Boogie TA-30 Channel 1 Noise.

[caesparza]

Hi,

Recently a client told me that he has a Mesa Boogie TA-30 that has some noise on channel 1. He brought the amp to me, it's completely different from the old Fenders I use to work on, been probable the Prosonic and Blues JR the only modern ones (PCB) that I have worked before. I found the document attached which is a Tech Bulletin from Mesa, here you can find the flow chart and schematic for the amp.

Since the noise is in channel 1, I'm discarding channel 2 components as possible suspects (channel 2 is dead silent), but I'm having a hard time trying to determine the whole path for channel 1 since the amp has some new to me switching options.

In the attached vídeo the noise can be heard, it comes and goes. When you play the guitar loud basically you don't have any problem because it's a background noise, but the owner wants to know if it can be removed.


What I have done so far:

1. Checked the tubes (the all test ok).
2. Swapped tubes (the noise is still present).
3. Chopstcked the amp for loose or colder solder joints (everything seems to be fine).
3. Trace the signal on V1A/B (Which is exclusively on channel 1).
3.1. Signal goes clean after the volume control after V1A/B Parallel Tubes (Page 3).
3.2. Signal goes clean after tone stack for channel 1 (junction of R34-R35) (Page 4).

I would think that those are the paths related with channel 1 but of course I can and probably are wrong. The only part I see on the flow chart is the Reverb, but It works on both channels and in channel 2 there is no noise present with reverb.

Any pointer on how to proceed on this one would be appreciated, this Mesas seem to be a PITA to work on.

[Jon Snell]

Not much to fail there. Anode load resistor, valve, that's about it.

[Chuck H]

Well there's j175a. Is it possible that hash on that switching circuit is being injected into the cathode? caesparza reports having swapped the valves and plate load hash doesn't usually sound choppy and intermittent.

[caesparza]

Hi Chuck, thanks for the answer.

The output at the volume after V1 is clean, so I guess the problem should be located in another place. I'm not familiar with those J175As.

[Chuck H]

The output at the volume after V1 is clean,..

The volume pot immediately follows the coupling cap for CH1/V1. The video clearly shows the noise being affected by the volume pot setting. So I would say the V1 output is not clean OR the volume pot is causing the noise. Which seems unlikely considering the nature of the noise. But maybe not impossible.

[caesparza]

I'll check it again and post back.

I've to go out of town a couple of days but surely will be posting the result.

[caesparza]

Checking the amp again.

I noticed that the channel 1 Top Boost doesn't have the noise, it has some hiss (I guess because it's way louder than the normal).

I can't see the way of getting the noise to appear on the scope at the viper of the volume pot, I don't know what I'm doing something wrong (don't have much experience with this devices, but want to learn).

Yes the noise gets louder with the volume which suggest that it's present before that control. So Chuck your argument is absolutely valid, but I'm kind of clueless here.

[Chuck H]

Ok. I'm just spit balling here, but it might be hash from the switching matrix or the 12V supply for the FET (J175A) riding on top of R77 (6.8k) in the cathode circuit for V1A/B. Though this seems unlikely since if there were a problem with the switching matrix or 12V supply I would expect other problems with other circuits using that supply. And there are plenty.

With the "Top Boost" switched off check the voltage supplied to FET J175A gate to see if it's clean. This is a DC supply and should be filtered and virtually free of AC. So you're looking for AC voltage on that gate. You probably can't see the noise now because your scope is set more for audio range signal levels and frequency. The noise is hashy and of a relatively low voltage so you'll need to increase sensitivity and look for higher frequencies. You can probably find a *outube tutorial on looking for noise with a scope.

Chances are you won't find any noise there... I might just replace J175A and see how it goes.

[caesparza]

Chuck, thanks for the insights,

Measured the AC Voltage at the gate of the J175A (at the 475 ohm resistor terminal) it's 0.04mV (so no AC voltage at all). DC Voltage reads 11.42.

[Chuck H]

Try replacing the FET then.

[g1]

Top boost does not have the noise? Fet is off in this condition. (12V at gate)
So fet is on when noise is present. Fet 'on' shorts across R77.

Tack a jumper across R77. Check for noise with top boost OFF. If noise gone, replace Fet. If noise still there, Fet is not at fault.

(edit: incorrect statement, see post #13)

[Chuck H]

Top boost does not have the noise? Fet is off in this condition. (12V at gate)
So fet is on when noise is present. Fet 'on' shorts across R77.

Tack a jumper across R77. Check for noise with top boost OFF. If noise gone, replace Fet. If noise still there, Fet is not at fault.

I'm not an SS guy so I was a little confused by the FET switching. I had thought that what you describe is what the schematic said, but was unsure considering many Mesa schematics have errors (so much so it seems intentional). With the FET gate grounded (Top Boost on, right?) could there still be a fault in a connection or component in that circuit. C49 or the switching relay (not moniker indicated in the schem), etc.? With no activation of the FET could this influence the closed switch?

But this has to be where the noise is coming from since the volume control affects it. I just hope it's not some other dirt in the switching matrix getting in somehow because that's a real maze.

[g1]

With the FET gate grounded (Top Boost on, right?) .

You are correct, I had it backwards. Noise is with Fet off, not on. (normal mode)

Removing jfet or disconnecting D or S legs should tell if the fet is at fault. If noise remains with fet disconnected I'd suspect R77 or it's connections.

[caesparza]

Ordered the J175 replacements today.

Don't want to try this without spare parts at hand.

Will come back with results.

Thanks!

[caesparza]

Ok I disconnected the leg to the C12 cap (Drain), turned on the amp and the noise was still present.

Then I reattached the leg of the FET to check the resistors and now the noise is gone. It's been working fine for half an hour. I don't know what to conclude form that.