Mark IV Problems

[EvenKnudsen]

I'm currently experiencing problems with my mark IV amp. When playing everything works fine for a while until the volume for a short time increases (without touching the volume knob of course), then the volume drops to almost zero and all tone is lost. When switching the amp on and off again it somtimes fixes the problem for a short while, and other times it don't. My first thought was that this could be related to damaged tubes so I changed all tubes (both power- and preamp tubes) but the story is still the same.

Anyone here that have experienced anything similar or have any idea what the cause for this might be.

All tips and help are greatly appreciated

Even

 

[swbo101]

I had a similar situation a couple of years ago and like you changed the tubes to no avail. Ended up being my low voltage regulator. That was my case, may not be yours, but just so you know.

Good luck

 

[TOOLGUY]

Gonna keep an eye on this thread.
I have a 2:90 power amp that I have never had repaired that does the same thing.
From what I understand the 2:90 and Iv power amp sections are pretty similar.

I gave up when I could not find a local tech that I trusted that was willing to deal with a pcb amp with a random problem. It's now my mono backup. :|

My amp will jump in and out of this problem with a standby flick or a volume pot crank.
I have replaced the ldr's,tubes and a questionable jack.
If you find the problem please post it.

swbo101.....If you had a schematic,would you be able to point out the part in your amp that failed? Just curious if the 2:90 has the same part.

 

[swbo101]

TOOLGUY - Here is a link to a thread where some one posted the schematic to my MarkIV, with the failure points.

Good Luck

http://forum.grailtone.com/viewtopic.php?f=27&t=49104

 

[EvenKnudsen]

I must add that when this problems occurs the channel switching leds are also fading out.
Any ideas, anyone?

I'm currently experiencing problems with my mark IV amp. When playing everything works fine for a while until the volume for a short time increases (without touching the volume knob of course), then the volume drops to almost zero and all tone is lost. When switching the amp on and off again it somtimes fixes the problem for a short while, and other times it don't. My first thought was that this could be related to damaged tubes so I changed all tubes (both power- and preamp tubes) but the story is still the same.

Anyone here that have experienced anything similar or have any idea what the cause for this might be.

All tips and help are greatly appreciated

Even