Fisher 400 mod questions

[CZ4A]

I picked up a Fisher 400 (serial 22###) last week at a local antique store. The case in fair condition and the chassis is in good condition. All the tubes are original except one of the 7868s and all tested strong for emissions except two of the 7868s. The rear section needs cleaning, the dial bulbs are blown, and a couple of brass knob brights are missing. This receiver was so original it even has the factory service tag on the power cord! I hooked the Fisher up to my variac and Minimus 7s and it worked and sounded great! So I've begun looking for longevity mods and have keyed on on these two threads.

http://audiokarma.org/forums/index....er-fm-stereo-mpx-tuners-and-receivers.708033/

https://antiqueradio.org/Fisher800B.htm

I've already replaced the EM84a with an EM87 and changed the associated resistor on the socket. I have a new silicon bridge rectifier on hand, and a CL-80 and the items I need to convert MPX V100 and V102 to 12AT7s are on the way. I have a couple of questions though.

1) On the Antique Radio site, it says to attach safety resistors to the cathode sockets of the power tubes. Does safety resistor = fusible resistor in this case?

2) Does this chassis have one "death cap" or two? On the schematic of the 20000-29000 series, C83 definitely looks like a death cap to me, but is C80 also a death cap?

 

[larryderouin]

1.) No and Yes. The Cathode resistors are replacing the straight wire that is grounding the cathode. This is for 2 reasons. 1.) So you can directly determine cathode draw and convert the reading from mV to mA. 2.) If the tube dies, it will blow the resistor and shut the tube down, saving the OUTPUT Transformer.
NOTE: on the 400, 2 of the tube will have a 330 ohm resistor from the heater pin to the cathode ground. These need to be moved to a direct chassis ground.
1/4w 10ohm metal film resistor. These will pop like a firecracker if/when they go.

2.) 1 "death cap". C83. C80 is NOT a death cap.

The Forum Stickies have quite a few threads that deal with 400 upgrades and mods. Look under Technical threads, and the main list. They would be more pertinent than the 800b thread.

 

[CZ4A]

Thanks for your reply, Larry! I've been looking at other 400-specific threads here too. The 800B thread was linked on the Antique Radio website as the author said many of the mods on the 800B can be applied to the 400 (accounting for the different pinouts of the 7686, of course).

https://antiqueradio.org/Fisher400Receiver.htm

 

[larryderouin]

The mods he did on the 400 are mainly the basic stuff. He does good work too. We tend to get into some more detailed stuff, like splitting the AUX/Tape Monitor inputs, correcting the wiring on the Tape Mon so it's at the same input level as Aux, EFB, and some other stuff that I'm having trouble remembering. Look at Dave Gillespies "improving the 400" thread. It's geared more for the later version of the 400 (48001 serial and up) although 90% of the upgrades can be done to the early model without modification of the upgrade. Reason being is the later version has a different IF strip setup, 10K ohm impedance transformers and the attendant changes in the NFB circuits. If you want to do all of the upgrades, get with Dave Gillespie and ask him to detail the changes between the early and later versions as they pertain to the upgrades and what changes need to be done. One thing to note reference the 800b (and on the 400) is the FISHERDOC modifications. DO NOT REMOVE the 19kc filters. Dave goes into why not in either the MULTIPLEXER thread or the improving the 400. Dave is an Engineer with about 40 years of experience in Audio Gear repair and research, while FISHERDOC isn't. And we'll leave it at that.

Additional links:
http://audiokarma.org/forums/index.php?threads/improving-the-fisher-400.511867/
http://audiokarma.org/forums/index....dividual-bias-adjustment-modification.601583/
http://audiokarma.org/forums/index.php?threads/400-eyetube-adjustment-procedure-wrong.687346/
http://audiokarma.org/forums/index....r-800-500-early-version-400-receivers.539265/
http://audiokarma.org/forums/index.php?threads/fisher-400-owners-please-read-important.682273/
http://audiokarma.org/forums/index.php?threads/fisher-400-service-bulletin.671281/

 

[CZ4A]

Thanks again, esp. with the warning on the 19kc filter. I plan on keeping my mods simple. My 400 worked really well despite its age so I don't want to mess with it too much.