HELP! Can't adjust voltage with trimpot!!

Started by SuperGeo, July 26, 2005, 07:23:23 PM

Previous topic - Next topic

SuperGeo

Hi, I'm having a strange problem with my May Queen http://www.runoffgroove.com/mayqueen.html

It didn't work first, then I removed all transistors and put sockets in it's place.

But when I was going to bias the Q3 (with no transistor in place), I saw that when moving the trimpot, the voltage changes insignificantly, like from 9.27v to 9.02 volts :(

that happened with only Q2 in his place and before removing the transistors, I was able to set 5.5v normally with the trimpot.

I have absolutely no idea of how to solve  this


EDITED: I measured the resistance to drain of Q3 to +V, and it goes from 0 to 10k normally
please help!!

transient

I think you should do the biasing with all the transistors in their place. After all, variations in FETs are why those bias trimmers exist.

.
e

MartyMart

Some advice :
1 - always, always, always use sockets  !!
you may well have damaged the transistors when you soldered them in
and/or when you removed them !!!

2 -  You wont get any sense from a trim pot without the transistor in the
socket -- because there is NO CONNECTION to "ground" at the emitter
of the transistor ( the sockets three pins dont connect without the tranny
in place )

3 - Always "test" trim pots before using them, I've had "duff" trimmers
a couple of times.
To do this, take your new trim pot, connect your DMM red lead to an
outside "lug" and connect the black lead to the wiper ( centre lug )
Set your DMM to read "Ohms" say 20k for a 10-20k pot and watch the
meter as you turn the trim pot with a small screwdriver.
You should have a reading from "0" to "10" or "20" k depending on the
value of the pot ( within 10% )

Cheers,
Marty.
"Success is the ability to go from one failure to another with no loss of enthusiasm"
My Website www.martinlister.com