Re: My amplifone monitor... update

> > I had hoped that the new transistors (Q16, Q17) had solved the problem.
> > After watching the sparks (plasma) fly, I figured Q14 and/or Q15 were
> > the problem. Sure enough, I pulled out Q14 and it was shorted all the
> > way around (I think Q15 had problems too). The transistors on the other
> > side of the board (Q4, Q5) seem to be OK. So, where can I find these
> > parts: MPS-U07 and MPS-U57? The Woodcock/Scheive material I have looked
> > at tells me that NTE188 and NTE189 are equivalent parts.
>
> > Just to ask a silly question, aren't these mounted with mica insulators?
> > Did you put those back in (and heat sink compound)? (I've worked on a
> > couple of monitors that people omitted that step and wondered what went
> > wrong.. *poof*)
>
I had not touched these until I removed them after I re-fried the board.
The were mounted with insulators but...

When I removed Q14 I noticed that whoever installed it mostly missed the
screw hole in the insulator, punching a new one right next to the original.
I'm pretty sure the pressure from the screw head was enough to push the
tab in contact with the heat sink where the original hole was. I assume
this is where it had originally failed.

BTW, I just ordered the replacements from Circuit Specialists. The did not
have the insulators so I am going to have to find at least one somwhere else.
They also didn't have any 3.3 ohm, half watt resistors.

Thanks.

Dan
Received on Thu Nov 20 10:55:21 1997