A 400C Transformation

Discussion in 'Fisher' started by dcgillespie, Jun 15, 2014.

  1. HiFiHarv

    HiFiHarv Active Member

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    So, no one else has been through this? I don't know if I am blind, or is the schematic wrong, or did Fisher really have this cap in one channel's circuit but not in the other's. We do know there are errors on this Sam's sheet. I guess when I open her up I can chase through the circuit and find out.
     

     

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  2. HiFiHarv

    HiFiHarv Active Member

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    I am finally getting onto this project. Dave, if you are listening, I have a question regarding the moving of the 100K resistor on pin 2 of the phono preamp sockets. Just so I know I have it correct and don't screw this up... am I right that the end result will be the 100K between ground and the EQ selector switch and the 10K between pin 2 and the EQ switch? It's busy and crowded in there and I can't even see one of these resistors to verify the color code. I think by being "creative" I can do this without removing the EQ switch assembly completely. Not too eager to attack it that way!Thanks in advance for any insight. By the way, mine turns out to be an early version with no resistors on the RIAA 1 jacks. I would have sworn that RIAA 1 sounded better than RIAA 2. Meter reads 110K on both jacks (unpowered). This fooled me at first because I forgot to set the switch appropiately when testing each input and they measured differently.
     
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    HiFi -- Sorry I missed your questions on this unit!

    10-28-2014 -- The corresponding cap to channel A's C28 in channel B is C49. Both should be removed with a wire then replacing the caps.

    Today -- Your understanding is correct regarding the 100K resistors. By moving them as I indicated, they will no longer then form an attenuator circuit with the associated 10K resistor they work in conjunction with, but will still allow the 10K resistor to act as a stopper and provide RFI protection as well. This allows the phono preamps to provide more effective gain, with no increase in noise level to achieve it.

    Good luck with your 400C!

    Dave
     
  4. HiFiHarv

    HiFiHarv Active Member

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    Thanks Dave, I did eventually find C49. Not sure why I was missing it at first. Looks obvious now. Can I assume that whether or not the other cap values are changed as you advise, that 49 and 28 should still be eliminated? I'll probably have other Q's as I get deeper into this, thanks for being there.
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Regardless of how you proceed, the C28 and C49 should be bypassed out of the circuit.

    Dave
     
  6. HiFiHarv

    HiFiHarv Active Member

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    319
    This has been a tough one. Since someone else restored the unit, it has taken alot of time just getting the lay of the land and finding the location of all the caps. Sam's barely helps since everything looks different now and the caps are marked with cryptic #/letter codes in tiny print that really challenge the old eyesight. Usually you can get a pretty good idea of how a code relates to the actual value, but these are tough. Spent alot of time going back and forth with the schematic to double check and ensure that I don't make a goofy mistake. I'm confused by the way the schem. deals with tape monitor and recorder. C45 and 23 are clearly connected to the record out jack, but the schem. labels it as tape monitor. just out of curiosity, what does that symbol mean where the two meet near V3 and V6. You know the one I mean? Looks like a coil, and/or switch. I'm not familiar with this symbol. Regardless, can I assume C45 and 23 don't affect the sound unless you are using record out? Or, do they have some interaction with the circuit that I don't understand.
    I see in your picture how you routed the shielded cable from the silicon rectifier to it's cap. Much of that space in mine is taken up with under chassis PS caps and a bulky cable would be tight. I understand the goal is to get away from channel A territory, but what's worse, draping over/under the PS caps, that large bundle of wires that go front to back, and close to the aux AC outlets? Or, routing just in front of the rectifier socket and PS cap location even though it is closer to channel A circuits. Thanks Dave for any advice on this.
     

     

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  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    HiFi -- Post a few pics of the underside of your unit if you will.

    Regarding the well-peppered-with-mistakes Sams schematic:

    1. In fact, the jacks labeled Recorder and Tape Monitor are switched in identity in both channels. Correct your schematic accordingly.

    2. The symbol you are referring to is in fact the Tape Monitor jacks for both channels -- but they are very unique jacks. With nothing plugged into the jack, the jack is a "shorting" jack, so that the signal presented to the jacks via C23 and C45 passes right through the jacks, and on to the volume control. The schematic shows the jacks in this condition.

    However, when a plug is inserted into the jack, the center pin pushes the two contacts apart, so that the center pin makes contact with the terminal going on to the volume control, but is insulated from touching the contact receiving the signal from C23 and C45 because of the insulator strip (the piece you said looks like a coil). In this way, the signal chain is broken when a plug is inserted, with the signal from the plug now going on to the volume control. That's how the plug works to effect a tape monitor function. If you look at these actual jacks in your unit, you will see the insulator mentioned, and the operation of the jacks will become clearer to you.

    You've got to remember that this preamp was the FIRST stereo device of any kind that Fisher designed, so it was developed in the earliest and headiest days of stereo. At that time, tape monitor SWITCHES were not a part of audio equipment, so the jacks had to perform that function. Besides, it was all great fodder for Avery to later announce equipment of his that could monitor tape recordings, "without having to switch cables to do so"!!!! Space age stuff for sure! :)

    Dave
     
  8. HiFiHarv

    HiFiHarv Active Member

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    Thanks so much Dave for clearing that up. I had an idea the record and monitor labeling on the schematic was backwards. Never looked hard enough at those jacks to notice the switching function. Now this makes sense. Any insight on my question about the rect. to cap cable location? I'm guessing you will say to route it as far away from channel "A" territory as possible even if it contacts all those other areas I mentioned. Worrying about having enough clearance to install the bottom cover with a fat, shielded cable over the PS caps. Pictures not possible for me right now. Will have to enlist help for that, but I'll see what I can do next week. That's one reason why I'm determined not to make any goofy mistakes on this. I've seen you quickly clear up others errors simply by looking at the pictures...incredible. Thanks so much
     
  9. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Kinda like a Pocket Transistor radio that kills the speaker when you plug in the Earphone?
     
  10. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Yup.

    Dave
     
  11. HiFiHarv

    HiFiHarv Active Member

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    I'm still plugging along... at a snail's pace. Broke a resistor while de-soldering to move the grounds, so I have to wait for that to come in. None at our "Last Stop Texaco" Rat Shack. Sorry about no pic's. Got them in the camera, but my young and lovely wife can't locate the doo-hickie that gets them into the computer. I'm tired of badgering her about it. Dave, I see you used a two conductor shielded cable from the bridge to the cap. Is that because of the "antennae" effect you mentioned. I would think just shielding the + would be all that's needed, but I know just enough to be dangerous. I have some two conductor Mic cable, but it's small guage... less than 22 I think, and not as well shielded as some single conductor I also have on hand. Also,would you mind commenting on my question about C-45 and C-23? Am I right that they only matter if I use the record out jacks? Thanks for any advice on this.
     

     

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  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    HiFi -- You might remember that when I got my 400C, it was quite literally in many, many pieces, being a long forgotten project from good AKer friend. At any rate, at the time I got it, he had already re-stuffed the DC heater supply can with new caps, so with the rectifier over under the power transformer, and the caps located in the middle of the chassis, it cause some rather long wires to exists between the rectifier, and the first filter cap. Those leads carry very high ripple currents, such that combined with the long leads, the antenna effect was well entrenched. The nearby sensitive circuits of Channel A easily picked up the radiated noise as hum.

    Were I doing a restoration from scratch, I would leave the can in place for show, but locate two new discrete caps, along with the rectifier (very close proximity to each other) over where the original rectifier was installed. Today's smaller cap sizes will easily allow that to be done. Such a configuration won't require the shielded cable, and will be every bit as quiet (I've done both) as when the cable is used when the distance exsists between rectifier and caps.

    I hope this helps!

    Dave
     
  13. HiFiHarv

    HiFiHarv Active Member

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    319
    I hate to drag this out Dave, as I'm sure you stay very, very busy. Yes, I did notice in your pictures that it even looked like every resistor was new. You apparently had to put a lot of work into that chassis. Mine has already been restored, so I'm stuck with the caps and bridge located where they are. So, my main question was whether or not it is necessary to shield both - and + from the bridge, or just the + lead. If the answer is obvious, then my appologies for being so dense, but it looked like you used a two conductor cable. On purpose?? Thanks again.
     
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    No apologies needed. Both the negative and positive leads from the rectifier carry the same high ripple currents (although they are opposite in polarity), so both can act as a transmitting antenna. But by shielding both of them within the same wire, they both act to help cancel the radiating effects of each other out -- just as tightly twisting AC leads has the same effect. When I initially tried shielding just the positive lead, it was notably not as effective as shielding both as shown in the pics.

    Dave
     
  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    HiFi -- C23 and C45 are very much in the signal path for any source applied to any input BUT the Monitor inputs.

    At some point on the Fisher Forum, I made mention of the fact that on the Sams schematic for this unit, the Monitor and Rcrdr jacks have been swapped with each other in each channel. Therefore, all inputs applied to selectable inputs ultimately appear at the cathodes of V3A and V6A, go through C23 and C45 respectively, through their respective Monitor jacks, and then on to the volume controls.

    Signal inserted into the Monitor jacks break the flow-through connection of these jack that allow the signals from C23 and C45 to flow through them, and allow the signals applied to the Monitor jacks to flow on through to the volume controls.

    I hope this helps!

    Dave
     
  16. HiFiHarv

    HiFiHarv Active Member

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    All of that is exactly what I needed to hear Dave. Thanks so muchfor clearing this up for me. I was mistakenly thinking of 23 and 45 as DC blocking for the recorder out jacks. I mentioned before that this area on the schematic with the "switching" monitor jack was confusing to me. On a related note, am I wrong to think of C-31 and C-51 as dc blockers for the Main outs? As soon as the signal goes through that complex multi section selector switch, I get confused. I will be looking for a heavier guage 2 conductor shielded cable now since I don't thing the guage of the Mic cable I mentioned is really large enough, and I would like better shielding as well. Thanks for all your help.
     

     

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  17. lemania01

    lemania01 New Member

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    Hi Dave,
    I own a Fisher 400C preamp and am really impressed with your work on your 400C. I hear a small amount of hum when i get foot or two from the speakers and am wondering if this could be causing the stuffy sound on mid grade vinyl pressing.
    I had the preamp recapped but not modified to the level you have described. My current set is Radio Craftsmen 500 mono blocks, Thorens 124, and Pioneer PAT 30X.
    I only listed to vinyl so the phono stage is my main concern. I know it's hard to give advice with the limited info i have given you but am looking for any recommendations.
    My current sound is good with top vinyl pressings but sound gets a little stuffy when i play mid grade pressings.
    Thanks much!
    Min
     
  18. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Thanks for the kind words! There were generally three versions of the 400C, as identified whether you have a single crossover setting, dual crossover settings (very rare), or no crossover settings, with these last units to my knowledge always employing the dual concentric tone controls (earlier versions employ single knob ganged controls). The point is, determining which of these units you have will also help to determine how much hum you have. The last generation had the most (as well as some other issues as well), while the earliest models had the least. Properly modified however, any version of the 400C will emit nothing but a soft hiss.

    Hum always interferes with detail presentation, most obviously of course at low levels, but in the 400C, there are other elements to effect that can affect the sound as well:

    1. The earliest units had both phono inputs reflecting a cartridge load of 100K. Later units had the RIAA 1 input reflect the newer 47K standard. This can have a significant effect with some cartridges.

    2. There are built in permanent LF filters -- besides the switchable LF filter -- that can hinder response as well. Now back in the day, it was hardly a concern with the changers and records of the day, and many of these units were used in console settings as well. Today however with the best mounts, tables, arms and pressings today, having full response throughout the system is important. Removing these filters is a big help towards improved definition.

    3. Fisher also generally tightened up the rather typical RIAA deviation of the day (that is, as designed into the 400C) in later units (after the 400C), but even then it was still loose by the best standards of today. There is a thread you can reference on the Fisher Forum here where I developed a highly accurate network for the Fisher preamp (same configuration, different values) that maintains a very tight compliance with the published curve with just a very small deviation (.25 db as I recall) over the full 20 Hz to 20 kHz bandwidth. This was confirmed BOTH by measurement, and by expert modeling work. That thread might be of interest to you as well, which you can find here:

    http://www.audiokarma.org/forums/index.php?threads/the-fisher-phono-preamp-and-riaa.600690/

    I hope this helps!

    Dave
     
  19. lemania01

    lemania01 New Member

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    7
    Dave,
    I can't thank you enough for you taking the time to share this tremenous knowledge. All this information is way above my skill level but i will re-read both threads and see if I can give a strong effort to update my phono section. These fisher 400C's are so nice to look at! Thank you so much for your kind reply. You certainley have made my day! Warm regards, Min.
     
  20. lemania01

    lemania01 New Member

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    7
    Hi Dave, i rerouted the shielded output lead as you recommned and the hum is gone! It's amazing the folks from Fisher did not implement this. The level of hum i was hearing before was detectable from 1 foot with your ears to the speaker at volume level 3 in phono section (more on channel A). I can now turn the volume level to 10 before hearing light hum in phono section. I have attached some pictures of my unit and it does appear i have the model with the dual crossovers.
    My next step is to address the phono section. I did some research on line and the two capitors in the phono section are greenies? I ordered some vitamin Q .01 uf and the Russian K40y-9 .022uf as recommended by some of the folks on this thread. Will i hear a dramatic difference by swapping out these capacitors?
    Any other recommendations would be much appreciated! Thanks Min.



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