minus 25 on output mosfet amp

[Jan Deckers]

Hi ,

I just build a project 101 (p101) from ESP. It all worked fine, until I decided to test it with ARTA....I made a terrible mistake and shortcut the output (while testsignal was on)...just some stupid thing with wiring and the soundcard....blew up my little speaker (kef.....is dead now ).
But when trying to repair it (instead of fuses I use 100ohm resitors) there is -25 volts on the output....seems like one of the output mosfets isn't working....

Has somebody some suggestions. I changed already the output mosfets (alright they can be damaged too). but...no result.

Any way to test the ouput mosfet?
Some suggestions?

A link to the article is here:

http://sound.westhost.com/project101.htm

[Zen Mod]

check/replace Q7 and ZD2 , D2

[megajocke]

Check ZD1, D1, D3 and VR1 too. R10 and R11 may be bad too if ZD1 and D1 opened instead of shorted. (there are internal zeners in the transistors)

A shorted output is likely to damage D1, ZD1, Q7 and/or D3 as the VAS isn't current limited.

[coder2]

If you can solder them out for checking, you can do this:

Switch your multimeter in Ohm-meter mode as for checking diodes. Do not touch the transistor leads by fingers during the tests and their input capacitance will allow to switch the transistors on and off by the multimeter probes.

Checking 2SK1058: (Pinout: 1-GATE, 2-SOURCE, 3-DRAIN).

Check G-S and G-D resistance: It should be "infinite" regardless of probes polarity.

To open the transistor connect probe- to lead 2, probe+ to lead 1. Then re-connect probe+ to lead 3 and the multimeter should display resistance about 0 Ohm.

Then close the transistor by connecting probe+ to lead 2(source) and probe- to lead 1(gate). Then check if it has been closed by connecting probe+ to lead 3 and probe- to lead 2: the resistance should be "infinite".

Checking process of 2SJ162 transistors is the same, but swap probes polarity before.


Hope it will help.