Word of warning concerning Fender Hot Rod DeVille 410

[Stratfordade]

Finished servicing one of these recently and it sounded fantastic with chassis on bench. Moved everything into hallway to reassemble it into cabinet and replace panels, reconnect reverb and speakers. Flicked it on to admire it again, to be met with a loud motor boating noise and the occasional shriek! Sighed and moved it into room where I store amps, intending to strip it down again soon to find the new fault (it had never motorboat before).

Day or so later I decided to start work on it again. While it was in storage room powered it up and it sounded fine with no sign of motor boating. I don't trust intermittent faults so decided to disassemble it and try to provoke whatever had caused the fault. Removed the chassis and on bench went through all the power supply capacitors including the crucial one supplying HT to V1. They are all pretty new F&T caps and all were perfect. Amp sounded fine.

Scratched my head and took chassis back in to hallway to reassemble amp. Powered it up, off standby, and it began motor boating. At this point light dawned and I shut off power at wall to the wifi router standing right behind the amp. Golden silence!

Did a bit of experimenting and amp could pick up some interference from the router even when it was a couple of feet away. Maybe Fender should have used one or two metal shields on the preamp 12AX7s?

May help anyone here with a mysterious fault, and save you all the time I spent looking in the wrong place. Check for possible interference sources including routers and 'smart' devices!

[mikeydee77]

Thanks for sharing... Yep quite a few of my clients bring amps along with noise problems only to find it is an issue particular to their environment - DECT phones, ethernet over powerline, baby monitors, central heating wireless controllers, dimmer switches, refrigeration or anything else with a motor. Always worth keeping an open mind.

[Tom Phillips]

I once worked on an Amp that had an intermittent slight tremolo like effect sound. Very minor but noticeable. Long story short...the cause was using the same circuit in an old house that also powered the washing machine. It only happened when the washing machine was running and in the wash or rinse agitation phase.

I recently got a call from a customer with an amp that I had just repaired due to common crackling noises. He said that new noises were heard at his last gig. Turned out that he had a cell phone sitting right on top of the cab using the phone to play background music during break time.

[Mick Bailey]

Electric fence energizers can be a source of a regular ticking noise and the fence wire makes an excellent aerial for transmission.

[Bill Moore]

Dimmers also! I built a DR on my (kitchen table) work bench, and on initial start up was great, left it until the evening to set bias. I had a terrible hum that I spent some time on, but didn't find it, the next day it was gone, 'I played my Tele through it for a while to check bias, and all was great. My wife came in, flipped on the light and asked why I was working in the dark, my hum came back!

[R.G.]

Yep - we all have to wire defensively. Vintage amps did not have to contend with the very crowded RF spectrum and nasty AC power we have today. I suspect that modern amps do not test for EMI >resistance<< only emission.