怎样修好一台无线遥控接收的

Bowman

       公司每年要配置几十台无线遥控器在生产的设备上，有些遥控器已经是十几年前的了，有失效了需要返回检修的。由于是第三方产品，我们并没有图纸，所以维修比较困难。唯一的办法就是找一台好的进行对比维修，试试运气。
" a: v' u( f: R# ]. p# E2 S       最近维修了一台遥控接收机，把经验和教训交流一下。经验是：对比后首先从明显的不同处下手(在该例中，主要一个发光二极管显示有误)；查找相关器件的资料，弄清楚一些关键功能和逻辑关系；方便的话做一些适当的代换以缩小故障查找范围以致最后锁定故障元件。教训是：任何时候先确定供电电源是否正常（在该例中，犯了没有首先查供电的低级错误以致走了弯路）。下面是写给工头的检修记录供参考：
! n: S7 T- ?$ b4 _# t: x& P- jMemorandum of troubleshooting on Alpha 560S receiver
* S# l4 l/ e; c, p2 v4 a3 q/ m8 xBad circuit board showed: LED13 red light; LED2 (Status) constant red light; LED4 extinguish
& `) ?/ N( P# |Good circuit board showed: LED13 red light; LED2 red blink, LED4 constant red light
( S& m4 v; x; Q& f& U) wStep1: de-soldered shield covers on both good and bad circuit boards, metered two ends of LED2, bad one: 0V and 1.7V, good one: 4.8V and 5.0V. All major chips are: U7 (HEF4021B), U4 (74HC14), U9 & U10 (ULN2003A), U3 (LM324), U5 (W78E52B-40), U6 (3373P), U1 & U2 (LM7805), Y1 (crystal) and U8 (cannot see the info, only marked EEPROM on the board). LED2 is associated with U7 and U5, and U4, and some deferent volts were showing on pin30 of U5, pin13 of U4, crystal, and etc when comparing.
$ q5 C) y4 m* c0 ^$ eStep2: exchanged U5 and crystal, problem remained. It is good news to convince U5 is good since the 40pins U5 is out of date and cannot get info from its manufacture Winbond and distributor Digi-Key.
Step3: Read through data sheets of U7, U4, U9 U6, and U3 and metered more pins on those Ux and focused on U7 because the manual of Alpha560S tells if LED2 constant red light, could be incorrect receiver ID code setting. According to data sheet of U7 (it says “when PL is LOW, data on DS is shifted into the first register position and all the data in the register is shifted one position to the right on the LOW-to-HIGH transition of CP ”), metered its pin9 (PL) on bad board, which is a little high 1.08V while the good one is 0V.
Step4: analysis: All those Ux are associated with each other, it’s hard to determine whom is fault causing the wrong voltage to others without schematic. For this particular issue, the higher volt on PL of U7 is the key causing LED2 constant red light.
5 ]  S: b* e3 O1 QStep5: Trace the circuit and found the power supply is strange: bad one: 4.09V and good one 5.0V. Further found volt on filter capacitor: bad one 11.9V, good one 16.6V. Suspect the CAP. Disconnect power, metered the CAP: bad one had no charge/discharge activity. Borrowed the good CAP onto the bad board and powered on, everything became OK: the CAP becomes dry and loss its function.
% s, i) \0 s  e/ NLesson: Why ignored checking power supply at the first stage was mainly because no short circuit and lights worked properly only wrong position, which led to focus on tracing signal rather than power supply. Later on, noticed the circuit board was produced in 2001, and it is a long time for some CAP’s life time.

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