Mashall Lead 100 Mosfet -> bad sound

[Flo1K]

Hi! im new and home u can halp me.

I got a Marshall lead 100 Mosfet for a few bucks but the sound is very unclean even in the clean channel.
The problem must be in the last stage power amp cause the sound of the DI is ok.
here is a shematic of the stage:

An externally hosted image should be here but it was not working when we last tested it.

i dont know how to find the problem so im happy is u post some tips for me to fix it.
thx
newb Flo

 

[jan.didden]

Pray tell, what is a 'DI'?

Jan Didden

 

[Flo1K]

its the direct output of the preamp (for mixer in for example).
here is the complete shematic http://flosen1000.zapto.org/Mar_3210.gif

 

[hitsware]

Is the speaker (s?) possibly blown ?

 

[jan.didden]

Can you do some simple measurements, like DC supply voltage, DC at the speaker output? Compared with the 'good' channel. If you say the sound is unclean, what exactly do you mean?


Jan Didden

 

[Flo1K]

ok, voltages and speaker are ok.
hmm the sound is muddy dont know how to explain .. sometime the level goes totaly down and after a while it comes up again ...

 

[rljones]

There may be a short circuit and the current limiters are activated (TR10/11). Or, possibly a bad cap in feedback loop.

If there is definitely no short circuit on the output, you could temporarily remove TR10/11 from circuit (lift one leg of D10/11) and see if this helps. (These things would be better checked on a test bench, but if you must connect a speaker for testing, use one you won't might destroying.)

If there is a short circuit at the ouput and TR10/11 are activated, the heatsink for the outptu devices should get rather warm soon after powering up the unit. But if the devices are misbehaving, they can create a distorted output.