MKIV tone worries... Help needed!

[kungfujews]

A desparate question from a MKIV short body combo user...I've recently noticed a worrying trait on my amp. When I'm playing, after dialing in my tone I've noticed the graphic seems to fade out and the tone changes to a 'boxy' 'honky' middley tone. I've switched the graphic off - changed channels and switched it back - only to find the graphic - and nice tone return. The volume doesn't decrease though. This has happened a few times in the past week and it's causing me sleepless nights. I've tried changing the valves - replaced the V1 as well as swapping the 6L6s from my younger F100 - I thought this had cured the problem, but it has since happened again. Can anybody advise me? :?

 

[Boogiebabies]

It most likely sounds like the LDR that opens the EQ to ground is going bad.If your using the EQ from the
foot switch, you may want to isolate the cable as a problem first. As for any volume fluctuation, depending on
your 750 settings the volume may not change at all with the EQ in or out.

You need to determine if the amp is a MK IV A or B, then either the board members can
help you or you can call Mesa.

 

[kungfujews]

Thanks for the response Boogiebabies - it's a MKIV B and the footswitch cable does look a bit suspect now you come to mention it. I'll try to get this checked out and in the meantime keep the graphic engaged via the switch on the back! I might sleep tonight!