Cosmic

General information

Job Number	0210-62
Equipment type	HiFi 3-in-1 home stereo (CD, Cassette, Tuner)
Made in	n/a (china?!)
Client Info	Donald Parsons
Date in	10-Oct	2002
Date of repair	19-Oct	2002
Date out	22-Oct	2002
Comments

Symptoms observed

BAD	1	Buzzing noise on CD, Tuner and Cassette
	2	Cassette triggered to stop after 2sec on Play, FF and Rew (not reported)
	3	CD not reckognized giving an Er message (error)
GOOD	1	Powers up,

Repair notes

Full repair description and observations
1	The first suspect fell on the power amp. There where some other Ics, so I had to discover which was the Audio Amp, but it was not difficult, since only one chip had the heat sink, the KA2206. The other chips where a microprocessor, a tuning IC, and an EQ IC (KA2228) Datasheet for KA2206 (Ctrl+click to see) 1
2	This 16 oin IC was replaced, and there was no buzzing noise anymore. Also the click sound from the speaker could be heard on powering. However still no sound output. I checked the voltages on the pins, and there where strange voltage redings, such as 4 – 8V on inputs and outputs and other pins. The Ic was removed and was found that it got broken, when compared the pin resistance readings to that of a new one.
3	On checking the voltages with the IC disconnected, there was only found a 15V on vcc (pin16). Checking the datasheet, it states that VCC max must be 12V
4	Another KA2206 was replaced, and fter few mins the buzzing sound was heard again – The IC broke again, with high voltages again on several pins!
5	The power supply was checked, and the reservoir capacitor was found to be broken. All 5 or 6 electrolytics in the power supply were replaced. The Vcc voltage on pin 16 of the Power amp IC was now raised to 17V. It was concluded that this high voltage was causing the breakdown of the IC.
6	There was not much to dig in the circuit. The17V were a directly obtained from full wave rectification and filtering from the transformer ac output. Instead of replcing the transformer I decided for a different and much betetr alternative modification
7	I connected a 12V voltage regulator to the 17V output. I was sure that the voltage should be 12V since the IC required a 12V dc, and the dc motor of the cassette deck operated by 12V dc. An IC base was now fitted under the Ka2206 amp, and a new IC was fitted in. The sound output coupling capacitors were also changed.
8	The click sound on power up was heard, and there was no buzzing. When the switch was turned to radio, sound was coming out perfectly !!!
9	The CD was found that it was not giving Eror anymore, probably it was recieveing a bad voltage supply from the bad powersupply.
10	The poweramp IC heatsink could not be fit due to the base, but by a little modification, it was applied and held firmly with the IC
11	The next problem was the cassette. It stops after few seconds and tangle the tape at the pinchroller.
12	The stop triggering was found to be due to a spring wire out of place. When placing back, it used to go out of place again. A little modification had to be done to prevent it from slipping. It involved soldering it at the chassis at one end, and bending it over the plastic lever at the other end so that it would not slip.
13	The other problem of tangling the tape on playback was in part fixed by the above fault (the spring wire). However this was enough. The pickup reel was loosing contact with the gear beneath which was turning normally. A spring fitted between this gear and the pickup reel was loose and made back strong. This completely cured the problem and the cassette was working back to perfection.
14	The unit was closed, and the only problem remaining, which cant be done much to solve it was the CD sometimes failed to reckognize and play the CD. Perhaps it needs some adjustment at the laser bias and power presets, but probably the laser is the defect amd that would be an expensive option to repalce.

Repair summary

Power Supply providing a high voltage (12V -> 18V) casue PowerAmp Ic to break up.

Other measurements and technical comments

1

The 3 pin voltage regulator comes in the nomenclature L78Sxx or L79Sxx where the 78 IC gives positve voltage, 79 gives negative voltage and xx is the voltage value given. Pin 2 is the ground, pin one is the input, while pin 3 is the steady voltage output. Look datasheet below for more info. L78xx Voltage Regulator series Datashhet (Ctrl + Click to view)