490-T IF

Discussion in 'Fisher' started by Thunderclap, Jul 24, 2017.

  1. Thunderclap

    Thunderclap New Member

    Messages:
    14
    I need diode CR1 in the parts list is is designated as Diode type 1112.Does any one know what the sub for it is?
     
  2. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

    Messages:
    19,509
    Location:
    Glen Burnie Md.
    What's wrong with the 49-T??

    Diodes that are in the IF Strip are not replaced unless proven inoperable. The 1112 is a germanium diode. Which makes it doubly hard to replace. Over on Sansui and Pioneer 1112's are replaced with a pair of series'ed 1n4148's when in the amp or preamp sections. I don't know if this is the case here or not.

    Hold off until we see if Dave show's up.
     
  3. Thunderclap

    Thunderclap New Member

    Messages:
    14
    I believe the 1N1112 and the 1N4148 are small signal silicon diodes.This is a 490-T.This is CR1 which is listed as a 1112 diode,The color code on the body of the diode corresponds to it.There are two other diodes found in the ratio detector.The f to b ratio of the diode in question is 1:1 presenting a value of resistance in the circuit which does not belong there.This caused R41 to smoke.I was wondering if anyone would know a sub for it.
     
  4. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    Looking at the schematic I do not believe that CR1 would cause R41 to burn. CR1 appears to be the AM detector diode for the AM IF signal. There is C35 between CR1 and any B+. If CR1 is indeed shorted, then check C35 to see if it is shorted. Some of the voltage developed by CR1 is apparently AVC/AGC for the AM RF amplifier being fed to the signal grid of V2 and V3 through feedthru C17. There would normally be a very low voltage appearing in this circuit due to its isolation from B+.

    Joe
     
  5. Thunderclap

    Thunderclap New Member

    Messages:
    14
    Hi Joe,
    Thanks for the input.There is a path thru the secondary of Z4,thru R47 thru R41 to the primary of Z4 thru the primary of Z3 right to B+ 160.5VDC.
    Normally you would find the secondary of Z4 at about.7 VDC.I replaced the diode with a 1N914 the unit is up ang going.
     
  6. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    Thunderclap;

    True, there is a path, but the resistance is pretty high at 150K ohms for R47, and R45, a 100K ohm is connected to R47 making a voltage divider. The apparent intent is to produce a bias voltage at that point. Now that I look at the schematic closer, I am wondering if there is a mistake on the schematic. The +170VDC feed gets to the junction of R47 and R45 through R49, a 22 Meg ohm resistor??? I don't think so. I would need to check the actual wiring, but passing B+ for an IF stage through a 22 Meg ohm resistor makes no sense. It makes me wonder if the R49 value has been swapped with the value of R47 or possibly some other value of resistance for R49, but not 22 Meg ohms. It would not be normal for B+ to be present on the detector secondary winding of an AM detector circuit. There is a somewhat similar detector arrangement used in the 800C, but I do not see the same thing used in the Electra IV or V AM circuitry. That said, a 1N914 can stand a repetitive reverse voltage of 75V and a non-repetitive voltage of 100V.

    I will be out of pocket for the next few days. Once I get a chance I want to investigate the actual wiring in my 490-T to see if the wiring is actually as the schematic says. Passing B+ to an IF gain stage through a 22 Meg ohm resistor just does not make any reasonable sense to me.

    Joe
     
  7. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    31,767
    Location:
    Southern NJ
    A 1N34A is the general replacement for germanium detector diodes.
     
  8. Thunderclap

    Thunderclap New Member

    Messages:
    14
     
  9. Thunderclap

    Thunderclap New Member

    Messages:
    14
    Hi Joe,
    Did you ever look at your 490-T and see if it follows the print.The one I repaired did follow the print.
     
  10. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    Thunderclap;

    No, I have not been as active as I could have perhaps. My energy level is not up to par since the surgery I had, but is coming back slowly. I did look through a significant number of schematics of other Fisher units plus other manufacturers and have not spotted a similar arrangement elsewhere. This may be a one year design approach. Regardless, I will see if I can look closely at mine some time tomorrow and get back to you on it.

    Joe
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    8,676
    Location:
    Ball Ground, GA
    The schematic is in error. The bottom of R41 does NOT connect to the junction of C43, R49, R48, and R47. Rather, the bottom of R41 connects to the BOTTOM of R49.

    For R41 to burn, it would imply a short at terminal #4 of Z4. Were the short within the coil of Z4, Z3, or V4 00 or even C35, then the primary coil of Z4 would have burned open rather immediately. I suppose a simultaneous short in C40 and C39 could do it as well -- but I'd buy a lottery ticket before betting on that.

    Dave
     
  12. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    Dave;

    You beat me to it! I had just looked at the 490-T as I had promised and indeed there is a schematic error. The B+ feed to the V4 1st FM/AM IF amp. circuit is coming from the red wire B+ distribution +170VDC or so. The red wire connects directly to one end of the R41 1K ohm B+ feed coming to that 1st IF plate/screen circuitry. There is no function switch between the B+ and that point, just the direct wire connection back to C74C electrolytic can filter section. There is no connection between the full 170VDC B+ end of R41 and the junction of R46 (100K ohm) and R47 (150K ohm). The current jumper shown on the schematic showing that connection should be broken and the full +170VDC B+ end of R41 instead connects to the other end of R49 22M ohm resistor which connects to the red wire.

    A short in V4, even if intermittent in its plate circuit, could be enough to burn R41. I don't think a short in the screen grid of V4 would be as likely because there is R38 a 27K ohm resistor in series with the B+ from R41 going to the screen which only gets about +90V applied to it. You might check R38 just to be sure. The disc ceramic bypass capacitors in this area seldom short intermittently. They usually go bad completely and may even display a burned spot on their surface somewhere.

    A 22Meg ohm resistor is such a high value of resistance that very tiny amounts of current will ever flow through it.

    Joe
     
    Last edited: Aug 6, 2017
  13. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    Thunderclap;

    I just corrected the schematic which I have ( a composite on one page) as either a *.jpg or a *.pdf format. If you would like a copy, contact me by PM and I will arrange to send it to you.

    Joe
     

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