MESA NOMAD Fifty Five

[plexi50]

I removed the chassis from the cab and found a transistor with the two outer leads disconnected and up in the air. Whats this about? The amp is producing ghost notes.

[Dave]

My guess would be that the transistor died and the resistor was put in to make the amp function. If you put it back to stock it might work better.

Dave

[plexi50]

I think it is a mod to make the reverb stronger. These amps are known for having a weak reverb. Also my comment about ghost notes is revised. It sounds more like a chimey vibrato as the notes fade out. They vibrato out. (Thats a new one) I have to replace all the tubes. The original Mesa tubes are in it. Some will read good and others just drop off the scale on my tester. My tester will not test some EH,Mesa tubes. I have a B&K Dyna Quick 600. I have known for years i have been needing a nice 747 or Hickock.

[phsyconoodler]

The PC board is labeled "J175" which is a jfet for channel muting.
    There is a mesa bulletin out about adding a diode to one leg of the jfet to prevent the amp from switching out of the 'tweed' mode.
It's not likely your issue here.It may be for the reverb muting in distortion mode.

[Jack1962]

I know this may be a stupid question , but have you tried reconnecting the transistor?

                                                      Rock On

[plexi50]

No im not going to do any more work until the tubes arrive. But reconnecting the jfet has crossed my mind. I can do it with the tubes i have now and see what happens. Better yet (Leave alone for now)

[Jack1962]

aren't most of the jfet's in the Mesa Nomad fity-five for switching?

                                                  Rock On                               

[phsyconoodler]

No,they are for muting.

[plexi50]

I tried to reconnect the Jfet and one leg fell off. The reverb sounds fine. But the footswitch is not operating the reverb on or off. The reverb stays on.

[phsyconoodler]

The jfet MUTES the reverb in this case.
it is still not your issue,but it is AN issue.
 

[plexi50]

Yes thanks phsyconoodler.

[plexi50]

That Jfet has the number 174 C012AB. Is this a common part i can get local?  All i can find online is using the number 174

[Geezer]

Look for "J174"

http://www.alldatasheet.com/datasheet-pdf/pdf/57185/CALOGIC/J174.html


http://www.mouser.com/Search/Refine.aspx?Keyword=j174

[plexi50]

Look for "J174"

http://www.alldatasheet.com/datasheet-pdf/pdf/57185/CALOGIC/J174.html


http://www.mouser.com/Search/Refine.aspx?Keyword=j174

There is a sub i found that is J176. I ordered that part from Audio Labs Of Georgia along with other parts i needed. It's next to impossible to get everything you need from a single vendor anymore.  Thanks Geezer! Hey Geezer? Im a Geezer to! I feel like one today.

[phsyconoodler]

Take a close look at the pc board .It says J175. That is what Mesa uses.

[plexi50]

Take a close look at the pc board .It says J175. That is what Mesa uses.

I saw that  phsyconoodler on the board but the part that is in it now is a J174 and it looks to be original. Though i suppose they may have put whatever they had at hand at the time. I could be wrong. The J175 is a sub of the J174 which i only found on one site that as well had all the other parts i needed for other personal projects.  Do you think i should specifically use the J175?

[phsyconoodler]

I don't know for sure if there is any difference.I find the 175's to be rather unreliable.All the circuitry in Mesa amps is rather over-engineered.IMHO.

[plexi50]

How was this j175 originally hooked up? Do the hanging leads go to each side of the the 3.3K resistor?

 

[Shrapnel]

r3mov3 r3sistor, ins3rt l3gs of jf3t.

r3sistor is a substitut3 for holding down on3 l3g of th3 jf3t.

in the style of numb3rs. One of the worst shows ever.
I'm sorry it is "numbers", with an "E".
Here is the corrected version.

remove resistor, insert legs of jfet.

resistor is a substitute for holding down one leg of the jfet.

Whew!!!

Thought you went l337 on us for a sec.

[plexi50]

It looks sorta russian to! A leg fell off the jfet i had. The new one. Have to get another today. Man /

[plexi50]

I just got a bunch of J175 jfets. I removed the resistor and installed the j175 in the same direction that the old one was installed. The footswitch works now and brings the reverb on and off. BUT! the reverb is crackling breaking up and sounds terrible. Could i have the new 175 installed backwards? The crackling breakup is on all channels. Channel 1 is the worst

[VMS]

Did you check the schematic:

http://www.schematicheaven.com/boogieamps/boogie_nomad55.pdf

I'm just guessing here, but it says 3M3 R10 on the PCB so I think there should be a resistor there. And the jfet legs probably go to those empty holes near the middle leg.

[plexi50]

Thats what i was thinking this morning. The resistor stays and the 175 goes to the 3 inline j175 hole on the board

I will try it later today.

[plexi50]

It works and is finished. It was a 3.3Meg resistor on the board in it's place along with the j175 to the three holes next to each other. The schematic says 3.3K but the board says 3M3. Thanks for all you guyzizz help again. I could not do it without you!

[phsyconoodler]

Are we to assume that the ghost notes are gone too?

[plexi50]

Are we to assume that the ghost notes are gone too?

After replacing all the tubes in the amp i am not hearing the ghost notes any longer. I suspect it was a microphonic or internally bad preamp tube that was causing that problem. I have seen a bad PS capacitor cause ghost notes before