SV20H volume up and down at 16 ohms

[gbirocco]

I've been playing the excellent SV20H for a couple of months, I use it at rehearsals with an 8 ohm 1922 cabinet: it works well and is perfectly loud. I use an attenuator (SPL Reducer).
At home I use it with a 16 ohm 1960AV cabinet and here is a problem. Often, as soon as I start playing, the volume is noticeably low while it gives "thumps" at a higher volume, like coughing to clear the throat: a continuous rise and fall in volume. Most of the time, after a few minutes of work, it settles at the lowest volume but every now and then it throws a few "coughs"; other times it settles on the highest volume, which I think is the right one.
The problem occurs at both 5W and 20W and on all four inputs.
I got another attenuator (Bugera PS1) but the result is the same.
At this point all I can do is send it to the dealer for repairs under warranty.
However, I'm curious (I don't have any technical expertise) if anyone has any idea what it might be.

 

[dtier]

If its happening only with the 16 ohm cab on the 16 ohm output jack I would clean those jacks (cab and amp) with electrical cleaner on a cotton swab first, if you haven't already. Then check inside the cabinet speaker connections. Clip on connectors can be problematic. Two of the four speakers could be cutting in and out?

 

[armandodiaz]

Do you have a switcher in the back of the 1960? If yes. Try using each jack. Maybe you need a new switcher for the cab?

 

[gbirocco]

I followed your advice and focused my attention on the 1960 switcher. After several mono/stereo tests I decided to remove the switcher panel. I unplugged and replugged the speakers connections and sprayed everything with electrical contact cleaner spray. I reassembled the panel and "magically" everything started working without problems; also the mono connection at 16 ohm. Thanks guys for the suggestions