MK preamp

[Rapisme]

So I received this and for some reason the PP pot does not engage the passive mode. I did notice that a black ground wire was a little frayed so I replaced it and still no passive it just mutes the bass.
Any thoughts folks?
I did notice as far the diagram that the volume pot isn't stacked?

 

[ABtheBassPlayer]

It seems to me that they wired the push-pull pot incorrectly.

The output of the blend pot should go to the volume pot, and the output of the volume por should go to the switch. From there, it gets switched between the output jack and the input of the preamp.

However, it looks like this preamp uses an active blend, which may make it impossible to incorporate the preamp bypass without making modifications to the PCB.

 

[fig]

I'm pretty sure the push-pull switch is an eq bypass, not an active/passive.

My bass had the same MK1 preamp, but I pulled it about 10 years ago. Mine used a toggle switch for eq bypass. I'm thinking they changed the same switch function to a push pull in your case.

Here's the original diagram I marked-up for emergency passive mode if the battery or preamp died at a gig. The wire colors on plug P1 correspond to the left-hand plug in your wiring pic. All the colors match between yours and mine that go to the switch and the blend pot.



Ignoring the red lines and my mark-ups, here's regular operation:

In eq bypass, 2 pickups go directly into the MK1 where they're buffered and amplified a little, then sent back out the MK1 to the blend, then thru the bypass switch to the volume pot, then out. Totally ignored the eq section of the MK1.

When the eq is not bypassed, the 2 pickups go directly into the MK1 where they're buffered and amplified a little, then sent back out the MK1 to the blend, then thru the other position of the bypass switch and back into the MK1 to be eq'd this time. Eq'd signal exits the MK1 back to the bypass switch. From here, to the volume pot, then out.

So, it's never passive at any time because even if the eq is bypassed, the pickups still hit that active first stage in the MK1 (the buffer that does a little amplifying).

Whew!...

That's my story and I'm sticking to it LOL

By the way, in emergency bypass with the two jumpers (purple in my diagram), the eq bypass switch becomes a convenient kill switch. Should work the same way in yours if needed.

 

[fig]

...just mutes the bass.

Now that we know the switch is for EQ Bypass, which mode is muted?

When muted, do you mean very low volume or totally no sound?

When it's working (not muted), do the controls still work?

Normally when the switch is working and Bass, Mid, Treble controls are centered, both modes have about the same volume.

 

[Rapisme]

Now that we know the switch is for EQ Bypass, which mode is muted?

When muted, do you mean very low volume or totally no sound?

When it's working (not muted), do the controls still work?

Normally when the switch is working and Bass, Mid, Treble controls are centered, both modes have about the same volume.

No sound.
Yes all knobs work.

 

[fig]

No sound.
Yes all knobs work.

Cool, thanks. So the EQ'ing route is working, which means the MK1 is good. Mute seems to be when EQ Bypass is selected (volume pot pulled). And that narrows it down alot.

On one hand, you currently have full eq capabilities (pushed) and a kill switch (pulled). Not a bad thing. Maybe it was intentionally done.

Or the bypass side of the switch is bad.

I can't make out all the switch/pot wiring in the pic you posted. Can you verify the wiring to the switch is like this diagram I made? I used my 'emergency' diagram as reference, since yours doesn't show a switch. Just pretend the switch is on the volume pot, wiring would still be the same.:

Right now, to me, everything points to the bypass half of the switch being bad or no jumper connected.

 

[Rapisme]

Cool, thanks. So the EQ'ing route is working, which means the MK1 is good. Mute seems to be when EQ Bypass is selected (volume pot pulled). And that narrows it down alot.

On one hand, you currently have full eq capabilities (pushed) and a kill switch (pulled). Not a bad thing. Maybe it was intentionally done.

Or the bypass side of the switch is bad.

I can't make out all the switch/pot wiring in the pic you posted. Can you verify the wiring to the switch is like this diagram I made? I used my 'emergency' diagram as reference, since yours doesn't show a switch. Just pretend the switch is on the volume pot, wiring would still be the same.:
View attachment 4682608

Right now, to me, everything points to the bypass half of the switch being bad or no jumper connected.

Hey Fig,
Sorry I haven't responded in a minute. The seller decided to accept the return on the preamp as they wanted to send me their diagram but where away and gonna return on "The 14th."
So just wanted to give you my sincere thanks in trying to help me with this, you have a great knowledge base on these things and hopefully I be bending your ear on some things.
I really appreciate you!
Thanks,
Rap