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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. |
