我检修/装制分立件功放的实用方法

小鬼头

我检修/装制分立件功放的实用方法

    在装制/检修分立件功放中，烧毁功率管是一件恼人的事，既费劲、费钱，又费时。我在实践中摸索了一套方法，这些方法可以视情况组合使用，能较好地避免再度烧毁功率管、快速检查和排除故障。

    下面介绍的这些方法主要针对常见的负反馈型三极管输出功放线路的（注意：有些功放并不一定适用）。你可以灵活运用于其它情形。

一、将功率输出管与电路分开

    将功率输出管与电路分开，可以避免许多意外的情形而再度烧毁功率管。如果你有足够的经验，当然也可省略掉这一步。但将两者分开处理并不是一件难事，请看附图的1。

我检修/装制分立件功放的实用方法

    该图是典型的分立件负反馈式功放示意图。其中Q1至Q4是功率管。将功率输出级与电压放大级分开的要点是断开功率管，保持负反馈的连接，以便进行检修。若果不保持负反馈连接的话，正常放大器也会因失调电压问题而出现高的直流输出电位，导致给放大级的检修带来障碍。
    1、将所有功率管（Q1至Q4）拆走，或全部断开电气连接。可以断开这些三极管接线中的两条，但不推荐使用只断开基极一条的做法。
    2、在B1点与输出端Vout、B2点与Vout之间各接一只330欧姆至1K欧姆左右的电阻，以让电路的负反馈生效。
    经此处理后，就可以放心检查前面的电压放大级。其检修就类似于检修一台分立件前置放大器。在修好前面的电压放大级、确认偏置电压Vbias压无误后，才恢复至正常连接（必要时，可使用第五即附图3的方法，以进一步保护功率管）

二、检修电压放大级

    检修电压放大级正常与否，有两个重要标志：一是Vout点的直流电压。正常的机子应在100mV以内、更常见的是在50MV以内。二是功率输出级的偏置电压Vbias的大小。见附图的2。

我检修/装制分立件功放的实用方法

    表中列出了我大概估算的正常机子Vbias范围。表中，还列有经第一项处理后的VBIAS的大致范围（正常的Vbias经处理后也会增高一些）——这些值是我估算的，仅供参考——若果机子有问题，这个VBIAS常常会偏差较大，还是比较容易分辨的。
    在检查电压放大级（含电流推动级）中，我一般是综合运用第三项的“断电在路测量法”和第四项的“通电测量法”来检查的。其中，重点是检查电路中的放大元件（三极管）。通电测量法比较简单，但往往可通过通过它、再加上电路分析找到问题存在的硬故障点。

    下面围绕三极管的检查，介绍这两个办法

三、断电在路测量法

    这种方法不是准确的测量法，但它可以在机子断电情况下进行的检测，在方便安全的情况下，有助于排除一些明显的故障。
    此法需要一些技巧和思考。使用的工具是一块模拟指针式万用表。推荐使用电阻档为3V电池供电的万用表（1。5V的表也可用，但指针偏转角度不够明显）。我小鬼头常用的是上海第四电表厂的MF368型（这个表的可靠性和耐用性不是很高，但他的电阻档电流比较大，X1档最大输出约150MA，检修东西甚方便，比如检测LED、检测达林顿管、双表检测可控硅等，很能派上用场）。



     1、模拟指针万用表的LV刻度
    这个LV刻度显示的是在用电阻档进行测量时，被检测两点上的电压降。见附图的4

我检修/装制分立件功放的实用方法

    有的万用表本身就带有这个刻度，比如上面的MF368表，有的没有。由于这个LV刻度是线性的，没有LV刻度的可以自己在心里头给它标上：以3V表为例（仅指使用3V电池时的电阻档位，如X1、X10、X100、X1K等，不包括使用“高压”电池时的最高电阻档），指针满幅时（即0欧姆时）是0V，指针不偏转时是3V（准确地说，等于内部电池的实际电压），中间刻度为1。5V。
    我在测量时，X1、X10、X100、X1K档都使用（读取LV刻度），其中以X10档用得最多。

    2、在路测量的基本法
    在实际线路板中，经常是电阻、三极管（二极管）存在着电气连接。由于三极管在测量时实际上是等效为二极管的组合。见附图的6。

我检修/装制分立件功放的实用方法

    因此，知道二极管与电阻并联时如何测，就基本可以解决问题。见附图的5

我检修/装制分立件功放的实用方法

    图中，左边为正向测量与电阻相并联的二极管。选取适当的电阻档，万用表的LV指示应为0。6至0。7V（硅二极管或硅三极管的PN结，下同）。这个电压，就是二极管PN结的正常压降。
    右边为反向测量与电阻相并联的二极管。按欧姆（电阻）档的刻度，可以读出电阻的阻值，因为二极管这时是反向偏置，漏过的电流极少。也就是说，万用表将显示电阻的阻值——开动脑筋，可以利用它来估算实际电路板上的电阻是否正常——因此，只要选取适当的电阻档，万用表的指针将停在刻度的左边，而不会到过右边的0。6到0。7的LV刻度区。
    那么经上述两次正反向测量，就可以立即查出有开路、短路故障的二极管以及内部PN结开路、短路

小鬼头

四、通电测量法

    见附图的7

我检修/装制分立件功放的实用方法

    这种方法很简单，就是测量工作时三极管的be极电压。因为功放（以及前置）中的三极管（除保护电路等少部分外、这里的测量通常也不适用于末级功率三极管）均是处于放大区的，所以正常的硅三极管be极电压是0。6至0。7V。
   不是0。6至0。7V的，若果是高于此值的，通常是此管已损坏。若果低于此值的，则不一定是此管坏，但可以判断属于不正常工作了，可循此进行追查。。。需电路分析了，呵呵。
        
五、调节功放管偏流的安全方法

    这种方法供需要调节偏流时使用，或者在对电路没有把握的情况（比如怀疑出现自激现象）下使用，而且仅适合于总偏置电流约200MA以下的情形。
    详见附图的3

我检修/装制分立件功放的实用方法

    在功放管的Q1、Q3与正电源之间、Q2、Q4与负电源之间，各加串一只27至51欧姆2W的电阻（1W也可以，注意勿工作时间过长），来保护功放管免遭意外而损坏。特别是电路可能存在振荡时。



——全部附图，写在一张纸上[em11][em06][em06]

我检修/装制分立件功放的实用方法

—— 这是我在国庆节前夕，综合运用了前4种方法修复的马兰士PM-6功放图片。此机功放管、电压放大管、电阻（比较多的时候通过外观上可观察到有损坏嫌疑的电阻坏）均有烧毁。。。。还不少呢。呵呵


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HQ

我的方法也跟你差不多啊.[em11][em11][em11]

4059

好贴！

小鬼头

以下是引用HQ在2004-11-15 19:42:00的发言：
我的方法也跟你差不多啊.[em11][em11][em11]

难道贵宾。。。。。。。。待遇就比版主高？。。。。。。。。。。。。[em10][em10][em10]

aeon

先下了好好看。抓你的尾巴就拍。[em11][em11]

fly05

这贴更强。

孤心冰封

看不清楚啊，可不可以重新贴一下啊？

seabreeze

经验丰富啊

0002300182

顶啊

0002300182

我感到老法师多喜欢用指针表，我也喜欢用指针表啊，但是我不是老发师。
  现在的小年轻修东西多用数字表，好象是有点赶时髦式的。但是数字表是有它的优点，量电压特好携带方便。

catlike

学习学习再学习  [em12]

new

学步会了！！！

hengkong

鬼版强文，收藏了，建议置吧！
我的方法类同，还有一项不同的，就是你的第5项，最好是用小容量变压器代替原电源变压器，因为就算以上几步有疏忽，接上功率管后有自激或电流过大都不怕，顶多找个小变压器做替死鬼。[em11][em11][em11]我就从中受惠过！

DarkHorse

看看也好，到时候自己做的AMP出问题了，也可自己弄一弄……[em11][em11]

DANIELYANG

[em11]

大胡子船长

慢慢看。

小鬼头

http://www.diyaudio.com/forums/showthread.php?s=&postid=535159#post535159

X.G. []
diyAudio Member

Online
Registered: Sep 2003
Location: GuangDong China


Share my methods to repair/build a discrete power amp

Blowing up the power devices is a one of the most trouble.This thread I post is my experiences to prevent/troublshoot this problem in pratice.Wish you can see what I say(sorry for my poor English) and get the helpful infos

Note: My methods are mainly for the bjt power amp of NFB,but you may apply it (or its parts) for other SS amps(FET AMP,pre-amp...etc)

1)Seperate the power devices from the circuit and 'protect' them

This method I use is for the 'big' or complicated trouble(for example,the amp had blew up the power devices)

look at the picture below

The pic show the diagram of the bjt power amp of NFB.Q1,Q2 are the power transistors.The key of my method is keeping the NFB active,while disconnect the power transistors.if you don't do that,you can not check correctly the front circuit.

The mothod of keeping the NFB active is soldering two resister(300ohms to 1000ohms,1/4 w or 1/2 w)  which between  B1  and Vout,B2  and Vout.

After disconnect the power devices and keep the NFB active,you can easily troubleshoot the front circuit,liking a SS pre-amp.

When you finish the troubleshoot of the front circuit,you should confirm the Vbias voltage is not fault before re-connecting the power devices.....if you are still suspicious of the front cicuit,you maybe apply the method below of 5) to continue.


2)troubleshoot the front circuit

the front circuit (dual rail PS) which work have two obvious marks
a.the DC voltage of Vbias is below 100MV in general.
b.The Vbias voltage is in the correct range.

The list of the below pic is my estimate of the correct Vbias voltage range,just for hint,not for roference....If your amp trouble on there,the Vias voltage will abviously out of this in general.

I apply the next methods of 3) and 4) to check/troubleshoot the front circuit including input,VAS,drive...etc.

3)check on-board

This method is in the turn-off state and isn't precision,but can be easily (and safely!) to check the BJTs,Diodes(even resister),then mostly find the bad component.

The tool of check is an analog multimeter(not DMM.I think the people which have much experiences mostly like to use it for troubleshoot in my country).a majority of analog multimeter have 1.5V battery for the Ohm functions(and "high" voltage battery for the max Ohm function),but I would recommend the analog multimeter which have 3V battery for that,because it will show the check-on-board result more clearer and can check the LED,darlington tr,even triac(with another multimeter).

This method maybe a bit complicated....let's go step by step

a.the LV scale of analog multimeter

look at the picture below

The LV scale indicates the drop-Voltage between the two terminals of check on the Ohm function.(some Chinese analog multimeter have this scale)

Because the LV scale is line.when the multimeter set the Ohm funtion(X1,X10,X100,X1000 in general,I mostly set on X10 in practice) ),the full-scale is 3V(the precision vale is the actual Voltage of inner battery),the indicator at centre show 1.5V....the indicator at start show 0V.

(to be continued)
[em06][em06][em06][em11]

小鬼头

b.the basic of check on-board

the amp is made of resisters,BJTs,(diodes)....and other components which connected each other.the BJTs in checking state is compound of two diodes.

look at the picture below

the pic whow the equivalent of BJT for check

so,if we know how to check the  compound of a diode and a resistor,we can check the transistors(including diodes of course) on-board.  

look at the picture below

The pic shows how to check the diode/resistor which connected each other in parallel.set at suit Ohm funtion(X10 in general,but if the resister is small for example a few decades ohms,it should be at X1).when check in positive direction,the LV value of indication is 0.6V-0.7V if the diode is not bad(not short and not disconnect inner),and you can read the resistor value of indication which is non-effective. when check in negative direction,you can read the resistor value of indication(if the diode in not short),and can read the LV value which is useless.

if you set correctly the Ohm funtion and the diode is not bad,the indicator is on the left of the scale when check in positive direction,and on the right of the scale when check in negative direction.so,you can easily justice the diode is good or bad.

because the method isn't precision,you should re-check out-board if you found a bad diode(BJT) on-board....I believe that you had known how to check the devices out-board which method introduced in many books.

I found that we are lucky in my experience,the bad BJT ordinary behaves that: a))the P-N junction is short or disconnect,b))Between the e pin and the C pin are short.

sometimes,the method on-board can not find the trouble BJT(especial be 'soft' bad) and other bad components,then we can go on with the method of 4)

4)check the Vbe of BJT

This method is very simple,but is much helpful.Because the majority BJTs work on line amp area(except for the power BJTs in general and the BJTs in protect circuits ...etc),their Vbe is 0.6V-0.7V.

look at the picture below

If the Vbe of the BJT is higher than 0.7V,the BJT must be dead in general.

If the Vbe of the BJT is lower than 0.6V,maybe the BJT is dead,maybe the other components which relative to it is in trouble. Although it can not tell me the answer, we can trace that to troubleshoot....need to analyse the circuit....;)

5)adjust the idle current of power stage in safe

look at the picture below

Note:this methods which show in the picture just suit for the idle current is below 200ma

the two resistors are 27ohms to 51 ohms,2 watt(1w as well,but don't work continue in a long time),they can 'protect' the power BJTs.I often use 47 ohms 1W.
The thread of Chinese verion which I posted last month is on here

http://www.hifidiy.net/dispbbs.asp?BoardID=2&replyID=42514&id=5000&star=1&skin=0

This is the link of the whole picture including the 7 pictures above


I  repaired  the marantz PM-6 integrated amp in succeed two months ago,using my methods.:cool:




Those are the dead components I found and desolded.I found dead resistors just by their appearance ;)  


(the end)


Good luck and enjoy in DIY audio

X.G.

[em02][em02][em02][em05]

DarkHorse

虽然我只看懂一半，但是还是受益匪浅……顶一下！[em11]