TL Audio VTC User Manual
Page 38
38
oscilloscope. Note that high impedance points such as the valve grids and anodes should not be
probed directly, as the probe impedance may cause a misleading change in operating conditions.
Some pointers to possible faults are listed in table 1 below:
Table 1: PSU / Valve PCB fault symptoms and causes.
Symptom.
Possible
cause.
Hum on one or more signal paths.
Check HT filtering components.
A
short
circuit
transistor
(or
open
circuit
resistor
in
base
circuit)
may
cause
excessive
HT
current
drain,
detectable
by
a
higher
voltage
drop
across
the
filter
resistors.
Signal asymmetric distortion.
Check anode transistors for open or
short circuits.
(One half of waveform suffering
distortion at lower level).
Check valve biasing (approximately 1V on
cathode
and
130V
on
anode).
Faulty valve.
Intermittent noise or buzz.
Poor connection of valve in socket.
Noisy
trimmer
in
buffer
circuit.
Low level signal. Faulty
valve.
Loss of metallic deposit at the top of the
valve
envelope
indicates
loss
of
vacuum,
requiring
valve
replacement.
No heater supply to valves.
WARNING: DO NOT REMOVE OR REPLACE VALVES WITH THE POWER SUPPLY ON, AS
A HIGH VOLTAGE SPARK OR POTENTIALLY DAMAGING TRANSIENT MAY OCCUR.
2.3. Valve Replacement.
When removing a valve, use a straight pull to extract it from the socket. Do not rock the valve from
side to side, as this may strain the socket pins and cause intermittent connection or noise problems.
Similarly, when replacing a valve, ensure that the pins are all straight before inserting the valve into
the socket, with a straight push. Avoid unnecessary pressure, as this may mean that a pin is not
correctly aligned. If it is necessary to straighten a valve pin, apply gentle pressure only as the glass
envelope can easily be cracked, resulting in loss of vacuum and eventual failure of the valve.
Small signal valves such as the twin triodes used in the VTC console are usually very reliable. The
valve parameters are not highly critical as local feedback is used to control the circuit performance.
The valves should have a life of several years normal operation, and do not require regular