inpi etlkiiil:i thtt clt~sired f?rtqur~ricy. 14?01- example, if ltalc+cl loat for Idh t tit1 ILloclt4 20OA and 200B is
it is desired to selrct 400 cps, set the frcv~uvncy dial 500 otliiis rosist ivc.. llr~rtic~ loads rill1 tw uscd but
to ?40? and the IiANGE switch to ??~10?. rcbsult in ib coi~rc~s~~oiitlirrg loss of power. Lower
The niagnitutle of the output voltage is deter- loads ;ire riot recommc~rrcled because they cause an
mined by the setting of the AMPL control. By increwsc in distortioii and a reduction in the output
means of this control voltages up to app:*oximately voltage. However, no dtimagc. to the instrument
25 volts can be selected whcln the instrument is will occur through thtb use of loads of less than 500
working into its rated load. () hms.
MA NTENANCE
horizontal dcffrcting plate input.
Replacement of lamp R7
Next chthck the calibration of the Model 200 by
If the 3-watt lamp (R7) should require rtqdace- mwns of 1,issajous figures on the oscilloscope; de-
ment, it is necessary for proper operation to check twrnint? M ticithvr thc. frrquencies gtmtmted by the
the ac voltage between the junction of C4-Kl9 and Rlodel 200 iirt? higher or lower than the dial calibra-
ground with the new lamp in the circuit. As meas- tion. If the instrument is in nerd of recalibration,
urvtl with a high-impedance (1 megohm or more) the! dial calibration will show a definitcl trend either
ac viiciium tube voltmeter, this voltage should be higher or lower than the true output frequency for
within the range of approximately 20-22 volts. If each range. The various ranges of the Model 200
the voltage is not within this range, it may be cor- do not nrcrssarily change in the same ?direction?.
rrcttd by chrtnging thc value of R11. Increasing the If thtb xt r?angt? rtyuirc.5 rutijristing, change the
value of this resistor will cause an increasth in the sinall rwistors in serics with lil and 124. If the out-
subject voltage, while decreasing the value will (le- licit frcqiicliicy is hinhvr t tian thv (1i;iI ciilit)ration,
crease the voltage. A change of 50 ohms will ctianxo iiicrcvisc> this ViiIti(1 of ttitb smilll rthsistor; if thv out-
the voltage ahout 1 volt. If it is necessary to usv it iriit frctlucwy is Iowcbr th;iii the dial c.;ilil)rat.ion, de-
resistor of more than 820 ohms, rojtvt the lamp in Cr(?iis(b tlich valiitb of thin siiitlll rcbsistor. A change of
favor of another. The design of the circuit is such about 100,000 ohms will chang:c thc calibration
that not more than an 820-ohm composition resis- about 12?; . I)o Iiot (hinge the value of K1 more
tor should be used. then 100,000 ohms without making a correspond-
ing change in R4. It is destrable to keep the value of
Frequency Calibration H1 and It4 as equal as possible, btlcause the oscil-
lator teiids to become unstable if the resistors are
If a change occurs in the frequency calibration
after a long period of use, the cause of this change too unbalmced. After each change it is necessary
probably lies in the aging of the frequency-deter- to rep1ac.e the dust cover before checking the out-
mining resistors in the oscillator positive feedback put frequency.
network. These resistors each consist of tl precision The same proccdrrre should b~ used if necessary
resistor in series with a small one-half watt resistor to adjust the calibration of the x10 and x100 ranges.
which is selected to give a very accurate frequency Ori the x10 rmxc a chanp grrater than 10,000
calibration. Therefore, adjusting the frequency cal- ohms should ottt l)t~ marl(8 in 122 without ill?io rhang-
ibration requires that the value of these small re- irig R5. On the x IO(J r:iiit:t* a ch;ingv grt.;lttar than
sistors be changed as necessary to obtain proper houltl [lot he, rriatl(* in IC! without also
calibration. changing it&
4 |
ACEC
