Mark III Simul, Buzzing

[megavoice]

Hello my mates,
Now it seems so the devil has taken me by the balls "too".

After buying my second red about three years ago, first, when switching from "standby" to "power" "too early" I had a buzzing in all three modes, very clearly to perceive in the "Clean" mode.
That means, switching earlier than 3 o 5 minutes to ""Power" I had to switch back to "Standby" and waiting a while again.
I had sometimes to do this a several times till I succeeded in having the amp working fully.
The whole thing worsened in the last time so that when playing 2 o 3 minutes, amp is fully working, that buzzing is coming back and doesn`t get away all the time after.
Just to show you a comparison: On my first red there are only one or two minutes needed to warm up the tubes for fully working.

On the wrong I did the advised MESA procedure by swapping all the preamp tubes step by step with the spare one I have but with no success.
I also put all nearly new tubes, also the power tubes from the first red in the wrong working one, also with no success.
TV and fridge and all other electric gear is placed far away enough.
I also tried different outlets, and it`s NOT that typical radio humming often caused by wrong audio cables.

So as it shows it`s NOT a tube problem, does anybody have an idea what it can be ??? Maybe some wrong caps ?
It`s very known that caps get broken when being permanently exposed to heat, and this for a longer time.....

 

[MrMarkIII]

Not wrong caps, but old caps. Caps also go bad and dry out when they are NOT used.
An amp that has sat in the closet unused for 20 years is almost certainly going to need more cap work than an amp used every weekend.

 

[megavoice]

Not wrong caps, but old caps. Caps also go bad and dry out when they are NOT used.
An amp that has sat in the closet unused for 20 years is almost certainly going to need more cap work than an amp used every weekend.

Seems to be pretty reight what you`re telling as the preowner as he claimed let that thing stay for a long time in in his bureau unused. Don`t know if it`s true.
I checked more intensively yesterday and the not well working seems to be a bit more clear now.
When glowing the amp in the "standby" mode 10 or 15 minutes there is no buzzing after and it works fine steadily then.
But glowing only 3 or 5 minutes there`s buzzing immediately or it`s coming up after some minutes. Switching back then, it all depends, 1,2,3 or 4 times back into "standby" and then powering on after it works.........

 

[megavoice]

Just to precise the issue a bit more, there`s only buzzing with the played notes, like a nasty unusable distortion.
When no playing notes there isn`t any noise or buzzing or humming to perceive....

 

[McBarry]

Def needs diagnosis, and agreed caps are high on my list of suspects...
Check bias and cathode caps also - they all age equally...

 

[megavoice]

Def needs diagnosis, and agreed caps are high on my list of suspects...
Check bias and cathode caps also - they all age equally...

Yes, that`s completely right what you`re telling, taking a look at the caps in general after the tube check first, bur I suspect
that bloody optocoupler thing after all my experiences with my first III.

 

[McBarry]

Agreed it might be an LDR.. BUT there's other possibilities and some of them are sinister/ominous.

I'd strongly suggest not using until the diagnosis is made.

If its signal chain, no biggie.
But if it's something leaking DC where it shouldn't, you're risking a world of pain, much of this with a $ sign in front...

Let us know how U get on.. these threads are terriffic education for all concerned..
Dave

 

[megavoice]

Agreed it might be an LDR.. BUT there's other possibilities and some of them are sinister/ominous.

I'd strongly suggest not using until the diagnosis is made.

If its signal chain, no biggie.
But if it's something leaking DC where it shouldn't, you're risking a world of pain, much of this with a $ sign in front...

Let us know how U get on.. these threads are terriffic education for all concerned..
Dave

Thank you again, of course Ì`ll take this all seriously, but I still focus the the pin. Let`s wait about this.

 

[BlackBoxy]

hi,

i would first cut reverb stage by removing reverb tube !

if it still buzzing, so it would be leaky filter caps to change!

 

[megavoice]

hi,

i would first cut reverb stage by removing reverb tube !

if it still buzzing, so it would be leaky filter caps to change!

Thank you, I`ll gonna try this as well.

 

[megavoice]

hi,

i would first cut reverb stage by removing reverb tube !

if it still buzzing, so it would be leaky filter caps to change!

BlackBoxy ! You`re my mate DIRECT HIT !!!

One week ago I had been advised by a friend who only roughly knows about guitar amps to go out from "Effects Send" into
a speaker system to exclude the power amp section.
So I did and buzzing still prevailed. It seemed to be clear then that the bug is in the preamp section.
Now yesterday before pulling out the chassis I pulled the reverb tube and no buzzing occured anymore.
I put then the spare tube into and again - buzzing. The final step was then turnig the reverb pot to zero and also no buzzing. Of course I cleaned the pot and there was no scratching to hear before and after.
I told all this my friend a he said I should check out again while pulling the two reverb cales ((grey/white) one after the other.
I did, in all three mods, that means also with no cables plugged in, the buzzing was there.
As the final conclusion my friend said there must be something wrong in the recovery amp.
Again: I want to underline that the reverb itself ist intensiv, loud and clear to perceive and NOT distorted. (BTW its the finest spring I ever heared in my life)
The rest of the amp is working fantastic.

Now has anybody an idea how to go on now and what might be wrong there ????

 

[McBarry]

V4B is reverb recovery.
So.. as I understand, pull tube = no buzz, and tube changed = no difference.
I'd check if buzz persists with tube out, tank return unplugged and rev volume on full. If so, I'd suspect the coupling cap and chase the source.

If coupling cap OK you're really gonna have to chase voltages around V4B.

At least you've narrowed it to a small area. Cool !!

Let us know for future readers info also..

 

[loylo69]

So, as far as I can understand, it is not a bad solder on the LDR after all, as I suggested. :wink:

I would totally agree with McBarry about a coupling cap failure. Except in this case, I think the buzz would still be present even with the reverb tube pulled out (because of bad DC filtering).
Could you confirm that there is no buzz anymore with the tube pulled out, and the reverb pot still on 10? Does the amp still buzz with no cable plugged from the reverb tank ?

Could you try to listen what the signal sounds like out the reverb send stage?
You'll need to plug out the reverb tank and plug in an 8 ohms speaker in the reverb send socket.
You'll also need to keep a speaker plugged at the output of the amp, so you'll not fry the output transformer. But you can keep the global master on 0 in order to listen only to the signal from the reverb send.

 

[lovetoboogie]

bad ground on the reverb return cable(the shielded one)...all mark series amps are notoriously finicky with this. the RCA jack on the underside of the chassis after time fails to make a good ground contact with the chassis...

you can check it with a DMM to confirm...take a resistance reading from the end of the shielded reverb cable, the outer part, where it connects with the tank, and place the other lead of your meter on the chassis at a speaker jack or some other exposed ground point...

if you don't read 0 ohms then your "buzz" is most likely in the ground path...

 

[loylo69]

I hadn't mentionnes this idea as Megavoice wrote the buzz is still there with no cables plugged at all.

 

[lovetoboogie]

I hadn't mentionnes this idea as Megavoice wrote the buzz is still there with no cables plugged at all.

with the reverb cables unplugged the buzz will even be more exaggerated since the reverb pot by design will still be in circuit and never truly closes the circuit even on zero(0)

i admit i didn't read all the posts but i saw reverb and buzz, and when it comes to mark series amps, those two words are almost always synonomous with a ground path problem...

of course, PT/OT transformer proximity and all those usual suspects come into play...

i notice some cats have flipped their tanks to change the noise associated with that, not recommended...but that is more a hum than a buzz anyway

i know how noises will drive us all crazy, besides the noise we actually are making!

 

[loylo69]

I can't see why the buzz would even be more exaggerated with the cables unplugged.
Please don't get my comment the wrong way. :wink:
I'm sincerely questioning this technical relation. Because it seems that the buzz induced by bad ground on the reverb return cable is due to the fact that the reverb tank loose the ground connection. So the cables and the tank are no longer insulated from the beam of the PT and other interferences.
As a result, there could be no buzz induced by bad ground on the reverb return cable if no cables are plugged to the chassis.

But Megavoice has tested this configuration, and there is still a buzz. That is why the source of the buzz seems to come from somewhere else.

 

[lovetoboogie]

I can't see why the buzz would even be more exaggerated with the cables unplugged.
Please don't get my comment the wrong way. :wink:
I'm sincerely questioning this technical relation. Because it seems that the buzz induced by bad ground on the reverb return cable is due to the fact that the reverb tank loose the ground connection. So the cables and the tank are no longer insulated from the beam of the PT and other interferences.
As a result, there could be no buzz induced by bad ground on the reverb return cable if no cables are plugged to the chassis.

But Megavoice has tested this configuration, and there is still a buzz. That is why the source of the buzz seems to come from somewhere else.

You are correct in a sense but let me clarify... I don't troubleshoot reverb circuits without a known good tank. And removing the tank from circuit will introduce unwanted noise from the recovery grid, the reverb pot is tied to it. it becomes an RF issue at that point and there is no way to isolate one buzz from the other. Even with the reverb control at zero, some noise can and often will be introduced with no tank tied to the circuit via the cables...ever turn up a reverb control without a tank plugged in? It ain't pretty...

But you have also reminded me of another problem i come across which makes a buzzing mess. The reverb pot...needs a good chassis ground. With the pot mounted on the back of most Mark series amps it can get a fair share of abuse and sometimes needs to be removed and reseated. So all of these things play into a buzz....

You can always question my technical method

...I am crazy but somehow I get things fixed. Seriously though, only trying to be helpful.

 

[megavoice]

Thank you for all your replys and ideas. I`m going to think all about next week with my tech, I`m in a hurry now.

Again, as it ain`t be clear:

NO BUZZING when reverb tube is pulled, or reverb Volume put to Zero.
Having the tube IN and reverb Volume Full and pulling one cable after the other, buzzing does NOT go off.

 

[McBarry]

"NO BUZZING when reverb tube is pulled, or reverb Volume put to Zero."
Does this mean no buzzing when a) tube out (it's pre-tube) and b) tube IN and rev vol zero (it's either pre or post tube)
or no buzzing when a) tube out (it's still pre-tube) and b) tube OUT and rev vol zero (it's post-tube eg DC leak, verified with DMM)

Totally agree re a known good tank, and even better is known good grey leads, or at least check continuity/resistance as per lovetoboogie.

Suggest you check INSIDE the tank for the miniscule leads to the RCA sockets - they get loose/free/break strands.... PITA to resolder as wafer-thin and I protect with 1mm heatshrink on all my amps and those I repair.

Also, inside the tank check the insulation where the spring anchor wire with the hook end passes through the retaining bracket. I've seen crud here giving signal loss AND introducing hum (not strictly a buzz like yours, but hey.. it's subjective..)..

So there's a few more ideas that U can check without the tech...... but for definite diagnosis U need it on a bench.

Shotgunning parts might work, but it's an imprecise repair method..