Turning down the Fisher X-202

Discussion in 'Fisher' started by dcgillespie, Feb 16, 2017.

  1. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,623
    Location:
    Ball Ground, GA
    In response to a recent thread by RS Steve regarding one of these models that would not quiet appropriately in the right channel when turned to minimum volume, I took the symptoms of his unit, and using his test conditions, checked to see if my own unit, as well as that of a client's would display the same behavior. While not to the extent that Steve's unit did, they too both display the same characteristic: When turned to minimum volume, the right channel is distinctly playing at a level that is louder than the left. And what's more, the symptom was not consistent from unit to unit. Hummmm.

    The facts surrounding the problem are addressed in post #99 of Steve's thread, which can be found here:

    http://audiokarma.org/forums/index.php?threads/finally-acquired-a-x-202-but-it-has-an-issue.754793/

    Also discussed is one possible solution, which involves using external attenuators to reduce modern digital source signal levels to that which is more appropriate for which the unit was designed. This has the added benefit of ensuring that the input of the amplifier will never be overloaded as well. Others however will see that as a band-aid, wanting to address the root cause of the problem, so as to keep the signal path as pure as possible. As with most things in life, the answer is usually somewhere in the middle, as I suggest it is with this issue as well: A minimum of 3 db external attenuation is recommended to ensure that overload will never rear its ugly head, but addressing the basic cause of minimum volume signal bleed is a good thing as well -- and particularly for those units that are inclined to display this problem more prominently than others.

    The basic cause is due to the fact that there is not enough appropriate shielding in place within the build, to ensure that when large signal levels are applied to the unit, every unit will properly "quiet" when turned to a minimum volume position. What follows is four modifications to provide that shielding, which will result in extremely quiet minimum volume levels, and virtually eliminate all signal bleed under that condition. The modifications include:

    1. Adding a ground plane shield between the Bass control, and the Hi/Scratch Filter switch.

    2. Adding a ground plane shield between the Cathode Follower section and tone control section.

    3. Adding shielded cable as appropriate.

    4. Relocating two components in each channel.

    The work to be done will be presented as a series of pics showing the results of the modifications installed into an early version unit. Since the modifications do not change the circuit in any way, the only components you'll need to make the modifications are 2 feet of miniature single conductor shielded cable, and a small (4in X 10in) sheet of #254 Easy Solder Tin. The latter can be had from Amazon and other outlets. The tin is quite easy to form an solder to, so it makes for ideal shield material. No dimensions are given for the shields; they are simply formed to fit in place which will be self evident from the pics provided. The pics will start with the stock look of a just restored unit for the target areas, and progress from there into the four modifications installed, basically to provide before and after pics for reference. Pics include:

    BELOW: For reference, a pic of the finished restored unit before all this nonsense started. While irrelevant to this discussion, this unit as shown has already had as part of its restoration, the following modifications installed:
    1. The old damping control circuits removed to free up a total of four terminals along the rear terminal strip.
    2. Individual 10 Ohm cathode resistors installed with the freed up rear terminals now re-purposed as test points for setting the bias controls.
    3. Output tube Screen Stability resistors installed.
    4. All permanent LF filters removed.
    5. An AC line current limiter installed.
    6. The old selenium bridge rectifier converted to silicon.
    7. EFB(tm) installed to improve performance and tube life and reduce operating temps.
    SAM_1880.JPG

    BELOW: C58 (Sams) in stock position that will be relocated.
    SAM_1888.JPG

    BELOW: C31 (Sams) in stock position that will be relocated. It is the horizontally shown yellow cap. Also shown are R102 and R50 (both Sams), which are 47K resistors that connected between the balance control and the T-Strip. These two resistors will be relocated as well.
    SAM_1887.JPG

    BELOW: Shows the relocation of C58/R102 to the back of the balance control, and C31/R50 just above it. The caps are secured in place with some silicone adhesive. Also shown is the shield cable connecting the these components back to the tube pin they were disconnected from. Electrically, nothing has changed. The components are simply relocated to a more "secluded" area relative to available radiated signal for pick up.
    SAM_1898.JPG

    BELOW: The other end of the shielded cable coming from the relocated components that connect to V10 (Sams). The cable is only grounded at the balance control end on the ground terminal of that control.
    SAM_1893.JPG

    BELOW: The other end of the shielded cable coming from the relocated components that connect to V5 (Sams). The cable is only ground at the tube socket end at the center ground ring of V5.
    SAM_1900.JPG

    BELOW: Lead #5 from K2 (Sams) as been sniped back close to the body of K2, and shielded cable used to make the connection from it back to pin 7 of V4 (Sams) where this lead originally connected to. The shielded cable is only grounded at the tube socket end, on the T-strip terminal where the cathode resistor from pin 8 (R49) connects to for its ground connection.
    SAM_1903.JPG

    BELOW: A new shield is installed between the bass controls, and the Hi/Scratch Filter switch. It is secured by soldering it to an unused ground lug terminal on the T-strip below it.
    SAM_1895.JPG

    BELOW: A new shield is installed between the Cathode Follower circuits, and the tone control circuits. It is secured at each end by soldering it to T-strip ground terminals used for existing connections. The shield should extend from the floor of the chassis, up close to the bottom cover (within ~ 1/4 in) when installed.
    SAM_1899.JPG

    BELOW: The finished under chassis look with the new modifications installed (in addition to those previously mentioned).
    SAM_1902.JPG

    And that's it. It does take some time to install, but for those units with excessive signal bleed under minimum volume conditions, these modifications are just the ticket for peace and quiet.

    Dave
     

     

    Please register to disable this ad.

  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,623
    Location:
    Ball Ground, GA
    Since my client was nice enough to allow his unit to be used as the development model for the modifications presented in the preceding post, it surely deserves a nice frontal shot as well. It is an absolute beaut!

    Dave

    SAM_1904.JPG
     
    AlTinkster92 likes this.
  3. RS Steve

    RS Steve Tube Junkie Subscriber

    Messages:
    7,473
    Location:
    Jupiter, Florida
    Nice work Dave, leave it to you to take what Fisher did with these wonderful amps, and improve them even more. I hope to address these changes to the units I have at some point.
     
  4. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,763
    Location:
    Glen Burnie Md.
    MY X-202 does NOT have (sams) C-31 or C58. R50 and R102 are intact. Serial # 33689D. This is one of 3 that Steve had, and 1 of 2 that he sold. FISHER lists the parts as C-44,C45 and R84 & R85. Caps are .01/250v and Resistors are 47K/ 1/2W. Resistors are ok. So I presume I need to install said caps, and do the mod while I'm at it. I can do everything on the list except the EFB. It already has the individual bias points, thermistor, and I'm installing a silicon bridge. I have a question on the 5AR4 I get the resistors, although I think 2 1n400x in series may not be enough to handle the current. I'd be of a fashion to change them out for 1n5408's on each side. I curious as to what is the purpose of the jumper from Pin 4 to Pin 6. You have the HV from the transformer(370VAC each side for a total of 740V) coming in to pins 3 and 5, with the diodes coming off each and going to pins 4 & 6. Then there is a jumper between pins 4 & 6 coming directly off the cathodes of the diodes. Seems counter intuitive to me. I think I understand that the diodes are preventing back flow and all but why tempt fate and have a jumper across 2 370V AC leads if the diodes break down and allow AC Leakage. See pics below

    Jumper across pins 4 & 6 on 5AR4.
    100_5379.JPG

    C-58 & C31 missing. Looks factory to me.
    100_5380.JPG

    Balance pot @ about 40Deg.
    100_5381.JPG

    Balance pot straight on.
    100_5382.JPG


    X-202 series 30000-39999 Balance circuit with pertinent components. C-44&R84 and C45&R85. These correspond to the part #'s on the Sams photofact of C-31, C58, R50, and R102.
    X-202 Balance circuit 30000 series.jpg
     
  5. RS Steve

    RS Steve Tube Junkie Subscriber

    Messages:
    7,473
    Location:
    Jupiter, Florida
    I believe I asked Dave about those missing capacitors, and he said they probably
    decided they weren't needed. It's been too long ago to fully remember.
     
  6. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,763
    Location:
    Glen Burnie Md.
    I checked the 30000-39999 manual, and they are listed. Oh well it'll give me something to do.
     

     

    Please register to disable this ad.

  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,623
    Location:
    Ball Ground, GA
    Larry -- The caps represent the 4th (as I recall) permanent LF filter installed in the X-202 -- but they are also partly responsible for picking up stray audio in and around the chassis interior, which contributed to the lack of the 202's ability to fully "turn down". Note that in the unit I did for a client, I raised the value of the cap appropriately to effectively eliminate it from acting as a LF filter, and also relocated it to the back and top of the balance control, using shielded cable to then connect the output of those components to their respective driver tubes. This removed them from the area where pickup was the strongest, which in conjunction with the other measures taken, allows the unit to fully turn down. The cap does serve a secondary purpose of preventing any DC from reaching the Balance control should the driver tube become even slightly gassy. This ensures that the Balance control will always operate quietly. On the other hand, Fisher simply elected to remove the cap in later versions -- no doubt finding as I did that it was an antenna picking up any extraneous signal floating around inside the chassis. This was a simple expedient to implement on the production line, versus moving the cap to a "quieter" location.

    Dave
     
  8. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,763
    Location:
    Glen Burnie Md.
    That solves that query. Any comments on the Rectifier jumper? I'm extremely hesitant to even get the plug near a AA battery.
     
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,623
    Location:
    Ball Ground, GA
    That appears to be a modification I recommended for another brand, wherein the silicon diodes are doing the actual rectification, and the 5AR4 is simply acting as a time delay. By paralleling the plate, the tube's diode sections are then paralleled making for twice the current handling capability, and half the normal voltage drop. It's a good mod when using foreign manufactured 5AR4s in equipment that operate the tube near the limit of the original manufactured piece. The X-202 does not do that, and if your 202 still has the original Mullard 5AR4, it will outlast you and me. If not, then the mod is particularly helpful if running (for example) a JJ 5AR4, which is proven to be quite good in applications that do not demand maximum performance from the tube, and quite poor in ones that do.

    Dave
     
  10. RS Steve

    RS Steve Tube Junkie Subscriber

    Messages:
    7,473
    Location:
    Jupiter, Florida
    Yeah, that mod was done before I owned the unit.
     
  11. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,763
    Location:
    Glen Burnie Md.
    OK. I'll leave it in, as the 5AR4 is a new Tung-Sol (russian). It just looked funny with the jumper in it. I've got a Mullard as a backup.
     

     

    Please register to disable this ad.

  12. walyfd

    walyfd Well-Known Member

    Messages:
    718
    Location:
    Northeastern PA
    Oddly sounds like the exact issue I'm having with my 500 B...

    If I gotta go through this with the 202...
     
  13. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,763
    Location:
    Glen Burnie Md.
    Seeing that this 202 didn't have the 4th LF Filters, I went ahead and got some Sheet metal Tin that Dave recommended above. Used the photo's that Dave supplied above and made 2 shields and incorporated them into the unit. I haven't put it in service yet (waiting on tube's, although I can raid the TA-600 for them, but I can wait.).

    The larger of the two, between the Cathode Follower circuits, and the tone control circuits was extended out to the far end of the 4 section cap, just out of laziness with the Tin Snips. Both shields were soldered in multiple places to the chassis and "T" Strips as shown in the pics.

    Pics later as the battery needs charging on my camera. Maybe an hour or two.
    Larry
     
  14. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,763
    Location:
    Glen Burnie Md.
    Pics. The battery was charged while I was writing above post. Go Figure.

    100_5383.JPG


    100_5385.JPG
     
  15. RS Steve

    RS Steve Tube Junkie Subscriber

    Messages:
    7,473
    Location:
    Jupiter, Florida
    You should of just hooked it up and enjoyed it for a while. :rockon: I'm sure you'll have it singing in no time.
     
  16. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,763
    Location:
    Glen Burnie Md.
    Tubes are due early next week, so I'll wait.
     

     

    Please register to disable this ad.

  17. fred soop

    fred soop Super Member

    Messages:
    1,965
    Larry, at least cut the corners on that sheet metal round or bend them over. You are going to end up with blood in the unit.
     
  18. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,763
    Location:
    Glen Burnie Md.
    At least I'll have a DNA sample to ID it and prove it's mine, if someone steals it.:biggrin: Corners cut off.
     
    AlTinkster92 likes this.

Share This Page