Mac 500 wont power on

[dmoorcroft]

Hi all,

 

I have a mac 500 which wont fire up - did all the usual checks - cable, fuse but found these to be fine.

 

Opened up the unit and removed the motherboard to check the fuses and found that one had gone (F602) which was 4A. Replaced with a 3A fuse and then a 5A fuse however when I connect all modules back to the motherboard, the fuse blows instantly. I dont have a circuit diagram to hand unfortunately.

 

Any ideas whats most likely to be causing this? (its annoying as the unit is bought second hand and serviced by one of the big hire companies!)

 

Cheers,

 

Dave

[DrV]

Hi all,

 

I have a mac 500 which wont fire up - did all the usual checks - cable, fuse but found these to be fine.

 

Opened up the unit and removed the motherboard to check the fuses and found that one had gone (F602) which was 4A. Replaced with a 3A fuse and then a 5A fuse however when I connect all modules back to the motherboard, the fuse blows instantly. I dont have a circuit diagram to hand unfortunately.

 

Any ideas whats most likely to be causing this? (its annoying as the unit is bought second hand and serviced by one of the big hire companies!)

 

Cheers,

 

Dave

F602 is in the +12v supply line. It is immediately after the bridge rectifier (D602) and before the reservoir caps (C606 and C607) so any of those 3 might have gone short. Another possibility is C610 which might well be a tant (I haven't got a 500 PCB here to look at) and is probably just next to IC601 which is an LM2596 regulator. this regulator creates the +5V line. I can't remeber what else +12v does apart from supply the +5v regulator so if there's no dead short across C610 then the fault must be on the +5v rail - good luck wth that!

 

Dave

 

E2A +12v drives the fans through a ULN2803. However, the fans have +12V going directly to them so it could well be a fan wire which has got trapped/burned through and shorted to the chassis

[stuart.thompson]

On Martin PCBs its normally the Diode Transil that goes to protect the circuit. Check that hasnt blown and if it has replace it the get the unit working again.

[dmoorcroft]

On Martin PCBs its normally the Diode Transil that goes to protect the circuit. Check that hasnt blown and if it has replace it the get the unit working again.

 

Thanks all for the advice - fortunately it transpires that I didn't replace with a 5a fuse but a 0.5a fuse as they had been put in the wrong container - served me right for thinking it would be a good idea to open up a mover after a long day rigging and lack of sleep!