Mesa Boogie Subway Rocket - Low output

[stereomonostereo]

Hey guys,

I'm working on a Mesa Boogie Subway Rocket with low output. All of the tubes are good, and my tube voltages seem correct too. Running a signal tracer, I'm not getting anything at my Master Volume for either channel, and a weak signal to the grid of the P.I. (Pin 2). Looking at the schematic I've found, it shows a Presence control before the grid of the Phase Inverter. I'm not seeing a Presence control anywhere on the amp itself. So, I'm stuck as to where I'm losing my signal. (Also getting nothing at the Effects Loop).

Thanks in advance!

http://www.webphix.com/schematic%20h...bwayrocket.pdf

[guitician]

Have you checked the switching circuits for functionality? Do you have anything at the recording output?

[stereomonostereo]

The channel switching works. I don't know for sure that the Contour is working

[guitician]

The signal goes through the contour, so that may be where the problem is. I'd check the output on V3a on either side of that 680K resistor.

[stereomonostereo]

I'm getting signal through the tone stack and all of the pots are working

[guitician]

From looking at this schematic it really doesn't switch channels, it adds a gain stage and sends the signal through a stage with a "Lead" tone structure, instead of the normal stage. If one of the optoisolators has failed, it may still switch the relays giving the appearance of functionality. I would suggest tracing an input signal along the whole signal path until you get to the failed component.

[Chuck H]

The amp you have may be the Subway Rocket (reverb). These were simply named "Subway Rocket" on the face plate but there is a reverb control where the presence control would be on the non reverb model. If this amp has reverb that would be another circuit you may like to have a schematic for in your quest. I couldn't find one on line and the one at Schematic Heaven isn't for the reverb model (even though it says it is). Mesa is pretty good about helping with these things. Email them if this amp is the reverb model and they'll probably get a schematic to you that's accurate.

[Mick Bailey]

Mesa is pretty good about helping with these things. Email them if this amp is the reverb model and they'll probably get a schematic to you that's accurate.

I've found them to be really helpful too. Sometimes it can take a little while to get a response (they must get lots of requests) but I've never been disappointed.

[soundguruman]

Yup.
1. Bad relay contacts. I often install sockets, which make it much easier to pull and test mechanical relays.
2. Bad FETs in mute circuit. The mute is stuck ON. remove FETs and test again. The effects loop mute FETs can go bad.
3. Bad LDRs.
4. Bad jacks, with corrode oxidized internal switch contacts. The internal switch MUST close, it's not an option.

These amps have several points of failure because off all the switching and mute circuits.
You have to cover ALL of it.
It can be quite time consuming, about 5 times the labor, compared to other conventional amps.

That's why sockets for relays, and ICs, can be extreamly helpful, in reducing the maintenance time required.

[Chuck H]

I agree with all that as it relates to the MB amps. Another good reason to have the right schematic. The OP hasn't been back to tell us if he has the reverb model or not.

[soundguruman]

It takes a certain kind of tech to work on these amps.
Very patient and thorough. Maticulous...

Most techs can't stand the pain.

[Chuck H]

It takes a certain kind of tech to work on these amps.
Very patient and thorough. Maticulous...

Most techs can't stand the pain.

To be accurate, MB amps are repaired by common repair techs every day. They just don't like it the way you do.

[mikeboone]

Find the JFET in the channel switching that mutes the circuit, cut that bad boy out (will work without it, it will just pop a little when you switch). I fought a Rectum Fryer for quite a while before I stumbled across another thread besides mine that suggested this idea. Worked like a charm. Mouser has the part.

[soundguruman]

Find the JFET in the channel switching that mutes the circuit, cut that bad boy out (will work without it, it will just pop a little when you switch). I fought a Rectum Fryer for quite a while before I stumbled across another thread besides mine that suggested this idea. Worked like a charm. Mouser has the part.

Yes that was my idea. Thank me later.

[soundguruman]

To be accurate, MB amps are repaired by common repair techs every day. They just don't like it the way you do.

Yes, they repair them, but they usually don't keep going very long...
even when repaired at the factory.
Oh yeah, I'm compulsive obsessive about long term reliability. Just kill me now.