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Another Microtube 100 question

Started by deorai77, January 30, 2018, 06:51:57 AM

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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....
PLAY LOUD!!!

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,
Studio Rig: Stuff; Live Rig: More Stuff; Guitars: A few

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..
PLAY LOUD!!!

MarshallJMP

#3
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

#4
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

PLAY LOUD!!!

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)
PLAY LOUD!!!

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.