658 
aie) 
constant. The ratio of the response for time constant 9 to the response for 
infinite time constant is plotted against the reciprocal of the time constant 
@, as shown in Fig. 7. Over the range of interest, the curve is approxi- 
mately a straight line. This curve can be compared with a specified stand- 
ard of performance, and also provides 2 convenient means of determining 
the error in recording a negative exponential of stated duration, The dashed 
curve is calculated from Eq. (6), assuming an equivalent amplifier time con= 
stant Uf (ope 0.36 pSECe This value is obtained frdém the frequency-response 
curve for the same instrument, which shows a 30-percent drop in response at 
a frequency of lO ke/see [see Figs (6)]. The agreement between the two 
curves is fairly good, in view of the simplified analysis leading to Eq. (6). 
The compensated low-frequency response is checked by a voltage step on 
the test film, which has rarely shown any variation greater than 1 peroent 
over the 25-msec interval used. On a typical unit, the time to fall 30 per- 
cent is 160 msec, corresponding to a frequency of 0.99 cycle/sec. 
The sensitivity is 50 mv dec. per inch at maximum gain and may be dc- 
creased in steps of approximately 35 percent to a minimum of 5 volts per 
inch deflection on the tube screen. The linearity is measured from the test 
film which has steps of g-in, 1-in., 13-in., and ‘2-in. deflection on the 
screen, To pass the test, scopes must have the ratio of deflection to volt- 
age agree to 1 percent up to 14-in,. deflection, The gain stability is tested 
during use and is usually better than 1 percent provided the a-c line is 
stablee 
The troubles encountered with the t,rpe 208 oscillograph are usually due 
to tube or mechanical. failures. The most common are: noisy input 6F8*tube, 
bad focus of the cathode-ray tube due to age, a broken wire in the output- 
stage inductances, loose cathode-ray tube socket, and occasionally a very 
high-frequency oscillation in the regulated 150-volt supply. This last 
trouble may be cured by installing a 0.01-pf condenser from the plate of the 
input 6F8 to ground. 
Up to this time, 18 oscillographs have been modified as outlined in the 
foregoing, and have been in use for periods up to three years. 
(ii) Four-channel oscillograph. This unit was designed to fill the 
need for a compact multichannel oscillograph recording on moving film, ‘It 
has a flat frequency response from direct current to 100 ke/sec; a maximum 
