Repair Sony Portable Radio, Cassette Tape Player CFS-904

Date: July 2012

Problem:

This portable radio, cassette tape player from the early 1990's would only operate with batteries, but not when it is plugged into the AC-DC adapter power source.

Diagnosis:

Since all functions are working when operating on battery power, it has to have something to do with the DC power source from the AC adapter. First, I tested the 9V AC-DC adapter and made sure it outputs 9 to 12V (these adapters are not precisely regulated, and can output up to 12V when there is no load).

By eliminating the AC adapter as a source of problem. I then check the circuit board. By tracking the circuit board from the DC jack when it is plugged it, testing the current flow using a voltmeter of each component, it only took the second component to realize there is zero voltage. The second component is a zener diode. The first component looks like a transitor, but in fact it turned out to be an ICP or IC Protector. A type of fast-acting fuse used to protect delicate digital circuits.

As with all individual component testing, the component has to be removed from the circuit first. So I unplugged the power and desoldered the zener diode, and the diode tested fine (it should only conducted in one direction). Then I tested the ICP. Using a continuity test on it showed there it is an open circuit (blown fuse), which means it must be replaced. 

Repair:

The IC Protector is a fuse packaged like a transistor to be mounted on the PCB without taking much space. To the untrained eye, it looks like like a transistor. By looking up the service manual, it shows a part number of ICP-F38 with a specification of 50V, 1.5A maximum. The F in F38 means Fast-blow, there are also N for normal-blown and S for slow-blow. Generally slow-blow fuse is only used for electrical appliance using a motor to prevent blowing the fuse when starting the motor, which draws much more current than the normal operation after start-up. Fast-blow is usually used for very sensitive digital electronics.

 

This part number has long been discontinued and difficult to find locally, not wanting to wait too long to test the repair, I found a fuse made by Little Fuse that is 220V, 1.6A. When it comes to this type of replacement, ideally you want one that is the same type (fast fuse) and has the same current rating (1.5A), the voltage does not matter as long as it is higher than the original rating. The current is the more important factor as it should not be below the original rating, otherwise the fuse will blow quickly under normal load; and it should not be higher than the original rating and the extra tolerance for excessive current may overload the circuit and cause damages. In the end, the extra 0.1A is unlikely to pose a problem in practice. It is not always easy to find a fast-blow replacement, so if necessary, a normal-blow fuse substitute can be used, but not a slow one.

Result:

One the new fuse is installed, the radio, cassette tape player can operate when plugged into the AC power source. This is another relatively easy repair that one could do for under $8. Eventually I found an equivalent of the ICP-F38 and ordered it, but it took some time to arrive.

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