发布时间 : 星期一 文章IC-751A 说明书更新完毕开始阅读f5a06d0548d7c1c709a1452d
少. There may be other failures. Unit is not repaired until listed voltages at D47 diode are correct. 8、可接收强信号，但灵敏度偏低。 Symptom: Distorted不难理
解 RX on strong signals. BC band RX sensitivity may be low or marginal.边缘 Probable Cause: Q15 transistor on the RF unit has become leaky.
Cure: Check and replace Q15 transistor. If Q15 transistor is leaky, there may be other problems on the RF unit. See cure for RX problem listed above. 9、当连接有室外天线时，不能接收，测量天线坐的芯，对地电压有6V。
Symptom: RX disappears when outer conductor of coax is connected to antenna jack. Inspection reveals 6 volts DC at the center conductor of the antenna jack. Probable Cause: Shorted DC-blocking capacitor C40 on the ANT SW unit. Cure: Replace the capacitor.
Remarks: There are probably other problems on the RF unit if this capacitor is bad. Check the voltages at D47 diode on the RF unit. Radio may have been struck 罢工by lightning or sustained持续 a large RF overload through the antenna jack. 9、当安装有UX-14组件时，在旋转大钮时，经常接收减弱。
Symptom: RX cuts消减 out randomly when turning VFO knob in IC-751A with UX-14 installed. Probable Cause: Incompatibility不兼容 between UX-14 and 751A is causing the PLL to unlock. Cure: Turn off transceive mode on UX-14 (bank S1, switch #3 to OFF position). 10、接收到杂乱无章的、象鸟叫的声音。 Symptom: Hash noise in RX, birdies.
Probable Causes: 1) Voltage regulators on PLL unit are oscillating, producing noise on VCC line.
2) Overheated components and/or loose screws are causing the power supply to run noisy. 3) Noisy DC-DC converter on display unit.
Cure: Replace overheated C129, C132, and C137 regulator filter capacitors on PLL unit. Inspect power supply for overheated components, tighten the screws that hold down the reg. unit. Don't forget to tighten the screw hidden beneath之下 the wire bundle
捆 where the DC output lines leave the case. If none of these help, and radio is a IC-751 (not A version), then the display DC-DC converter may need modification. Contact Icom 4
installed already. Inspect the D4 area of the ANT SW unit and replace any burned components. If none of these solutions solve the problem, disconnect the J10 connector on the RF unit and see if the problem goes away. If it does, then there is a problem with the TX/RX switching on the RF unit. If disconnecting J10 connector doesn't help, then most likely either the drivers or PA transistors are bad, especially if the ALC meter deflection偏转 is low.
Remarks: The spurious output may also be caused by incorrect band switch voltage to the low-pass filter unit, or even faulty/burned components on the filter unit itself. The above causes should be ruled out first since low-pass filter failures are uncommon in the IC-751(A). 17、在FM模式下工作，IC-751存在断续的、低功率发射问
题。 Symptom: Intermittent断续 low FM TX output power in IC-751. Probable Causes: Bad relay RL1 on FM unit. Cure: Replace bad RL1 relay (221-D009-M).
18、在RTTY 、CW模式下没有发射，在SSB 、AM模式下正
常。 Symptom: No TX output at all in RTTY or CW modes. SSB and AM are OK. Probable Causes: Bad carrier generator crystal on main unit. Cure: Replace bad X1 crystal on main unit.
19、无显示，仅仅有小数点显示，问题反复出现，与元件过热影响有关。
Symptom: No frequency display, only decimal points. Problem may be intermittent or temperature related. Probable Causes: Display DC-DC converter has noisy噪音 output.
Cure: Replace all of the following dried-out electrolytic capacitors on the display unit: C17, C18, C20, C21, C22, C23, C24, and C25. (These capacitors are light yellow when they are new. Capacitors that have turned brown are old and dried-out).
Remarks: While servicing the DC-DC converter section, be on the lookout for cracked solder joints, as this section of the radio tends往
往 to run hot. Also, it would be a good idea to reprogram the RAM card after servicing the DC-DC section.
20、IC-751在SSB模式下，极低的载波功率输出，与调整不恰当有关。
Symptom: Poor carrier suppression抑制 in SSB mode in IC-751. Will not adjust properly. Probable Causes: Main unit needs factory update. Cure: Contact Icom Technical support for update bulletin. 21、在SSB模式下发射，声音低而浑浊不清，甚至显示屏关闭。
Symptom: Low garbled浑
浊 audio sound in speaker/headphones in SSB TX, even with monitor turned off. Probable Causes: RF feedback. 1) Inadequate station ground.
2) External 3rd party power supply needs RF decoupling. 3) Main unit needs factory update (IC-751 only).
Cure: Check station ground. If a 3rd party power supply is being used, add RF decoupling capacitors to output terminals to prevent regulation problems. If the problem occurs in an IC-751, a factory update may need to be installed to the main unit. Contact Icom Tech Support for this information. 22、设备看起来一直工作在“SACN”模式
下。 Symptom: Radio seems to be stuck in \chip on the logic unit. Cure: Replace IC2 integrated circuit (RP5G01-007). 23、设备内部发出大的高音调的叫声。
Symptom: Loud, high pitched 斜度squeal尖叫 emanates发出 from inside radio. Probable Causes: DC-DC converter transformer has a loose core.磁心松动 Cure: Replace T1 transformer (TO-9) on display unit. 24、VFO调节不稳，但缓慢调节大旋钮时，显示数字不增长。 Symptom: VFO tuning is erratic不
稳, sometimes skips when changing frequencies or bands. Display does not increment smoothly when VFO knob is turned slowly. Probable Causes: Dirty rotary encoder.
Cure: Remove white plastic cover from back of rotary encoder and spray clean the carbon track with Blue Shower solvent. Spin VFO knob vigorously while spraying. Remarks: Do not let solvent saturate the display unit or allow it to come into contact with the plastic display lens. Stuff塞 a paper towe毛
巾 between the rotary encoder and display unit to catch spray run-off. 25、显示屏上反复出现缺字段或模式指示符的故障。
Symptom: Missing segments or mode indicators on display. Problem may be intermittent. Probable Causes: Bad connection between logic and display units.
Cure: Check the connectors J5 and J13 on the logic unit where the flex cables to the display unit are attached. If they look OK, and none of their connector spring contacts flattened, replace the flex cables. 26、低MIC增益的问题。 Symptom: Low mic gain when using Heil mic element.
Cure: Install modification: In 751, change value of R264 resistor on main unit to 200 K. In 751A, change value of R21 resistor on AF-VR unit to 47 K.