Hi everybody, I have an amp-related conundrum that I've as of yet not found a solution for: when my HT-5R's effects loop is engaged, I get a very audible radio frequency interference (RFI) from an AM station here in Maryland. This happens even when I take the lead plugged into the FX loop "send" into the "return". The RFI is present at the same level whether or not a guitar is plugged in.
I have tested all the pedals in the loop, the power source for the pedals, and replaced all patch cables and longer 1/4 inch cables running from the amp to the pedal board and back to the amp. I have purchased a Furman power conditioner, which did nothing to squelch the RFI. I have replaced the tubes. I have even brought it to an amp tech at a Blackstar approved shop. He examined the amp, found nothing wrong with it, and even demoed it for me--it was pristine and quiet even with all the pedals plugged in and the overdrive channel engaged.
I'm at my wit's end, having tried seemingly everything. It could be a problem with the mains in my apartment, though the recommended remedy--the power conditioner-- didn't solve it, and I'm not sure what else I could try. Alternatively, it could be the amp, despite it performing normally at the repair shop. Is it at all possible that something in the amp is awry and makes it susceptible to picking up RFI? Would love some input! Thanks everyone.