My guess is that you have a funky pot. While the joined legs will always be the same, the single leg has to change when you turn the wiper. Try another pot.
Fisher 400 startup issues after rebuild
- Thread starter tonefoto
- Start date
Ok, so I'm moving on and going to install an IBAM. I don't want to mess with the single 10K pot at this point. I'm changing the screen resistors to 200K 1/2W and changing coupling caps from .047 to .1uf 630V Westcott and Russian PIO. I'm hoping someone checks it all out before I start up on DBT and variac to test. Will post progress.
I finished building and installing the IBAM, 200K grid resistors, .1uf 600V PIO caps. Photos show where IBAM was connected with yellow wire on the left to the negative side of the bias and -rectifier, and the black wire on left to ground lug on terminal strip. Each trimmer goes to the 4 new 200K resistors and trimmers mirror the position of each tube they control.
Can I test the IBAM on variac without output tubes?
Attachments
Tested OK on DBT and variac. Went off DBT and was able to adjust pin 2 to -17 VDC on each socket with IBAM trimmers.
Ok, you’ve verified you have a functional IBAM. What you want is to bias your output tubes to a safe 30ma until other aspects of the restoration is complete. You can then pop in your best tubes and set them to your preferred bias.
I adjusted each tube's bias to 320mV for testing. Sounds good so far, except for a scratchy volume pot on right side only. Its noisey just past turning power on then quiets down. Cleaned it thoroughly with deoxit and faderlube
Yeah, noisy pots are a pita! But looks like you’re good to go .
Thank you for the good advice and all the help in getting this 400 back on the road safely. Larry, thank you for shepherding another one to the flock.
Its a labor of love rebuilding these, almost feel like a cultural preservationist to be honest. I just found a pair of Fisher XP-9 speakers that I'm thinking of mating with this 400.
