Power soak options randomly switching

Hi all, I got my gd40 just over a year ago and having spent a fair while sorting the trebliness out (agree with recent post on dialling down presence) . Having sorted that, I was in a gig and in the last song the amp started to randomly jump from 1W, to 20W, to 40W. It didn't seem to jump to 0W. There have also been a couple of rehearsals where it doesn't detect the speaker, until I plug the line out through the PA and then plug the speaker in and it finds it ( I did check it wasnt the lead).

I was wondering if it was the tubes playing up but am loathed to replace them if not that and think it's still in warranty.

If anyone has some ideas on what to try it would be very much appreciated.

Thanks

Hi Jenew. Sorry to hear about your problem. What are you using to control the GM40? Is it the H&K FSM432 foot pedal unit or is it a third party MIDI controller, or are you actually only making changes by hand on the amp itself?

Hi bordonbert, I am using the H&k FSM432. However, I tried the amp out a few hours after the gig without the foot pedal and it did it again

I left it for a week and played on it for an hour and it didn't do it.

It just makes me a bit nervous, so any ideas on things go try if it does it again would be great.

Thanks

Jenew

Unfortunately, if it is doing it without the foot pedal as well then that restricts the problem to something inside the amp or to the speaker connection. I only have schematics available for the GM36 and TM18. The GM40D should be very similar to the GM36 in most aspects like the power soak but I can't be absolutely certain about that. It feels to me as though the most likely culprit is an intermittent relay and if that is the case then the only option is to check out the PCB tracking to it from the control point and then if that is ok, a replacement.

The only bit of useful info I can some up with is to ask you, what do the power soak LEDs show when the power soak has flipped to another setting? Do they show the correct display for the new setting or do they stay with the old setting? If they follow the new setting or you get odd results like two being lit at the same time then it could point to a problem in the control circuitry. If the LEDs stay with the original setting then it could be a problem in the relays themselves.

If you have no experience working with modern high concentration surface mount PCBs then you will not be able to do anything with the amp's insides. H&K amps are Hi-Tech devices and they are no place for inexperienced hands wielding red hot pokers I am afraid. A good tech will be able to easily look into this for you. H&K do have a list of "recognised" repair shops that they authorise to carry out warranty work for them so look into that side first.

I'm sorry to leave you with nothing you can try yourself but this is such a simple go/nogo setup that that seem to me to be the most likely cause. If it were just dropping into its Silent Mode setting then it could be just speaker connections as the protection system in the amp detects no speaker connection and automatically protects the output by applying the full power soak as a load. But to jump between different settings is not down to that and it points to something in the switching path, either the relays or the logic controlling them.

Thanks so much. As you say, when it jumped the led lighting was correct. Will cross fingers for the moment but sounds like an h&k warranty job.

This is an old post, but were you aware the power soak can change depending on the patch?  There's an option to set the power soak globally, which is NOT the default setting.  So if you've made patches with the power soak on a separate setting, changing to that patch would cause the soak to change to the previously saved value.

I've changed my power soak setting to Global.  The instructions are on the bottom of the G40D, and in the manual as well.

Hopefully that helps!