No Chl 1 on my Mk IV

[Hammer On]

All of a sudden the first rhythm channel all but disappeared (very faint and very buzzy). I'm hoping it's a tube problem. If so, any idea which pre-amp it would be? Thanks

 

[chipaudette]

There's no tube that Ch1 uses that the other channels don't use as well. So, if the other channels sound unaffected, then it's not a tube.

To me, it sounds like one of the relays or LDRs died on you. These two elements are used in different places within your amp to switch the circuitry between the different channels. If one of them died, but the rest still work, you can definitely get odd residual sounds.

Looking at the schematic, my guess would be LDR6, which is supposed to activate in Ch1 to let the signal go from the Ch1 gain knob out to the rest of the circuit. If this is the case, then the Ch1 master volume and the Ch1 presence should both still have some effect on thesound. Do they?

Also, try plugging your guitar directly into the Return ont he FX loop. Does it sound like a guitar (though pretty clean sounding, no overdrive, somewhat quiet, but still like a guitar)?

Chip

 

[Hammer On]

There's no tube that Ch1 uses that the other channels don't use as well. So, if the other channels sound unaffected, then it's not a tube.

To me, it sounds like one of the relays or LDRs died on you. These two elements are used in different places within your amp to switch the circuitry between the different channels. If one of them died, but the rest still work, you can definitely get odd residual sounds.

Looking at the schematic, my guess would be LDR6, which is supposed to activate in Ch1 to let the signal go from the Ch1 gain knob out to the rest of the circuit. If this is the case, then the Ch1 master volume and the Ch1 presence should both still have some effect on thesound. Do they?

Also, try plugging your guitar directly into the Return ont he FX loop. Does it sound like a guitar (though pretty clean sounding, no overdrive, somewhat quiet, but still like a guitar)?

Chip

Yea it plays through the loop return. Oh well, I was hoping it was something simple.

 

[chipaudette]

It's good to know that the amp works via the FX return. That means that you don't have any super serious problem. You still might need to take it to a tech, but the problem is pretty well limited to the preamp, which is good.

Just to make sure that its not a tube, turn off the amp and pull V3. V3 is dedicated to the lead channel, which we won't be using for this test. Instead, swap that tube into V1 (and put the original V1 into the V3 slot), turn on the amp and play the clean channel. Does it still have the problem? If so, I guess the original V1 was the problem. Get a replacement.

If it still has a problem, turn off the amp and put it back the way that it was (original V1 into V1, original V3 into V3). Then, pull V2 and swap it with V3. Turn on the amp again and try the clean channel. Did the problem go away? If so, the original V2 must have been the problem. Replace it.

Otherwise, return the tubes to the original positions. It must be a problem with the circuitry inside (such as that LDR that is my guess). You'll need to take it to a tech to get it fixed. Sorry.

Chip