find, polarities are correct, etc. I re-reflowed the solder joints on all the cable connectors (including the motherboard), reflowed the solder on the flyback, etc., but sadly there's no change in the behavior - I still get a chime, the display builds some static, but otherwise the machine is dead.

Do you get any voltage on the floppy connector?

Just tested this and found something interesting: the voltage hovers around around 3.9v - 4.1v when testing via the floppy port. However, carefully adjusting the voltage potentiometer seems to have no effect at all - turning it to the limit in either direction doesn't have any impact on the voltage reading. WTF does that mean!?

Agree with this. @Huxley, if you have an oscilloscope and feel safe using it in this situation, it's worth checking to see whether you have video signal and sync at the connector where the cable goes off to the LB. If you do, you can follow the signal through the signal path until it all gets too scary with the aid of the 'macintosh analogue board' PDF that's sloshing about. For what little it's worth, I had symptoms a lot like this and it turned out to be the 74 series inverter (? or perhaps a NAND gate being used as an inverter, I'm working off memory and can't remember) near the connector which had failed, which I only found out by tracing the video signal until it disappeared...

Sadly I don't have an oscilloscope (or the know-how to use one). I'm totally open to more suggestions but I'm right at the limits of my skills / knowledge and I'm at the point where I'm feeling like it won't get fixed without outsourcing the work to someone who knows what they're doing...

 

[Fred1212]

Voltage is too low, should be around 5v, check your two boards in the working 128k. Have a look at the circuit of analogue board and look for faulty components or joints around the voltage adjustment potentiometer.

 

[chrisrueckert]

Sounds like a video signal issue. Try reflowing the pins plugs on the logic board plug that goes to the analog board. Also worthwhile to try swapping logic to analog board cables with the 512k.

My 512k had the same issue and reflowing of these pins helped to fix it.

 

[finkmac]

Sadly I don't have an oscilloscope (or the know-how to use one).

Well, you've got the perfect opportunity to learn here! A scope and small logic analyzer are an integral part of an electronics repair lab.