Job Number

0009-01

Equipment type

Hi-Fi Stereo – Cassette / Radio / Longplay

Maker + Model #

SILVER – Model System 70

Made in

UK

BAD

1

No Sound output

2

Cassette section non-operating

GOOD

1

Radio backlight display OK

2

Long play operates (but no sound)

Repair summary

OPEN FUSIBLE RESISTOR AT INPUT OF A VOLTAGE REGULATOR IN THE POWER SUPPLY SECTION

Full repair description and observations

1

When opened, the system was divided into 3 parts, - (i) the power supply and amplification system at the rear base, (ii) the cassette, radio and sound controls at the front face, (iii) the longplay at the top.

2

The long play was easily disconnected by only 2 pairs of wires, but the power supply and the front panel was connected by a number of wires (mostly soldered in PCB) so it could not be separated apart for better inspection.

3

After some voltage testing, it was noted that the cassette motor was receiving a mysterious 3.6V instead of 9-12V as indicated on the motor.

4

The source of the 3.6V was located at the power supply, which was coming from a 3 pin IC, probably a Voltage regulator and fed the cassette part by a red wire.

5

The input of the Voltage Regulator was about 4V only.

6

The input track was followed and it was found that a fusible resistor in series with the input contained 24V on one side and 4V on the other side.

7

The fusible resistor was marked 6.8 W, but after being removed, the resistance was checked and found to be - 140 kW, hence high.

8

Not having a fusible 6.8 W resistor, it was replaced by a common 1/4W 13 W resistor.

9

The set was powered up and the cassette operated and sound was given.

10

Later on, I had to make a cable from stereo jack (source) to the HiFi’s  AUX socket (dest). The aux socket was a 5 pin female type A. The middle pin served as common ground, while pin 3 and pin 4 were the left and right sound input. Pin 1 and 2 were unused.

Precautions, fouls and advises

1

When checking the voltage at the pins of the power transistors, the Voltmeter probes were resting on the chassis, and when the probe touched some pins having more than 24V, there was sparks given off. This was a risky short circuit, which could have been fatal to the transistors

2

While playing the longplay, the needle, which was extremely corroded, broke off in my fingers.

1

Long play, Cassette, Front panel light voltage supplies = 12V

2

Audio power transistor voltage supply = 24V