MKIII constant background hum

[Musikminister]

Hi guys,
this is a question to the special matter experts with experience concerning to the electrical circuit details.

I have a serious problem when trying to record via mic. from my MesaBoogie MkIII because there is a constant background hum which grows even bigger when I turn up the presence control.

I've changed all tubes already and adjusted the bias via the ´special Mesa resistor´ - in addition tried to double the 220MFD capacitors after the rectifier diodes in the power supply section, but this stupid hum stays constant as a rock. I wonder if anybody has an advice for me....

P.S.: I've tried out all these ´simple´ first ideas in vain, like:
Does it hum when nothing is plugged into it? >>YES
How about when nothing but the guitar is plugged into it? >>HUM
Are you using any effects? >>NO
Are they all powered off one power supply? >>N/A
Does the hum changed when you switch channels? >>NO
Does the hum change when you move the amp into a different room? >>NO

The hum seems to come out of the power amp/power supply itself and gets bigger with presence control which gives a direct feedback from the output transformer to the driver tube...
Hope you can help.

 

[Boogiebabies]

Hi guys,
this is a question to the special matter experts with experience concerning to the electrical circuit details.

I have a serious problem when trying to record via mic. from my MesaBoogie MkIII because there is a constant background hum which grows even bigger when I turn up the presence control.

I've changed all tubes already and adjusted the bias via the ´special Mesa resistor´ - in addition tried to double the 220MFD capacitors after the rectifier diodes in the power supply section, but this stupid hum stays constant as a rock. I wonder if anybody has an advice for me....

P.S.: I've tried out all these ´simple´ first ideas in vain, like:
Does it hum when nothing is plugged into it? >>YES
How about when nothing but the guitar is plugged into it? >>HUM
Are you using any effects? >>NO
Are they all powered off one power supply? >>N/A
Does the hum changed when you switch channels? >>NO
Does the hum change when you move the amp into a different room? >>NO


The hum seems to come out of the power amp/power supply itself and gets bigger with presence control which gives a direct feedback from the output transformer to the driver tube...
Hope you can help.

Als Sie sagen, dass Sie das Vorurteil geändert haben, hat die Gleichrichterdioden verdoppelt und hat versucht, die Kappen zu verdoppeln, annehme ich, dass dies ein Problem war, bevor Sie die Arbeit machten?

Ich kann sicher Sie erzählen der, wenn Sie Geräusch mit keinem Kabel hören, das wird angeschlossen, dass Sie reparieren müssen, oder muss den Eingabewagenheber ersetzen damit es Kurzhose zu erden, wenn es keinen Stöpsel im Wagenheber gibt. Es ist repariert ein leichtes und Sie werden sehen, dass der shorting Ansatz vom Wagenheber die Eingabe nicht berührt.

Ihre Anwesenheitsteuerung löst von Ihren Ausgangshähnen vom Ausgangsumformer. Wenn Sie drehen die Anwesenheit ab macht Sie noch das Summen erhalten? Es sei denn haben Sie ein inneres das kurz ist in Ihrem OT ich zweifle, dass der Anwesenheitkreislauf von irgendeinem Problem wäre. Es kann zufällig sein. Ich habe schlechte Töpfe gehabt, die summen würden, weil sie so dass der Kreislauf würde sterben getragen wurden, oder würde summen, als es kein resistive Element gab, das an einem gewissen Punkt in der Streife verlassen worden ist. Nur für Spaß würde ich mit allen meinen Knöpfen an 0 anfangen und würde sehen, wenn Ein von ihnen der Schuldige ist. Es würde nicht verletzen, das A zurück zu stellen, zu führen, als es das Problem nicht repariert hat. Ich benutze riesige Kühlkörper wenn Lötdioden. Sie sind sehr launisch zu erhitzen und ich, unbedingt das ein zu prüfen, das darin jetzt ist, sicher Sie zu sein, habe Ein und exaserbate das Problem nicht gekocht.

Meine Großmutter und Onkel leben in Furth und Bayern.

Ich bin immer froh, aus einem Mit Deutsch zu helfen.
Meine Mutter, die zu den USA in 1963 bewegt wird, aber mein Deutsch ist sehr arm. Sie hat größtenteils an mir verflucht.





When you say you changed the bias, doubled the rectifier diodes and tried to double the caps, I assume this was a problem before you did the work?

I can certainly tell you that if you hear noise with no cable plugged in that you need to repair or replace the input jack so it shorts to ground when there is no plug in the jack. It's an easy fix and you will see that the shorting lug of the jack is not touching the input.

Your presence control works off of your output taps from the output transformer. If you turn the presence off do you still get the hum? Unless you have an internal short in your OT I doubt the presence circuit would be of any problem. It may be coincidental. I have had bad pots that would hum because they were so worn that the circuit would die or hum as there was no resistive element left at a certain point in the sweep. Just for fun I would start with all my knobs at 0 and see if one of them is the culprit. It would not hurt to put the amp back to stock as it did not fix the problem. I use huge heat sinks when soldering diodes. They are very temperamental to heat and I would be sure to test the one that are in it now to be sure you did not cook one and exaserbate the problem.

 

[Musikminister]

Hi "Boogiebabies" thanks for your reply, there are some good ideas and I will check them out. Following are some answers to your questions:

When you say you changed the bias, doubled the rectifier diodes and tried to double the caps, I assume this was a problem before you did the work?
>>NO, the bias correction was necessary because I've changed the 6L6 tubes into non Mesa originals. With the rectifier diodes I've done nothing. I've doubled the caps because this is done in the Simul Class version with 4 6L6 tubes (mine has only 2) and thought that this would quieten up the hum a little.

I can certainly tell you that if you hear noise with no cable plugged in that you need to repair or replace the input jack so it shorts to ground when there is no plug in the jack. It's an easy fix and you will see that the shorting lug of the jack is not touching the input.
>> If I plug in a plug into the receive jack from the effect loop (which cuts the pre-amp section from the main-amp section) the hum does not change a bit, so it seems to come out of the power amp.

Your presence control works off of your output taps from the output transformer. If you turn the presence off do you still get the hum?
>> It gets more and more quiet but never ceases completely.

Unless you have an internal short in your OT, I doubt the presence circuit would be of any problem. It may be coincidental. I have had bad pots that would hum because they were so worn that the circuit would die or hum as there was no resistive element left at a certain point in the sweep. Just for fun I would start with all my knobs at 0 and see if one of them is the culprit. It would not hurt to put the amp back to stock as it did not fix the problem. I use huge heat sinks when soldering diodes. They are very temperamental to heat and I would be sure to test the one that are in it now to be sure you did not cook one and exaserbate the problem.[/
>> OK, I'll check that out.

Thanks again for your reply (BTW the German part alone was sometimes very funny but not to understand, I assume you've used a translation application?). Just for fun, I've re-translated it again as follows, but I would no go so far to critisize you for the German part ,-) it was very polite of you to do this extra effort.

Funny re-translation of your German text (with babelfish): When you say the fact that you changed the prejudice, have the rectifier diodes doubled and has tried to double the caps do I assume that this was a problem, before you made the work?

I can tell surely you to that, if you hear noise with no cable, that am attached that you must replace repair, or must the input jacking equipment with it it to short trousers to ground, if there is no plug in the jacking equipment. It is repairs an easy and you will see that shorting the beginning does not affect the input of the jacking equipment.

Their presence price increase solves from your output cocks from the output transformer. If you turn the presence make you still humming off received? It is has you internal a that is short in her OT I doubts that the presence cycle would be from any problem. It can be coincidental. I had bad pots, which would hum, because them so that the cycle would die were carried, or would hum, when there was no resistive element, which was left at a certain point in the patrol. Only for fun I would begin and would see mine with all buttons at 0, if from them the guilty one is. It would not hurt to place A back to lead when it did not repair the problem. I use enormous radiator boxes if soldering diodes. They are to be heated up very moodily and I to examine absolutely who am now to be surely you in and the exaserbate problem did not cook.
My grandmother and uncle live in Furth and Bavaria.

I am always glad to help from a with German. My nut/mother, that is induced to the USA in 1963, but my German is very poor. It has to a large extent at me verflucht.
Regards Musikminister

 

[Boogiebabies]

I told you it was bad. She taught me the curse words the best.