Thread

Mac Plus not starting up

68kMLA Troubleshooting 9 posts Apr 9, 2009 to Apr 10, 2009

#1 Thu, 9 Apr 2009 - 06:21 68kMLA

Hi,

I have been given a Mac Plus 1Mb however it does not power up. I have checked visually the components on the boards, there do not appear to be any blown caps, I have unplugged and re-plugged all connectors. When I turn it on, it makes a "chirping" noise. Am I assuming correctly that this is a power supply related issue. Thanks for any directions to try get this computer up and running again.

cheers,

Wayne

#2 Thu, 9 Apr 2009 - 06:51 68kMLA

This is most likely a problem with you analog board, I've heard that the barrel rectifier can make chirping noises.

Have you ever have it running? If so, did it have problems with the display moving in and out?

#3 Thu, 9 Apr 2009 - 07:16 68kMLA

Thanks Damian, I had actually been given 2 Mac Plus's, both were not working when I got them. However, one of them has obviously been on a workbench many times with poor re-soldering of some parts on the board(don't hold much hope for that one). But this "chirping" machine looks very clean on the inside. I'll check the barrel rectifier.

cheers

#4 Thu, 9 Apr 2009 - 10:06 68kMLA

Damian, I'm not sure what you are referring to by "barrel" rectifier. I have gone through the 1N4007 diodes in the little bridge rectifier but they seem fine. I guess I'm going to be working my way through the board component by component...

cheers,

Wayne

#5 Thu, 9 Apr 2009 - 16:59 68kMLA

Damian, I'm not sure what you are referring to by "barrel" rectifier. I have gone through the 1N4007 diodes in the little bridge rectifier but they seem fine. I guess I'm going to be working my way through the board component by component...cheers,

Wayne

What you want to check is the barrel rectifier at location CR5, this is the number 1 culprit related to chirping. No need to work through everything as 99.9% of these problems have been identified. See this for additional details: http://68kmla.org/files/classicmac2.pdf

#6 Thu, 9 Apr 2009 - 19:03 68kMLA

Hi wayne, I'm pretty sure the location of the "barrel rectifier", is as this image. http://farm4.static.flickr.com/3389/3426387195_d8010b7e4e.jpg

I hope this helps.

#7 Fri, 10 Apr 2009 - 05:45 68kMLA

Thanks for pointing me in the right direction. The rectifier CR5 seems fine, I should replace it but I can't find a supplier in Australia for them. Although it is listed on the schematics as a GI854, the actual diode on my board is a GI824. The soldier points around Q3 (labeled Q2 on my board) seemed tarnished so I removed Q2 ( BU406 ) and it seems to test faulty. As for the second Mac Plus: I can see arcing on the Flyback Transformer ( I guess it is shot!).

...to be continued

#8 Fri, 10 Apr 2009 - 07:29 68kMLA

...well, I swapped out the CR5 and Q2 from the now "donor" MacPlus. It started with a chime :-/ , then disk drive access :?: , then the smell of hot electronics 8-o , and tripped my mains voltage earth leakage switch xx( .

I think I have tow donor Macs now and I'l go on the hunt for a working machine :lol:

#9 Fri, 10 Apr 2009 - 09:56 68kMLA

It is likely that you have a shorted flyback. The horizontal drive transistor is pretty robust, so one might assume that something serious caused its death. If you plan to give it another go, I recommend performing some resistance checks of the flyback windings.