MARKBASS CMD121P Need Help!

[KOH76401]

I own a secondhand Markbass CMD 121P that I absolutely love! My only issue was the tweeter was very noisy and I had seen on separate threads that simply unplugging and taping off was fine. I did that and noticed my amp switched off during a rehearsal one day. I switched it off and back on no issues. A few weeks later I set up my amp, got coffee, came back and the amp and my Morely Hum Eliminator were off.

Steps I took to determine issues:
1 - Morley Hum Eliminator did not come back on after plugging into multiple outlets on different breakers.
2 - Replaced fuse and blew three manufacture spec fuses.
3 - Took apart and noticed some burn marks around the motherboard and crossover.

Environment:
I play at my church every week using the same outlet for the past 6 years and never had any issues prior to now. I play a MIM Fender Jazz Bass through a pedal board into the CMD121P. Amp has been kept in a dry shelving unit at my church only being used for rehearsals and sunday service.

I will post some pictures of what I found. I will be honest and tell you all I am not a knowledgable amp user. I understand the knobs and what they do but as far as the insides I have no clue. Whatever information anyone has I would appreciate it.

 

[Old Garage-Bander]

If you have a cab with a tweeter and it utilizes a crossover, you may have upset the cab's total impedance as seen by the amp when you disconnected the the tweeter.

 

[abarson]

Those are very telling photos, and I think you have correctly identified areas of concern. Unfortunately, those kinds of issues can only be addressed by a qualified technician.
I would recommend not using that amp further as that may cause additional damage.

 

[DaDo625]

Those are very telling photos, and I think you have correctly identified areas of concern. Unfortunately, those kinds of issues can only be addressed by a qualified technician.
I would recommend not using that amp further as that may cause additional damage.

+1 - I would not power this up again - it needs an Amp Dr.

I've been running my CMD121 with the tweeter disconnected and taped off for years - weekly band practice etc. - in the "for what its worth" department I'm thinking that was not the cause - that looks like a magic smoke/current surge/part failure event...

Good luck and let us know how it turns out...

 

[KOH76401]

I have a thought and i'm hoping that I can get some confirmation on this. I understand that most Markbass head units will fit in the slot what the amp head normally goes in for the CMD121P (at-least the LM3). I am wondering if anyone has heard of swapping out the head unit with a new one but also at the same time bypassing the crossover completely after the swap? The proprietary speaker inside does say that it is an 8 ohm speaker so it makes me think that this would work.

 

[Sparky Mark]

Yes, all Little Mark series heads will fit into the CMD121P slot and needn't be connected to the tweeter. When I owned a CMD121P I swapped the tweeter for a Monacor HT-30 mini horn which was much less hissy. It will fit without having to modify either the CMD121P or HT-30 but with only 3 screws and it is a tight fit.

 

[Nebadon2000]

First we all know tweeter's don't hiss Tweeter may have bump in frequency of amp's residual noise.

I have 2 CMD 121P's[1 class A/B 1 class D] and have no problem with tweeter only hear hiss when right in front of amp never standing by amp or a couple feet away and never when actually playing solo or with band.

I can hear Fan in quiet bedroom not playing even a few feet away again not when playing a few feet away.

I respect some are OCD about this, best/safest way IMO is to do what Sparky Mark did or attach thin piece of foam over tweeter and not leave disconnected

 

[Sparky Mark]

First we all know tweeter's don't hiss Tweeter may have bump in frequency of amp's residual noise.

I have 2 CMD 121P's[1 class A/B 1 class D] and have no problem with tweeter only hear hiss when right in front of amp never standing by amp or a couple feet away and never when actually playing solo or with band.

I can hear Fan in quiet bedroom not playing even a few feet away again not when playing a few feet away.

I respect some are OCD about this, best/safest way IMO is to do what Sparky Mark did or attach thin piece of foam over tweeter and not leave disconnected

I agree, and the hiss was only ever an issue when I was playing at home at very low volume. Foam in front is probably the simplest solution or just sit off axis when playing quietly; it's no biggie. Strangely the same tweeter performs better in my two NY121P cabs, perhaps the L-pad attenuator set at its mid position is how the tweeter works best?

 

[KOH76401]

My amp must be the exception. I can hear the tweeter from the back of our church with my amp set up far upstage on the altar. My amp also sits off axis from where I sit. Maybe it's the year it was made but this tweeter has been the absolute death of me. The 90yr old married couple that sit across the sanctuary from me have complained to the music minister about it.

I have looked into the Monacor upgrade but I think by swapping the amp head and removing the crossover entirely may be the best bet for me. Or maybe the new amp head will be quieter through the tweeter? We will see.

I really appreciate the input from everyone!

 

[SJan3]

I agree, and the hiss was only ever an issue when I was playing at home at very low volume. Foam in front is probably the simplest solution or just sit off axis when playing quietly; it's no biggie. Strangely the same tweeter performs better in my two NY121P cabs, perhaps the L-pad attenuator set at its mid position is how the tweeter works best?

Sparky, I've only ever owned separates in Markbass gear, never a combo so I have the attenuation control on my cabinets. That said, wouldn't introducing a bit of VLE or now Old Skool quiet the tweeter on the combo amps?

 

[Zooberwerx]

Bottom line: you've cooked some components, blown fuses (repeatedly), and sitting alongside the road with the hood up. Definitely needs some quality tech time if there's any chance of it being salvaged. Never a good idea to defeat design features.

Riis

 

[agedhorse]

Your tweeter is likely damaged also.

Generally, it’s not a good idea to unplug the tweeter, but in this case it’s also possible that an amp failure caused this damage.

The amp has had a catastrophic failure, it can be repaired by a qualified technician who will also investigate for possible causes of the failure so that it doesn’t happen again.

Internal fuses are not intended to be replaced by end users.

 

[Zooberwerx]

Your tweeter is likely damaged also.

Generally, it’s not a good idea to unplug the tweeter, but in this case it’s also possible that an amp failure caused this damage.

The amp has had a catastrophic failure, it can be repaired by a qualified technician who will also investigate for possible causes of the failure so that it doesn’t happen again.

Internal fuses are not intended to be replaced by end users.

I thought the tweeter was the only component not wired into the harness at the time.

Riis

 

[Nebadon2000]

I think by swapping the amp head and removing the crossover entirely may be the best bet for me.

; Yes and will keep 90yr old couple happy also but in my case I like tone with tweeter since I have used mine at 100's of Gig's/Rehearsals/Auditions with no complaints of unusually loud hiss; which as I said don't hear at home off axis or the few feet away not playing facing speaker at ear level volume set at Gig level.

Amp may have been source loud hiss if nothing else was connected [pedal etc.] So go ahead and modify with new amp seemed to work for DaDo625

 

[Sparky Mark]

Sparky, I've only ever owned separates in Markbass gear, never a combo so I have the attenuation control on my cabinets. That said, wouldn't introducing a bit of VLE or now Old Skool quiet the tweeter on the combo amps?

Yes, using the VLE a little will take some of the hiss away, but this issue is such a common observation by owners of this particular combo that it doesn't appear to be quite that simple. It's a real pity that the combo doesn't include the L-pad found on the cabs. I'm certainly not a fussy user but that piezo tweeter in that particular combo did irritate me somtimes.

 

[SJan3]

Yes, using the VLE a little will take some of the hiss away, but this issue is such a common observation by owners of this particular combo that it doesn't appear to be quite that simple. It's a real pity that the combo doesn't include the L-pad found on the cabs. I'm certainly not a fussy user but that piezo tweeter in that particular combo did irritate me somtimes.

I don't yet know whether or not the MB58R cabs have the L pad attenuator. I can't find a picture of the left side or back of these cabs. The specs make no mention of it.

 

[agedhorse]

I thought the tweeter was the only component not wired into the harness at the time.

Riis

It may have been damaged before, during of after.

A failure (short circuit) of that xover cap on piezo tweeters can result in an illegal load to the amp, and that particular fault can be catastrophic to amps. I do not see the protection circuitry required that isolates the tweeter from the amp in the event of a series cap failure.

 

[KOH76401]

I try to blend in the VLE quite frequently but it does tend to bury me into the live stage mix. I know i'm being a little too particular but I did purchase this amp with the intention of always being able to stand out when necessary.

Riis: Yes the tweeter was unplugged and the wires leading to it were taped off. I also taped off the spade connections coming off the tweeter since I am a worrier. I however did not unplug the wires leading to the crossover. That is when the trouble started and the amp randomly shut off one time. Turned back on just fine with no issues until this past Sunday.

agedhorse: I am a numpty and do not know the inner workings of bass amps all that well. Could you message me and elaborate on what I would to isolate the tweeter from the amp?

 

[KOH76401]

My other concern is if I am to replace the head unit will it cause more damage since the crossover has some obvious damage as well?

 

[agedhorse]

My other concern is if I am to replace the head unit will it cause more damage since the crossover has some obvious damage as well?

At this point I would absolutely be concerned.

My recommendation is to find a qualified service tech to determine what else is damaged.

Thos is the sort of thing that tends to happen when changes get made without understanding how everything works together.