Markbass combo mysteries

[PaulHornBass]

So, I have two Markbass Mini CMD 121p (the old versions). I keep one at home, and because I left the Midlands, where our band is based and gigs, and don't have a car, I keep the other in my bandmate's music room. The one I gig with in the Midlands has no issues - played with it last night and it was great. The one I have at home has a couple of quirks, though. There is some crackle on the volume knob, and it needs some working back and forth sometimes to get rid of a distorted signal.

 

Something else that I've noticed is that the gain the two amps require is considerably different. Whereas my Midlands gigging amp only needs the gain set to 8 o clock with my P Bass with SD quarter pounder pickup, my home amp needs it set to nearly noon. In both cases I am setting it to just before the amp starts to clip. Can anyone suggest why this might be? I have a bass purchase lined up, but should I be thinking of getting a new amp instead? Ideally I'd like to join another band where I live (Wallasey, Wirral), and want to be set up for local gigs and rehearsals.

[Jakester]

12 hours ago, PaulHornBass said:

Whereas my Midlands gigging amp only needs the gain set to 8 o clock with my P Bass with SD quarter pounder pickup, my home amp needs it set to nearly noon. In both cases I am setting it to just before the amp starts to clip. Can anyone suggest why this might be?

Could the knobs have been taken off one and put back slightly differently? Or it could be they were just like that from the factory?

[sandy_r]

hi Paul

...a couple of observations here...

 

the clip led can be triggered from several different stages along the signal path in the preamp section; there are also a few Trim pots amongst those stages

 

its quite possible that your gain-adjust/clip test, between your 2 amps, is not comparing like-with-like (ie it could just be a difference in setup whenever/wherever the two amps were commissioned)

 

there is obviously an issue with the Master Volume (not ruling out a link with the relative gain setting issue) - the Vol pot could be affected by wear, dust, or influence by possible degrading (if it is an early amp) of the electrolytic capacitor feeding the signal to the pot

 

so the best scenario is that you just have a 'dirty' Vol pot (which might be cleanable with Deoxit type spray) and the gain difference is just build/setup variation - a more serious scenario is that the Vol pot and/or electrolytic caps (possibly many) have started needing to be replaced

 

the second scenario will need to be confirmed and repaired by a Mark bass authorised tech (preferably)

 

you might just be able to gain enough access to the Vol pot body to apply cleaning spray to see if its a scenario 1 issue, first of all

(the construction of the amp hardware and combo installation however makes access look to be er, 'challenging'!)

 

hope this helps your decision-making

 

[PaulHornBass]

3 hours ago, sandy_r said:

hi Paul

...a couple of observations here...

 

the clip led can be triggered from several different stages along the signal path in the preamp section; there are also a few Trim pots amongst those stages

 

its quite possible that your gain-adjust/clip test, between your 2 amps, is not comparing like-with-like (ie it could just be a difference in setup whenever/wherever the two amps were commissioned)

 

there is obviously an issue with the Master Volume (not ruling out a link with the relative gain setting issue) - the Vol pot could be affected by wear, dust, or influence by possible degrading (if it is an early amp) of the electrolytic capacitor feeding the signal to the pot

 

so the best scenario is that you just have a 'dirty' Vol pot (which might be cleanable with Deoxit type spray) and the gain difference is just build/setup variation - a more serious scenario is that the Vol pot and/or electrolytic caps (possibly many) have started needing to be replaced

 

the second scenario will need to be confirmed and repaired by a Mark bass authorised tech (preferably)

 

you might just be able to gain enough access to the Vol pot body to apply cleaning spray to see if its a scenario 1 issue, first of all

(the construction of the amp hardware and combo installation however makes access look to be er, 'challenging'!)

 

hope this helps your decision-making

 

Thanks for this - that is all quite useful to know. Hopefully then, if I can clean the dirty vol knob and resolve that issue, the gain-setting difference may be neither here nor there if the resultant output is the same. 

 

As you say, the challenge will be accessing the vol pot body to apply the contact cleaner. I think when I last checked, the closest (possibly only??) Markbass authorised tech was in Sheffield. 

[BassBunny]

1 hour ago, PaulHornBass said:

As you say, the challenge will be accessing the vol pot body to apply the contact cleaner. I think when I last checked, the closest (possibly only??) Markbass authorised tech was in Sheffield. 

From what I remember the amp section comes out pretty easily. The mounting screws are clearly visible going through the cabinet. The beauty is that once removed it is just a Little Mark without the lid, so you can easily see the workings.

Unfortunately Real Electronics in Sheffield is the only authorised MB service centre and I am pretty sure they don't repair to component level. A faulty gain pot would be a new pre-amp board.

Edited October 30, 2023 by BassBunny

[sandy_r]

31 minutes ago, BassBunny said:

From what I remember the amp section comes out pretty easily. The mounting screws are clearly visible going through the cabinet. The beauty is that once removed it is just a Little Mark without the lid, so you can easily see the workings.

 

Getting access to the preamp controls is a little more involved than removing the head from the combo - just sayin'