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PK-88 OPERATING MANUAL
Short circuits on the address and data lines can also appear as lack
of activity on the control bus lines, especially device select lines.
•
Verify each of the 16 address and 8 data lines for activity. Any lines
showing a lack of activity are not operating properly.
•
Remove all memory chips if you suspect problems with address or data
lines. Each address and data line will now show a distinct pattern. The
address lines should be (possibly distorted) square waves whose periods
increase by a factor of two on successive lines as you move line by line
from A0 to A15.
•
If you use an Ohmmeter to look for short-circuited lines, make sure it is
a low-voltage, low-current test instrument. Most modern digital
voltmeters are adequate for such tests. Disconnect the power source and
remove any integrated circuits connected to the lines being measured if
in doubt.
•
If you suspect a short circuit, verify the high-density areas of the
printed circuit board for the problem.
6.3.2
Symptom: Transmitter cannot be keyed
If the transmitter cannot be keyed and the DCD LED is not lit:
Verify the watchdog timer circuit at TR3 and TR2.
Verify PTT driver transistors TR1, TR5 and TR4.
Verify timing capacitor C18.
6.3.3
Symptom: Transmitted signals not copyable by other stations
If stations are unable to decode your transmissions when using VHF FM:
Verify your transmitter's deviation or modulation index. Verify that peak
deviation at any tone does not exceed 4 KHz.
Verify that your transmitter's modulation limiting circuits are not being
driven into limiting or clipping, and that both tones are being
transmitted in the correct amplitude ratio. A standard deviation meter
should show the higher (Space or Start) tone to be higher in amplitude
than the lower (Mark or Stop) tone.
Adjust the PK-88's AFSK output level with the internal adjustment
potentiometer VRl to produce the correct transmitter deviation.
Verify that your TXDELAY is long enough for the distant station's
receiver to recover from transmit mode and squelch. Increase your TXDELAY
in increments of 10 (100 milliseconds) to see if the distant station's
decoding improves.
6-4
TROUBLESHOOTING
PK88-178
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