Fetzer Valve Problem

[ildar]

I completed a Fetzer Valve, my 1st perf build after many PCB builds, including the ROG 18 and TC, so I know how to bias FETs. However, when attemting to bias the FET in the FV, I get a reading that initially starts high-7.5v or so and dwindles down, or a reading that starts low- .3v and climbs. So, WTF???

[MartyMart]

Sounds like a bad fet or bad trim pot.
I had a very similar problem a few days ago, it was a bad 100k trim pot and worse.... I had TWO bad 100k trim pots !!!!
Drove me nuts !!
You should get a stable reading when all is well.

Marty.

[ildar]

Well, I tried a couple different FETs, two gave me no reading and one gave me the problem I've described. I'll change the trimmer and see what happens.

[triskadecaepyon]

I just built one, and I am having the same problem!! Stupid trimpot must be the reason.

[petemoore]

If a trim is connected to a socket at say drain, try pulling the transistor, then check to see if the pot's working with DMM.

[ildar]

I tried pulling the FET and got a voltage reading identical to the battery-the one I was using read 8.9v, and that's whatI got from the trimmer after pulling the FET-EACH TIME I pulled different FETs.
:x

[B Tremblay]

Solder bridge?

[Alpha579]

I tried pulling the FET and got a voltage reading identical to the battery-the one I was using read 8.9v, and that's whatI got from the trimmer after pulling the FET-EACH TIME I pulled different FETs.
:x

that would be because theres no current running through the trim pot when you remove the fet...isnt there?

[ildar]

Solder bridge?

Well, no-at that point in the circuit it's 9v+ > lug one trimmer > lug 2,3 trimmer > FET drain- hard to have a solder bridge with three solder points.

that would be because theres no current running through the trim pot when you remove the fet

There is indeed current going through the pot, and pin one of the FET socket, exactly equal to the voltage straight off the battery.

[petemoore]

What I mean is pull the transistor, then the only thing connected to the pot is the output cap [IIRC]. Under these conditions in the Fetzer I think you can get an accurate pot reading without having to pull it out.
 without power, test the resistance[s across the pot with DMM, turn the pot, see what kind of range you have, I sometimes check pot function, pre-install, but get tired of that...still I have to find and dig bad ones out of the circuit occasionally.
 I think they sometimes bend EZ...
 Could be anything, is Jfet orientation...re-verified?

[B Tremblay]

Solder bridge?

Well, no-at that point in the circuit it's 9v+ > lug one trimmer > lug 2,3 trimmer > FET drain- hard to have a solder bridge with three solder points.

Yes, but what if there were a solder bridge between the Drain and Source pins of your FET socket?