User guide

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