Analysis and repair: the radio 74 series TTL 3-8 universal digital IC segment control decoder 74LS138, cyclophosphamide-four feet for the input control, sadly, Krispy Kreme ~15 feet to the foot of decoding output, decoding functions as listed in table 2. Table 1 represents a high level, 0 represents the low level. Pressing band selection button, thejNKservice manual download site
jNKservice manual download site
CPU33, and 32 feet output by 00, and 01, and 10, and 11 cycle changes, and by received plug-in of A9, and A2 Terminal plus to 74LS 138 ①, and II feet (a,, and b), due to ③ feet grounding (C=0), and ⑥ feet received are power (G=1), and 3), and ⑤ feet grounding (G1=G2=0), so continuous by band select key Shi, 74LS138 of 15 feet (Y0), and 14 feet (Y1), and 13 feet (Y2), and 12 feet (Y3) will followed by output low level, Control switch Q11, Q6 and Q7, Q8 and Q9, Q4 and Q5 conduction respectively to complete the segment.jNKservice manual download site
jNKservice manual download site
Inspection, pressing band selection key and the 74LS138 voltage of ② ① feet, according to 0011 order changes, but found that the output side 15 pin to 0V, and 14 foot ~12 foot 4.3V (high level), and does not change with the band selection button, indicating 74LS138 internal circuit damage, lose their normal decoding function, replace the band switch back to normal.jNKservice manual download site
jNKservice manual download site
Discussion: 1. If the band does not work, first among 16, h foot 74LS138 to check whether the power supply is normal, four feet high, ③, ④, ⑤ feet is a low level, such as normal you can determine 74LS1.38 damage; 2. If the band switch health, 74LS138 15~12 foot can output low in turn shows 74LS138 CPU outputs 33, 32 feet and band selection key contacts are normal if a particular band does not work, you should check the control switch or the tube is not damaged.jNKservice manual download site