Activating effects loop kills the sound

Wondering if anyone else has had this issue and if anyone knows how to fix it
Had my GM 36 for 18 months , only really used it about once a month since as have quite a collection of amps but finally got around to setting it up as my main gigging amp .
Problem is every time I press the fx loop button on the front of the amp the sound cuts out , press it again so it's not activated and sound returns , now I love a kill switch as much as the next guy but out of the two I'd rather have the fx loop kick in
Tried it with pedals plugged in to the loop and without (analog and digital) tried it manually and by using the iPad app via midi , same problem every time.
Even tried the factory reset ( which caused the buttons to flash six times not three as stated in the manual and the speaker off button to activate ??) again no joy .
Guessing this is going to have to be returned under warranty but hoping someone on here can help.

As a simple first step try cleaning the Loop sockets thoroughly. It is amazing how many times problems like this occur simply because of dirt on contacts.

If that gives no joy then you need to explore the loop with another amp.  There are 3 tests you can do which would help tie this down.  First make sure the Noise Gate is switched off and is also set to its least sensitive position.  This is obvious but needs to be said, make sure that no Mutes are engaged.

Get a jack lead and put it in the Loop Send and Return sockets and engage the loop.  You have no pedals in the loop, even switched off with true bypass, so if the problem still exists it is the amp and the amp alone at fault.

Assuming the problem still exists during the previous test, take a signal out of the Loop Send and feed it into another amp via its Loop Return, (or its input if it has no loop but turn the volume down low).  If it doesn't work then you can assume it is the Send part of the GM which is at fault.

If the Loop Send works as it should then take a signal from the other amp's Loop Send or Preamp Out into the GM Loop Return.  If it does not work like this then the Return setup is faulty.



The GM Loop is pretty simple though its switching is hidden in a mysterious circuit block named the "GAIN PREAMP _EQ SECTION"!  All we can see is a pair of lines to that block from the loop driver and sockets.  The Send line is also attached to the Mute/Noise Gate circuitry which shorts it to ground when needed to kill any signal and that is why those controls must be off.  It could actually be a fault in that section which is causing this though why it would only happen with the loop engaged is a mystery so far.

Failing all of this showing anything then it could be a bad connection in the tiny interboard connector which carries the Send/Return signals from the controller board to the rearboard. That would probably only need disconnecting and reconnecting again to fix. Sadly a fault in the controller area to do with the switching of the signal lines or the engagement of the Mute and Noise Gate would also cause it and would most likely mean a replacement board.

Thanks for that , only got as far as step one
Cleaned out the contacts and then daisy chained the return and send together with a patch cable , wanted to keep anything else out of the equation just in case , turned on the amp while doing a reset and she's now working fine .Even with the noise gate engaged so god only knows .
Not sure exactly what cleared the fault or indeed if it's intermittent and is going to return but it's behaved itself for a day now and I've tried to replicate the fault and it's working as it should be .
Have been speaking to tech support at H&K as well , if it does return I'll get on to GAK and arrange for it to go back for a repair.
Again thanks for taking the time to reply

Good news there then. Glad it turned out to be a simple fix.

Do try to remember that simple setup for testing the loop with a jack lead and another amp, it comes in handy quite often. Loop sockets are always a weak point as the switches in the sockets are often directly in the signal path and frequently degrade with time.