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PostPosted: Thu Apr 20, 2017 8:58 am 
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Mark II

Joined: Tue Mar 25, 2014 10:14 pm
Posts: 71
Location: New York City
those voltages are just about spot on...

v1 seems high because the schematic references voltages are shown with the fetron engaged. what you have is a 12ax7 so you are good there.

v2a is good... v2b you take your measurement off the cathode, pin 8, should be in the 260v range....

v3a(reverb recovery) and v3b(lead gain) are correct...

v4 is the reverb driver and v5 is the phase inverter. both of those readings are correct...


so with that said, your lead hum sounds 60 cycle to my ear. you could still have another preamp tube that has some capacitave coupling going on and the lead channel is picking it up. with a known good 12ax7 do some preamp tube swapping and see if the hum follows it. the other things to address are relay, ground, film cap, pot or lead-dress related....i see some modified trace work that doesnt look like original Mesa work coming out of the relay....

you will have to meticulous work your way through the schematic matching up the trace runs on the preamp board. i can only guess that whoever modified this previously was looking for a certain sound.

here are clean Mark II schematics;

http://soundup.ru/index.php?option=com_ ... &Itemid=78

the first and third from the top are both IIA's...

do you have signal on the lead channel even with the hum?

reverb and presence controls should be at zero(0) while you are doing the troubleshooting.


Last edited by lovetoboogie on Thu Apr 20, 2017 10:43 am, edited 1 time in total.

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PostPosted: Thu Apr 20, 2017 9:03 am 
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Bottle Rocket

Joined: Mon Apr 17, 2017 5:46 am
Posts: 17
Location: Denmark
Thank you for your reply @lovetoboogie. I will be going through the board one step at a time. Hell i just bought an old osciloscope;).

To answer your question: Yes there is sound through the hum on the lead-channel.


lovetoboogie wrote:
those voltages are just about spot on...

v1 seems high because the schematic references voltages are shown with the fetron engaged. what you have is a 12ax7 so you are good there.

v2a is good... v2b you take your measurement off the cathode, pin 8, should be in the 260v range....

v3a(reverb recovery) and v3b(lead gain) are correct...

v4 is the reverb driver and v5 is the phase inverter. both of those readings are correct...


so with that said your lead hum is going to be relay, ground, pot or lead-dress related....i see some modified trace work that doesnt look like original Mesa work coming out of the relay....

you will have to meticulous work your way through the schematic matching up the trace runs on the preamp board. i can only guess that whoever modified this previously was looking for a certain sound.

here are clean Mark II schematics;

http://soundup.ru/index.php?option=com_ ... &Itemid=78

the first and third from the top are both IIA's...

do you have signal on the lead channel even with the hum?

reverb and presence controls should be at zero(0) while you are doing the troubleshooting.


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PostPosted: Thu Apr 20, 2017 11:22 am 
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Mark II

Joined: Tue Mar 25, 2014 10:14 pm
Posts: 71
Location: New York City
Here's a fairly good birdseye view of a 79' IIA. You might find some use comparing the wire runs and circuit trace on the preamp board...


Image


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PostPosted: Thu Apr 20, 2017 11:33 am 
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Bottle Rocket

Joined: Mon Apr 17, 2017 5:46 am
Posts: 17
Location: Denmark
Thanks a lot for the image. I've been googling for this for days without finding anything usable.



lovetoboogie wrote:
Here's a fairly good birdseye view of a 79' IIA. You might find some use comparing the wire runs and circuit trace on the preamp board...


Image


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PostPosted: Fri Apr 21, 2017 5:03 pm 
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Bottle Rocket

Joined: Mon Apr 17, 2017 5:46 am
Posts: 17
Location: Denmark
@lovetoboogie, i started going through the board, one small step at a time. I bumped into my first problem. According to schetatic, im missing a 470K resistor in parallel with the large cap in the preamp board. See my image below.
What really throws me off is that i found another image on the net that has the same resistor missing.

Have a look;)

My image:
Image

And i found this online:
Image

The question is what is right and what is wrong?


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PostPosted: Sun Apr 23, 2017 3:48 pm 
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Bottle Rocket

Joined: Mon Apr 17, 2017 5:46 am
Posts: 17
Location: Denmark
A minor update from me regarding this messy board;).

I've been focussing so hard on those diodes around v3 tube on the preamp board. Actually i thought that they should not be there at all and were from a mod or something.
Lesson learned: Read the whole schematic!!! Those diodes are there. Way down in the bottom of the schematic.
The thing that threw me off were the pictures from the net of the circuit board. These diodes were not there around V3. I am assuming that Mesa changed the layout several times.
But the conclusion is that the diodes must be there and until now the preamp-circuit looks to be correct. I still need to check all the resistors, caps etc. But it all looks right.

And the noise is still there. So when im done checking the board i'll start to measure voltage again. And those old original caps will be changed. I will be resoldering a lot of the board, since it's pretty poorly done. Generally see if i can remove noise as much as possible.

If any of you m2a owners out there has any good pointers to share, i'd love to hear from you. I am a Mesa newbie and i'd love to learn.


;)Lars


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PostPosted: Mon Apr 24, 2017 2:56 pm 
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Bottle Rocket

Joined: Mon Apr 17, 2017 5:46 am
Posts: 17
Location: Denmark
The task's i've done for today is:

All new caps on power-side. Works fine. Voltage is the same.
I put in the missing 470k resistor as mentioned above. Measured voltage around the tubes. No change. The voltage on pin 1 and 6 has dropped. But only very little.
Replaced a couple of the ceramic caps that were very microphonic. I know @lovetoboogie said it wouldn't do any change. I tried it anyway. But as always @lovetoboogie is spot on;).

The noise is still there. I am beginning to assume that every owner of the m2a out there has this noise, though i'm not giving up.

One thing that throws me a bit off is preamp tube V3B. Pin 6 is the grid for the B side of tube 3. It is very very microphonic. If i tap on the grey cable that goes from pin 6 on tube 3, i can hear it so clearly. The grey wire lead goes to the Lead-drive potmeter on the center pin.


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PostPosted: Mon Apr 24, 2017 6:25 pm 
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Mark II

Joined: Tue Mar 25, 2014 10:14 pm
Posts: 71
Location: New York City
Hi There...

That extra 470k serves to bleed off a little of the B+ voltage to the first stage of the preamp. If you line up 10 Mark series amps from the era, some have em" and some don't! Nonetheless, good work on the troubleshooting so far.

Let's go back and re-evaluate that noise on the lead channel. You can check your control pots to make sure they all have a good ground to the chassis. On pots that have one of it's lugs tied to ground you will use your DMM to check resistance. You should be seeing clear paths to ground where appropriate. Less than 0.5 Ohms.

I have seen instances where the reverb pot is bad or has a bad ground and the lead channel is prone to really exascerbate a reverb issue even when the reverb control is at zero(0).

If you could be so kind as to shoot another short video clip with both no signal present and a guitar signal present, it might help us pin it down. The standard setting for the lead channel would be, VOL: 7-8, TREBLE: 6-7, BASS: 2-3, MIDDLE: 5, MASTER: 2-3, LEAD DRIVE: 7-8, LEAD MASTER: 0-3...

Reverb & Presence at zero(0)...

With that said, a short clip with no guitar plugged in and then a guitar plugged in with a few strums. You can also bring up the reverb and presence control to demonstrate any changes. And you can switch between Clean & Lead channel via the Lead Drive Push/Pull pot.

Some microphonics on the shielded cables from the tube stages is totally normal as long as it not the ringing type or some type of oscillation.


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PostPosted: Wed Apr 26, 2017 2:48 pm 
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Bottle Rocket

Joined: Mon Apr 17, 2017 5:46 am
Posts: 17
Location: Denmark
Thank you for your reply @lovetoboogie.

I've checked every pot for ground. Everything is fine. Almost no resistance to ground. All good. I bought some cleaner for pots. Every single pot got a treatment. They all work fine.
I still need to do the video for you. That'l be a job for the weekend.

Meanwhile, i found this very very interesting post in here: http://forum.grailtone.com/viewtopic.php?f=27&t=54614. On page 5 somebody wrote:

Quote:
5) Very important, grounding reorganization. Pots, cathode and filter capacitor share the same ground bus on the preamp board. This introduced a loud bass hum.
a) Pots are grounded on a separate part of the chassis. I just used a hole near by and used an screw that fitted there to attach a soldering point for the ground.
b) Left the ground points from the cathodes of the preamp tubes are on the ground bus.
c) Preamp filter capacitor is now grounded on the same ground of the rest of the filter capacitors. This is the responsible for most of the loud bass hum.

6) Heater voltage on the preamp, replaced it with a twisted pair of cables. The heater voltage runs on two separated bus on each side of the board. This can introduce mayor hum on the cables near by. To prevent this I replaced this bus with a twisted pair where the noise from one cable cancels the hum from the other.

To do this mods I had to cut some of the board bus and this was a major decision for me. But do not regret it, now the hum level is extremely low for a 33 year old high gain combo.


I have started to do this process. I have moved all the pot-grounds from the preamp-curcuit-board and directly to the chassis-ground.
Since i am not 100% of what exactly a "preamp filter capacitor" is on the board, i have not touched anything here. But the capacitors that runs parallel with the transistors (if thats the one he call this) are also on the preamp-curcuit-board-ground-section.
I wished i fully undestood what he is writing about. I would be able to do the tasks correctly.
I am plannig to execute #6 which is about the voltage for the heaters.
A stupid question regarding this - why am i only measuring 3,5V on the heaters of all the tubes? Is this right? Should it not be 6.3V?


The above URL is full of really great information for noise reduction on M-II-A amps. Seriously great stuff that i hope you good m2a people will read.
Also the guy created a huge image with so many tips and pictures for you to enjoy.
Here it is: http://img11.hostingpics.net/pics/643310boogieMKIIAimprovementspic.jpg.



And by the way... The hum is still there...




lovetoboogie wrote:
Hi There...

That extra 470k serves to bleed off a little of the B+ voltage to the first stage of the preamp. If you line up 10 Mark series amps from the era, some have em" and some don't! Nonetheless, good work on the troubleshooting so far.

Let's go back and re-evaluate that noise on the lead channel. You can check your control pots to make sure they all have a good ground to the chassis. On pots that have one of it's lugs tied to ground you will use your DMM to check resistance. You should be seeing clear paths to ground where appropriate. Less than 0.5 Ohms.

I have seen instances where the reverb pot is bad or has a bad ground and the lead channel is prone to really exascerbate a reverb issue even when the reverb control is at zero(0).

If you could be so kind as to shoot another short video clip with both no signal present and a guitar signal present, it might help us pin it down. The standard setting for the lead channel would be, VOL: 7-8, TREBLE: 6-7, BASS: 2-3, MIDDLE: 5, MASTER: 2-3, LEAD DRIVE: 7-8, LEAD MASTER: 0-3...

Reverb & Presence at zero(0)...

With that said, a short clip with no guitar plugged in and then a guitar plugged in with a few strums. You can also bring up the reverb and presence control to demonstrate any changes. And you can switch between Clean & Lead channel via the Lead Drive Push/Pull pot.

Some microphonics on the shielded cables from the tube stages is totally normal as long as it not the ringing type or some type of oscillation.


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PostPosted: Sun Apr 30, 2017 11:50 am 
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Bottle Rocket

Joined: Mon Apr 17, 2017 5:46 am
Posts: 17
Location: Denmark
An update from Denmark;) It's spring here. Weather is good. Wife want me in the garden aka gardening;), so time is limited fixing this amp. And yesterday i was at a concert with the mighty mr. doyle Bramhall II. My biggest influence both guitar'ish and vocal'ish;).

But... I've been watching some videos from Uncle Doug. Learned a thing or two. That guy is so good at teaching.
Anywayz, he has a video about hum-noise and the frequencies. I always thought mine was a low-hum at around 50Hz(europe), US is 60Hz. So i downloaded an app which is a sine-wave-emulator-thing. I synced the app with the actual noise. It seems my noise is at 100Hz (120Hz is US equivalent). According to Uncle Doug i should then take a look at the Capacitors. So tonight I will do exactly that. All the caps around Tube 3A will be taken off and measured. See if that gets me anywhere.

Have a great day/evening dear good forum-folks. It's spring - don't forget;)


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PostPosted: Mon May 01, 2017 5:18 pm 
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Mark III

Joined: Thu Jun 02, 2011 4:00 pm
Posts: 264
Location: France
Hi Alagami,

Some years ago I posted a thread on the forum about my MKIIA 60/100 Rev. Eq. and how I could get rid of the adverses noises, humzz, buzzzz there was in the amp factory-stock, because mine was untouched.

And it wasn't the tubes, not really the caps, but mainly the wiring, the erratic GND arrangement, the heater imbalance due to relay, the induced reverb transformer, several useless or obsolete components in the input circuit, etc... It's a tough job, but very rewarding in the end.

I suggest you to have a look to it : who knows ? It may help you...

https://forum.grailtone.com/viewtopic.php?f=27&t=54614

The essential is below :

Image

And feel free to ask me informations, if required.

The amp today - I no longer use the nice hardwood cab I prepared for him : too heavy and delicate. I stick with the original Boogie Tolex cab, fitted with an ALTEC 417-8H.

Image

A+!

_________________
MKIIA from March 1980 (S/N° 4915) - dixit Daniel "igfraso" (thanks !)
Tolex ALTEC 417-8H and Hardwood Bubinga EVM12L Black Shadow combo cab option
Silverface Fenders
My DIY/designed tube amps and stompboxes : guilhemamplification.jimdo.com


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PostPosted: Wed May 03, 2017 4:28 am 
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Bottle Rocket

Joined: Mon Apr 17, 2017 5:46 am
Posts: 17
Location: Denmark
@mark2boogie thankyou for your info. I allready knew it. Actually I printed your forum-thread, and i printet your good long document about how you got your m2a back to good noisefree shape. And i have read your document many many times. And i am slowly working my way through your recommendations.

I do have a hard time with it. My preamp-curcuit-boards seems different than most other i've seen. The diodes that comes from the 6.3V trnasformer and that has to do with switching in the relay and footswitch, they're on my preamp-curcuit-board while most other boards i've seen has it on a seperate board.
If you take a look at this image you'll notice that the extra pcb-board is missing. The diodes are actually around the relay-switch. Link: https://dl.dropboxusercontent.com/u/109969/musik/1979-mesa-boogie-mark-ii-b/fullsize/mark-ii-b-fullsize-4134.jpg.

Last night i removed all heater-routs for all preamp-tubes. I cut the lines on the preamp-curcuit-board and i soldered twisted cables instead. I must have made a mistake somewhere though. When i pull the "Lead Drive" now, the realay switch keeps goind on/off/on/off. So i need to see what i am forgetting there.

Till now i've spend so many hours on this one. Sometimes i'm almost giving up. Then i start to think of how mych i am learning from this and how much fun it actually is. And then i keep on goíng. But it's a tough one to start out with. An old Fender amp would have been easier. But i'll keep on until i am happy with it.

Without the help from you good people in here, I would be nowhere.


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PostPosted: Thu May 11, 2017 5:08 am 
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Mark III

Joined: Thu Jun 02, 2011 4:00 pm
Posts: 264
Location: France
Quote:
Till now i've spend so many hours on this one. Sometimes i'm almost giving up. Then i start to think of how mych i am learning from this and how much fun it actually is. And then i keep on goíng. But it's a tough one to start out with. An old Fender amp would have been easier. But i'll keep on until i am happy with it.


Indeed ! I was in the same feeling, alagami... But the result will be worth the expense !

Quote:
do have a hard time with it. My preamp-curcuit-boards seems different than most other i've seen. The diodes that comes from the 6.3V trnasformer and that has to do with switching in the relay and footswitch, they're on my preamp-curcuit-board while most other boards i've seen has it on a seperate board.
If you take a look at this image you'll notice that the extra pcb-board is missing. The diodes are actually around the relay-switch. Link: https://dl.dropboxusercontent.com/u/109 ... e-4134.jpg.


I'll look to that accurately when at home. But I had the relay PSU on the preamp board also, not separated, so like yours...

EDIT :

Yes, I confirm that the relay supply on my MKIIA was on the preamp board, at the power transformer side. It was the same on my MKIIB. Looking at your pictures, I don't see any missing boards except the 2 Eq. PCBs, but that's normal since your amp isn't an Eq. version. IMHO, you have the normal PCB set for a non-Eq. 60W MKIIA inside, and the preamp PCB seems to be identical to my 60-100 Rev. Eq. MKIIA.

Quote:
Last night i removed all heater-routs for all preamp-tubes. I cut the lines on the preamp-curcuit-board and i soldered twisted cables instead. I must have made a mistake somewhere though. When i pull the "Lead Drive" now, the realay switch keeps goind on/off/on/off. So i need to see what i am forgetting there.


I suspect your relay to be now "chattering" : you're probably fronting the problem that I described in E - Supplying the relay correctly of my document. Finally, I choose to built a separate supply for the relay, with two advantages : perfect filament hum balance, reliable relay operation.

A+!

_________________
MKIIA from March 1980 (S/N° 4915) - dixit Daniel "igfraso" (thanks !)
Tolex ALTEC 417-8H and Hardwood Bubinga EVM12L Black Shadow combo cab option
Silverface Fenders
My DIY/designed tube amps and stompboxes : guilhemamplification.jimdo.com


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PostPosted: Wed May 31, 2017 3:58 am 
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Mark III

Joined: Tue Sep 20, 2011 11:15 am
Posts: 163
Hi.Just wondering if you've had any luck with sorting the hum problem on the lead channel and getting this back to stock yet?


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