EVH 5150-3 issue

[Valvehead]

Ok.... ive got 2 scems ..1 is a 2006 and other is a 2011 - I think ?? My amp has a 2014 pcb. This amp is a 50w combo with 6L6 and doesnt have the concentric pots.

So.....When im on BLUE or Green mode.......the Gain pot of Ch3 RED , affects these 2 channels...it should not. Red channel operates normal.

I suspect Q1 ..it looks like a Mute . J113 transistor . What do you guys think ?

I can even find Q1 in the amp.......frustrating...

[Valvehead]

WELL-----i found that Q1 is REMOVED............wow.

i am looking thru my bin and found a J111 ---that should work yes ?

[Valvehead]

Installed a J111... green)clean) is ok....Blue CH is still affected by ch3 Red Gain pot...

[Jon Snell]

J111, 112 and 113 are very similar and should be interchangeable according to onsemis data sheet.

[Mick Bailey]

The cutoff voltages are different and given the range of variation within Jfets you could get unexpected operation. A J113 may work as a substitute for a J111, but not necessarily vice-versa, depending on the specific device cutoff voltage.and the circuit voltages. Check the circuit VGS to be sure.

EDIT: Looking a the schematic and the voltages should be fine for a J111 in that position, though I wonder why Peavey chose to use a J113 when elsewhere in the amp they used J111. You'd think that they'd be wanting to rationalize parts.

[Jazz P Bass]

Installed a J111... green)clean) is ok....Blue CH is still affected by ch3 Red Gain pot...

That really makes no sense.
Green & Blue are a totally separate circuit from Red.
If Q1 is working (mutes Red) for Green, then it should also work for Blue.
Something else must be amiss.

[Valvehead]

II dont know if this helps , but when on the Blue ch. , turning the Gain pot of Red ch. it acts like a wide sweeping treble...not a gain control . I will post a video of me turning the red gain .


https://youtu.be/SCV2IZcHkFA

Could it be Z1 / Z2 ? Maybe a relay ?

[Jazz P Bass]

So Blue is not ok.
Good.
Q1 circuit is still the prime suspect.
It's not muting the Red channel.
What is the gate voltage doing when Red is Off & then On?
I would check every part & connection of that circuit.

[Valvehead]

Gate is 0.4V blue mode , -07.1v Red

i now have a 2011 evh here to compare on the bench

The readings are the same on both amps at Gate of Q1 .

I took readings on both amps at Z1 / Z2 . heres a pic of that--the bad amp is highlighted =

[Jazz P Bass]

Verify that 1 is functioning.
Pin 2 at V2 A should have no signal when Red is off.
If so, then the issue is before that.

[Valvehead]

Verify that 1 is functioning.
Pin 2 at V2 A should have no signal when Red is off.
If so, then the issue is before that.

amp on in Blue mode , V2 - pin 2 measures 1mV dc , and .06VAC

clipped out Z1/Z2 = no change