How i repaired Kenwood automobile sub amp

Dear repair friends ,
Welcome to  Electronic repair article. Today again I would like to reveal how I repaired  Ken wood automobile sub amplifier system.
Before that I would like to put one word on audio repair. Since my repair carrier started via audio I never forget the time spending to resolve the critical issue during those days in 1990s .
People usually ask me to give extra sound effects with their system .But to repair an audio systems it consumes much time and extra attention is required when compared to CRT TV. Later on I will definitely share on this Topic now let us come back to Kenwood Sub amp repair.
Pls go through the system one came to my bench for repair.
 

Before that why sub is required?
Simple answer is to enhance the bass effect. Is only sub woofer & there amp will run the music effectively? Answer is NO .Because we need to add something to this bass .ie HF & mid

The problem is unit is dead. Don’t suspect I am telling fuse open or cap's bulge the problem is some what extraordinary. I will try my level best to make you understand. Before that one should know what is dc-dc converter and its application in sub amp ckts. Pls go through the example circuit below.

                                Typical DC-DC Circuits

With the help of my meter I checked the vcc12  input volts found ok but dc-dc out put 48volt missing(Note: above circuit is an example circuit mentioned 28volts dc ). I traced the reason why the out put is missed even when switching transistors or ok, now I made my oscilloscope active to check the output wave from pwm ic. Before putting an oscilloscope probe I checked all the necessary voltage powered the pwm ic upc494 & its surrounding components’ found ok.
UPC 494 IC FOUND IN KENWOOD AMP

      

Pulses are not released at these conditions hence this is the reason why 48v is missed at dc-dc output. Suspected  pwm ic faulty replaced same and re-checked the pulses got good result  happy to see the signals in my scope.

Now the time to check the dc-dc output 48V again i am un happy to see the required voltage is not coming from this sections. Now my main task here is to make the dc-dc circuit output active & i need to find the reason why the out put is not coming .I Goggled an internet & searched the similar circuits & started to DE-bug the culprit ultimately i successes in finding a fusible low ohms Resistance open .

Find out the similar circuit attached.In this repair case VCC Fusable resistance & pwm ic marked found defective .Gained the life after replacement.

This is the main vcc Resistances connected to center tap step up dc-dc
Pls find out the faulty components marked  below.

After replacing the fusible 1 ohm resistance 48 volts released &  re-life the equipment & worked fine. Costumer felt very happy after deliver .This is how I repaired the Ken wood amp.
That's it coming back next with one more interesting Topic.

  With Best regards,
    Basavaraj :BM
basavarajradiotech @gmail. Com