Tuesday, April 3, 2012

Fisher FM-90X output caps

Also been thinking a lot about a recently purchased FM-90X.  The front panel looks awful (from simple corrosion) but the back is pristine looking, tubes are original, even rectifier, as if it was never used. This is the pinnacle of Fisher's pre-stereo models.  In the past, I've like the sound of Fisher 60,80,90,100.  This one seemed to have sonic potential, but thin sounding.  Then I discovered there was 100Vdc on the output.  Fisher used Goodman paper capacitors in critical places for euphonic sound.  These caps are the first to go bad, and it looks like mine has gone bad.  it is a 0.22uF driven by 12AU7 plate coupled amplifier.  It's not clear what the other section of the tube is for, I need to see the schematic.  But it seems to me like a 12AU7 with plate of 47,000 can drive as much as 1mF or more, it's ridiculous to use 0.22 which yields low cutoff of 70Hz into 10K load.  OTOH, increasing the cap could, in worst case, cause motorboating.  Or perhaps the tube could be reconfigured as cathode follower.

Clearly it was designed to plug into preamp with very high impedance like 100k.  (In fact, inferior 90T model with built in preamp used 1uF on the output, driven by 12AU7 with same plate resistor.)

Sure, I could use an outboard buffer before plugging it into the 10k load of one of my ADC's.  But the tuner already has the perfect buffer tube in it, just as good as my buffer, it's just not being used very well.

I had spent all night thinking what kind of 0.22 cap to get (drooling over V-Cap teflon, merely $96.58 in 300V or $134 in 600V).  But now I don't think I want a 0.22 at all, although i did think of a trick if it turns out I can't use a bigger cap.  I can put 20k in series with the output after the cap.  Into 30k load, the cutoff falls to about 20Hz.  And then, crank up sensitivity on the other side (which has S/N to burn) rather than forcing more output through the 12AU7 (which would increase it's distortion.) 

Most likely I'll get polyproplylene/foil (not metalized) cap, several brands in play in my mind now.

KPAC-FM is still on low power.  Can't get good stereo on anything.  What sounds nice right now is the old Marantz 20B switched to mono.  I brought that out after it was clear I couldn't use the Fisher yet. With 300 ohm connection to 300 ohm antenna, the sensitivity is OK, especially in mono.

[Just now ordered FM-90x service manual, original old stock, on ebay.  Nobody I've found lets you peek at the FM-90x schematic for free.]

Here's a guy raving about his recently restored REL Precendent Mono FM tuner.  These highly collectable tuners sell for thousands of dollars even without restoration, said to be the very best of the very best.  Guess what, he doesn't bother with stereo adapter, he doesn't like how they sound, so he uses a Y adapter for pure glorious dual mono.

I won't claim that my Fisher FM90X will equal a REL, but the 90X is a highly sensitive mono tube tuner that was also considered a top unit in the late 1950's (ant still today!).  Given some restoration, and possibly even some astute modification, it might be hard to hear the difference, and I'm thinking that with all teflon caps in the audio section, the 90X might be very special indeed, and one might even prefer it.

1 comment:

  1. Hi there, would you be willing to share a copy of your FM90-X service manual for a few bucks? I have a SAMS but not a factory manual. Thanks~

    Regards, Larry Steinhauer