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Recapping the AO-29

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  • Recapping the AO-29

    I think my caps are starting to fail. I am a pretty decent solderer and I can sort of read schematics. Since I'm pretty broke right now and unable to drop my two ao-29's to a hammond tech, I'm considering taking on the job myself. I am aware of the high possibly lethal voltage. Firstly, how many capacitors will I be swapping out and second...as long as I discharge the capacitors by shorting the leads with a screwdriver and measuring the voltage, will I be ok?

  • #2
    Re: Recapping the AO-29



    sounds like you're good to go. the best thing to do is remove the ao-29 and make a list from visual inspection. you'll need to replace the electrolytic caps in the can(s) up top; write down the values written on the metal jacket (100mfd @450VDC--that kind of stuff). then write down every onesy cap on the underside. you have a choice--you don't necessarily have to change all of them. the signal caps are less likely to be a problem, but they are cheap....of course, check the schematic to make sure of the values.
    </p>

    go to antique electronics and order your caps. then just replace them one by one, taking note of the polarity in the electrolytics. hopefully, antique will have a multi-unit electrolytic that matches what you need. if not, you'll have to add caps on the underside. good luck.
    </p>

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    • #3
      Re: Recapping the AO-29



      I went through the schematic and for certain models there are a c62 capacitor rated for 50mfd/450v. On another there is a c62 and a c62 a both with the same rating. You mentioned the can capacitor to be rated for 100mfd/450v. Do I just wire 2 50/450 capacitors together on a channel strip or do I use a 100/450. And will I face problems if I am using non-polarized caps? partsexpress.com has micro electrolytic caps and I was planning on using those...http://www.partsexpress.com/pe/showd...umber=020-1476</P>
      <P mce_keep="true"></P>


      as for the smaller caps (ie.. .00002, .33,.047) I'm going to leave those alone unless the pose as a fire hazard.</P>

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      • #4
        Re: Recapping the AO-29



        Can capacitors are single or multi-section, meaning that one can may have several very different values. It's too early for me to dig out a service manual or pull my M-143 away from the wall and see just what value mine has. I think it's best to do the whole amp while you're at it. It requires just basic soldering skills and minimal equipment. It is OK to clip the old axial electrolytics out and use a portion of the original lead to solder in the replacement. Pay attention to polarity and be as neat as possible with your connections. Pull the amp, take a good photo or two, and make a list of every capacitor underneath the amp, both mfd. value and voltage. I just re-capped an old Wurlitzer 4100 with excellent results and am in the process of re-capping a Hammond H-182. The H has a big power supply separate from the main amp. Suffice it to say that I'm replacing a LOT of capacitors, many of which are simply not available in the exact value or form as the original. A cap with a higher voltage rating than the original is OK, butdon't go lower.Stay as close to the original capacitance rating as possible. While you may replace a can with a less expensive axial or radial (or two or three for multi-section cans), you should be able to find a new can in the value your AO-29 uses at Antique Electronic Supply: www.tubesandmore.com. </P>


        John</P>
        <P mce_keep="true"></P>

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        • #5
          Re: Recapping the AO-29

          As I continue studying the schematic and gaining an understanding for how the tube amp works, I am likely to replace some of those caps.  I rarely ever use vibrato so in that section, I'm not really concerned.  I just replaced the 6v6 pair and am thinking of replacing the 5u4.  Keep in mind that I am doing 2 M3s.  One is a field coil and one is a permanent magnet.  The tone of each sounds awesome.  The chop has different degrees of hum depending if someone bangs on it.  My biggest concern is fire hazard or damage to other sections of the amp hence the focus on high voltage caps.

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          • #6
            Re: Recapping the AO-29



            you'll see at the antique site how they sell the multi-unit cans. they should have something close to what you have. if not, you'll have to get more creative, and add some on the bottom. and yes, you must replace polarized caps with polarized caps, going to the same poles. take pictures and careful notes, and go methodically. check your work after each cap--right cap, right position, right polarity, good solder job? not being an electronics genius myself, i know from experience how much harder it is to troubleshoot a dumb mistake than it is just not to make it.</p>

            tube wise, don't change tubes if you don't know they're bad. i have a tube tester, and i've tested many a hammond's tubes, and they rarely go bad--especially rectifiers like the 5u4. in fact, once a tube has survived its first 100 hours, its chances of surviving the next 5,000 hours are excellent. most tubes, especially newly made ones, will fail sooner than later. and unlike a guitar amp, in the organ circuit you're not going to hear a big difference as long as they are functioning.
            </p>

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            • #7
              Re: Recapping the AO-29

              yes my research has brought me to the conclusion that electrolytic capacitors are almost always polarized unless clearly indicated. I did some shopping at mouser but haven't checked out yet. Am I ok using slighty different values (ie. 47uf for 50uf...33uf for 25uf...) so long as I keep the voltage equal to or greater than?

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              • #8
                Re: Recapping the AO-29



                I wouldn't recommend it.... use two or three in-line to get as close to the original value as possible hondro1025.</P>

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                • #9
                  Re: Recapping the AO-29

                  The problem is that some of those values are not available. I've read some of the literature on antique radio restoration and they state that as long as you stay withing a 20% + or - and staying asclose as possible. I saw a website where a guy recapped his A-100 with 247uf caps in parallel to make up fot the can capacitor.

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                  • #10
                    Re: Recapping the AO-29



                    A 20% variance should be OK in most Hammond circuits. From what I understand, the correct microfarad value is more important voicing circuits. Although axial and radial electrolytics aremuch cheaper, I still strongly suggest using the correct part, especially if you haven't done this before.</P>

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