MT100 too hot

[Thorgen]

Hello, I just got an MT100 whitch works perfectly in stereo, but becomes really really hot in bridge mode (can't put my hand on it).
I can't insert a picture, so here is the link :

https://servimg.com/view/10057491/638

it is the component circled in red that heats up

Thanks for your answers  :)

[rnolan]

Hey Thorgen, So the bit you have circled in red are the 2 Mosfets (which provide the output current) for one of the 2 channels.  That one pair get excessively hot compared to the other 2 indicates something isn't quite right  .  Have you read the manual (attached).  In bridged mode (using mono input and bridged output jack), the speaker impedance must be a total of 8 ohms minimum or 16 ohms.  If your overall speaker impedance (at the bridged mode output jack) is 4 ohms, while it may work (for a while) it "may" cause the Mosfets  to get too hot.  What speakers are you using? and more importantly what impedance are they.  If you have got the impedance right (8 or 16 ohms), maybe MJMP will have some ideas why it's getting too hot on one pair of Mosfets, although this seems strange as I assume (I may be wrong), in bridged mode, one pair does the +'ve side of the signal and the other pair the -'ve (push pull).

[Thorgen]

thank's for your answer : the loudspeakers are 8 or 16 ohms (2 v30, or 1 electrovoice).
I'll take it to the technician who i one of my friends, but I'm also interrested in the answer of MJMP  ;)

BTW, is it possible to find a schematic sowhere ? The tech asked me ?

Thank's

[MarshallJMP]

Is it only one pair that gets hot or both pairs? Does it sound ok in bridge mode? (try it with a clean tone).

Mail me and I'll send you the schematics.

[Thorgen]

Hello everyone, first of all, sorry for the late reply.
Then, to MJMP, great thanks for your proposal of schematics.
Now, the explaination : my friend found where the problem came from: the amp had been "bricolé" (sorry, I don't know the word in english) by an incompetent person. The output jack had been changed and was metallic, therefore, was shorted. My friend soldered a plastic jack, and everything is now back to normal. I am just waiting for a 220v transformer so that the amp is perfect for me

[rnolan]

Hey Thorgen, common issue with some ADA amps, e.g. the B200s and B500b/s output jacks don't earth to the case and cause really bad hum if you do.  However, lots of amps (I suspect most) do earth to the case so it's an easy mistake  .  You may need 2 new transformers ?  I know the MT200 has a power transformer and a separate toroidal transformer which IIRC both need to be changed from 117v to 240v.
Good to know you have it fixed though 

[MarshallJMP]

The MT100 has 2 transformers a power transformer for the power stage and a small high voltage one for the tubes. The MT200 has a SMPS or a switching power supply.

As for that jack you got lucky it didn't blew the output Mosfets. I think it had enough resistance to not blow it up. Output jacks from bridged amps always have to be isolated from the chassis. If not you ground one amp and it will blow.

Bricolé means that he actually did a crappy job in fixing it.