Job Number

0204-51

Equipment type

3 in 1 portable stereo (CD/cassette/Tuner)

Maker + Model #

Schneider SP120

Made in

Germany

Comments

This item was brand new and to be sold from the shop.

Previously  seen by another technician and did not fixed it up.

BAD

1

 No power up, completely dead

2

 2ndry fault: sound with hum distortion and noise especially on changing volume

GOOD

1

 CD, cassette and radio operations all work fine

2

 Turns on with remote control

Full repair description and observations

1

On opening the item,  a small part of the main board was found broken. All the broken links were connected back, but this did not solved the problem. On further Analysis, more hidden open tracks where found in the main power switch section. Once these were closed back, the item powered up normally.

2

However, there was a defect in the sound, it made distortion bassy hum, and noise especially when changing the volume (digitally controlled).

3

 After many time of testing and investigation, the following was found.

1)       When the hum was made all the main voltage supplies dropped from 13V to about 7V

2)       The current would increase to about 1.2Amp

3)       When one channel was disconnected (left or right) the hum fault was not made

4)       When applying an external power supply (via the batteries compartment) the hum fault was not made

4

Since of the last finding, I assumed that tehfault was in the power supply. I changed rectifying diodes and corresponding spike capacitors, but without success

5

Then the power supply regulation section was tackled, and changed all smoothing capacitors and transistors, but also in vain

6

There was no other component in the power supply left else the mains transformer. This was a quite expensive component, but there was no other way to tell if it is faulty or not else to replace it. Unfortunately, it was not the faulty component, and replaced back the original transformer in place.

 7

At this point I was a bit hopeless, but decided to give a final attempt. I checked many transistors and capacitor at random but no fault was found. Then I began to work on the finding that if one channel was disconnected, the fault did not show up. So the main sound amplification chip was replaced (AN7148).  The fault was solved up! No Hum at all!!

 8

The stereo was closed up but to my horror I found that the sound was coming only from one channel !! L

 9

I inputed an external sound to the output of both channels and both were amplified, thus the outputs were oK. Then I fed the sound input directly to the left and right input of the IC. Only one of the channels worked. This meant either there was no input entering the IC on one of the channels, or the IC was faulty.

10

I exchanged the input feeds entering the left and right input pins of the IC. I fed the good channel input to the faulty channel input. No sound was produced. This concludes that one of the IC channels was faulty.

11

To confirm that the inputs were ok, and the IC was faulty, I fed the input which previously was on the faulty channel to the pin input of the good one, and the result was that good sound was amplified. Hence both sound inputs are present and the IC is faulty.

12

On replacing a new AN7148, the fault remained the same!! Only one channel worked. At this point it was too much. Probably MAP had a batch of faulty Ics? Or there is another fault in the stereo which cause one of thechannel to break up immediately.

13

Hence there was nothing else to do, then connecting the output of the good channel to the output of the other channel so that both speakers are driven.

Repair summary

OPEN TRACKS IN BOARD, SOUND AMPLIFIER CHIP FAULTY

Precautions, fouls and advises

1

By mistake, due to tiredness and lack of concentration, I applied a 12V powersupply to the speaker wire instead of the battery ve /-ve wires. As a result one of the speakers was damaged. To find a pair of replacement speakers prooved to be a very difficult task. I checked about 12 electronics/ hifi / car stereo shops around malta (mostly by walk) but all in vain. Mirald electronics fortunately had a pair J