HEWLETT PACKARD (HP) 185B
Manual de Servicio

Model 185B (3) The voltage across staircase capacitor C236 increases, and is applied, through SCANNING switch S203C/D, to unity amplifier V201AQ210, increasing output of the generator by the same amount; this output is the horizontalscan voltage. (4) After comparator blocking oscillator pulse decays, diode CR215 becomes reverse-biased again, and prevents charge on capacitor C236 from leaking off. (5) DENSITY contrbl R255B is in the charge path of staircase capacitor C236. DENSITY permits limited adjustment of charge-path resistance, thereby determining amplitude of staircase steps by controlling amount of charge added to staircase capacitor each time blocking oscillator fires. (6) The output of unity amplifier V201A-Q210 is applied to transformer T203 in such a way that a bootstrap action results. Thus for a given density, the same amount of charge is added to staircase capacitor C236 each time, which results in staircase steps of equal amplitude.

Section IV Paragraph 4-84 (7) When voltage across staircase capacitor C236 reaches about +15 volts, reset blocking oscillator Q209-T204 fires and discharges the capacitor. The staircase is thus reset and starts over again. 4-84. RESET BLOCKING OSCILLATOR. When the staircase voltage exceeds voltage on collector of Q209 (approximately 15 volts), diode CR217 becomes forward-biased, current begins to flow through collector winding of transformer T204, and capacitor C236 effectively becomes the voltage supply for blocking oscillator Q208-T204. The effect of this current flow is transformer-coupled to the base of Q209, which causes blocking oscillator to fire, essentially dropping collector of Q209 to ground. The resulting current-flow through CR217 and QZOQdischarges capacitor (2236, thereby resetting the staircase. It might be pointed out here that the amplitude of the staircase signal remains constant. The height of each individual step can be varied by adjusting R255B a s mentioned in paragraph 4-83, step d (5). The result of varying the height of each step is to change the number of steps per staircase, which changes the number of samples per sweep on the crt.

t25OVF

-IOOVF

tIOOVF

1

1

1

t12.6VF

t

8

L'Cg38

1
0209
?hi450

lo.01
=

4 R 2 t B IR253 13.7K

RESET BLOCKING OSClLL ATOR
1 -FEEDBACK

-12.6VF

r----R255E 30K

i TIMETEASE I OU WT
-'SA!! J202

I

R267
300

R272
HORIZONTAL SIGNAL
c

I8K

L

J

0t . C,
DENSITY

r
so-,
.I

52038

TO COMPARATOR AND HORIZONTAL AMPL.

Figure 4-10. Staircase Generator 01255-1 4-11