Marshall mb30

[opacheco]

Hi,

Do someone have the Marshall mb30 schematic?

This have some component toasted and I can see it’s values!

I will appreciate any help.
Thanks anticipated
opacheco

 

[Khron]

Have you tried taking a look what they're connected to? Or at least which "area" of the circuit they're in?

 

[buckethead]

Here's a link with some infos about the MB15, which more or less has the same output stage - those are diy schematics, so beware:
https://github.com/8TN/Marshall-MB15-Bass-Amplifier-Ampli-Basse

You can also reference infos from MG30, or bass 30 schematics, I think they all use the same TDA2050 output stage. The resistors are for HF roll-off of the pwr amp output to keep it from oscillating  - you can more or less "guesstimate" values there. The TDA 2050 datasheet proposes 2,2Ohms / 470nF, marshall seems to use something around 2,5-10Ohms / 220nF in their designs.

The question is, if those resistors went up in flames because of their low pwr rating or some other problem. If it's just the low pwr rating then I would probably go a little higher than 1/4W there and mount them with some space to the pcb.

Hope that helps
Best regards,
Mathias

 

[opacheco]

Have you tried taking a look what they're connected to? Or at least which "area" of the circuit they're in?

Khron, thanks for your replay! They are connected to the amp section

opacheco

 

[opacheco]

Here's a link with some infos about the MB15, which more or less has the same output stage - those are diy schematics, so beware:
https://github.com/8TN/Marshall-MB15-Bass-Amplifier-Ampli-Basse

You can also reference infos from MG30, or bass 30 schematics, I think they all use the same TDA2050 output stage. The resistors are for HF roll-off of the pwr amp output to keep it from oscillating  - you can more or less "guesstimate" values there. The TDA 2050 datasheet proposes 2,2Ohms / 470nF, marshall seems to use something around 2,5-10Ohms / 220nF in their designs.

The question is, if those resistors went up in flames because of their low pwr rating or some other problem. If it's just the low pwr rating then I would probably go a little higher than 1/4W there and mount them with some space to the pcb.

Hope that helps
Best regards,
Mathias

buckethead,
Thanks a lot for your great information and comments! Let tme to try to test and change these resistir to the suggested values in that schematic part!

Thanks again,
opacheco

 

[analogguru]

....
Let tme to try to test and change these resistir to the suggested values in that schematic part!
.....

Beware, those resistors only burn out when the amplifier (chip) is oscillating (at high, inaudible frequency)

 

[Khron]

I had a bit of a suspicion that four identical(?) resistors would be grouped up like that anywhere else than in a Zobel network on the output of an amplifier. If those got cooked like that, odds are good the amplifier broke into oscillation at some point.

Since uber-extended high frequency response is not necessary in instrument amps, as a preventative measure, I'd also add a capacitor (1nF max, maybe lower) between the TDA's output and the inverting ("minus") input, to roll off some more top end and reduce the chances of oscillation down the road.

 

[opacheco]

I had a bit of a suspicion that four identical(?) resistors would be grouped up like that anywhere else than in a Zobel network on the output of an amplifier. If those got cooked like that, odds are good the amplifier broke into oscillation at some point.

Since uber-extended high frequency response is not necessary in instrument amps, as a preventative measure, I'd also add a capacitor (1nF max, maybe lower) between the TDA's output and the inverting ("minus") input, to roll off some more top end and reduce the chances of oscillation down the road.

Khron interesting notes!, I am considering that.

Thanks a lot
opachecp

 

[Whoops]

You can get the schematic here:

https://www.dtforum.info/index.php?topic=164045.0


For those 4 resistors to have burned up something is really wrong with other parts in the circuit.
I would not replaced it ever with higher wattage ratings as was suggested here neither would I use any other values than the original ones

 

[buckethead]

IF the amp has only been played at bedroom levels, something might be oscillating. BUT if it is beeing used as in my experience amps like that are often beeing used, in a quite torturous way, then a lot of times the output will be clipped hard and hence have a lot of HF content making the zobel lowpass filter shorting quite an amount of power to GND - one explanation of why those resistors would get really hot and will get hot in the future. The speakers inductive load at rising frequencies won't make that amp's situation any easier...

But what actually is the problem with the amp, besides the burnt resistors? There must be a reason why you opened it up in the first place. Maybe knowing that would drop the rate of "wild-guessing"

 

[Whoops]

one explanation of why those resistors would get really hot and will get hot in the future.

The situation is that the resistors didn't "get hot" the resistors "completely burned", so there's definitely something wrong in the amp that burned those resistors