Crown IC 150 Preamp popping noise - source of problem?

[jimkarl]

Have a Crown IC 150 preamp, 14,000 series serial number so last revision made, that has a odd popping sound under very specific circumstances. Initially thought it was just dirty controls but after cleaning convinced it's a part failure, just not sure which one.

The problem occurs on the RIGHT channel only (no source audio connected to rule out source issues), and only when one of the two phono preamps are selected (either) and only under these 3 conditions:
1) Engaging or disengaging the loudness control and the volume is set to approx 75% or less (if volume is higher, the engage/disengage does not cause a problem)
2) If the volume control is moved in the 80-90% range (approximately), no other controls touched
3) If the volume is approx 75% or higher and the phono input is DE-selected over to another non phono source input type. Watching the pot as I turn the control, it occurs the moment both the Phono input and non phono input are simultaneously connected by the control which is just briefly as you turn the control and the fin moves from the phono terminal to the non phono terminal.

The popping noise does not seem to occur with any other input type selected, only the phonos.

Visually, C8 on the main board (100pf brown mica) appears darkened, much moreso than it's counter part on the other channel, almost like it has been burnt. Given it's place in the circuit (see pic - between the loudness and the volume control) does it make sense that this might be the culprit? Should like ONLY be change out with another mica, or is a different cap type acceptable?

Or should I be looking at a different spot for the problem? I have already changed the 220uf caps all out, both on the main board and the phono board, as they were old and some out of spec. No change. Thanks is advance for any helpful suggestions!

Main board excerpt
crown150-main_board part.jpg

Phono board

crown150-phono board part.jpg

100pf cap
Crown IC150 Capacitor.jpg