MX50SE problem please help

[Deleted member 550749]

I think it's pcb problem , u can see there capasito connection present with collector but base connection missing 🤔

 

[emin818]

This is the pcb

 

[Deleted member 550749]

Ok check ur pcb first is that connection base to collector ok or not

 

[emin818]

On which transistor?

 

[Deleted member 550749]

Is that double sided printed pcb??

 

[emin818]

Yes I believe it’s double sided

 

[Deleted member 550749]

101 cap with bc546

 

[Deleted member 550749]

Ok, so pcb is ok, change that cap with other one , like npo cap or same one same value

 

[Deleted member 550749]

Also check dry soldering, sold properly all caps,

 

[emin818]

Everything looks properly soldered! Unfortunately I don’t have any spare parts to swap in I’ll have to make do with what’s on the board

 

[emin818]

But here is the thing both boards did the same thing they burned up the 10ohm resistor!

 

[Deleted member 550749]

Hmm, try to find out ur problem here , https://www.diyaudio.com/community/threads/ljm-mx50-kit-amp.216609/page-31

 

[Deleted member 550749]

Check if newer version schematic available there, to see what components r used and what u r using, check components properly

 

[Mooly]

It’s very strange could it be a very high frequency oscillation above the audible range ?

99.9% that is what is happening.

I'm going to say you should start using a DBT (dim bulb tester) to limit current and further damage. The board should draw minimal current if powered up on its own with the inputs shorted.

It is unlikely (imo) to be a faulty cap as both boards behave the same way.

 

[emin818]

Well strange update I desolder the blue cap I think it’s 100pf and and resolderd it and now amp has been running trouble free for 10 min go figure that one out . The amp with no load pill like 20ma it’s very minimal amount of current so I’m very very confused lol 😂

 

[emin818]

The heat sink is warm to the touch at very low volume is that normal ? Not hot but warm !

 

[chermann]

yes it is normal, which rail voltage did you use?
start with lower voltage , e.g. 20V each rail. if that is fine...then go to the recommended voltage at the supply rails.

 

[chermann]

this will help too.....link

 

[Deleted member 550749]

Try to get these caps and replace , much better than that blue one

 

[emin818]

The blue ones I believe are 2kv 100pf does it matter if it’s 1000v or 2000v isent it a bit redundant for it to be such high voltage ?

By the way one of the 100pf measures at 79pf would that be enough for it to oscillate?

Now I need to get the second amp going I need to order output transistors the 100pf caps and 0.1 ohm 5w resistor and 10ohm 2 w resistor and see if I can get the second amp to not oscillate!

The rail voltage is at 39+-. What I really need is to make is a dim bulb so I don’t have to keep changing parts out lol .

But so far we have progress one amp is running now to get the second one to run .