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Hi everyone.
I am interested to know how the tonality of the percussion effect on the very earlist B3, C3 and RT3 organs built between 1954 until sometime in 1955, with the very first version of the AO28-1 (with no code letter at the end) ciruilt differed from the percussion tonality of the subsequent AO28 preamps.
According to the service manual schematic, the very first version of the percussion circuit was only used in the AO28-1 preamps (with no code letter at the end) .
Someone who I have been corresponding with has a 1955 C3, and this C3 has a AO-28-1B preamp which has the revised or the second version of the percussion circuitry so therefore this indicates that the very first version of the AO28-1 preamp (with no code letter at the end) with the original percussion circuitry was only used for a very short time between December 1954 until the revision to the AO28 preamp circuit which occured sometime in 1955 with the introduction of the revised AO-281B circuit.
The service manual schematic shows that the original AO28-1 (with no code letter at the end) had a different percussion output high pass filter circuit between the points "P" and "D" in the AO28 preamp.
As well as that, the orginal AO28-1 (with no code letter at the end) had the R52, R53, and R54 47K resistors and the C32 0.0082 uf volume reducing and treble cut circuit wired after the secondary winding of the T5 percussion transformer whilst these R52, R53 and R54 47K resistors and the C32 0.0082 uf capacitor were omitted from all subsequent versions of the AO28 preamps.
By looking at the schematic circuit of the earliest version of the percussion output high pass filter, I suspect that this particular percussion output high pass filter circuit would cause the percussion signal to have more bottom end response compared to the revised or second version of the percussion high pass output filter.
My own 1962 C3 and 1965 C3 as well as a friend's 1959 C3 all have the "H-AO-28-2" version of the AO28 preamp and these all have the same second version of the percussion circuit and percussion output high pass filter circuit as that shown in the service manual schematic for the revised AO28 circuit.
Several years ago l installed the Trek II CPR-28 board into my 1962 C3, and this has the very last or the third version of the percussion output high pass filter circuit which includes the additional high pass filter effect created by the C35A 820 pf series capacitor and the R63A 820K grounding resistor as was used in the 1970's era H-AO28-5 preamps.
I noticed that the C35A and the R63A caused the percussion tonality to sound thinner so therefore l dissabled these by shorting out the C35A 820pf series capacitor and by disconnecting the R63A 820K grounding resistor thus recreating the specs of the second version of the percussion output high pass filter as was used in the H-AO-28-2 circuit.
For those who have a stock 1970's era H-AO-28-5 preamp and for those who have installed the Trek II CPR-28 board and who feel that the percussion effect sounds too thin, I definitely do recommend dissabling the additional high pass filter by shorting out the C35A 820 pf series capacitor and by unsoldering and removing the R63A 820K grounding resistor.
All the best.
Kon.
I am interested to know how the tonality of the percussion effect on the very earlist B3, C3 and RT3 organs built between 1954 until sometime in 1955, with the very first version of the AO28-1 (with no code letter at the end) ciruilt differed from the percussion tonality of the subsequent AO28 preamps.
According to the service manual schematic, the very first version of the percussion circuit was only used in the AO28-1 preamps (with no code letter at the end) .
Someone who I have been corresponding with has a 1955 C3, and this C3 has a AO-28-1B preamp which has the revised or the second version of the percussion circuitry so therefore this indicates that the very first version of the AO28-1 preamp (with no code letter at the end) with the original percussion circuitry was only used for a very short time between December 1954 until the revision to the AO28 preamp circuit which occured sometime in 1955 with the introduction of the revised AO-281B circuit.
The service manual schematic shows that the original AO28-1 (with no code letter at the end) had a different percussion output high pass filter circuit between the points "P" and "D" in the AO28 preamp.
As well as that, the orginal AO28-1 (with no code letter at the end) had the R52, R53, and R54 47K resistors and the C32 0.0082 uf volume reducing and treble cut circuit wired after the secondary winding of the T5 percussion transformer whilst these R52, R53 and R54 47K resistors and the C32 0.0082 uf capacitor were omitted from all subsequent versions of the AO28 preamps.
By looking at the schematic circuit of the earliest version of the percussion output high pass filter, I suspect that this particular percussion output high pass filter circuit would cause the percussion signal to have more bottom end response compared to the revised or second version of the percussion high pass output filter.
My own 1962 C3 and 1965 C3 as well as a friend's 1959 C3 all have the "H-AO-28-2" version of the AO28 preamp and these all have the same second version of the percussion circuit and percussion output high pass filter circuit as that shown in the service manual schematic for the revised AO28 circuit.
Several years ago l installed the Trek II CPR-28 board into my 1962 C3, and this has the very last or the third version of the percussion output high pass filter circuit which includes the additional high pass filter effect created by the C35A 820 pf series capacitor and the R63A 820K grounding resistor as was used in the 1970's era H-AO28-5 preamps.
I noticed that the C35A and the R63A caused the percussion tonality to sound thinner so therefore l dissabled these by shorting out the C35A 820pf series capacitor and by disconnecting the R63A 820K grounding resistor thus recreating the specs of the second version of the percussion output high pass filter as was used in the H-AO-28-2 circuit.
For those who have a stock 1970's era H-AO-28-5 preamp and for those who have installed the Trek II CPR-28 board and who feel that the percussion effect sounds too thin, I definitely do recommend dissabling the additional high pass filter by shorting out the C35A 820 pf series capacitor and by unsoldering and removing the R63A 820K grounding resistor.
All the best.
Kon.
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