Sunn Alpha Four PA Hum/testing/will it destroy my speakers

[icecub]

I just bought an Sunn Alpha Four PA.  It is rated at 4ohms 100 watts maximum.  It is supposedly not working.  Prior owner said loud hum.  
I'm somewhat electronically challenged with regard to the type of speaker I can safely connect to this PA without causing further damage to the PA or damage to the speaker

1.  I need to test the unit.  If I plug in a speaker, is there a chance the components in the speaker will be fried if there is something wrong with amp?

2.  I understand that the total resistance of the speaker should be at a minimum 4 ohms, but as for wattage, can I use a speaker rated at less than 100 watts?

3.  What happens if you turn a PA on without speakers attached?

4.  Is this a "slave" PA?  If it is, what exactly is that?  Can I plug a mike cord directly into the imput on the Alpha Four?

Any help would be appreciated

[Don T.]

Hello:
 Loud hum can be a few things. I would try to find out before plugging in any speakers. It might have bad filter caps or rectifier or transistor. It's best to look at it with a scope to see what kind of wave form is coming out. There are some backyard test methods but they take a reasonably good knowledge of electronics.

[icecub]

I do not have a scope.  Is there another way to test these components.  I have a multimeter.

[icecub]

I have taken the cover off of the amp.  I looked on the power supply and the filter capacitors look ok(no bulging or leakage.)  The fuse holder has come loose at F3 so I will resolder that before turning it on.

[Don T.]

Hello:
 After you turn it on check the AC and DC voltage at the speaker leads (no speaker in circuit). If there is DC at the output is it at the voltage of the power supply? Is it + or - volts (will indicate a bad output transistor or predrive).
Check the power supply voltage too. AC & DC Are the + and - voltages the same? Is there any AC voltage? A few millivolts of ripple is OK.

That should get you started.

[Anonymous]

Plugged it in and F3 fuse (6 amp Fast Blow)  immediately blows.  Any ideas?

[loudthud]

Probably one or more shorted power transistors. Check all of them with an ohm meter. Check for burned resistors.

[icecub1]

Are the power transistors the six big ones attached to the heatsink?

[Isaac]

Yes. Do you know how to test them?

[Anonymous]

Yes, does anybody know what the values are for the transistors? The schematic is illegible