Here’s some photo’s from a recent repair of a VOX Night Train amplifier.
The amplifier was producing no sound.
Curiously the HT voltage on the first pre-amp valve was only 95 VDC, way below spec.
I removed the valves and the HT on the first valve stage was still only 95 VDC. On further investigation I found that there was only 10k of resistance from the HT to ground on the first valve, ie something was shorting to ground.
Finding the problem was an interesting exercise.
The filter cap was removed, tested and given a clean bill of health.
I then traced the HT circuit around the PCB, NOT helped by the brown gunge liberally spread over the board.
After mucho careful scraping I found resistors R23 and R24, which lie over a PCB track with HT on it.
On lifting these resistors from the board, the resistance to ground was unmeasurable, and of further inspection, you could see that there was a short between R24 and the PCB track underneath.
I replaced R23 and R24 with 1W MF resistors (all I had in the relevant values), and stood them off from the PCB trace to avoid any arcing/ shorting in future.