NOISY mesa boogie NOMAD 55

[peter jeurissen]

Hi you all,

I am having trouble finding the issue on this mesa nomad 55 amp.
The amp functions all well, but it makes a lot of noise (or hiss) on all 3 channels.
The power tubes have recently been replaced and are OK.
The first 12AX7 is a bit microphonic so I changed that one without result.
There's also quiet a bit of hum which increases turning up the volume.

With all the controls on 5 its really very noisy without any imput.
The ground on the imput jack functions well.

When I pull the V4 or the V5 the amp is quiet so the power stage must be OK.....?
On the PCB there are 2 large filter capacitors (220uF/300V) which date 0024 so they are 15 years old.
This might be the cause of the hum but can it also cause the noise?
The PCB is dated 1999.
I've checked the capacitors by measuring the voltage directly after switching off and within 20 seconds the voltage drops from 300 to zero.

I hope any of you have any experience on this amp and had the same problem.

I also cleaned all the pots and contacts with no result.......

Any help or suggestions are welcome

I will attach the schematic below.....

Thanks in advance!

Peter

[Pedro Vecino]

It could be that the output capacitor to the send (1uF/50V) is leaky. Usually tantalum but other times is an electrolytic.
If I remember correctly this capacitor is located on the pcb two or three centimeters above the connection wires of vintage/modern third channel selector.

[peter jeurissen]

Thanks I'll check it out

[Jazz P Bass]

Blank file on Post#1.

Here is a repost of a valid file.

boogie_nomad55.zip

[Jazz P Bass]

Quote:"There's also quiet a bit of hum which increases turning up the volume."

Are you refering to the Master volumes or the Gain control of each channel?

I would start by verifying that the power supply filter capacitors do not have excess ripple (which would indicate a failing cap)

[peter jeurissen]

The increase is on ALL controls
gain and master on every separate channel and on the overall end master control.....

I do suspect the caps in the power supply, but can it also be the cause of the (hissy) noise?

I also changed all the pre amp tubes with known good ones> no result

[peter jeurissen]

I changed the output capacitor as Pedro Vechino suggested
Nothing changed......

[Alex R]

is the input jack grounding ok?

[MarkusBass]

When I pull the V4 or the V5 the amp is quiet so the power stage must be OK.....?

Peter,

You need to conduct some more tests in order to deliver more information. Pulling out V5 does not make much sense since this is a part of the power amp. With V4, when pulling it out makes the amp quiet, the next thing that I would check is plugging in an external preamp into the RETURN jack of the FX LOOP. You can also verify V4 by shorting the hot pin of the RETURN jack to the ground. Such test would tell you a little bit more than you know at the moment.
Also, quessing what could be wrong (capacitors in the power supply) is a bad idea. You need to verify this. And you can simply verify it by soldering two new capacitors to components that are connected to the power supply rails. You can do it without removing the old capacitors and without removing the PC board from the amp. This will make your life easier .

Mark

[peter jeurissen]

Yes it's the first thing to check

[drewl]

I have kind of the same problem with a B52 AT100 this week.

Customer complains of noise when the gain if either channel is turned up.

Waiting on some new low noise 12AX7's that should be in today along with crappy construction and parts I may be able to get the noise down a bit.

Have you scoped the dc filament voltage on the first couple preamp tubes?

I'm seeing the four 1N4007 rectifier diodes for the dc heater starting to go bad in older Mesa's.

Look for heat damage on the board under them.

[peter jeurissen]

Hi,

I lifted the PCB (it was a PITA) and all the component on the boards seem OK
I ordered new filtercaps since the old ones are 15 years or more..........
So a recap would be necessary anyway.....

[ThisLifeILead]

My 94 mesa DC-5 had bad 100ohm Filament ground resistors and made my whole amp very noisy. They measured fine too but since they were carbon comp i replaced them and to my suprise it quieted the amp. if its DC filaments check that power supply out.

[jesusm1]

Hi you all,

I am having trouble finding the issue on this mesa nomad 55 amp.
The amp functions all well, but it makes a lot of noise (or hiss) on all 3 channels.
The power tubes have recently been replaced and are OK.
The first 12AX7 is a bit microphonic so I changed that one without result.
There's also quiet a bit of hum which increases turning up the volume.

With all the controls on 5 its really very noisy without any imput.
The ground on the imput jack functions well.

When I pull the V4 or the V5 the amp is quiet so the power stage must be OK.....?
On the PCB there are 2 large filter capacitors (220uF/300V) which date 0024 so they are 15 years old.
This might be the cause of the hum but can it also cause the noise?
The PCB is dated 1999.
I've checked the capacitors by measuring the voltage directly after switching off and within 20 seconds the voltage drops from 300 to zero.

I hope any of you have any experience on this amp and had the same problem.

I also cleaned all the pots and contacts with no result.......

Any help or suggestions are welcome

I will attach the schematic below.....

Thanks in advance!

Peter

hi:

the problem for this case was a a tantalum capacitor in one nomad 55 i was repairing. it is located on pin 3 of V1. the value is 1uF to 50v. replacing it solved the problem

regards