• Transmit pulse rate
• Transmit pulse width
• Integrator sample delay
• Integrator sample pulse width
• Secondary integrator sample delay
• Autotrack speed
HammerHeard PI PC Boards
Parts List (Opt. 1)
Resistors (5% 1/4-watt)
R5, R6, R15, R37, R43,
R44, R45a, R47, R48, R5010k
R12, R24, R25,
R28, R36, R391k
R13, R16, R17, R191M
R17, R19, R26, R27, R29,
R32, R33, R34a, R38100k
R2, R18, R31100k
R3, R42, R4610k
R415k Audio Taper
C4, C5, C6,
C7, C14, C15, C2747µF elect.
C8, C24, C25, C2610nF poly.
C18, C22, C230.1µF poly.
C19, C20, C210.47µF poly.
D1-D4, D6, D71N4148
Q1, Q5, Q6, Q72N3906
Q2, Q8, Q9, Q112N3904
IC278L05 (+5v ref)
IC3, IC479L05(-5v ref)
IC5, IC9NE555 (Timer)
IC74066 (Bilateral switch)
IC11, IC1274HC221 (Multivibrator)
SW1, SW2SPST switch
Search coil (see text), 12v battery,
battery clip, housing, knobs, coax, etc.
Troubleshooting and solutions
Here are some common problems and potential solutions.
Output voltage of the 7660 is not right.
• Check the polarity of the diodes.
• Check the polarity of the electrolytic cap C3.
• Try disconnecting C11.
No pulses or strange pulse widths coming out of the 74221’s.
• Make sure they are CMOS versions of the 74221 (C, HC, AC). TTL
versions such as the 74LS221 will not work.
• Check polarities of D6 & D7.
Coil pulse is ringing badly
• Oscilloscope probe capacitance is loading the coil. Use a low-
• Incorrect damping resistor value.
• Excessive cable or shield capacitance.
Valid pulse on the coil, but no output on opamp IC6:
• Misadjusted offset R18 can saturate the output.
• An incorrect (low) value for R16 can saturate IC6.
• Bad clamping diode D3/D4 can fry the opamp.
Calibration of IC8a via R23 doesn’t work:
• For this procedure, R44 & R48 should be installed, but Q8 & Q9 should NOT be installed. If Q8 & Q9 have been installed already, simply short R44 & R48.
• R5 (or R15) should not be installed. If they are installed and Q3 is being pulsed, then the coil should be disconnected. There will be a slight (but acceptable) error in calibration.
• Is R22 installed (low-pass filter) or shorted (no filter)?
• If R22/C16 was installed as a highpass filter, temporarily hort C16.
Audio is not working or is erratic:
• Make sure TP9 is close to zero volts for no target.
• Try reducing sensitivity via R35.
• If testing indoors, try going outside, well away from noise sources.