Fisher 400 FM sensitivity: seems low - what should I check for?

monkboughtlunch

Super Member
My Fisher 400 has struggles to pull in most stations in my area free of distortion in stereo. I'm wondering if I have an out of spec part or parts -- or if this is normal sensitivity for this receiver.

Most of the FM station transmitter towers in my city are about 30 miles from my house -- but I'm in the primary broadcast radius.
I have a Marantz 18 receiver that doesn't have the same struggles.

If this is a out of spec part issue, would a tube be a likely culprit?
 
Last edited:
Does it play normally in MONO???

What's the serial # of the 400.

Hi Larry

It's a 67000 serial. I believe it's a 1966 manufacture based on transformer codes. Yes, it plays more stations in mono free of noise. It really struggles with stereo stations (lots of static and flanging artifacts on many stations). But a few stations play ok in stereo.
 
Last edited:
1st off check the separation pot on the MPX board. That's the blue one on top. Make sure it's set to near center no more than an hour arm on a clock angles a 1/2 hour either way. Then check the 3 12ax7's on the MPX to make sure they are ok. Then check all the 6AU6's in the IF Strip, although with good mono they are probably OK. IT may need an alignment. If you take it in to a shop make sure you note on the copy of the manual that in the alignment instructions on step 1 the Z5 adjustments are REVERSED! The manual is WRONG!
 
The 400 receiver is plenty sensitive for equipment from that day: 20 db of quieting can usually be had with only 1.5 uV of signal, which is right up there with some of the better tuners. What kind of antenna are you using?

Dave
 
The 400 receiver is plenty sensitive for equipment from that day: 20 db of quieting can usually be had with only 1.5 uV of signal, which is right up there with some of the better tuners. What kind of antenna are you using?

Dave

Hi Dave,

I'm using a standard dipole. I have a 1969 Marantz Eighteen receiver to compare it with, and the Marantz is pulling in stations fine in stereo with the same antennae in the same room. I'm guessing something is amiss with my 400.
 
Last edited:
1st off check the separation pot on the MPX board. That's the blue one on top. Make sure it's set to near center no more than an hour arm on a clock angles a 1/2 hour either way.

Hi Larry,

Is this the pot you are referring to? Which side of the blue pot should be at the center. There are grooves in the blue pot at the 4 p.m. position. Should the middle of the groove be at the 6 o'clock position or noon position?

fm.jpg
 
Last edited:
Ok, I rotated the blue pot to the norma 12 o'clock position. In the picture, it was turned to about the 10 o'clock position. No discernible difference when rotated to 12 o'clock.
 
Then check the 3 12ax7's on the MPX to make sure they are ok. Then check all the 6AU6's in the IF Strip, although with good mono they are probably OK. IT may need an alignment.

I don't have a tube tester. But the 3 12ax7s on the MPX have the normal glow. Same with the ones on the IF strip. Could a leaky cap cause this as well?
 
Did this just crop up all of a sudden after you rehabbed it (provided it has been rehabbed) or was this showing up when you got it and tested for the 1st time. Your location helps us with more than just locating a nearby AK'er. With FM problems the terrain is a factor in detemining whether you have problems due to hills, knolls with bldgs on it that is higher than you are and possibly blocking your reception. Remember that Stereo need 4 to 5 times more signal strength than a mono signal. So a 30 mile radius with Good Mono, can sometimes result in fair to poor Stereo reception with a lot of HISS and some static. A dipole isn't really the best for 30 mile reception unless you get it up higher (inside the attic away from power lines in the attic) or an outside antenna (if your HOA allow's it).
You can put two of the identical model, finalized and aligned by the same tech, and their spec will be different from each other by the time they get to the customer. It's very subtle, and not noticible by the customer, unless he has all the needed test gear.


The pot won't turn with the slot at the top. The bottom is the spot you want for it. If it's at 6:00 it's plenty close, but you'll need a scope to set it correctly (just off 6:00). Tubes glowing normlly only tells us and you that the heaters are glowing normally. That's all. It could be a leaky cap (did you change the 1uf 350v lytic(C-210) sitting off the MPX unit? another that would cause problems is the 8uf 50v(C41) in the discriminator circuit ). If you've changed those then either a 12ax7 is going (mono is good so the IF Strip tubes are ok.) or the Tuner/MPX needs an alignment. Find some known good 12ax7's and rotate them one at a time for the V100 and V101. I doubt the 3rd one is causing the problem, but what thehell, rotate one in there anyway to cross all T's and dot the i's. As the 12ax7 is a dual triode *basically two tubes in one) you could have a failing section, while the other is fair to good.

To summarize;
1.) Check C41 and C210. Replace if not done already. A Film cap 1uf 400v will work well here and not need replacemant for a long time(longer than a lytic).

2.) Dig thru your stash, and find 3 known good 12ax7's. Swap out one at a time and test. Note any improvement or no improvement.

3.) Just for the hell of it, if you have good spares for the IF strip swap them out one at a time starting with V6 and work back to V3.
 
Remember too that the Fisher needs a good antenna. With a steel chassis fully enclosing the electronics, it only has what is presented to to the antenna screw terminals to work with. More modern gear that uses circuit boards that exposes the circuits and non-metallic cases will certainly appear to be more sensitive, but with a good antenna provided to both, the Fisher will then show its capabilities. The big thing SS electronics brought to the table was not more ultimate gain, but quieter gain. That is, more gain could be had with less noise in sensitive applications, than is possible with vacuum tubes -- be it in phono preamps, or FM tuners. The Fisher will require a good antenna -- but with a good antenna, should perform quite well. I don't know if the Marantz uses PLL (phase locked loop) circuitry for the stereo decoder or not -- but it was coming in around that time and offered an advantage in quiet stereo decoding as well. It would be good if you could get another tuner or receiver of similar vintage and construction as the 400 to compare against if the Marantz does not fill that bill.

Dave
 
I don't know if the Marantz uses PLL (phase locked loop) circuitry for the stereo decoder or not -- but it was coming in around that time and offered an advantage in quiet stereo decoding as well. It would be good if you could get another tuner or receiver of similar vintage and construction as the 400 to compare against if the Marantz does not fill that bill.

Dave

I don't think the Marantz Eighteen has phase locked loop. (The PLL came on later Japanese made Marantz receivers). The 18 was manufactured from 1967-1970 in New York. I've attached a picture for reference. This was Marantz's first receiver effort and the only one released before Saul left in 1968. The FM topology is solid state and based on the 10B design (according to the manual).

IMG_2790.jpg
 
Larry - on a side note, thank you for your great tutorial on servicing Miracord turntables. I've completed my tear down, cleaning and re-lube of an Elac 10H that I intend to use with my Fisher 400. The Elac 10 and 10Hs go with the Fishers like bread and butter. It's a great period correct synergy.

IMG_3824.jpg

IMG_3772.jpg

IMG_3778.jpg

IMG_3814.jpg
 
The Elac 10 H was used by FISHER in most of the line between 1963 and 1967. It was repainted an off white (very little off) and renamed the 10"F". The tonearm back weight has "THE FISHER" on it vs. ELAC Miracord or whatever else Miracord put on them. Paired up with a Pickering V15 (ATE TYPE) it's one of the better mid level turntables. I think you'd like th 50H also. It's got a better arm and cartridge holder which is slightly adjustable back and forth. Mine are all in consoles, and work flawlessly. A hell of a lot better than a Garrard Type A Mk II or a AT-60.

As for the antenna. See if you can run the antenna up into the attic, away from the power lines up there. Nail it to a main truss member and then see what your reception is on both the Marantz and the FISHER. You should see better reception from both. If the stereo on the FISHER is still distorted, I'd take it to a shop that does alignments including the MPX.
 
The Elac 10 H was used by FISHER in most of the line between 1963 and 1967. It was repainted an off white (very little off) and renamed the 10"F". The tonearm back weight has "THE FISHER" on it vs. ELAC Miracord or whatever else Miracord put on them. Paired up with a Pickering V15 (ATE TYPE) it's one of the better mid level turntables.

I've examined a few of the white Elac "10F" models used in Fisher consoles, and I believe that Fisher did a bit of cost reducing when specifying the turntable components they wanted Elac to use.

I think the Elac model that Fisher utilized was really just a rebadging (with different paint) of the cost reduced "Elac Miracord 10" which itself utilized the cheaper shaded pole induction motor manufactured by Perpetuum Ebner.

I don't believe the "10 H" (H is short for hysteresis motor) was ever used in any Fisher consoles (although I could be mistaken). The superior 10H model was different from the "10" and "10F" in that the 10H utilized a superb Papst hysteresis synchronous motor which locked onto the 60Hz mains frequency to achieve extremely precise rotational velocity not influenced by voltage variations.

It's really remarkable that a changer used a Papst and is what makes the 1960s Elacs so exceptional vs other changers. When the Papst motor is used with the Miracord's 6 pound platter, the result is a very fine turntable indeed.

The Papst motors were used in many of the top transcription turntables of the day, including Empire, Rek O Kut and Fairchild. If you put a 10H in your Fisher console you'd have superior performance (but they wouldn't be white in color) than what the 10F delivers. Or you could buy a parted out Papst motor and retrofit it to your 10F (along with a motor run cap) if desired to maintain the original white chassis.

Here's a few pics I found online of a 10F which shows the cost reduced PE motor found in the standard "10" models. Note also that Fisher further cost reduced the 10F model by forgoing the chrome pushbuttons found the 10 and 10H.

s-l1600-3.jpg

s-l1600-2.jpg

In contrast, below is the superior Papst motor used on the 10H. Note in the white 10F model above, there are empty holes to mount the motor run cap to run a Papst. As far as I have been able to research, the 10H model was manufactured from about 1961-1966.

elac10_H-60hz.jpg
 
Last edited:
2.) Dig thru your stash, and find 3 known good 12ax7's. Swap out one at a time and test. Note any improvement or no improvement.

Thanks Larry.

I didn't have any known good 12ax7s. So I just tried swapping the V100 and V101 tubes with the V10, V11, V8 and V9 tubes. When I swapped in the 12ax7s from other locations to V101, an electronic hum noise not previously present was heard in FM only (not in FM mono, phono or Aux) . When I put the 12ax7 tube that was originally in V101 back in the V101 socket, the hum introduced by the test tubes went away. Does this mean I have weak 12ax7s in the V8, V9, V10 and V11 positions if they were causing hum when inserted in V101?

Is there an inexpensive tube tester that's recommended to better analyze the tubes (and test strength) in this unit? I may have multiple issues at play.
 
Last edited:
All of the V8,9,10,11 tube in the V101 posit caused a hum in FM STEREO or just one of them?? Could be a weak section (remember the 12ax7 is a dual triode (2 tubes in one)). Cheaper to get some new tubes. Suggest a pair of 12ax7LPS for the phono and anything but the Mullard Re-Issues. They sound like shite. I like the Tung-Sol's and the EH's but not necessarily in that order. They both sound about the same in the line stages and P.I. Positions.
 
All of the V8,9,10,11 tube in the V101 posit caused a hum in FM STEREO or just one of them?? Could be a weak section (remember the 12ax7 is a dual triode (2 tubes in one)). Cheaper to get some new tubes. Suggest a pair of 12ax7LPS for the phono and anything but the Mullard Re-Issues. They sound like shite. I like the Tung-Sol's and the EH's but not necessarily in that order. They both sound about the same in the line stages and P.I. Positions.

Thanks. Where's the cheapest place to buy the tubes? Also, are the pins on the Sovtek, Tung-Sol or EH 12ax7s oversized and prone to stretch the sockets?
 
Probably Antique Electronic Supply(AES) in Az., or Jim McShane. The only tube with slightly oversized pins from Russian tubes is the EH7868's
 
How does 12ax7LPS sound in the line stages and PI positions vs the Tung-Sol and EH? Since the 12ax7LPS is cheaper is there any downside if I used this tube in all nine 12ax7 positions on the 400?
 
Back
Top Bottom