Any tech gurus know Transistor numbering?

Discussion in 'Amp Tech Center' started by schmee, Mar 14, 2019.

  1. schmee

    schmee Friend of Leo's

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    I'm working on a Gibson Lab Series L3 SS amp. The two big output Transistors are numbered:
    RCA
    R 7821
    NPN T03

    I cant find any of that number anywhere.
    What designates what? I know/think the T03 is the mounting style. But what do I buy?
     
  2. PCollen

    PCollen Friend of Leo's

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    RCA is the manufacturer

    R7821 designates the type of transistor (semiconductor) that it is..Google R7821 datasheet. Hint...it's a MOSFET.

    NPN stands for No Pus....no wait, it stands for the polarity of the substrate materials: NPN is one of the two types of bipolar transistors, consisting of a layer of P-doped semiconductor (the base) between two N-doped layers (the emitter and collector).
     
  3. schmee

    schmee Friend of Leo's

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    Thanks, but yeah, nothing shows up on googling R7821... other than those little black rectangular transistors which is the problem. These are the big round ones with two dogears for mounting.
    These look like this with numbers on them. About 1.5" overall size
    [​IMG]
     
    Last edited: Mar 14, 2019
  4. Dennyf

    Dennyf Tele-Afflicted

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    Your part number is suspect. Check this and this.
     
  5. Dennyf

    Dennyf Tele-Afflicted

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    PS, that form factor is called a TO-3 package. Some transistor types are available in a couple different packages.
     
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  6. galaxiex

    galaxiex Tele-Holic Silver Supporter

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    The R7821 is probably a Gibson "house" part number.

    Many amp makers would spec a transistor from the transistor manufacturer, and want it labelled with their own part number.

    It's probably a common TO-3 like a 2N3055 with Gibson part number.

    Please note I'm NOT saying it IS a 2N3055, just that it might be.....
     
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  7. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    maybe? helpful:

    http://music-electronics-forum.com/attachments/25939d1382636822-lab-series-l-3-service-manual.pdf

    Look for Q9 in the text and part description.

    Then cruise to Mouser, https://www.mouser.com/Search/Refine?Keyword=R+7821+NPN+T03+Power. They show 5 options with different Amp and Volt ratings. Using the schematic (lame scan: https://music-electronics-forum.com/showthread.php?t=34602#post321809) or some other look-up method, you might be able to figure out which of the 5 on the Mouser site would work.
     
  8. schmee

    schmee Friend of Leo's

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    Yes I have the schem etc. There are two of these... so it's not Q9 which list "1 each". And Q9 Just states "PNP TIP30" Thus my question; Is that type of designation an actual part number?
    Trouble is, the sheets just spec T03 or etc for Q6, Q8 etc. No real part number.
    ???
    I thought maybe they were Q1 or Q2 which list MPS8598, but that number is different than the amp transistors labeled R 7821
    I have to admit I dont read this schem very well, there are only two of these big Transistors and the rest are the little black squares inside the amp. Maybe someone can tell from the schem which are the big ones? I'll post that part of schem in a minute.
     
  9. ICTRock

    ICTRock Tele-Afflicted

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    rca manufactured 21st week 1978 on those transistors ... just in case you were wondering what R 7821 means
    also they are very much NOT the 401 transistor from the L5 series, moog wouldn't create two different part numbers for the same part.
    as discussed, the 2n3055 is suitable. it can handle the voltage and has twice the amp rating of the 401.
    I would replace them as a pair, just like vacuum tubes.
     
    Last edited: Mar 14, 2019
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  10. schmee

    schmee Friend of Leo's

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    I think I found the replacement # while looking for the schem to post here. In the middle of a 10 year old thread on electronics forum!
    Thanks all. Wish me luck.
    2N3055
     
  11. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    don't breath the fumes!

    (or, at least filter them through a lit Marlboro when inhaling)
     
  12. schmee

    schmee Friend of Leo's

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    Thanks much. I didnt realize transistors had those date codes like other parts.. and no part number! I've got a couple 2n3055 NTE coming. It's a shot in the dark really. I got this amp for almost nothing, and I've replaced all the E'lytics. But there is a bit of distortion that I dont believe should be there. Seems to be more prevalent when drive harder by picking hard or etc. Just a shot in the dark on those outputs from a couple things I read online.
    What did you mean regarding the 401's? You mean Moog didnt use those in the L3 or...? (I've had a couple of the big Lab Series... before)
     
  13. PCollen

    PCollen Friend of Leo's

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    Well, I get all kinds of hits Googling R7821 datasheet....keep in mind that RCA Semiconductor is no longer in business....sold in 1991 to one of my former employers, Harris Corp.
     
  14. schmee

    schmee Friend of Leo's

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    Yeah, so did I, but on investigating they are the little black squares. Maybe they are the same function in a different format..?
     
  15. ICTRock

    ICTRock Tele-Afflicted

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    the schema for the l3 and l5-based series have house part numbers (so dealers and service techs could order replacements) the house part numbers for the power transistors are different between the two series. it is unlikely they created two part numbers for the same part.
     
  16. galaxiex

    galaxiex Tele-Holic Silver Supporter

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    Transistors work or they don't. They are not like tubes that wear out.
    Not likely to be the cause of the distortion you are hearing.

    Got links to those things you read online?
     
  17. schmee

    schmee Friend of Leo's

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    I don't know about that, my old Marantz SS stereo got weak and a bit fuzzy in one channel. The tech ended up replacing the huge output transistor on that channel and all was well.
    But yeah, I dont have high hopes for this fix. Just a shot in the dark as it's not expensive. There are a ton of the smaller Transistors in this thing if I start to go there! I think I counted 15 of them. No burned resistors or evidence of such, values seem in spec. Then there are the Integrated circuits, which may be more likely to be the culprit? Bottom line is the amp isn't worth the $ for a repairman, like most "throwaway" SS amps. I think my Bassist has 3 bass amps that aren't worth repairing. My ex drummer had 2 PA heads ...ditto!
     
  18. ICTRock

    ICTRock Tele-Afflicted

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    honestly transistors aren't where I would start either. at this age, I'd be going after every single electrolytic capacitor in that thing
     
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  19. schmee

    schmee Friend of Leo's

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    Done, noted it in earlier post.
     
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