Heathkit 342 Owners Manual

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HEATHK'IT LOW CAPACITY PROBE KIT #342

When investigating high frequency, high impedance, or broad bandwidth circuits such as en-
countered in television service work, it is essential that the capacity of the probe used be sut-
ticiently low so that distortion or loss of gain in the circuit will not occur. The Heathkit Low
Capacity Probe.used with anoscilloscope having moderate to high input capacity.will permit ac-
curate oscilloscope display oi the wave form under observation. A variable capacitor in the
probe furnishes the degree of flexibility necessary to permit probe use with the input of prac-
tically any oscilloscope. It is only necessary to adjust the trimmer tor the instrument with
which the probe is to be used. The probe can then be used repeatedly without further adjust-
ment. When the same probe is used with other instruments, it will be hecessary to readjust the
trimmer with each instrument change.

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PROBE CONSTRUCTION

The pictorial clearly shows the assembly detail of the Low Capacity Probe. The assembly will
be further simplified ii the resistor and bare wire are first assembled to the trimmer condens-
er and the shielded test lead connected to the solder lug closest to the mounting bracket of the
trimmer. The two probe ends can be readily identified by their color, red polystyrene for the
tip end and black polystyrene for the shielded wire end. The red probe end should be assembled
to the probe body using a sell-tapping screw. The solderless tip can then be iirmly screwed in-
to the probe end, the bare wire dressed through the probe body and the opening in the solderless
tip, then pulled through until the trimmer condenser assembly is approximately aligned with the
drilled openings in the probe body. Surplus bare wire should be trimmed of! and the collar of
the solderless phone tip iirmly tightened. The trimmer condenser is secured to the probe body
by using a 6-32 screw, lockwasher, and nut. The black insulated probe end should then be slip-
ped over the test lead. The flat flexible braid also should he, slipped through the opening in the
probe end, and the braid soldered to the shielded braid covering of the test lead. When the in-
sulated probe end is attached to the probe body, the short length of shielded covering on the test
lead should be dressed along the flat portion on the shank of the probe end in such a manner that
the actual mechanical assembly of the probe and to the probe body will result in Jamming the
braid against the probe wall. This procedure will properly ground the probe body and also pro-
vide strain relief for the test lead. The probe end should then be secured with a self-tapping
screw. Final 3 bly involves the installation of the spade lugs on the end of the shielded
cable, the amflip to the end of the flat braid. and slipping the 1/ " rubber grommet in
place in the pr y. -

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TEST AND ADJUSTMENT

The variable capacitor in the probe can be adjusted in a number oi ways, and as a matter of
convenience either of the following methods can be used. If a square wave generator is avail-
able. connect the probe tip to the generator output and the ground clip oi the probe to the ground
terminal of the generate The spade lugs of the probe cable should be connected to the oscil-
loscope vertical input te als. The exact frequency output oi the square wave generator is
not too important, and any frequency between 10,000 and 50,000 cycles can be used. Adjust the
oscilloscope so that two 0 three cycles of the square wave form are observed. Then with a
screw driver (insulated gnment type preferred) adjust the trimmer in the probe body for
. the ideal square wave re uction. Of course, the oscilloscope used must be capable oi rea-
b sonably good square wave response. improper adjustment of the probe trimmer will be reveal-
ed as badly peaked or rounded square waves. When the adjustment is properly made for the
correct square wave shape, the probe is ready for use with the oscilloscope. Ii theoscllloscope
i has a step attenuator, it may be necessary to readjust the probe each time attenuation is alter-
| ed. thereby changing the scope input impedance.
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In the event a square wave generator is not available, the probe trimmer can be adjusted by

utilizing the saw-tooth output oi the oscilloscope multivibrator sweep generator. The Beathkit

Models 0-7 and 0-8 Oscilloscopes can be used with the Low Capacity Probe in the following
[ manner: Connect the spade lugs to the vertical input, set the frequency selector in the switch

position which will allow a sweep frequency in the range of 180 to 1800 cycles. The frequency
1 vernier control should be turned to zero. The tip of the Low Capacity Pr 3 then connected

to the input grid #2 of the 12AT7 tube used in the horizontal push-pull on stage. A wave

form similar to that shown in Figure A or B will be observed by regulating the vertical and
' horizontal gain controls at the oscilloscope. The probe trimmer should now be adjusted until a
f straight line as shown in Figure C is obtainable. The probe is now correctly compensated ior
i the input circuit of the oscilloscope.

A B C

TRIMMER CAFACITV TRIMMER CAPAClYV TRIMMER ADJUSTED
HIGH LOW DROPERLV

Since the Low Capacity Probe presents a high impedance input along with a low capacity input,
there is necessarily some degree oi attenuation present. it is necessary to obtain an approx-
imate indication of the degree of attenuation, and this can bequickly determined in the following
manner. Using a calibrated graph screen with the oscilloscope, the Low Capacity Probe should
be connected to the oscilloscope vertical input terminals, and the probe tip connected to the out-
put oi a sine wave generator source. Next connect a jumper wire from the oscilloscope vertical
input to the same output terminaloi the sine wave generator source. The exact frequency of the
sine wave generator is not critical, and usually a l,000 cycle sine wave will be satisfactory.
Advance the oscilloscope vertical gain control until a vertical deflection of a given convenient
value is obtained. For example, deilection covering ten vertical squares of the graph screen.
Assuming that ten squares are used for a reference and with the vertical gain control carefully
l adjusted, the jumper wire should beremoved and than without touching any of the controls, note
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the number of vertical squares now covered by the wave form. If, for example, only two squares
are covered, the ratio of attenuation is of course 5 to l. If one square is covered. the atten-
uation is 10 to 1. The Low Capacity Probe will prove a useful oscilloscope accessory and can
be used quite satisfactorily up to frequencies at 12 to 15 megacycles.

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