Let's play "Faulty Preamp or Main Amp?"

[elgecko]

Okay armchair amp techs, let's play "Faulty Preamp or Main Amp?". I'm getting a fuzz-like distortion from my Markbass CMD121P combo amp (approximately six years of moderate to heavy use). It doesn't happen when I first turn it on but pops up after a couple of minutes of playing and is most audible in the tweeter. Here are the facts:

1. It happens with all basses and cables
   
2. Plugging straight into the amp = FUZZ
   
3. Plugging the combo head into a separate cabinet = FUZZ
   
4. Plugging a separate amp head into combo cabinet = NO FUZZ

So far, I seem to have isolated the problem to the combo head. Under the advisement of VST Services, I've tried the following

1. Plugging the combo head effects out into a powered PA speaker = NO FUZZ
   
2. Plugging the combo head tuner out into a powered PA speaker = NO FUZZ
   
3. Plugging the combo head DI into a powered PA speaker = FUZZ

VST tells me it's $125 to swap out the preamp board and $165 to swap out the main amp board. Which is it?

Furthermore, what the hell? I've never had an amp go bad on me. The amp was NEVER driven hard or abused in any way.

Thanks for playing!

 

[Downunderwonder]

Need intimate knowledge. It depends where the DI is coming from. The MB effect loops are parallel or some odd configuration.

 

[Primakurtz]

Sure sounds like the main board.

 

[elgecko]

Need intimate knowledge. It depends where the DI is coming from. The MB effect loops are parallel or some odd configuration.

I'm not sure.

Sure sounds like the main board.

The more expensive one.

 

[TJBass]

Open it up and see if the effects loop cable to jack comes off thr main board, if so, its prob a faulty preamp board.

 

[David Jayne]

Long shot, but have you run a plug in and out of the effects return jack a few times?
Beyond that, every time I've had a similar problem it was a broken solder joint on a pot.

 

[elgecko]

Long shot, but have you run a plug in and out of the effects return jack a few times?

Yup, first thing I tried.

 

[Downunderwonder]

Somewhere between the fx loop and the DI it goes bad. It might well be an easy fix, not requiring a whole board.

Normally you'd try plugging into the fx return but I believe MB don't work like that.

 

[DigitalMan]

All the cables are 1/4" TS and shielded?

 

[elgecko]

All the cables are 1/4" TS and shielded?

Yes, except a mic cable to connect the DI to powered PA speaker.

 

[elgecko]

I dropped it off at VST yesterday and given the symptoms, they thought it was the preamp board and mentioned that was fairly rare.

I've read some less than stellar reviews for VST on TB but I have to say they've been great so far. My initial email was answered in less than an hour and they gave me some diagnostic pointers. When I asked what their current turnaround was they said approximately one month but Markbass is their number one priority and they'd be into my amp early next week.

Although $125 isn't too horrible to get my amp back working, from this point forward, it'll always make me go when I use it. I don't think that I'll ever be able to "trust" it again and that's a shame since up to now, it's always been the perfect amp for me.

 

[David Jayne]

Eh, I'd let it go if I were you. It's a bummer, but MB stuff generally is extremely reliable. Failures are rare. I'll bet it goes at least another decade or so without problems.

 

[bgavin]

Although $125 isn't too horrible to get my amp back working, from this point forward, it'll always make me go when I use it. I don't think that I'll ever be able to "trust" it again and that's a shame since up to now, it's always been the perfect amp for me.

I'm glad you do not apply this logic to 1) cars, 2) Windows.

Car repairs are a lot more than $125, even for minor work.
So is the cost of getting rid of Ransom-ware on a Windows machine...

The quality of the fix, is the quality of the tech.
The fast and apparently professional response you received, says a lot about the quality of the shop.

 

[JimmyM]

Swapping the main board used to cost over $400. I'd call this progress.

 

[elgecko]

I'm glad you do not apply this logic to 1) cars, 2) Windows.

Car repairs are a lot more than $125, even for minor work.
So is the cost of getting rid of Ransom-ware on a Windows machine...

The quality of the fix, is the quality of the tech.
The fast and apparently professional response you received, says a lot about the quality of the shop.

With cars, you are correct. I understand that cars have consumable parts and replacing those parts are part of car ownership. As far as I know, solid state bass amps don't have any consumable parts.

Swapping the main board used to cost over $400. I'd call this progress.

Had it been $400, I would've figured out how to crowbar my Walter Woods into it or simply turned it into a speaker cabinet.

 

[RockNRollAl]

I'm glad you do not apply this logic to 1) cars, 2) Windows.

Car repairs are a lot more than $125, even for minor work.
So is the cost of getting rid of Ransom-ware on a Windows machine...

The quality of the fix, is the quality of the tech.
The fast and apparently professional response you received, says a lot about the quality of the shop.

I've traded in a car before because of a cracked distributor cap.

I've even traded a car in while it was getting it's first oil change.

 

[elgecko]

ANOTHER ONE BITES THE DUST!

A little over 2 years ago, I scored a second CMD121-P on the Guitar Center used site. I took the head out of that one and slipped it into my existing combo and that's been my main amp ever since. A few weeks ago at a gig, the sound guy was complaining that he was getting distortion from my DI signal (I didn't hear anything weird coming from my amp) so I gave him the DI from my Zoom B3 and that fixed the problem. Figuring it was a fluke, I gave the sound guy the Markbass DI at a 4 night gig this past weekend and all was good until the last night when all of a sudden, the output from my amp dropped dramatically, accompanied by severe distortion. During the set break, I switched over to the DI from my Zoom and turned off/turned on my amp and everything was magically fixed for the rest of the night!

Today, I swapped my old, repaired head back into my combo and now, the faulty one has apparently given off the magic smoke! It provides a second or two of super low, super distorted output then drops out completely, along with no output from the DI. Seems like another preamp board. What a bummer!

At this point, I'm pretty much done with Markbass. I'm even reconsidering buying the Markbass synth pedal, which I've been looking at. I'll run the old, repaired head into the ground because I absolutely love the form factor of the CMD121-P but if and when IT decides to take a dump, I'll retrofit my old Walter Woods into the combo.

 

[BassmanPaul]

I'm glad you do not apply this logic to 1) cars, 2) Windows.

So is the cost of getting rid of Ransom-ware on a Windows machine....

I solved the Windows dilemma Bruce - bought a Mac!

Just a story: I wanted a MAC Mini to replace my wife’s aging IMac. I saw a couple up for bids on Ebay so I bid on both thinking I might get one. You’ve got it - i got both! I now have three nice little Minis and the IMac is still going!

 

[BassmanPaul]

@elgecko a thought just crossed my mind, a rare event now-a-days, was the Phantom Power engaged on the PA channel you were plugged into? It’s a natural thought that any DI from any manufacturer should be safe with Phantom Power but you never know.

Does the MB unit have a separate pre-amp board or is it all on one PCB?

Edit: I’m sorry for your troubles.

 

[elgecko]

@elgecko a thought just crossed my mind, a rare event now-a-days, was the Phantom Power engaged on the PA channel you were plugged into? It’s a natural thought that any DI from any manufacturer should be safe with Phantom Power but you never know.

Does the MB unit have a separate pre-amp board or is it all on one PCB?...

It's possible but unlikely in my opinion. Both gigs were at casinos that typically employ competent, full time sound guys.

From my conversations with VST, Markbass amps utilize separate preamp and main boards.

Edit: I’m sorry for your troubles.

Thanks...it is a third world problem though.