Mesa Rectifier Recording Preamp acting up.

[hollisdevillo]

Hi all,
The amp is cutting out power (volume) intermittently, especially on attacks, and has very little clean and much distortion. The "record out" is worse than the "line out." The lead channel is much quieter than the rhythm channel. All the voltages on the tubes seem relatively ok, however. the plate and grid of the two last stages of the second (lead) channel, are approx. 188vdc, 20v lower than the schematic. they are hooked directly together. this is the furthest out. I replaced a j175 transistor off the grid of the 3rd stage of the first channel that was the initial problem of squealing, I believe, but since then, these new problems have arisen. I'm at a stand still at the moment. Any help would be much appreciated and thanks in advance.

-h

[hollisdevillo]

it must be before the last two tubes that feed the line out and record out circuit. I suppose its more likely a transistor. although i suspect anything. anyone have any thoughts?

[a432a432]

mesa recto recording preamp

i am having some intermittent problems with my rectifier recording preamp. I have taken it to many amp techs, and they cannot seem to figure it out. I saw that you mentioned that you had the schematic. i was wondering if i could get a copy to try and diagnose this problem myself.

it must be before the last two tubes that feed the line out and record out circuit. I suppose its more likely a transistor. although i suspect anything. anyone have any thoughts?

[soundguruman]

Look at the circuit board.
You see resistors laying on top of circuit tracks?
Check to see that a resistor has not burned down to a circuit track (arcing between the resistor and the track it is sitting on top of)
lift the resistors, so that there is a space between the resistor and the board. Look under the resistors. See any burned spots?
Inspect very closely with a magnifier. See any dark spots in the fiberglass?
Arcing is most common between the plate track and the heater track, OR between the plate track and the grid track.
Once the arcing occurs, it will form a carbon thread in the fiberglass. The carbon MUST be entirely removed, it is acting like an extra resistor in the circuit.
When these types of symptoms arise, it is frequently a burned carbon spot on the board, and most techs fail to recognize the problem.
Sometimes the carbon spot is so small, that the defect track can be found with a ohm meter. Remove the tube, and other components, and check for resistance between the plate and the grid tracks, or between the plate and the filament tracks. The carbon burn in the board may measure about 2 megaohms, between tracks in the fiberglass.
Sometimes it's so small that it can only be detected with an ohm meter, measuring between the circuit tracks.

Well OK, if you give up, I can fix it for you. I have fixed lots of those problems in Boogies.

[Dave Curtis, dB AudioTech]

"When these types of symptoms arise, it is frequently a burned carbon spot on the board, and most techs fail to recognize the problem."

Sheesh,
No, it's not "frequently" a burned spot. Much more common than a charred PCB is "The Dreaded Switching Jack Problem".

How did you get to "burnt circuit board" from his statement: "i am having some intermittent problems" ? ?

You seem to be assuming that a432a432 is a tech; he may not even know what a resistor is.


I'm sure he's going to pack his amp up and ship it from NY to California so you can find out there's no burn spot on the board, and all it needed was a good jack cleaning.