Another Microtube 100 question

[deorai77]

Hi Team,

I have similar issue like El Chiguete, the Ch A works fine . The Ch B fuse is blown, when I replaced the fuse worked but got lose of tone and some volume drops for 1 sec to 1/2 sec coming on and off.

I replaced the tube then worked fine got the tone back the volume drop is gone, then all of a sudden the volume came down like going to mono from stereo. And this is all only channel B as I am only plugged in channel B.

When I bought the amp the transformers was not tightly screwed . But I had tightened it. I wiggled the transformer it made some crackling sound and the tone and volume came back. I wiggled some more , more crackling sound then the volume came down, I wiggled it the volume came back to normal wiggled some more , the fuse blew. What do you recon ?

This is not the same issue as El Chiguete is it? Do you recon its a sort transformer? If so how do I fix it?  Thanks in advance guys.

My MT100 has more transistors than El Ch's, pic attached....

[rnolan]

Well if your chan A still works fine, maybe use that instead for now ? MJMP may have some ideas, which trany are you wriggling ?, IIRC the MT 100 has 2, a standard and a toroidal,

[deorai77]

Hi Rnolan, thank you
I was wiggling the toroidal tranny, I got more fuse and placed the 3A fast blow fuse, it popped, I found out that I blew the mosfet as well, now no more sound In channel B, the fuse is blow and the mosfet IRF9530 and IRF530 is shorted . I would like to know what caused that and fix that issue before I replace the mosfet and the fuse.
thanks to replying..

[MarshallJMP]

Don't think the transformer is the problem since both channels share the same power supply. I "think" it might be a bad contact on the pcb since the wires from the tranny are very stiff they might move the pcb also. I would check the soldering on those big white resistors. Sometimes they desolder themself and if you get a bad contact it can blow the mosfet's. Anyway that's what I "think" 

But when you replace the mosfet's I would also change the drive transistors and check those signal diodes also.

[deorai77]

Thank you Mr JMP,
Now you meant the rectangle big white 3W resistors right? I will solder it,

Also some to the components are beeping while doing the contenting testing wiht my multimeter.

the 1st from the bottom toward the channel B large 4700uf CAP was beeping like it was sorted out but stoped and now shows reading1 after I removed the mosfet.

but the 4.7uf 50V electrolytic cap next to the L7912ACV still beeps but when I took it off and checked the CAP it seems to be fine,
the gold  cap next to the large cap give the same beep.
the 2 gray metal film resistors (think its 2W) next to the large white 3W resistor and  large electro 2200uf  35V cap.  (but the Channel A side also beeps)
another gray film resistor (think its 1W) next the the channel B tube next tot he brown cap also beeps.
Also the 8099 transistor slepping under a small diode next to the Channel B fuse does not give large reading but only shows 1 on both sides.

Do I need to replace these are is this the effect of the blown mosfet/ normal?

I will post some pictures later today from work.

Thanks Heaps

[MarshallJMP]

Did you set your meter on continuity test? Does it have a diode check test?

[deorai77]

Hi JMP,

Yes, the multimeter set on continuity/diode setting. (has the sideways wifi symbol and diode symbol)

[MarshallJMP]

Measuring in circuit is always difficult and not always acurate. Replace those mosfet's and all the driver transistors (MPSA...). Then it would be best to use a variac to slowly raise the voltage and check the current.