Hammond X-66 User Manual

X66 Vibrato Driver Preamp Dec.2010
dan.vigin

Page 1

Hammond



X-66

Renovating the

Vibrato Driver Preamp

by Dan.Vigin

Hammond® and Leslie® are registered trademarks of Suzuki Musical Instrument Manufacturing Co. Ltd.

Binche / Belgium Dec.2010

X66 Vibrato Driver Preamp Dec.2010
dan.vigin

Page 2

TABLE OF CONTENTS

- Forewords & Purpose of this project P. 3

- General Description P. 4
Original Vibrato Driver Preamp circuit

- X-66 Headroom concept P. 6

- New VDP circuit P. 8

- Transistor MJE340 P. 9

- Emitter resistor P. 10

- Schematic Diagram of new VDP P. 11

- Output levels P. 12

- Realization P. 12

- Conclusions P. 12

- PCB Layouts P. 13

- Datasheet MJE340 P. 14

X66 Vibrato Driver Preamp Dec.2010
dan.vigin

Page 3

Renovating the Vibrato Driver Preamp on Hammond X-66.

Forewords – Purpose of this Project

Several weeks ago, when playing an abnormal level of background noise and hiss was
detected and this excessive noise made me nervous enough to start investigations in
order to reduce this annoying noise.

This background noise was mostly audible in Drawbars use while being less significant
with either Tabs or Percussions.

Even with no key depressed, by checking the outputs of the Vibrato Driver Preamp
board, with the help of an oscilloscope, I found at once that one channel exhibited
much more noise than the two others. This faulty channel was the Drawbar channel.

So, I decided to pursue my investigations. Measured voltages were normal, capacitors
were previously replaced by new ones.

The conclusion is rather simple, the driver transistor creates this inherent noise in the
Drawbar channel.

The first reaction is to find and replace this faulty component by a new one.
Unfortunately, this driver transistor labelled 21221 in the Hammond parts list is the

equivalent of RCA 39699. This transistor was produced in the early 60's and is in the
line with the first generation of Si NPN transistors launched at that time by RCA, about
50 years ago !.

Needless to say that this component is now unobtainable, at least in Europe.
In view of this situation, I decided to redesign this board and re-install much updated

transistors providing better performances than the former RCA 396999.
This is exactly the purpose of this chapter.

X66 Vibrato Driver Preamp Dec.2010
dan.vigin

Page 4

General Description.

Original Vibrato Driver Preamp circuit.

The board AO-32297-1 is located at the back side of the scanner block, under the
Upper keyboard.

Personnaly, I don't like at all the way this board has been engineered. This is the kind
of board commonly used to create prototypes in lab's but not for production units.
This is a personal opinion.

Top view

Bottom view

X66 Vibrato Driver Preamp Dec.2010
dan.vigin

Page 5

The three Vibrato Driver Preamp's ( VDP's for short ) are located on the same board.
The concept of each VDP is the same however they all have some slight differences
that have to be taken into consideration.

In fact, the role of each VDP is to procure enought amplitude to 'drive' its
corresponding vibrato phase shift line that is linked directly to the scanner.

The VDP board is then composed of three sections:

- on the left, the TREBLE VIBRATO DRIVE PPREAMP

- in the center, the TAB VOICE VIBRATO DRIVE PREAMP

- on the right, the CELESTE VIBRATO DRIVE PREAMP
There is no so much to tell about the circuitry. Basic 'Common Emitter' configuration is

used with a partial decoupling in the emitter resistors.
Refer to original schematic diagram hereunder.

Fig 5-7 (partial) of X-66 Service Manual.