Resistors R156 ... 159 are best mounted horizontally. It is possible at different heights from the board. Cooldown better. The insulating tubes on them must be pulled to the very body of the resistor.
The instrument has been running for more than 2 hours without experiencing the problems that I mentioned previously. Before installing the shield and cover of the enclosure, I measured R156 to R159 (OHMITE MOX5002703GSE, 4 x 270 kΩ, 2 W/resistor, 100 ppm/°C, 155 °C max operating temperature) with an IR camera. The temperatures were measured at the base of each resistor. 75 °C was measured at R159, 85 °C was an average value and the maximum temperature was found to be 105 °C at R156. This is almost half the temperature that was observed previously when inspecting the original generic resistors. If one can source a high-voltage resistor of equal or greater power, then this setup should work fine. Your suggestion of positioning the resistors horizontally would improve thermal dissipation, but I wanted to avoid crowding the resistors and inhibiting airflow which also must be considered. Overcrowding is a problem with this Advantest design. Note that I applied your horizontal suggestion for most of the other power resistors.
Another alternative is to further divide the heat load amongst additional resistors
, but this might get complicated. Otherwise, if the OHMITE MOX (2 Watt) or Vishay VR68 (1 Watt) resistors are unsuitable, then one can try and contact Japan Fine Chemicals
https://www.jfine.co.jp/eng/e_c/resistors/lead/. These resistors were already incorporated in the original Advantest R8340 design and are rated for the kV range.
---
Surprisingly, all the taps that I measured previously have similar voltages to what was measured presently. If anyone has measured the voltages at these positions, I would appreciate it if you would share them for comparison.
At this point, I am assuming that the instrument is working properly and the overheating issues that were experienced previously have been dealt with. My guess is that the problem originated from the R156 tp R159 resistors operating at 185 °C and a linear regulator located at U50 that was showing signs of failure.
Eventually, I am going to have to recoat the board with some protective layer. As I have never seen coatings like this on circuit boards before, I would also appreciate it if someone with experience could provide a recommendation on a product that they have used.
Thank you all for your help and comments.