Fisher 400CX-2 potentiometer

Discussion in 'Fisher' started by vendo81, Nov 29, 2018.

  1. vendo81

    vendo81 AK Subscriber Subscriber

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    On Ebay there is a person selling a new version of the volume pot (with on/off switch) for the Fisher 400cx-2, 400c, X-1000, etc...

    Of the half dozen or so 400cx-2 preamp's I've owned, 3 had volume pots that wouldn't track correctly. One channels volume would track differently than the other. The switch was another problem but they are available with a little searching.

    I've been using one for several years that tracks really well but have one boxed up that tracks terribly. It tracks so bad that one channel is still heard when all the way down and they don't track the same until at the 11 or 12 o'clock position. Replacing the volume control without the correct taps would cause you to lose the loudness function. I did this on the boxed up preamp so the volume worked as it should but lost the use of the loudness switch. Hopefully the taps on this new pot would bring back the loudness function.

    Has anyone tried one of these for fit and for how well they perform? I thought I'd ask before ordering a couple.
     
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  2. bhamham

    bhamham AK Subscriber Subscriber

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    Thanks for posting this vendo. Just had a look at the ebay seller's site and his pots look very promising. I need one for a 500-B but wouldn't mind having an extra or two. I don't know what the loudness tap is for the preamps with reference to your post, but he has the tap for the amps/receivers at 125k. For some reason I thought it was lower than that, like 75k.

    I'll see what others have to say before hiting the buy button.
     
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  3. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    Keith, are you talking about that Capt Fant******** guy? Seems he has new pots for all kinds of Mac and Fisher gear, have never done business though. He has 2 Screen names on that site. Hope you find one! :) Al
     
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  4. fred soop

    fred soop Super Member

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    If the tracking is not extremely bad, you can play some games with a fixed resistor on the loudness tap. Disconnect the control completely. Connect a 9 V battery to the top end of both sections and its return to the bottom (ground) end of both sections. Now, set the control at your most common listening position, 10:00 for example. Measure the voltage between each wiper and ground. The section with the higher voltage needs to be reduced by adding a resistor between the loudness tap and the ground connection. Calculations are complex and iterative, so best just to experiment. Something on the order of 220K - 330K is a good starting point. Note that if the pot tracks well at higher volumes, this "fix" will actually make it worse but at higher volumes, the unbalance may be less obvious.

    [​IMG]
     
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  5. vendo81

    vendo81 AK Subscriber Subscriber

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    Hello Al. Yes that's the guy. If they are exact matches they would sure be needed by many. The volume imbalance issue makes things unlistenable. The fix Fred Soop posted looks like a great thing to try as well.
     
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  6. vendo81

    vendo81 AK Subscriber Subscriber

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    Great idea Fred! It's common to find pots doing this and many times impossible to find a good replacement. I'll make note of this.
     

     

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  7. vendo81

    vendo81 AK Subscriber Subscriber

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    Yes, the resistance of the taps and how smoothly the pots work are the items I'm questioning. I know several people that don't care about the loudness feature but I use it often.
     
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  8. rufleruf

    rufleruf Poor Impulse Control Subscriber

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    It's a shame Alpha or another big potentiometer maker couldn't be persuaded to make a line of these - lots of stereos through the 70's have these issues.

    The Alpha dual 500K audio pot is less than 5 bucks. With a longer shaft, switch and loudness taps, they should still be able to keep the cost under $20. I went through some experiments on one of the dual audio taper Alpha 500K pots, one can add loudness taps with some effort - not too bad, and pull a power switch from another new Alpha Pot. Shaft needs an extender, but they make another pot with the right length - it's just permanently assembled, so hard to swap over and have a robust, long lived connection.

    I once saw someone selling Dynaco amp pots modified as I describe on eBay

    Point is, they already make all the parts to do this separately except the loudness taps (they may have some pots with loudness taps, I just haven't seen them).
     
    Last edited: Nov 30, 2018
  9. vendo81

    vendo81 AK Subscriber Subscriber

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    Yes Matt I wish some big potentiometer maker would make these with quality/quantity and at a decent price.
     
  10. 1rebmem

    1rebmem AK Subscriber Subscriber

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    I bought one from Capt Fantastic for my 500B. The tracking was about the same as the original using an ohm meter so I installed it and it works fine.
    With the original, the volume would cut out briefly when I touched the control. Even after multiple cleanings.
    Jef
     
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  11. sgmlaw

    sgmlaw Well-Known Member

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    On all the ones I have encountered with genuine volume tracking irregularities, it is typically in an area of rotation that has a lot of mileage on it. All will generally track tighter and cleaner at further rotation levels, which is par for the design.

    On the CX-2 and other preamps, you can also usually adjust a little for panel volume irregularity by trimming the chassis pots on the secondary auxiliary inputs. Same for the phono/low level inputs.

    But the older preamps, and the CX-2 in particular, are extremely sensitive to input overloading which can mimic volume control irregularities. These models were designed for 200mV maximum line level inputs. Slap a modern 2v line level signal on a CX-2, and the overload will be so profound that one of the 7247s in the tone amp will literally broadcast into a nearby unshielded signal path past the volume control in one channel. That will exactly mimic the ‘one channel stronger’ result described above. Don’t ask me how I know this. You have to attenuate a modern line level output nearly -20 db to reach the input level these were designed for. Even a tuner output from the same era needs to be fairly padded down on them or the range of volume control is excessively compressed.

    But at those adjusted input levels, the old preamps have much better volume control ranging and tend to track fairly well up and down the range, and are dead quiet with over 60db separation, provided the pots are not worn to death.

    The same input sensitivity applies to the receivers, but to a much lesser degree. The easiest way to diagnose the volume pots on those without a signal generator is via the tuner, which outputs at a correct design level.
     
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  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Regarding the excellent comments that sgmlaw made, those models that employ active tone controls (400C, CX, CX-2, X-202, X202B, and EL34 X-1000) are in fact susceptible to significant circuit bleed -- most notably manifesting itself as a unit that simply won't fully "turn down". Oh, the body of the signal is properly reduced to a zero level, but a very tinny and sometimes quite objectionable sound level in one or both channels often remains. Not an overload in the classic sense, (the high level inputs of any of these models will easily handle the 2 volt output of a CD player without distortion), the issue is traceable to the fact that these models all employ a cathode follower circuit to drive the rather low impedance volume control circuit they all employ. With a low output impedance and typically physically rather large coupling cap at the output of these stages, this makes up a perfect transmitter/transmitting antenna system that can in fact radiate an audio signal throughout the chassis of these models -- with the high impedance grid at the input of the power amplifier section just waiting to pick this signal up. This results then in the very tinny sound sometimes heard from these units when the volume is fully turned down, and an unattenuated hot high level input signal is applied. Back in the day, a hot high level signal typically never exceeded about 0.5 volts, so by comparison, a 2 volt signal is in fact quite hot. With only (even) a 0.5 volt signal applied, the problem is usually quite unobjectionable, so it wasn't a problem on Fisher's radar. And for whatever problem there was, the various input level controls these units employ on some of their inputs could reduce the problem to a non-problematic level.

    For various reasons however, some examples of these models hardly display this problem, while others are particularly problematic. Some time ago, I took on a particularly problematic X-202 to investigate exactly where these leaks were, and what could be done to resolve it. The results were that when finished, with a 2 volt CD player applied full strength to a high level input with no attenuation capabilities, and connected to 101 db efficient speakers, the volume level does in fact diminish to zero when turned to a full down position. As a result, any channel imbalance observed then is solely due to either the volume control, or imbalanced circuits within the unit. You can find the thread on that effort here:

    http://audiokarma.org/forums/index.php?threads/turning-down-the-fisher-x-202.757457/

    Dave
     
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  13. notdigital

    notdigital AK Subscriber Subscriber

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    This weekend I popped my SA-100 and 400c into rotation to finally get some juice running through the Russian PIO's I inserted in in the amp. After despairing at the crackling and popping on the L channel, the caps finally settled down. I turned down the volume all the way only to hear the R channel with that tinny, noticeable sound, the volume control fully down notwithstanding. The bleed on the left channel is very, very slight and doesn't bother me at all. So I too will be digging into the pre with the hope of finally getting rid of this gremlin. Thanks for the link Dave.
     
  14. sgmlaw

    sgmlaw Well-Known Member

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    Just an aside, but when this bleed-through/cross-talk condition arises, it is often truly cross-talk. So everything relating to the entire stereo presentation is damaged at all volumes. It will still be there if you escalate the panel volume, but only at a level sufficient to disrupt and smear the stereo imaging. The usually excellent stereo separation of something like a CX-2 (the one still I keep in my personal system normally measures well over 65db) is destroyed. If you run a stereo channel ID check, you can quickly determine whether it is that bad. If it manifests as genuine cross-talk, it absolutely must be corrected.

    Until I first pinpointed the cause of such problem, such cross-talk in the preamps could be easily mistaken for a runaway element in a stereo dimension circuit or a drifting resistor value in a center channel amp circuit. But it is not.

    As Dave identified, it is an irradiated signal pickup into the tone amp section, several portions of which have unshielded elements inside the chassis (at least on the models fully equipped with full tone control packages). A line level audio signal has been amplified at least another 25db by the time they pass that last 7247.

    When I encounter such a 'broadcasting' preamp, besides attenuating the input levels to something more rational, I usually drop shielded leads into those nearby vulnerable areas just as insurance.

    Again, the chassis pads for the auxiliary line level inputs can help in more marginal afflictions of this. Otherwise, unless the line level source has built in attenuation, you can either build-in or buy in-line attenuators.

    But generally speaking, if you can get the input level down to around 1V, if you don't mind a little reduction in panel control range, there is usually enough padding on the auxiliary inputs to manage the difference.
     
    Last edited: Dec 11, 2018
  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Excellent point about the cross-talk element of the radiated signal, and therefore one more excellent reason to resolve the issue -- either through appropriate attenuation, or by installing the necessary shielding (or both!). All of these pieces sound way too good to have them produce stereo signals that are anything less that the best they are capable of!

    Dave
     
  16. sgmlaw

    sgmlaw Well-Known Member

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    It's good to see you are still active here, Dave.

    I don't drop in to the audio hobbyist sites too much anymore. And other than you and one or two others, it looks like a whole new crowd.

    Time marches on, but the gear survives on. Hopefully, the interest in it will persist.
     

     

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  17. notdigital

    notdigital AK Subscriber Subscriber

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    Well, to close my circle at least: ended up being a dirty socket on V-5. A good cleanup with Deox solved it.

    And sgmlaw: been a member of AA (under a different moniker)for almost 20 years and always appreciated your contributions there. Nice to see you drop in here, for sure :thumbsup:
     
  18. sgmlaw

    sgmlaw Well-Known Member

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    Glad to hear it was something simple. Always start with reseating the tubes and other basics.

    I am winding down now from this hobby. Most of my bench gear is gone. And other than a handful of special pieces for heirlooms, and some spare parts, all the rest I have cleared out.

    Not that I don't still enjoy the hobby, or occasionally coming to places like here and mouthing off. And I still run some gear daily and love my music. But at some point there are other things in life.
     

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