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Author Topic: switching the old caps out of my M3  (Read 2689 times)

absolutkj

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switching the old caps out of my M3
« on: November 17, 2005, 08:41:07 AM »

I suspect the most likely cause of my noisy 1950s Hammond M3 is that there are a few leaky caps somewhere in the amp or tonewheel section of the organ.  I plan on testing these with a multimeter to see which caps are bad and need to be swapped, but before I go touching the terminals with the metal prongs I want to make sure I have a couple of things straight.  

1.  Do I need to discharge the cap before testing it with an ohmmeter?  

2.  If so, what is the safest and easiest way to discharge a cap?  I've heard of a few different (and might I say crude) methods of discharging a cap, but keep in mind that I am performing the maintenance on the organ in the studio and not in a repair shop because the organ is damn heavy to move.  

3.  Once I do remove the bad cap/caps, where should I go to get a decent replacement?  I'm not looking to buy a super expensive electrolytic capacitor, but I want something that will last and get rid of the noise that now prevents me from using the organ for recording.  
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ssltech

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Re: switching the old caps out of my M3
« Reply #1 on: November 17, 2005, 10:18:37 AM »

If you're asking these kind of questions I (without knowing you) feel obliged to suggest that you might not know enough to do the job safely.

Some caps in there are charged to potantially lethal voltages, others aren't. You sound sufficiently unfamiliar with the process to alarm me that you wouldn't necessarily know the difference.

You mention testing a cap with an 'ohm meter'. This is one example of what alarms me: an ohm meter won't read good or bad. Ohm (resistance) meters work by measuring DC current flow; This is about as much use for testing caps as a chocolate teapot.

What is this "noise that now prevents me from using the organ for recording" of which you speak? is it intermittent crackle? -Hiss? -Buzz? -Hum? Most times when people bring gear to me and say "It probably needs a re-cap, I can hear one going bad" they are referring to a noise that is utterly unconnected with old caps.

By all means, re-capping an old Hammond may very well improve the unit, but you've told us nothing of the nature of the problem you're setting out to cure, and from your post, you've told us enough for me to worry that you'd be better off paying someone who knows what they're doing... in this case, you literally sound like you know enough to be dangerous!

No offence intended, but I'm genuinely concerned that you don't know enough about capacitors to be safe around them when they're working at or close to lethal voltages.

Keith
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MDM (maxdimario) wrote on Fri, 16 November 2007 21:36

I have the feeling that I have more experience in my little finger than you do in your whole body about audio electronics..

archtop

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Re: switching the old caps out of my M3
« Reply #2 on: November 17, 2005, 10:40:45 AM »

Hammond cap kit here.



http://www.goffprof.com/

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Richard Williams

absolutkj

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Re: switching the old caps out of my M3
« Reply #3 on: November 17, 2005, 06:37:45 PM »

I wasn't planning on swapping the caps right away, that's why I posted here, to see what the safest method of doing so would be.  I never jump right into anything without researching it first.  As for the use of the term "ohmmeter", my terminology is most likely off.  But isn't it true that a good capacitor will not pass DC, therefore the impedence will be infinitely high for a DC voltage accross the cap? If this is true, a sign of a leaky cap would then show a meter reading far less than infinity.  Maybe my question should be more generally phrased to apply to all vintage analog gear:  How do you check for bad caps?  I am well aware of the lethal voltages that can be stored in capacitors, and I am in no way an idiot.  I'm merely a student who wants to learn how to maintain his gear.    
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Sahib

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Re: switching the old caps out of my M3
« Reply #4 on: November 17, 2005, 08:26:01 PM »

absolutkj wrote on Thu, 17 November 2005 23:37

I wasn't planning on swapping the caps right away, that's why I posted here, to see what the safest method of doing so would be.  I never jump right into anything without researching it first.  As for the use of the term "ohmmeter", my terminology is most likely off.  But isn't it true that a good capacitor will not pass DC, therefore the impedence will be infinitely high for a DC voltage accross the cap? If this is true, a sign of a leaky cap would then show a meter reading far less than infinity.  Maybe my question should be more generally phrased to apply to all vintage analog gear:  How do you check for bad caps?  I am well aware of the lethal voltages that can be stored in capacitors, and I am in no way an idiot.  I'm merely a student who wants to learn how to maintain his gear.    


Seems like you already have the answer so why are you asking? I think Keith has been protective of you in his comments.

First buy yourself a good capacitance meter.

To check a capacitor, first remove it off the circuitry. Then discharge it by connecting a resistor or a resistive device such as a light bulb between its terminals. Do not discharge it by shorting its terminals.

Finally, as Keith said it is possible that the problem may well be caused by something else. And if you cared to tell us the nature of the problem then you could have had answers that would save you doing unnecessary work.

Anyhow, good luck with checking the caps.

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absolutkj

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Re: switching the old caps out of my M3
« Reply #5 on: November 17, 2005, 09:14:05 PM »

sorry bout that, I left that out in my last post.  the noise that I was originally referring to was a hum.  It doesn't really sound like a 60hz ground hum but may be a harmonic I guess.  I regularly oil the motor and a couple of months ago I replaced all of the tubes with NOS RCA's and the original AC cable was changed to have a ground, so my attention moved towards the capacitors.  
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ssltech

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Re: switching the old caps out of my M3
« Reply #6 on: November 18, 2005, 06:14:25 PM »

Hey, -Sounds like it might be the old favourite 120Hz rectifier hum, in which case you probably do need to replace a couple of caps.

Not the tonewheel caps in the link posted earlier on, but the rail electrolytics. These will probably be large in physical size, and will almost certainly have the highest voltage rating, as well as the most lethal voltage unfortunately...

However...

I doubt they're leaky, -they might be of course, but it's much more likely that they're just low value. A capacitance meter will tell you fairly quickly, but not all capactiance ranges on multimeters go all that high. -Assuming that they're low value, the reason it's huming is because they've lost sufficient capacity to be able to sustain a significant charge, and they aren't therefore able to smooth out the ripple. -the side-benefit is that they probably won't be able to sutain much of a charge to kill you. All the same, leave it off and unplugged for a good while before you work on it! Shocked (you can never be TOO safe!)

Bear in mind that all aluminum electolytics exhibit a tiny amount of leakage, and -as I've already suggested- there are other ways that a cap can go bad, the most common with this type of electrolytics is for the electrolyte to dry out, then they just go low value. Leakage in thses sorts of failures can often go down, so a simple leakage test won't give you even the slightest hint, and I'd bet at least an hour's pay that you've got low-value reservoir caps if you have a 120Hz hum.

You can try it yourself, but for goodness' sake, BE CAREFUL!!! Shocked

Note that I earlier wrote "unplugged" and not "switched off". If your soldering iron tip is grounded (They basically alla re nowadays) and you try and desolder a rail cap that is holding a charge above ground potential -which it is their job to do- then just toouching it with your iron will cause the immediate discharge that Sahib warned against. unplug the unit from the power AND ANY AUDIO CONNECTIONS to or from it, otherwise it could be ground referenced through these connections, which would be a problem.

Keith
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MDM (maxdimario) wrote on Fri, 16 November 2007 21:36

I have the feeling that I have more experience in my little finger than you do in your whole body about audio electronics..

RMoore

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Re: switching the old caps out of my M3
« Reply #7 on: November 19, 2005, 06:27:08 AM »

archtop wrote on Thu, 17 November 2005 16:40

Hammond cap kit here.



http://www.goffprof.com/





Great link - thats a real mothalode of Hammond related stuff,

I did notice they had a tonewheel filtercap replacement set for $90 - seems a tad pricey for 2 x caps, 1 x resistor, 1 x what looks like a tantalum cap:

http://www.goffprof.com/shopping.jsp?p=51

At my studio there are 2 x L100 tonewheels and various Leslie & Leslie-like boxes, all in varying states of disfunction but still producing pleasing sounds Smile

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