Mesa Boogie FUNTIME Happy hour !!!

[Valvehead]

Ok gents....Mesa Maverick time .....

Amp has a low freq hum and a buzz , depending on settings . I took a bunch of notes so here we go . This is going to be a long one.

boogie_maverick.pdf

In Rhythm mode = Output Level on 0 , hum , switch to Lead mode it turns into a Buzz on 0 without the hum
If Out level is turned up the hum re-appears along with a buzz (lead mode)

Pulling the PI tube kills all the noise, both channels .

Bias is perfect , not finding any odd voltages anywhere in the amp (yet)

no DC on v1 or v4 grids

no Dc on input jack

swapped all pre tubes . New el84 sovteks . output tube pcb looks good as do the resistors on it .

Chop-sticked the amp

Hit everything with Freeze spray


It has all the original caps. I did clip on the appropriate cap across each of the HV caps , 1 at a time . There are 5 or 6 30uf , and 2 220uf . No change at all with hum .

There are 2 radials that are not on the scem? a 1000uf/25v and a 150uf/100v. the 1k looks like the coating shrunk down due to being right next to the 10w bias resistor im guessing. I have ordered those 2 caps .

Theres also 2 diodes right in that area that i dont see on the scem. 1 has 63vdc at it and its getting to 130 degrees--is that excessive ?? Not sure where its going..trace is under pcb.

running the laser temp gun around i also see that 1 LDR is getting to 118 degrees. Too hot ? which Ldr? I dont know thanks to Mesa not labeling the pcb . I can try and trace that one if needed .

So , any advice would be great...i understand if you run far away ( Mesa)

[Jon Snell]

Removing V6 tells you the noise/hum is coming from a stage before it when the noise/hum dissapears after removing the valve.
What happens if you remove V3, V2 and V1 in turn.
Narrow down the issue in a logical way otherwise you will just confuse yourself.
I would check the -13v supply for ripple.

[Mick Bailey]

The schematic doesn't show it, but does your amp have the epoxy-embedded mojo module?

[glebert]

Since you get the hum with the channel levels at 0, but goes away with the PI pulled, it seems like the only circuit between those points is the effects loop path. I would probably try to jumper passed the whole effects circuit as a test.

Also, do you know if it is 60 or 120 Hz fundamental?

edit: I guess I misread, you said output at zero and I thought that meant the channel levels, but you probably meant the master. I would still probably jump the effects circuit, or pull tubes as Jon suggested to narrow down the area where the noise is coming in. I'd actually probably pull tubes first.

[Valvehead]

Pulling tubes - Output level on 0 -- Rhythm
v1 volume and hum drops, still there
v2 no change
v3 no ch.
v4 no ch.
v5 no ch.
v6 amp silent.

Lead
v1 no change
v2 less vol / hum
v3 less vol/hum
v4 lower vol /hum
v5 lower vol /hum
v6 amp silent

all tubes back in - lifted the 100k at v3 pin 7 = no change in hum

voltages at Q1 - Q4 are all slightly different from scem

example = Q1
-9.2v
-10.3v
-15.3v

Q4
-9.3v
-14.7v
-15.3v

so where it says -13v on scem i have -15.3v


where exactly should i check for ripple ?

ive got 2 videos of the noise , kinda hard to hear

hum output level 0

https://youtu.be/rMpW7Bv3n5g

hum/buzz output level at 3

https://youtu.be/IL8NTFIBfLQ

[Valvehead]

Here is what I have at the -13v spot above the Q1-4 on scem.

suggestions on scope settings ?

[Helmholtz]

Here is what I have at the -13v spot above the Q1-4 on scem.

suggestions on scope settings ?

Scope readout is nonsense.
Fundamental frequency is 60Hz, assuming that time base setting is actually 5ms/div.
So there's a lot of 60Hz ripple on the LV supply.

Where's the schematic part showing the LV supply and relay drivers?

[g1]

Where's the schematic part showing the LV supply and relay drivers?

more complete schematic package attached

[Valvehead]

Thanks G1 , i dont think any of them are fully accurate . Theres 2 " op amps " ? Buried that have 6 legs , not 8 ..probably not op amps but they look like that

[loudthud]

On the last schematic page there are two diodes labeled 4N33. This is a common six leg opto-coupler. The optos control relays on the same schematic page.

[Helmholtz]

On the last schematic page there are two diodes labeled 4N33.

You probably mean the two switch symbols.

https://datasheetspdf.com/datasheet/4N33M.html

[Helmholtz]

Ok, the low voltage supplies use half-wave rectification, so some 60Hz ripple is normal.
But the ripple level from your scope pics (over 4Vpp) is excessive. Check the 220µ filter caps. When replacing observe polarity.

[Valvehead]

Ok, the low voltage supplies use half-wave rectification, so some 60Hz ripple is normal.
But the ripple level from your scope pics (over 4Vpp) is excessive. Check the 220µ filter caps. When replacing observe polarity.

clipped those 220uf out to test and new caps = no change in hum.

also paralleled the 220uf/63v = no change .

all the 30uf/500 test around 30-36uf ....2 measured weird in circuit - test normal out of circuit. As mentioned , i paralleld all 30uf with a good 30uf cap = no change .

Tried a ground wire to all the pots = no change

Grounded V3b grid = no change

i re-routed the 2 Way too small heater wires and twisted them , greens that run to the power tube board , away from stuff = no change

I DID find that the main molex con got hot....of course at the heater wires --- see pic . I will be hard wiring all that tomorrow .

[Valvehead]

Removed burnt molex con and hardwired.

replaced 4 caps in Low Voltage supply = 2- 220uf , 1000uf , 150uf

No change in hum

[loudthud]

Did you check the DC going to V1's filament ? It's on that last page of the schematic.