Mesa Boogie Rocket 4Forty

[lowell]

Amp came in with a blown PT. Replaced it, amp works, but tubes are redplating and bias is -2v. Pulled power tubes, still -2v. Checked continuity of supply and bias diode. All good. Replaced the bias filter cap with no change.

This amp has something odd going on with the schematic, typical for boogie and likely to be incorrect. The phase inverter cathode connects to the bias supply via a 15k resistor. I see NO CATHODE RESISTOR in the schematic. It just say E on the cathode, and E is also the end of that 15k from the bias supply. Very weird. I get theoretically why they do it, for more PI headroom, but still just...different.

When I have the PI tube installed, the bias is -2v. If I pull it the bias very slowly over a couple minutes creeps up to about -8v at the EL84 grids. Still not right.

I've now replaced the diode, and lifted the anode and clipped in a new filter cap. The only load on the supply is the filter cap... Still super low at -8v. The schematic says it should be -160 at that point.

I'm really losing my patience with Mesa Boogie. The last 3 I've had in have been an absolute nightmare to repair.


Please let me know if anyone needs the schematic. It's unavailable online.

[lowell]

Here's another thread with the same issue, but definitely not the same diagnosis. Part of the schematics are in the thread as well.

http://music-electronics-forum.com/t16080/

The PI tube will only lower the bias supply further, so it's not related to that tube.

[The Dude]

Have you checked the level of the AC feeding the bias rectifier? If so, what is it?

[lowell]

I meant to post that. 358vac on the cathode of the bias diode.

Dont think this matters but the chassis is not circuit ground...even though the schematic shows it as such. I've taken all readings from the main filter cap negative end. All secondary grounds are indeed connected.

[lowell]

Welp, getting 490vDC on the filter cap. Very...weird. Especially being that the bias is low. Now I installed the PI and the bias is back down to -2v.

[The Dude]

I meant to post that. 358vac on the cathode of the bias diode.

Dont think this matters but the chassis is not circuit ground...even though the schematic shows it as such. I've taken all readings from the main filter cap negative end. All secondary grounds are indeed connected.

I've no idea without looking at a schematic, but it seems that could be your problem- something not grounded or floating.

[lowell]

These are in the thread I linked to.

http://music-electronics-forum.com/a...d-picture.jpeg

http://music-electronics-forum.com/a...picture-1.jpeg

[Enzo]

Of course we need a schematic. Don't worry about the schematic being "wrong". Not understanding something doesn't make it wrong.

If you are supposed to have -160v at some point, and you only have a few volts, then find out why. If 160 is only 8, then nothing after that can be right either. Is the diode screwed? Is the series resistor OK? Is the filter cap OK? Does it have a solid path to ground?

[lowell]

Yes to all 3 questions. That's what's driving me nuts. Resistor is 120k. Replaced the filter cap. Yes the cap is grounded. Replaced the diode ...lifted the diode and used an out-of-circuit cap...still only -8v. But again, the ac on the cathode is 358vAC!

[g1]

How about lifting the coupling caps from the PI.

[lowell]

Ooh good call. Doesnt change the oddly way way high B+ voltage issue but lets see if there's extra DC on those grids.

[Enzo]

So everything seems to be in place. SO disconnect the resistors to the bias and the other branch, leaving only the diode connected to the cap. Still get 8v? Or does it go up to something closer to 160?

[lowell]

I think I probably posted some posts while you were already further down on the thread. But yes I gone so far as to lift the anode of the diode and clip a cap onto the end of it...and ground the neg terminal. STILL -8v!

My latest update is that I put an 82k in place of the 120k and the bias goes to -13v. HOWEVER, when a brand new PI is put in....-2v.

[lowell]

I still need to try diconnecting those PI input coupling caps

[Justin Thomas]

Hell, it looks like the thing has a 70VAC tap on the PT... why not just use that (that is addressed to Messa Booger, not Lowell)? Unless of course it's just more added bits to a schematic that's intentionally wrong...

Justin