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Novice help transistors for TX-300

Discussion in 'Fisher' started by Tom Yocky, Nov 8, 2018 at 10:59 AM.

  1. Tom Yocky

    Tom Yocky New Member

    Messages:
    1
    Location:
    Snook, TX
    Hello all, I am very new to this stuff but am trying my best to read and learn and not just ask the same questions that have already been answered(although that may still happen). I could use some experts to check my work real quick.
    I am in the process of recapping a Fisher tx-300 amp. It comes out of a 1967 President 12 console that I found in poor shape and am restoring... but I digress. While investigating the tx-300 several of the 2 amp fuses were blown, and several of the 3 watt resistors attached to the fuses were cracked and discolored. My reading told me that the blown fuses likely caused by shorted transistors and after total confusion trying to check these germanium PNPs with a free harbor freight multimeter I got one of those lcr-t4 component testers which identified 7 of the 8 transistors as various single or double resistors...only 1 came up a transistor...meaning they are shorted and all 8 need replaced. All 8 are TR35144 which I can not find a data sheet for but comes up as replaceable by NTE121. These are too pricey for my budget at around $30 each and hard to find 8 anyway. So I started comparing transistor specs with what is in my service manual and those of NTE121 and I think I found a better priced option. BUT I would appreciate some confirmation first.
    Can I sub some motorola 2N1547 on ebay for around $5 each?
    Specs on both: and will attach page from service manual

    Cross-Reference Search Result (Equivalent Transistors)


    ↓ Type Mat Struct Pc Vcb Vce Ic Ft Hfe
    NTE121 Ge PNP 90 60 45 10 0.3 90

    ↓ Type Mat Struct Pc Vcb Vce Veb Ic Tj Ft Hfe Caps
    2N1547 Ge PNP 106 100 50 50 10 110 0.1 75 TO3

    2nd question...if I can sub these I need to try to pair them right? Will the measurements from my lcr t4 work for that. Should I buy 10 instead of 8 for a better chance to pair them?

    Last question(s): Is setting bias after replacing them something a novice like me should even be thinking about doing? I dont have any of the equipment that the manual mentions like oscilloscope or frequency generator. Are there other ways of setting bias without those pieces of equipment or inexpensive modern versions that will work? Can I destroy the amp if I do it wrong (read something about if bias adjustments are old and dirty they could short out new transistors if turned)? Or should I plan to have a repair shop do that and what is a fair price for that sort of thing? Thanks in advance.
     

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  2. fred soop

    fred soop Super Member

    Messages:
    1,990
    They really should be beta matched in groups of 4 (originally recommended by Fisher) and, if possible, the beta test should be run closer to real world conditions rather than the hFE function on a meter. Maybe a Vce of 10-20 volts and Ic of 1/2 amp. Keep the test short unless you have very good heat sinking. Otherwise, these should work ok. 2N2148 is also good and is essentially what was used initially. But, beware of eBay parts. You don't always know what you are getting. Also see some of the previous threads regarding those touchy bias pots or you may wind up with another pile of useless transistors. Also, NTE parts are generally disliked by those on this forum (so are tantalum capacitors).

    Someday, when I run out of other things to do, I should try this circuit with silicon transistors and see if it can be biased properly.
     

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