Mesa Rectoverb 50 Series 2

[lowell]

Doin' a mod on this thing. Working on the nfb for the different modes on the LEAD channel. This drawing MUST be wrong. It doesn't seem to make any sense. If you look on the page titled Preamp Part 2. When the Raw/Vintage mode is ON, if you trace it, the nfb doesn't go anywhere but to ground. It goes through the pot, and the 33k, but eventually it has nowhere to go but to ground. And the .003 cap, bottom end, gets grounded too. Trying to make sense of this and see if I'm missing something, or if someone knows the CORRECT drawing.

http://bmamps.com/Schematics/Mesa_Bo...0_Series_2.pdf

[mozwell]

Hi Lowell
Lets look at VIN mode.
FB in & to the wiper of the 100k, 33k in parallel with the 100k pot, & top of the 100k to 0V, this also connects the 0.003 to 0V

in MOD mode, FB is not connected at all, the bottom of the 0.003 now goes through the 100k (connected as a variable resistor) to 0V

That sort of all makes sense

in VIN mode, FB is connected & we have 33k & 0.003 to 0V after the tone stack
in MOD mode, FB is not connected & we have 33k & 0.003 & 100k pot to 0V after the tone stack

Looking at the POWER AMP page of the sch, in CLEAN mode we have 25k presence pot from the bottom of the 0.1 to 0V
In VIN mode we have 100k in parallel with 33k from the bottom the 0.1 to 0V
In MOD mode we have no FB at all

But i cant see where the FB goes elsewhere..... simply having 0.1 - 47k - 0.1 & a presence pot from the 8ohm output to 0V probably wont change much
The block diagram shows the top of the presence pot goes to V5A, but on the POWER AMP sch the grid of V5A goes to 0V through 4k7

I agree something must be wrong in the sch, so you have a small area to look at, trace it out & see where the error is, does the 4k7 on the grid of V5B go to the bottom of the 0.1 directly above the relay contact RY4A on the POWER AMP page ?

What mod are you trying to do ?

[lowell]

Ok thanks for confirming. I'm gonna have to trace it I guess. I'll report back. I'm attempting to reduce some sizzle from the MOD mode. The only way I see to do that without extra relays is to connect the FB via a smallish capacitor when in MOD mode. This requires some rewiring of a few relays as well, but I want to make sure I do it right because it's a major PITA to disassemble and reassemble this board. And doing it right requires knowing exactly how it is wired.

[lowell]

Even the CLEAN channel presence circuit is BUNK!! What gives? Haha. Seriously though it doesn't work as drawn. I found the series one schematic and think I have an idea as to how it's SUPPOSED to be drawn.

http://schematicheaven.net/boogieamp..._rectoverb.pdf

The clean presence circuit needs the RYFB COMMON pin to be drawn/wired to the 4k7/10k junction in the PI bias circuit. Then, with THAT there, the LEAD channel, channel 2, presence circuit works as drawn. It's rather clever actually. They utilize the presence pot in 2 different ways. When in MOD, there's no NFB, and the Presence draws the .003 cap closer to ground, reducing highs. When NOT in MOD, the TOP of the presence pot is grounded, and the other end disconnected from ground. The NFB is fed in just as in the CLEAN presence circuit.

Please confirm if you follow. Or if I'm mistaken. I'm trying to trace the board itself but my brain hurts from staring at this all day and night.

[g1]

Boogie schematics can be very problematic. Even if you know it is the exact correct one, it may still have errors.
If you are not sure it is the exact one, have a look at schematics for similar models, also for older or newer versions of the model.
Another option is contacting them with model and serial number, and requesting the proper schematic.