[HAM] Percussion & Matching Transformer

[David Anderson]

Jordan,

If you want to make your organ precisely like a 3-series organ, you  
have to add the 3.9M resistor (R1), a 1M resistor (R2), and a 470pF  
capacitor (C1) in parallel with the 1M resistor. It's all there in  
the service manual. Just compare the 2-series and 3-series wiring  
diagrams. There's a picture that shows you exactly where the  
additional components go.

Also, the Preset Percussion Switch under the Upper B preset key in 3- 
series organs guarantees that R1 (3.9M) is shorted when presets other  
than B are selected. If Percussion was left On with Percussion Volume  
at Normal, the volume of the Upper Manual would be reduced on ALL  
presets. (In other words, there are TWO switches that short R1  
restore full manual volume; a switch in the percussion switch  
assembly shorts it at certain settings, and the switch under the B  
preset shorts it when any other preset is pushed down.) This switch  
under the preset key is not present in organs before the 3-series,  
and retrofitting one could be a bit tricky.

Or, like Scott says, you can leave out the two resistors and the  
capacitor, and your percussion will work fine. These components have  
nothing to do with the triggering, harmonics, etc... You'll just end  
up with one shielded cable from the percussion switch that doesn't  
connect to anything and doesn't need to.

David

On Jan 10, 2008, at 8:54 PM, Jordan Kersten wrote:

>
>
> Thank you for that detailed explanation, it was exactly what I was  
> looking for.  I only have one last question: If I have a B2 with an  
> AO-10 D, E, F or G, then my matching transformer is the same that  
> is in a 3-series organ and the only mod I would have to make is to  
> add the 3.9M resistor for the percussion circuit?
>
>
>
> ----------------------------------------
>> From: thermionic27609 at earthlink.net
>> Date: Thu, 10 Jan 2008 16:36:38 -0500
>> To: hammond at zeni.net
>> Subject: Re: [HAM] Percussion & Matching Transformer
>>
>>
>> Jordan,
>>
>> The answer is a bit complicated, so bear with me.
>>
>> Page 2-25 of the Hammond service manual reads that the early 2-series
>> organs used "small core transformers" with AO-10 preamplifiers with
>> codes A,  B, and C.
>>
>> The later 2-series organs with AO-10s D, E, F, and G (plus all 3-
>> series organs) used "revised small-core transformers with smaller
>> stack and greater number of turns."
>>
>> The manual says these matching transformers types are not
>> interchangeable, but your mileage may vary. The two groups of AO-10s
>> do have somewhat different input circuits.
>>
>> The modifications to the matching transformer wiring to make it like
>> a 3-series organ aren't too complicated, though you may need more
>> solder terminals. What these resistors do is to lower the volume of
>> the entire upper manual at certain percussion settings. Some people
>> don't like this effect and jumper the 3.9M resistor permanently out
>> of circuit to maintain full volume on the upper manual at all times.
>> You might also want to add the special preset switch under the B
>> preset key, present in 3-series organs, but not in 2-series models.
>>
>> One effect you have to deduce from the wiring is that the cable
>> capacitance of the wiring used to short the 3.9M resistor becomes
>> significant due to the high value of the resistor. My calculations
>> indicate it should be about 400pF, and the effect is that the volume
>> is reduced less at higher frequencies since this cable capacitance
>> bypasses the 3.9M resistor. My theory is that the 470pF cap across
>> the 1M resistor is there to compensate for this cable capacitance,
>> and you could tune this 470pF value to taste, especially based on the
>> capacitance of the cable you have running from the percussion switch
>> to the matching transformer assembly. Lower capacitance cable might
>> make a smaller capacitor of 220pF-330pF useful.
>>
>> Bottom line: your percussion circuits will work just fine without the
>> extra resistors and capacitor. It all depends on whether you want it
>> to work exactly as Hammond designed it for the 3-series organs.
>>
>> David A.
>>
>> On Jan 10, 2008, at 3:48 PM, Jordan Kersten wrote:
>>
>>>
>>> Does anyone know exactly what the difference between a B2 and B3
>>> matching transformer is?  I know that the B3's have some sort of a
>>> resistor that has to do with percussion volume, and the B2's
>>> wouldn't have this.  Are the transformers themselves the same? Can
>>> you modify a B2's transformer with the percussion circuitry it
>>> needs? Or would you have to replace the B2 transformer with a B3-
>>> style one?  I am converting a B2 to a B3.  I now have tracked down
>>> the plate with percussion tabs, the AO-28 preamp, smooth drawbars
>>> and even a newer style vibrato line box.  I have herd that they
>>> have different transformers.  Can anyone comment on this?
>>>
>>>
>>> -Jordan-
>>
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>
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