Markbass LM250 defect (shuts down when I turn it louder)

[VoodooChild]

Hi,

got this markbass LM250 for nearly 10 years now. Its made in Italy and got 250 Watt, no extra volume pot for the DI out on the front. Last gig this thing did not work properly, i could play it only at very low volumes. As soon as i brought the volume to over 9 oclock the thing shuts down, especially when i play a low note like open E string. Preamp works fine, saved my ass on the gig cause we could go via the preamp out into another poweramp. My question is if this is a known issue? Can it be fixed? How much would a fix cost me? I can handle solder iron just fine and reading schematics is kind of ok too.

Thanks!

 

[BadExample]

Try a patch cord in the effects loop. If you can get a schematic, and you know how to be safe around pretty high voltage, maybe check power supplies before and during failure. Class D?

 

[/\/\3phist0]

Also, check your speaker cable and cabinet connections, because you say "shutting down" meaning no output from the speaker.... Not that it powers down....

 

[VoodooChild]

Thanks for the replies, i'm pretty sure it's not the speaker cause when i turn the amp off and on again (after getting it to shutdown) it works fine (at low volume) till i hit a hungry note like a open e sting.

And i think this is something like class D cause it is a rather small and light bass head.

 

[agedhorse]

This is also the symptom of a partially shorted (illegal) load. A qualified technician should be able to diagnose the problem fairly easily (depending on where you are and the skill level of the techs in your area).

 

[VoodooChild]

I meant to write "power hungy note" there.

 

[agedhorse]

Being pretty sure it's not the speaker is completely different than confirming that it's not a defective load (speaker, crossover, wiring, etc.) Ignoring a likely cause is not how you troubleshoot these things if you intend to get to the root of the problem.

 

[VoodooChild]

I checked my speaker with my aguilar head, works fine. Does it make sense to measure it via Ohm meter if it really got 4 Ohms? Btw. I am very thankful for your advice!

 

[agedhorse]

Try your amp with a different speaker, there are some specific faults that may affect one mp design differently than another. This is more common in class D designs, but possible in linear amp designs as well.

 

[Bassmec]

Transistors sometimes need to be rebiased just like output valves (tubes). drifting out of bias can cause an increase in distortion and heat for a given output power.
This can cause the amp to thermal out early even if the output load is correct.

 

[DirkP]

Maybe the fan has become dirty and dusty and doesn't work anymore and the security department decides to shut your amp down because of the heat?

 

[VoodooChild]

Checked with 2 more speakers, exactly the same behavior. Fans work fine, this happens right after turning on the unit on so don't think it is a heat issue. Might have to check the bias thing that Bassmec mentioned.

 

[BadExample]

Good chance you have bad outputs. Wait for advice from @agedhorse

 

[agedhorse]

Correct, this needs the be diagnosed by a proper service tech to determine exactly what fault is causing protection shutdown. There are multiple fault monitoring circuits involved, so the first step is to determine what fault the protection thinks it's seeing, then look at the circuit to see if it's really a fault or possibly a defective fault monitoring process. It's straight forward for s good tech, especially with docs.

 

[BassmanPaul]

Damn it Jim, I’m a doctor not an amp technician!!!

 

[VoodooChild]

OK, may i ask if those docs can be found somewhere?

 

[Emanuel Apascaritei]

I think that amp is AB class, but don't know for sure. It's possible the final transistors to be in their final countdown and need to be replaced but you won't know for sure until a tech opens it up and evaluate the situation.

 

[agedhorse]

I think that amp is AB class, but don't know for sure. It's possible the final transistors to be in their final countdown and need to be replaced but you won't know for sure until a tech opens it up and evaluate the situation.

That's a pretty unlikely scenario, transistors don't generally fail this way. But, it's an interesting speculation...

 

[Emanuel Apascaritei]

That's a pretty unlikely scenario, transistors don't generally fail this way. But, it's an interesting speculation...

Yeah, I know, but I remember (vaguely) a pa amp with this issue and it was the final transistors. But I can be wrong.