EVH 5150 III channel switching problem

[Pedro Vecino]

Hello everyone. I have a problem with channel switching in this amp. 5150 III 100w.
Everything works well and the three channels sound, but the circuit does not allow the activation of channel 1 when the channel 2 is previously activated. However it does allow his activation from the channel 3.
That is: activate channel 1 requires activate previously channel 3. This occurs from the manual controls on the front panel and through the footswitch.
As additional detail, powering the amp at first, LED of channel 1 shines ok and LED of channel 2 shines dimmed.
The circuit should be logical but escapes my knowledge completely. Does anyone have any idea where to start isolating areas that work well for example, or indicating susceptible components?
Schematic (second page): here
Thank you very much.

[Enzo]

Maybe a stuck switch circuit.

Any chance Q6 is hanging up?

Each number circuit has a flip flop that toggles from the switch closure, and each has lines to the other two to reset them. Something in that cross communication is breaking down, or one logic level somewhere is stuck.

[Pedro Vecino]

Thank you very much, Enzo. I removed Q6 transistor and measured with an Atlas Peak tester. It indicates that's fine. Any other susceptible component?
Thanks again.

[Jazz P Bass]

Next item (not component) is the switching circuit itself.

The LM339's on each switch. (1, 2 & 3)

Compare the inputs of the CD4027 flip flops when you toggle a switch.

The test points are marked.
There should be a 'pulse' when the switch (whichever channel you are testing) is closed.

The 4027 flip flop outputs should do just that.
On a valid 'going high' pulse, the outputs should change state (ie: high to low or low to high)

[Pedro Vecino]

Thank you. I do not know if I understand well but I measured clock inputs in the three flip-flops selecting each channel and voltages are as follows:
They are stable voltages and are accompanied by the pulse voltage when is different (they kept leaving the button pressed)

[Jazz P Bass]

Look at what is going on at the flip flop output pins.

When pin 1 is low pin 2 should be high.

[Pedro Vecino]

This is what I found (I have seen Egyptian hieroglyphs easier to interpret):



Thanks again!