Valves were from time to time, returned to either the main Mullard service department or one of the regional service centres. Reasons for return were varied and included, defective new product and devices that had failed during their guaranteed life. Today's blog entry will describe what happened to these naughty devices when received at the service centre.
On receipt, the suspect device would be unpacked and listed in a return register attached to a numbered report form on which the results of any tests carried out would be recorded. At this point, the report contained details of the valve type, manufacturers coding, parcel condition on receipt and result of a subjective visual inspection of the valve.
The device and report would then be passed to a service bench where a range of static tests would have been made using a test board. These tests were carried out in a definitive sequence and should a test device fail at any point, the sequence was discontinued. The test schedule comprised of the following tests: -
The first static test was always a filament check where both continuity and filament current draw were checked.
The second test was for insulation resistance where the cathode was energised to a positive potential and measurements of leakage current to other electrodes was measured whilst gently tapping the envelope with a rubber hammer to reveal possible intermittent faults.
The third test was a vacuum test where the valve under normal operating conditions was connected to a microammeter in series with the grid to measure any reverse grid current and hence check for a 'soft' valve.
The fourth test was an estimation of cathode emission where the valve was effectively configured as a diode to form a composite anode with an AC supply being applied to the anode and the total rectifed current hence being measured and compared to specification.
The fifth and final static test was a check of the anode current-grid volts characteristic curve at several different grid voltages.
If all static tests passed specification then the test device would be passed to another test area and operated for a period of 15 minutes in a suitable receiver - typically a Mullard or Philips receiver. This allowed a direct practical check of device behaviour in an actual circuit and additionally allowed a subjective assessment of any noise and microphony.
If necessary, the final test stage would involve valve disssasembly for further visual and microscopic examination, the results of which were recorded on the test report.
At the conclusion of testing, the report would be submitted to a claims department who decided, based on report contents whether or not the valve could be replaced under the terms of the guarantee
Today's photo presented below shows Doreen Snailpen operating a valve test board at the Mullard Waddon Service Department:-