Peavey Mark III Loud 60hz? Humming

[Morgo1984]

Hey all, new here, first post etc.. Btw google says this is a "harmful site" lol.
Anyway, working on a friends bass amp. When you power it up it just makes a loud buzzing noise which I'm assuming is power leakage to the mains. Does not blow a fuse, oddly enough.
After a little carousing through the board I found a transistor that seems to be closed circuit on all 3 terminals. I will venture to guess its the culprit.
Its a SAC187. Even at risk of sounding dumb, I tested in on the board soldiered in. Dont know if thats gonna make a difference or not.
And also, dont know how that one failing could make this much noise!
Thanks in advance fellas!

[Enzo]

Which power amp is on the rear panel, 400B/G? 400BH? SOmething else? I don;t usually see the SAC187 triac in MarkIII heads.

Disconect any speaker, power off, measure resistance across the speaker output jack on the rear. Does it seem to be shorting hot to ground?

Power on, no speaker. Is there substantial voltage across that jack? If the amp is all together, just plug a cord into the output jack, and measure voltage at the free end of the cord. No need to disassemble for this.

When you get the loud hum, l;ook at the speaker cone. Does it move one way and stay ther? Or does it sit mor or less at rest position?

If you try to play through it, can you still hear the music through the hum, or is the hum all you get?

LOUD hum is usually one of two things. Most common is a blown output putting DC on the speaker. If not that, then next is a loss of a filter cap. FIltercaps can crack free of their solder, which is easily repaired, or the cap can fail, and we rplace it.

[Morgo1984]

Enzo: Thank you for the lightning fast reply
Its a series 602 bass head. Is this the info you were asking about? I currently have it disassembled, what a P.I.T.A. it was getting the million screws out.
Anyway, yeah, cant hear ANY music when I turn it on. Shut it off quick before it blew a speaker. Before I took it apart, I ran a cord from the preamp out to a known working amp. Sounded fine.
Yeah there is a single SAC187 right next to where the power from the transformer comes in.

[Enzo]

Yes. That sounds more like a MArk VIII rather than a Mark III. Nonetheless, 602 it is.

The SAC187 is a triac used as a "crowbar." It guards the output, and shorts across the output when it detects DC. It usually gives its life in the process. The part is purely a protection item, the amp will fuction without it, like your car will work without its seatbelts. You can remove it during testing, just replace it before you finish the amp.

You do not need a speaker conected, I recommend disconnecting any speaker load until you have the amp sorted out. SO without the triac, is there DC on the speaker leads?

Verify the power supplies. The main rails are called out as +/-68v. The actual voltage is not very important, but both supplies ought to be up to about the same voltage as each other, and free of ripple. The +/-16v rails coming from the fused low voltage supplies are for the preamp, which is working. The ICs on the power amp have their own 15v zener supplies, so just check on the ICs on the power amp board for good clean 15v supplies.