Peavey Mark III / 400BH Died Last Night

[hypnostuff]

We were using the MarkIII with the 1820 cab... It was sounding great and loud, no fuzzy sounds or clicks or pops. Everything was working great and then just *nothing*. The power light went out and there was no smoke or frying smell (I've been through many amps, and would recognize something smoking)

So we waited a day for it to cool down, and still no power up.

Checked the main fuse, and it got a yes for continuity (0 resistance).

Pulled the back panel, and checked the two internal fuses, both good visible and on the meter.

Then (this is where my ability wanes) while the amp was unplugged I placed my meter leads on both of the primary wires for the Power Transformer and it was open (no current passing, a 1 on the ohm meter set to the 200ohm range).

Is this a bad PT, or could it be something ::ahem:: Less expensive!

Thanks,

[Chuck H]

Originally Posted by hypnostuff

I placed my meter leads on both of the primary wires for the Power Transformer and it was open (no current passing, a 1 on the ohm meter set to the 200ohm range).

Is that to say you got a reading of 1 ohm? That would be shorted, not open. But in this event I would have expected some fireworks and a blown fuse.

Chuck

[hypnostuff]

Thanks for the confirmation. There should be no resistance in the PT, therefore it's bad. Crap. Hopefully that's all that's wrong. But it is weird that there was no smoke or anything.

[Chuck H]

I'm not sure I confirmed anything... I was just saying that a reading of 1 ohm is could be a short and a reading of infinity would be an open winding. And though I can't say that your PT shouldn't have a primary reading of 1 ohm, that would seem very low to me.

If the wind were shorted there should have been smoke and blown fuses. If it were open the amp simply wouldn't turn on.

Chuck

[Enzo]

A low resistance reading is expected on the power transformer primary. If it were shorted fuses would blow, as CHuck points out.

DO this. Unplug the amp from the wall and hold the plug in your hand. Make sure the fuse is OK, which you did. Turn the amp power switch ON. Now read resistance betweent he two main prongs of the power plug. You should get the same reading as when measuring right across the power transformer primary. If you get open, then something is open in the mains wiring. That could be a wire off internally, a bad fuse holder, a bad power switch, a bad powr cord.

If that seems OK, plug the amp back in and measure AC volts at the wires to the transformer primary. If there is no 120v there, nothing else will happen.

If you connect 120v to the primary, then there should be other voltages coming out the secondaries.

Proceed from there.

[hypnostuff]

Thanks for the procedure. I took some time to try and read some more books and websites before I proceeded. I followed the procedure Enzo gave and I'm still looking at the same thing across the prongs and across the primary leads from the transformer. on my DMM I'm getting a 1 for infinity (sorry I didn't specify that before!). I've checked the switch with the meter and it's okay. I'm checking the grounding caps now, [ITW .01m 125vac DVL - Metal Film] and in circuit the cap across the switch and the cap from the switch to the speaker out ground jack are both reading infinity. Does that mean that the caps are bad, or would that just be normal for a DMM ohm reading?

I don't think that the caps look blown, and I'm not certain that these caps would cause absolutely no AC to get to the PT.

I did take a reading from the primary leads to the secondary leads (checked all 6 secondaries against both primaries) and am getting all infinity readings with no fluctuation. That's very strange that the transformer would die while it was in operation under max load, and no fuses would be burnt.

I'm going to try to replace the PT, it's part number 705 18715, do either of you have immediate knowledge if there is a suitable replacement part?

I truly appreciate the help that you guys have provided.

Thanks,
Steve

[Enzo]

The two primary wires (blue and black if I recall) disconnect from the restof the wiring by a Molex connector. Pull apart the connector. Measure resistance through the primary winding. if it is open, then the transformer is open.

If you need a transformer, CALL the parts deparment at Peavey on the phone. tell them you need a power transformer for the 400BH. They open 8AM central, and I recommend calling first thing in the morning. By lunch the phones get very busy. If they no longer have that part number, they should have one that will work.