Philips PM 3342 Manual page 44

2
FET INPUT UNIT
FOR DUAL-TRACE PM 3342
Make the following changes for channel A (For channel B select the corresponding parts).
a. At the decoupling unit near the 32 pole plug: remove the connection of the -150 V supply and
load the +6, 3 V supply with 10 52 (10 W) at the drift-compensator print.
b. Remove the connections of the 2 mentioned supplies from the decoupling unit to the amplifier
prints.
c. At the amplifier print, channel A: remove the -150 V supply and the +6. 3 V supply, and
connect C205 (68 nF; plate) between connection C223/R231/c TS202 and earth (C208).
Remove B201, GR201, GR202, GR203, GR204, GR206 and R201, R202, R204, R206, R207,
R208, ft209, R219, R223, L201, L202, C201, C202, C207, C213.
d. At the drift -compensator print: remove B202 and interconnect "1" and "9".
Remove R219 and R319. Change R211 and R216 for resistors 6800 S2 1/8 W 5 %.
e. Mount the FET-unit, using two 3-mm metal spacers and connect it according to the circuit
diagram shown below.
f. Plug the DUAL -TRACE Unit into the basic oscilloscope. Balance according to chapter
XI Bl and B2, and check the square wave response according to F1 and F2.
g. BFWll to be selected at ID
~ 2.5 mA, VDS ti 9 V for VGS ti 1. 7 V
Note: 1. for channel B it is advisable to use a mirrored layout of the circuit.
2. it is recommended to replace C401 by a trimmer 6 pF//5. 6 or 12 pF cer.
3. change the value of R238, R246, R338, R346, to 510 Sl and the value of R252, R257, R352,
R357 to 120 Sl.
4. change the value of R242, R342 to 82 S2