That yellow gunk is a sort of resin/glue ...which over time _can_ actually become conductive ..
It is simply there to secure the caps in place ..prevent vibration and fix the caps to the pcb ..
On old equipment I always remove this crap ...and if you actually need to physically secure a cap ..or for that matter any other component for example an inductor ..
Hot glue (hot snot) is the way to go ..
personally I prefer a clean pcb unless absolutely neccessary,,
rgds
VS

Thanks for reply. So i assume that the burning near the resistors is due to the conductivity of the compound, and that if just scraped off should be fine.

Is the compound applied like hot glue would be, ie to one edge of the cap and the board to fix, or would it be underneath the caps and require their removal to get it all off?

Older PCBS are actually often comprised of what is basically resin treated compressed card or paper ..
this can discolour over time due to heat generated by the components on the boards..
If the board is actually damaged you would see a sooty burn or even a hole ..
The discolouration can occasionally create a path of high resistance the charred area of pcb actually becoming what is effectively a high value resistor ..
a really sooty burned area a much lower resistor.
In this case a hole is preferable ...but for the most part you dont have to worry ..
To reduce this char effect you can fit components that stand off the pcb ..so less chance of heat damage.
Looking at your board ..just remove the components one at a time ..
scrape away the yellow resin ... clean the board under the components with ISO ..the components themselves ..and refit..
all should be good
rgds
VS

Thanks. Very helpful advice.

That black patch between the large capacitors is a bit more suspect ..as this could be leaked electrolyte ..
If when you remove those large caps for cleaning the black patch extends under the cap(s) to the pins ..then yes .. replace them..
rgds

keep in mind electrolytic caps can hold a charge for a considerable time after the power is removed ..
make sure to discharge high voltage electrolytics either with a resistor or by shorting the cap pins before working on them.