Customer Repairs

Alba R16 Tape Recorder

 The problem was it was dead.

The reason was a combination of two things.

First a leaky capacitor, even though the leak was almost nothing, and second poor safety standards.

How on earth can one reconcile this pair of events.

 To understand what happened you must first understand how a valve operates.

An output valve in Class A runs pretty hot because its standing current and its anode voltage equates to a power input about double its rated power output at those levels.

Valves draw more current as the grid is moved less negative.

If a valve grid is actually positive then the anode current can be very high and the valve runs exceedingly hot.

Most old radios that have not been serviced fall into this category. Admittedly they will work insofar as they will produce a nice loud audio output but at what expense?

The output valve will be losing emission hand over fist.

The coupling capacitor between the preceeding audio stage connects its anode which sits at say 100 volts to the grid of the output valve which is shunted to ground, typically via a 2 Mohm resistor.

The coupling capacitor develops a leak of say 1 Mohm.

All things being equal the output valve sees a grid voltage due to potentiometer action of the capacitor and its grid leak of 66 volts.

However you would not be able to measure this voltage because long before it could have been established the grid, once commencing to go positive, will be drawing current and this will reduce the voltage by virtue of the drop across the leaky capacitor. However the valve is well and truly biased into a high current region of its characteristic curve and will be exceedingly hot.

Now back to the Alba. There is a cluster of components near to the output valve which are getting extra hot due to the valve glass temperature. Also close to the valve is a hole in the circuit board through which passes a group of wires in a cableform coming from the separate power supply unit.

The HT feed resistor close to the valve starts to burn. The wires resting on the resistor get very hot. Their insulation starts to melt. A short starts to develop between a wire carrying HT and another carrying a ground connection. The HT current rises.

In the power supply another HT feed resistor starts to burn. In the process its resistance starts to drop from 680 ohms to 200 ohms. The first HT feed resistor near to the valve sees a higher voltage, draws more current, and gets even hotter. Just before it burns up it melts two other wires in the cable. These are mains wires from the power supply to the switch on the rear of the volume control.

The two mains wires short together. As they are mains live and neutral they go open circuit just like a pair of fuses. The fuse in the plug is OK because that's a 13 amp variety and it's a lot higher in rating than the thin mains wiring. The recorder goes dead.

Whoever thought of running mains wiring using miniature wires in a cableform carrying HT, heater and ground wires ought to brought to book. There was no third wire in the mains lead for safety earth and no fuse fitted in the tape recorder. A pretty dangerous state of affairs! In a slightly different scenario the live mains wire could have shorted to the ground wire and made the recorder chassis and all its metalwork, and microphone sit at 240 volts AC. What you might call a shocking state of affairs.

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