Rough TX-300

Discussion in 'Fisher' started by calman46, Apr 29, 2018.

  1. calman46

    calman46 AK Subscriber Subscriber

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    This is a little confusing . The connections to the selenium rectifier do not match the markings on it . There are 4 tabs 2 on the top row 2 on the bottom row . The AC is connected to 1 tab on the top row and 1 tab on the bottom row same with the DC. The marks on the rectifier case has both AC on the top row and both DC on bottom row .
    20180522_153600.jpg 20180522_153717.jpg The insulated wires are from the transformer .
     

     

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  2. Catmanboo

    Catmanboo AK Subscriber Subscriber

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    Transformer (white & blue) connect to the lugs indicated by the sine wave marks. Inputs to most bridge rectifiers are kittycorner from each other, same with the dc output. Shell ofcan body connects to neg.of bridge, lower lug close to side of chassis. Positive of bridge, (top lug, closest to can) goes to chassis ground, with a jumper to can positive. Look on back of bridge for more marks. BTW, flipping transformer leads on bridge won't change anything, as that's ac input to it.
     

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    Last edited: May 22, 2018
  3. calman46

    calman46 AK Subscriber Subscriber

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    The sine wave markings on the case suggest that they are both in same row ,not kittycorner. There is a molded small circle buy the + tab maybe that is all you need .
     
  4. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    If you have a 12v AC source, connect it to the ~ connections, then connect your DC meter to the DC connections. one way will show a + voltage, and the other will show a - voltage. The + voltage orientation is the correct one for DC connections.
     
  5. Catmanboo

    Catmanboo AK Subscriber Subscriber

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    It does appear that your bridge may be marked wrong, though. I missed that at first, couldn't get your pic clear enough. Hard telling on mine, as it's still in-circuit & kinda buried..
     
  6. calman46

    calman46 AK Subscriber Subscriber

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    Here we go , Power supply new diodes for SR1-SR4 new caps C10 ,C11 ,C13,C15 with nothing connected the voltages are -36vdc on EE, +34vdc on FF , +34vdc on GG , GG is high it should be +22vdc , will making its connection to the preamp bring it down to spec?
    new bridge rectifier for SR5 selenium. I have -48vdc off the negative terminal of bridge . This voltage will need to be knocked down. I am going to complete on this power supply next . I have to install the transistors . As always I'm open to suggestions & directions
     

     

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  7. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    If you haven't got anything connected to the P.S., voltages will be higher. Best to connect it to the complete circuits and then take voltages, and compare them to the schematic. if within 10% max you're good. FISHER Spec'ed 15% voltage variance due to part tolerances in tube stuff, and I would expect at least 10% variance as normal in S.S. stuff.
     
  8. calman46

    calman46 AK Subscriber Subscriber

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    I should knock the -48vdc of SR5. The schematic has -38vdc but that was selenium rectifier .
     
  9. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Is that with or without a load on it? (-48v on SR-5.)
     
  10. calman46

    calman46 AK Subscriber Subscriber

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    no load
     
  11. fred soop

    fred soop Super Member

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    Point GG will have exactly the same voltage as FF with no load because there is no current (and no voltage drop) in R26 and R27. The phono preamp will draw approximately 1.5 mA per channel. 3 mA through a total of 4.4 k will drop 13.2 V.
     

     

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  12. calman46

    calman46 AK Subscriber Subscriber

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    I was going to test it (power up) in steps as I reassemble it . But maybe I should complete assembly and leave the output transistors out and test it .
     
  13. calman46

    calman46 AK Subscriber Subscriber

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    I have Fairchild SE 4010 in my TX300 . Do you know the pin-out ? Looking from bottom with center pin on top thanks
     
    Last edited: May 31, 2018
  14. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Left to Right. E-B-C Should be in the manual for that board. Fisher was pretty good about detailing pinouts for the transistors.
     
  15. calman46

    calman46 AK Subscriber Subscriber

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    They're the older round type transistor .I wasn't sure on how to hold it . The pins are in a triangle pattern and off centered .. I think I have it ,I had taken a picture of the board . I didn't see any pinouts in SM , I'll check it , thanks . lookin for a cap to used for the 0.5uf @ 70v
     
    Last edited: May 31, 2018
  16. calman46

    calman46 AK Subscriber Subscriber

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    I'm ready to take a crack at wiring in the new trim pots . Will 24AWG be OK for the wiring ?
     

     

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  17. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    If you have 22awg go with that. 24 awg is a bit small when manipulating it, especially solid core. If not be very gentle with the 24awg.
     
  18. calman46

    calman46 AK Subscriber Subscriber

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    Have 22awg . I have 20awg but I thought it might be to stiff to manipulate ?
     
  19. calman46

    calman46 AK Subscriber Subscriber

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    Still moving forward with this I'm taking the re-wiring slow . I decided to go with the original bias layout but with new trimpots .My TX-300 has the two 22ohm resistors across the pot .
     
  20. calman46

    calman46 AK Subscriber Subscriber

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    DSCN2879.JPG All most there . I notice that there is a mod to my amp boards . C212 & C215 changed to 0.5uf/70v a 10uf/35 cap added with + on 2G and neg to a add on terminal strip also some added resistors 8.2k ?.At the bottom of picture Do you have any service bulletins for this mod ?
    >>>> What is a safe wattage for the 100ohm resistors replacing the output transistors ?
    If it passes the DBT and I power up, first I have to check voltage off the new rectifier SR5 (SR5 was originally selenium rect.) Any thing else ? besides smoke .
     

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