Fisher 400CX-2 resistor question

[wendelltate]

Hi all,
Thanks for the help in locating and replacing those diodes on my unit. I was able to do that with no problem. I am now trying to replace some of the resistors on my unit. I'm looking for the 220k that are numbered r26, r27, R75, r76. Are the 75,76 they ones that are going between the two tube sockets in the picture? Thanks

 

[dcgillespie]

Yes -- but why are you changing them? Unless they are off value -- which would be quite rare -- Fisher's glass enclosed carbon film resistors remain quite stable and quiet over the years. There is typically little need to ever replace these kinds of resistors.

Dave

 

[wendelltate]

I guess that I am just looking for stuff to do. I will leave those alone then. I have no reason to believe that they are off value. Are there any key resistors that I should replace? Thanks for saving me some time and effort.

 

[sgmlaw]

Why are you tearing into the phono section? Don’t fix what’s not broken. Unless they have drifted, leave them be. Most of the resistors in these are old Beyschlag carbon films. And they rarely drift and sound very good. Most I measure are nearly as tight as a new mil-spec Vishay metal film.

The only resistors that sometimes get replaced on these are the big 1w AB carbon comps, which can get drifty. And a lot of the time even they are still not too bad. An old trick if you are a crazy purist is to take them out, bake them in the Mrs. oven to drive the moisture out, and then reinstall them. But even I am not that crazy.

If you Dr. Stereo one of these too much, it won’t sound like a 400cx2 anymore. If you want to tweak the sonics, roll the tubes.

 

[sgmlaw]

Yes -- but why are you changing them? Unless they are off value -- which would be quite rare -- Fisher's glass enclosed carbon film resistors remain quite stable and quiet over the years. There is typically little need to ever replace these kinds of resistors.

Dave

Boy I wish they were ALL glass, Dave. I accidentally burned into one with an iron many years ago, and discovered that they are more likely just a hard enamel coat. Still pretty hard. It still measured perfect, so I touched it over with some hard clear coat sealer I keep on hand, and suffered the lesson.

 

[dcgillespie]

Very interesting! The worst thing I've ever had happen to these resistors was that an end cap and lead that's attached to it would (cleanly) break away from the body -- but you could always simply reinstall the cap, secure it with a dab of glue, and the thing was good as gold again. I agree on the hard enamel coat -- I just always assumed it covered a glass body underneath the exterior coating.

Dave

 

[sgmlaw]

There may be a glass core in there somewhere. But I don’t want to find out, if you understand my meaning.

But those old Beyschlags are terrific. Quiet, warm, and spot on. And hardier than the metal films. Unless I was intentionally changing values, or was using a metal film as a fuse, I would not see a need to touch them 99.99% of the time.