Mesa Boogie Subway Rocket - Low output

[soundguruman]

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

1. sometimes the coupling caps in the phase inverter short.
You will know this if the output tube has no bias, output tubes turning RED..

2. Resistors laying directly on top of circuit tracks- look for a resistor that has burned to the circuit track below it.

3. Look for burned spots in the fiberglass board. Sometimes the carbon shorts out the preamp audio function.

[mikeboone]

It was! I see that now. THANKS!!!

[soundguruman]

It WAS what???
a few details....throw us a bone?

[soundguruman]

It's too exciting.
I can't stand the mystery...

[mikeboone]

I was referring to a few years back when you directed me to the JFET switch. I had to go look at the old thread and just realized it is WAS you who led me there! Thanks! I got that amp for $100 from a guy who didn't want it because it wasn't working right. I clipped out the JFET and it started working. I bought a few from Mouser and got it replaced.

I traded that amp at Guitar Center for a Marshall 1965a (4x10) cabinet loaded with greenbacks! What a sweet deal. It now sits under my Plexi Clone that I built and sounds amazing. You did me a solid on that one, buddy!

Details! :-)

Didn't mean to drag the post off topic...

[stereomonostereo]

Hey guys, sorry about the delay. I'd been waiting for a schematic from Mesa, which I received yesterday. This is the Reverb model. Taking your suggestions and looking at the schematic, I'm seeing a J175 fet for the channel switching/reverb mute. I'm going to remove that fella and see where we're at. Thanks for all the insight thus far!

[stereomonostereo]

Well, I removed the two J175 fets and I'm still having the same issue. I think I'm going to order replacement relays and see if that does it. I hate to just throw parts at it, but it seems that's where I'm at.

[52 Bill]

I hate to just throw parts at it, but it seems that's where I'm at.

Then don't. Trace the signal, figure out what is working and what isn't, compartmentalize the circuit to limit the search down to find the problem.

No matter how complex the signal path is, it still has an in and an out. And it is made up of a lot of simple small circuits.

[stereomonostereo]

Agreed. Every now and then I need to be reminded to focus on this stuff.

Going over it again, it seems to point to RY2. I'm getting signal at my tone stack, and RY2 taps off of that signal path and then hits the master volumes.

[soundguruman]

It is not unusual for the relays to be bad.

Install sockets - it cuts hours of time, over years and years of service.

The last one I worked on had bad relays.
But there have been cases- of resistors mounted on top of circuit tracks- and the resistor was arcing down to the track.
Shorting out the audio path.

And there have been cases- where the fiberglass board insulation had turned to carbon-and shorted the audio path.

And there have been cases- where the effects send return jacks had bad internal switches- interrupting the audio path...

PLUG an audio signal into the effects return do you get audio from the power amp?

PLUG a patch cord between effects send and effects return. Does the audio come back?

TAP on the relay. Does the audio cut in and out?

There is all kinds of non-technical tests, that can lead you to a solution.

[52 Bill]

Focus on the signal path. You said that the signal was present at the tone stack but was missing at the master volumes. Based on the schematic that you posted, there are three tube stages that are between the tone controls and the master volumes.

The signal has to pass through either V1a or V1b and then through V3a. Is the signal present at the plates of V1a and V1b? How about at the input grid of V3a? If the signal dies between these two points, then you suspect relay RY2.

[stereomonostereo]

Plugging into the return of the effects loop gives me the same quiet output. I replaced RY2, and I'm still getting nothing. Double checked my plates on V1 and I'm getting signal, as well as on the grid of V3. Could it be an issue with the LDR?

[soundguruman]

Do you see signal at pin 7 V4B?
You have a mute relay at the input of the phase inverter...
Check plate resistors and cathode resistors V3B and V4 A/B
Go ahead and write down voltages of each pin on each tube. That may help spot a bug.

[stereomonostereo]

I was checking my voltages again, and stumbled upon a bad trace from a cap coming off the relay to ground. What a nightmare. Thanks for all the help and from keeping me from throwing this one out the window!

[soundguruman]

I was checking my voltages again, and stumbled upon a bad trace from a cap coming off the relay to ground. What a nightmare. Thanks for all the help and from keeping me from throwing this one out the window!

In case you don't throw it out the window, I have an alternative method, which I am quite fond of.