Version 3 Fisher 400 (S/N 80262) — Selenium Rectifier Replacement & Dropping Resistor Value?

[Audio fan1]

Hi all,

Long-time lurker, first-time poster. I recently found a Fisher 400 on the street and I want to start the process of restoring it. Serial number is 80262, which I understand puts me in the 48001-and-up (version 3) camp.

I'm getting ready to replace the selenium rectifier with a W04G silicon bridge rectifier and need some guidance on the correct dropping resistor value for my specific version. I've read that the bias supply rectifier was already changed to silicon at the factory for units above 48001 — does that mean I may only have one selenium to deal with (the main power supply), or should I be looking for both?

A few specifics about my situation:
- Unit appears to be all original inside, tubes still in place
- I have not powered it up yet... planning to use a dim bulb tester for first power-up
- I have a multimeter and can measure B+ voltage after the rectifier swap
- I've seen values ranging from 47 ohm all the way to 180-200 ohm mentioned across various threads — what's the right starting point for a version 3 unit, and what B+ voltage should I be targeting per the service manual?

Any guidance from folks who've done this specific version would be much appreciated. Happy to share photos if helpful. Also any other insight on what I should be doing next would be greatly appreciated I have purchased a tube tester.

Thanks!

 

[larryderouin]

Early or Late models. (10001-47999) or 48001 and up. ONLY 1 Rectifier. 4a 400v rectifier is plenty.

1st power up, PULL ALL TUBES! You're verifying transformer operation at this point. Because it's unloaded up to 10% higher than schematic is acceptable. NOTE: The Late model power transformer is known to produce lower voltages on the B+ for plate and screen voltages. 360-370vdc on the plate is fine.
Wait 5 minutes and physically touch the main Capacitors. Check for unusual heating. If substantially warmer or hot replace. No if's ands or buts. Hayseed Hamfest has a replacement can cap kit. It's a little pricey, but they work well. Last of my Fishers I rebuilt in 2018 is running at a friends and the earliest I put them in is a 800c (2011)I'm currently using. They made me a set for my Sansui 1000A in 2009 and still holding up very well.

Dropping resistor is on the rectified 12v circuit for the tube heaters. 12.6V + or - 5%. so 11.97 (call it 12V) to 13.2V) most guys like to keep them between 12.3 and 12.6. start with a 27ohm and work your way up until you get in range. Line (wall Votlage) at 117 per spec. Fisher declared due to variances in similar parts, etc. a 10-15% variance in voltages is acceptable. (This includes Plate, & screen ) Bias voltages will vary with plate and screens. Higher plate and screens will result in lower bias voltage. And vice versa.

That should get you started. There are Literally TONS of 400 rehab threads. Check the stickies. The best threads are there. Some parts listed may have been substituted or over-ridden by new stuff. If you decide to use a IBAM (Individual Bias Adjustment ) or Daves upgraded IBBA design (a couple guys fabbed them up and selling barebones or complete boards) NO NEED to Change the Grid return resistor or the output cap values. (300Kohm /.047uf) Matched tubes are ideal, but the IBBA/IBAM will accomodate mismatched to an extent.

The 1st MAIN cap(Rectifier can) that has the little black or puke yellow Raincoat on it is insulated from the chassis. Replace the insulator when you replace that cap. That cap is wired from AC and develops DC (2 sections) The 'ground" LUG ON THIS ONE is tied to the next cap as a rectified DC. Because the 2 sections in the CAN are in SERIES +to-to+to- One has a transformer lead on the + side of the cap with the - to the "ground lug. The other section has the + to the "ground lug" and - to the other lead on the cap. This creates a DC for the whole unit except for the bias section which is handled by the rectifer and the double can cap underneath. UNDER NO CIRCUMSTANCES MUST the Recifier can be left uncovered as it's carrying DC Votlage of about 200 to 300v (Im' not looking at a scat right now). You WILL get shocked, and we don't want you to become part of a "Lets see if I can duplicate an AED" experiment. DO NOT GROUND THE Rectifier CAN to the chassis or the Fire Dept WILL RESPOND to at minimum "Smoke in the bldg". No METAL from your fingertips to your shoulders. Silicone rings are ok.

Safety RANT OFF. Go SLOW, Ask Questions, POST PICTURES so we can see undersides. There may have been earlier repairs, that may or may not need to be addressed.

 

[Audio fan1]

Thanks Larry! This is all super helpful, you've given me a lot to review and hopefully I can kick off the process sometime this week. Will post pics next.

 

[kappclark]

Congrats on the 400 - please let us know how it goes.

 

[dcgillespie]

When Fisher changed to using a silicon bridge rectifier from selenium in the 400, they changed nothing but the rectifier itself. The main dropping resistor value was unchanged, while small adjustments to the bias dropping network made throughout the full production run of the receiver were more about changes in grid characteristics for the output tubes sourced than anything with the rectifier change.

Dave