SECTION 4
maintenance
4-7 CABINET REMOVAL has a designed life expectancy greater than the elec-
trical portion of instrument and will normally require
The two piece cabinet can be easily and quickly re- no maintenance.
moved as follows:
If cable drive system damage does occur, cable re-
a. Remove rear cabinet cover after unscrewing four placement can be accomplished by following directions
sheet metal screws holding cover. in -hp- Service Notes No. M14 in the rear pages of
this manual.
b. Place instrument with front control panel up.
4-4 TUBE REPLACEMENT
c. Loosen two 3-16 in. Allen screws located on bottom
of cabinet adjacent to front feet. Tubes in an -hp- Model 650A Test Oscillator can be
d. Lift cabinet from instrument. replaced without special considerations with certain
exceptions. Table 4-1 gives recommended tube re-
placements and tube functions. Instructions for select-
ing tubes. when required, along with any adjustments
1 or test necessary after replacing a certain tube are
Do not change settings of trimmer capacitors given in the following paragraphs.
mounted on or near tuning capacitor gangs.
These trimmers affect both calibration and
frequency response. Adjust these capacitors
only when proper test equipment is available
Power to instrument must be turned off prior and according to instructions contained in this
to removing any of tubes V1, V2, V3, V4, or
V16. This precaution will prevent ballast tube manual.
current regulator action from burning out
4-2 LUBRICATION OF TUNING CAPACITOR heaters of remaining tubes.
DRIVE MECHANISM
The tuning capacitor drive mechanism requires monthly
lubrication if instrument is in constant use. Remove Only type 5654 tubes are recommended for replace-
dirt or dust with a moderate air stream and place one ment of these five tubes. If 6AK5 tubes are used,
drop of lubricant at two points as shown in Figure 4-1. after replacing VI. V2. V3. or V4 it will be necessary
Recommended lubricant is "Lubriplate #3" (Fiske to adjust R42 to set ac heater voltage. Transconduc-
Bros. Refining Co., Newark. N.J.) or equivalent. tance of tubes V1, V2, and V3 should be nearly equal
and above 5000 micromhos with a preferred value be-
tween 5400 and 5600 micromhos. Although not critical,
4-3 DRIVE SYSTEM FOR OSCILLATOR impravedperformance will be obtained from low micro-
TUNING CAPACITORS
phonic tubes.
The two oscillator tuning capacitors effectively have
Transconductance of replacement for V4 should be
a common shaft since their shafts are mechanically above 5000 micromhos.
connected by a short supported shaft and two flexible
couplings. SET SCREWS IN COUPLINGS MUST NOT
Transconductance of replacement for V5 should not
BE LOOSENED. If shafts of tuning capacitors are be more than 4000 micromhos. A tube with high trans-
allowed to rotate with respect to each other, frequency
dial calibration will be lost. It may be necessary to conductance for V5 may cause low output from the
return instrument for factory recalibration. high frequency oscillator.
After replacement of all or any one of these five tubes,
The front panel FREQUENCY controls are coupled
to the oscillator tuning capacitor shafts by means of frequency response and calibration of the high fre-
a wire cable arrangement. This cable drive system quency oscillator should be checked. It will also be
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