Mesa Subway Rocket Trouble

[kramps]

I have a Subway Rocket of the bench that has, to put it lightly, issues when switched to the lead channel. The amp operates fine and quietly when switched to the rhythm channel but crackles and even goes into feedback when switched to the lead channel. It sounds like Linda Blair has taken possession. The amp can be switched by a front panel toggle or by footswitch and has no issues switching between channels but the whole board becomes microphonic when in lead mode. I've gone through it from front to back even though it appears only to be in the V2a portion of the circuit. I'm assuming the switching relays are fine because it switches fine but could it be in the vactrols that are part of the circuit as well?

Any help or advice would be greatly appreciated.

I seem to be unable to post the portions of the affected schematic so I'm including a link to the whole thing.

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

Much thanks in advance, KR Amps

[nosaj.seveer]

Have you swapped tubes? Your block diagram is not really a schematic, its like someone giving you a map of florida when you want directions to orlando.


jason

[nosaj]

Here are some schematics
Schematics: Mesa Boogie: Index : Updated July 11, 2011

Have you swapped tubes? Your block diagram is not really a schematic, its like someone giving you a map of florida when you want directions to orlando.


jason

[kramps]

Jason, the block diagram is just page one of 6. I believe page two shows the portion of the circuit that involves V2a. I have swapped tube, checked e caps, coupling caps, resistors, the power supply. With all the tubes pulled and no input the board is still microphonic and so loud I have to put it on a load box to attenuate the volume.

[nosaj]

Is it pcb? and if so have you reflowed the joints? Does a chopstick affect the signal at all in either channel make it worse or better?

Something I learned the other day was grounding your inputs to local the problem to which tube area the problem lies in. Like a 12ax7 grounding pin1 or 7.

Another great resource is some of the old radio forums thos guys work on anything and helped me through a prosonic issue.

jason

[nosaj]

Check this out and tell me what you think.

The Boogie Board • View topic - Subway Rocket Feedback Problems
Quote:
Looking at the schematic, there is no grid stopper resistor on the first gain stage (which is actually V2A for the lead channel), as there is (and should be) on almost all tube amps. This will make the amp unstable at high frequencies.

Turning down the guitar volume control just slightly effectively adds a grid stopper (the upper part of the guitar volume control track, in series between the pickup and the output jack), which is probably why it stops. Even though technically the 'resistor' is in the wrong place at the opposite end of the guitar cord, it will still change the circuit in a similar way.

The easiest way to fix it permanently is to add the resistor between the input jack on the amp and the circuit board, by unsoldering the wire from the board and putting the resistor in between, with some heatshrink over them to support and insulate. The standard value for guitar amps is 33Kohms (usually two 68Kohm resistors in parallel, on amps with two inputs).

I've seen these fitted as stock on several Mesa amps, so either they were routinely added to fix this problem, or it was a design change not shown on the schematics. If your amp does have one already, try increasing the value (eg from 33K to 68K).


This is a different problem to what I expected when I read the thread title . The one I had would feed back and 'motorboat' at a very low frequency when the Bass control was up all the way - actually just like the amp had tremolo! It was quite cool in some ways but could be annoying, and in the end I fixed it by adding a series resistor to the bass control in the same way.

[Enzo]

Is this amp made like recent Mesa amps with the smallish 16mm pots on the panel, with three wires each leading back to the circuit board?

If so, look closely at each control. Has the rear cover come loose on any? Grab with your fingers and see if it moves. If so, bend the tabs back down to tighten up the covers.

What happenes is that if something hits the knob out front straight on, it pushes out the rear of the pot. The rear cover is what holds the wiper assembly in contact with the resistive strip. If the cover gets pushed out, then the wiper can lose contact, leaving various circuit elements unterminated.

[AtomicMassUnit]

Something I learned the other day was grounding your inputs to local the problem to which tube area the problem lies in. Like a 12ax7 grounding pin1 or 7.

I think you mean 2 or 7.

[nosaj]

Yep your right.

I think you mean 2 or 7.

[Enzo]

yeah, grounding pin 1 would not be the best way to go.

However, if you want to ground points in the signal path to isolate the problem area, you cannopt just ground the B+ laden plate pin. However, if we are looking for the source of a noise, all we really need to do is ground off the noise SIGNAL. SO when there is no DC, like on a grid, ground away. When there is B+ present, insert a largish cap in series with your grounding probe. I usually grab a 0.047uf from the pile nearby. If grounding a point kills the noise, you know the noise is from an earlier stage. If grounding through a cap either kills the noise or substantially muffles it, you know the same thing.

And when I was a kid learning electronics in the 1950s, a very common piece of test gear was the "signal tracer", every shop had one. Now they have become rare. And a signal tracer is just the thing to follow the signal path. Back then we built an entire piece of gear as a signal tracer, even Eico and Heathkit and all the rest made signal tracers - even more than one model. But we tend to have extra am[plifiers sitting around, so all we really need do is create a signal tracer probe to plug into an amp input. Google "signal tracer."

[Jazz P Bass]

[QUOTE=Enzo;226878]yeah, grounding pin 1 would not be the best way to go.

However, if you want to ground points in the signal path to isolate the problem area, you cannopt just ground the B+ laden plate pin. However, if we are looking for the source of a noise, all we really need to do is ground off the noise SIGNAL. SO when there is no DC, like on a grid, ground away. When there is B+ present, insert a largish cap in series with your grounding probe. I usually grab a 0.047uf from the pile nearby. If grounding a point kills the noise, you know the noise is from an earlier stage. If grounding through a cap either kills the noise or substantially muffles it, you know the same thing.QUOTE]
Nice tech tip.
Thanks Enzo.

[nosaj]

I just took a pair of powered computer speakers, a 1/4 jack with 2 wires (ground with a gator clip, hot lead with a .01uf cap soldered into lead for the probe. Put tones on my mp3 player and put it into the input and follow the signal.

Got that from RG Keen on geofex. While we're at it got any other simple tools that can be made.


Thanks,
Jason

[Enzo]

Just reading all the stuff at Geofex will provide a lot of great tips. That very simple transformer tester is great.

[kramps]

I pulled the amp off the bench to get some other work done but will be back at it Monday. Thanks for all of the good advice and Enzo, I just picked up a signal tracer but I have just one big question that I hope someone can cover. When the amp is switched into the lead mode the whole board becomes screamingly microphonic. If I could find the cause of that I think the rest would be solved.

Thanks again for any help.

KR Amps

[Chuck H]

What tubes did you swap? It could be a microphonic preamp tube. And as Enzo mentioned, if a pot lead has come off it's trace the amp may think that pot is on ten all the time. FWIW even a little microphony will make the whole amp seem microphonic even if the amp doesn't squeal. It's just the nature of high gain preamps. Especially those with board mounted tube sockets.

I had a Subway Blues for a time and I remember the tube sockets being prone to losing pin contact. Maybe from oxidation due to bad plating or poor spring tension in the pin slots. Cleaning and re tensioning the sockets fixed it.