Hammond X-66 - Redesigned VDP Circuit

X66 Vibrato Driver Preamp Dec.2010
dan.vigin
Page 8
New VDP circuit.
Since very few information as far as the specifications of this VDP board are available
in the X-66 Service Manual, the original board AO-32297-1 board was dismounted and
put on the workbench for test.
As stated earlier, only one channel was defective (noisy), so the two others were
workable for measurements.
The original board AO-32297-1 was powered by +250 Vdc with an incoming signal of
50 mVpp @ 1Khz . The collector of transistor 21221 (RCA 39699) was loaded by a
12K/1W resistor to ground via one 1µF/400Vdc capacitor. See simplified schematic
herafter.
The three sections of the AO-32297-1 board were configurated so and results of
measurements were recorded in order to redesign a new VDP board.
With the help of a workbench simulator software, a new VDP circuit was redesigned
based on MJE340 transistor having much superior performances than RCA 36699.
VIBRATO
DRIVE PREAMP
12 K /5 W
82 O33 O
22 K
E
C
B
+
-
-
+
MJE340
100 µF
63 V
1 µF
63 V
1 nF
220 K
T1
E
B
C
2
1
3
TO-126
MJE340
Test-Point
1 µF
400 V
GND
50 mVpp
@ 1KHz
12K/1W
+ 250 Vdc
Load resistor
applied during
measurements
IN
OUT
To Vibrato Phase
Shift Line
Foot emitter
resistor