unable to bias prof tweed :(

[bentium]

hey
finally after a few weeks, i'm able to get busy again with some electronics

i had a prof tweed that i build a couple of weeks ago that didnt work
i was unable to bias q3, it jump from 0mv directly up to 9v (with bairly turning it)
q1 and q2 were nicely biased (4.5v and 4.6v)
i checked my wirings/ leads, no faults
so my guess was: bad trimpot
i swapped it for another one: same problem

could it be that q3 is busted?

[amz-fx]

could it be that q3 is busted?

Could be...  also possible that you have the pins oriented wrong on q3. What you describe could possibly happen if the gate were connected to the trimmer.

regards, Jack

[dpresley58]

I had a similar problem, except with Q1. Just couldn't get the biasing to behave. It'd been built on perfboard (the unpadded variety) and I had inadvertently made some sort of small connection between two of the socket pins Q1 was mounted in. Couldn't even see it, really, but it was obviously there. Sometimes I'd get a brief continuity on the meter, sometimes not.

Ended up taking that socket out and replacing it, paying attention to keep the solders clean. The 100k trimpots are pretty touchy, but I was able to get a voltage close enough to 4.5 for the tranny to be happy.

Just a thought for what it's worth. Hope it helps.

[bentium]

I had a similar problem, except with Q1. Just couldn't get the biasing to behave. It'd been built on perfboard (the unpadded variety) and I had inadvertently made some sort of small connection between two of the socket pins Q1 was mounted in. Couldn't even see it, really, but it was obviously there. Sometimes I'd get a brief continuity on the meter, sometimes not.

Ended up taking that socket out and replacing it, paying attention to keep the solders clean. The 100k trimpots are pretty touchy, but I was able to get a voltage close enough to 4.5 for the tranny to be happy.

Just a thought for what it's worth. Hope it helps.

i'm planning on taking the last section apart (when q3 starts until end) apart, and resolder it on the perf