Mac Plus does not start, no bong, no screen

[feltel]

Hello there, I got a Mac Plus of unknown condition recently and I'm attempting to revive it. The logic board does look good after a little deep clean. The analog board is what makes me trouble. I recapped it with Panasonic caps, replaced the Rifa and tried to start it. The moment I turn on the Mac Plus I hear a light crackling sound so I assume the high voltage starts. I see the fillament glow. But nothing else happens. No bong, no screen, nothing. I tried the brightness control, but no change. I dont have another Plus so I cannot check if the logic board is okay with another analog board or the other way around.



On the external floppy port I measured voltages. On the 5V line I get around 4.9V. The 12V line is only at about 3V. -12V is present and okay. Tuning the voltage pot I can adjust the 5V in both directions. On some site I read that U3 comes into play regarding wrong voltages.

 

[feltel]

Regarding recap I have to add that I did not replace the one bipolar cap at C1 cause I did not have such a cap "on stock".

 

[cheesestraws]

On the 5V line I get around 4.9V. The 12V line is only at about 3V. -12V is present and okay

This is really weird because the Plus power supply only actually regulates the 12V line. So the other two being about right suggests that maybe 12V is being regulated correctly, and something unfortunate is happening between the power supply and the logic board. Have you checked the solder on the back of the logic board connector and your AB <-> LB cable?

 

[feltel]

Yes I reflowed nearly all connections on the analog board. I did not check the cable. Will do it. The connector on the logic board side is okay. No cracked joints.

 

[feltel]

The cable harness is fine. It has continuity on all pins. My one has no inbuild fuse as some seems to have. I checked a few components which were related to the 12V generation. CR21 has continuity in both directions. Thats wrong. Also i measure a resistance of about 44 Ohm, also in both directions. I ordered a replacement diode, also the optocoupler at U3. Lets cross fingers that the shorted diode CR21 is the source of the problem with this analog board.

 

[wskjinfen]

Please check the voltage of capacitor C24 to confirm whether the analog board outputs 12V

 

[feltel]

I'm waiting for parts to arrive. As soon they are here I'll report back.

 

[feltel]

Parts arrived today. I replaced CR21 and also the bipolar cap which I did not had at hand first time. After reinstalling the AB I now get a checkerboard pattern. Progress Maybe it did this all the time but reflowing the AB a second time helped get it working. Unfortunately replacing the two parts didnt get the 12V working. Still at 3V. When I connect the internal floppy the voltage drops to as low as 2,4V. I dont get it.

 

[feltel]

Its working. Dont know what exactly I did but now it chimes and after a little while I see a floppy icon. Whoooo.

 

[feltel]

I cant believe it. Probably the fault was a dodgy solder joint. I reflowed around the transformer area and this did the trick. But there is work to do and this is what I'm afraid to do. The picture is shifted towards the right side and towards the bottom. I think I saw some videos on adjusting the screen on compact macs. But I have to re-view them again to not make it worse.

 

[Lord Refa]

Hi feltel, what program are you running on that last pic? Looks quite useful.

 

[feltel]

Thats Snooper 2, a Mac test suite.

 

[Lord Refa]

Thanks!

 

[nathall]

That image is more than just shifted; it is proportionally malformed, also. Looks stretched horizontally and possibly vertically.

 

[cheesestraws]

Probably the fault was a dodgy solder joint.

Those boards are very, very prone to dodgy solder joints, so I can believe it. Still very puzzled by how it manifested but - obviously working now!

Wrt adjustment - the only really hairy adjustment on those CRTs is rotation and you don't need to touch that. So hopefully you'll be able to get it to work by fiddling with the adjustments on the back of the board...

 

[bibilit]

yes just stretched, but nothing serious,

 

[feltel]

I think I'm okay with this screen. Maybe there are some geometry issues, but I dont feel confident fiddeling with the magnets to work out the issues.