Microtube 100 Blowing fuses

[kissfan113]

I just picked up a MT100 with a bad channel, Channel B immediately blows the fuse anyone have an idea where to start to cure this issue up, channel A works great.

[MarshallJMP]

Most likely cause is a blown power mosfet(s). For channel B these are Q106 and Q107 part numbers IRF9538 and IRF538. Sometimes you also have a defective driver transistor.

[kissfan113]

What is the location of the driver transistor on the schematic?
Thanks
Kevin

[kissfan113]

Also, what could have caused the original chips to blow? I don't want to replace parts and have them directly fail again.

[David Drake]

Hey I'll point ya in the right direction maybe? I'm not one of the brains 'round here. But I just got thrugh reading this the other day as my Micro Tube 100 had twice suffering from this reoccurring illness.

I think I know why the B channel gets blown when it's in bridge mode. Problem is that the output of the B side is connected to the sleeve of the speaker jack. Most of these jacks are metal so if something is touching the jack that is grounded it can blow up the B channel. So best is to isolate the metal part of the jack.

Does this also apply to the MT-100?

Yes it does.

Complete thread is here @ http://adadepot.com/index.php?topic=2587.

[rnolan]

Hey Dave, well spotted  :thumb-up: . I think allot of MT100 users ran their units in bridge mode.

[MarshallJMP]

Bridging can cause the problem like David said. 90% of the time it's the B channel.

The safest way to test it is using an external power supply and put a current limit on it of around 100mA this way you can't blow up the mosfets if something is wrong.
That's the way ADA did it too.

[kissfan113]

Thank you for that document MarshallJMP, I always appreciate the original tech literature for the gear I have it helps a huge amount when it comes to maintenance and service

[MarshallJMP]

No problem, just let me know if you need help.

[kissfan113]

Ok so finally got back into the shed today, tested the b channel mosfets they are both blown, new ones on order now. I'll keep you all posted on how the repair goes once I get the new mosfets in place.

[MarshallJMP]

ok keep us updated