Hum

[Guest]

My A-100 has developed speaker hum. Can I just go in and adjust the hum balance control trimmer between the 6BQ5's on the AO-39 amp? Do I need to take any precautions as the organ will be on? Appreciate any info on this subject. Thank You.

[Guest]

Re: Hum



I'm not familiar with the AO-39 amp (I didn't know they had a hum balance pot!) but there shouldn't be any precautions adjusting a hum balance pot - by their nature they're designed to be adjusted with the amp on! - just don't get burnt by hot tubes! The AC wiring should all be covered up - but just be careful...

A hum that wasn't there before could very well be filter caps though...which are more dangerous to replace...I mean it's 100% safe if done properly - it just needs to be done properly!</p>

Good luck,</p>

Jesse</p>

[fredy2]

Re: Hum



Make sure the hum balance control is exactly that and NOT the bias adjustment... If the hum is quite bad, one of the tubes may have developed an internal short. You might want to have them checked, especially for shorts. Replacement is the only cure for that.</P>


Filter caps dry out and are a common cause of hum. This can be decided using an oscilloscope. Also if half of the full wave rectifier goes that can cause hum. That will be 60 Hz compared to 120 Hz assuming you are on USA standard line freq. Bad caps result in 120Hz hum, bad rectifier is 60 Hz. Shorted or leaking heaters in amp tubes usually 60Hz.</P>

[jimmywilliams]

Re: Hum



Fred,</P>


Just out of curiosity - in general, if half the rectifier goes down, how will the resulting voltages even be correct? Willyou just lose the voltages on one side - ifan organ needs both negative and positive voltages for instance?</P>


Can't a ground loop also be a possible cause of 60hz hum?</P>


I have been trying to tackle a hum problem in my organ myself(not a Hammond like the original poster has) - had ALL filter caps replaced but still get a hum out of the speakers - can't detect any AC ripple voltage using a scope so I'm stumped. It's not that loud so I've resigned myself to living with it. If I get some more details together maybe I'll start a new thread - I don't want to interfere with solving the orignal poster's Hammond ampissue - just wanted some general info about causes of hum in general.</P>


Thanks for you very educational electronics-related posts.</P>


- jim</P>

[jimmywilliams]

Re: Hum



L A</P>


As per Fred's message, if you have a farily good ear you can sort of[:)] determine the type of hum. 60hz will sound almost like a "note" between Bb1 and B1. 120hz will sound almost like a note between Bb2 and B2 (i.e. one octave higher). In my experience the 120hz has more of a "buzz" quality as opposed to the 60hz which hasa more "muffled"quality - but I'm only a hobbyist, not a pro! If the hum is 120hz you really need to get the filter capacitors taken care of or else you run the risk of damaging the power transformer. I think most of the multi-section filter cap cansfor the hammond/leslie amps are actually still being made due to demand - they are not cheap though.</P>


- jim</P>

[fredy2]

Re: Hum



Depending on the load of the circuits on the power supply, if only half of a full wave rectifier is working, then the filter caps have to sustain the voltage during a half cycle all on their own... If a regulator follows the rectifier, the caps may not drop below the dropout voltage of the regulator during this half cycle and the output voltage will still be at the design voltage. The ripple current in the caps will be much higher aqnd shorten the life of the caps if this continues for very long...</P>


By half the rectifier failing, I mean that would be one diode of a full wave... for a tube unit, there would only be positives... for a full wave "bridge" circuit, with a single diode failure, either the plus or minus would have this higher ripple present... but still provide voltage... If the ripple is within what the regulator can handle, the voltages would appear normal.</P>


The HIGHER peak currents to charge the half that is busted can cause hum by magnetic coupling to audio circuits... This higher peak current would probably be 3 times the normal to recharge the cap during only the 1/2 cycle available.</P>


Hum problems are easy to solve... if you can find a schematic... Use the divide and conquer method. Go to the middle of the chain of audio and ground it... this will tell you if humis before or after that point... then go to middle of whichever side is known as the source again... to isolate where the problem is... USE A LARGE capacitor, say 100 mfd to ground to avoid shorting out biases... </P>


Sometimes you can unplug an internal audio cable to find a point that quiets the hum... Ground loops can cause hum... MAKE SURE all the equipment tied to the organ is fed from a single electrical outlet... </P>


Light dimmers are notorious sources of hum... usually with sharp buzzing type rather than smooth sine wave sound...</P>


Having a wall wart transformer nearby or a low voltage music light with transformer can generate magnetic coupling into the audio circuits.</P>


External audio connections can cause ground loops. Suggest you see if hum is present using headphones... that is a useful tool in isolating...</P>

[Hammondlover]

Re: Hum

There's no reason to pay the high prices for those electrolytic cans. If you're good at reading schematics and in anyway good with your hands, you can gut the old cans and put in normal electrolytics that shouldn't cost more than $1.50 apiece.

[Model E & Me]

Re: Hum

And unless you are a real purist and want the underside of the amp to look exactly like the original, you really don't need to gut the old cans. Just put a terminal srtip somewhere convenient and wire in the new capacitors to that. You will have to transfer <U>all</U> the wiring from the hot sides of the old cans, tho, to get the old units out of the circuit..

[Guest]

Re: Hum



This is a belated thanks to the folks (jd-f, fredy2, jimmywilliams, Hammomdlover, Model E &amp; Me) that responded to my originial post in April of this year regarding the hum in my A-100. I did adjust the hum balance pot and replace the 6BQ5's on the AO-39 amp to no avail.</P>


Having no skills for electrical operations it took awhile to seek out someone in this area with the knowledge about organs. Surfing the net, I came across MITA (Musical Instruments Technicians Association). This site list the names of members by state, zip, and country. The goal of this organization is Professional Technicians Organized to Improve the Electronic Music Instrument Service Industry.</P>


I found the Technician in April, but it took until June 25th to get him to my house because of his schedule.</P>


Well, the problem was filter caps in the organ and 122 Leslie. He pulled the 6BQ5's out so I can use the organ voicing thru the Leslie only. He check the amp in the Leslie, said there were new caps in it and its okay to use, even though he said it needs a filter. So now you guys really know I don't know anything about electronics. I can use the Leslie, but its still sick.</P>


The best thing though is that I can get back on the organ. I stop playing it completely in April when the hum didn't go away.</P>


I still have to wait for him to get back for the repairs, he said it would be sometime in July or early August because of his schedule and vacation time.</P>


This is a great forum and I read it all the time. Hopefully one day I'll be able to contribute.</P>
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[Subaru]

Re: Hum



Funny I have the same issue with my A100 the hum comes out of the speakers or the Leslie..</p>

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http://www.youtube.com/watch?v=ww3qgDJqE9U
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