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Thread: Carrotron C-821 Preamp Pedal Schematic or any information?

  1. #1
    Senior Member
    Join Date
    Dec 2008

    Carrotron C-821 Preamp Pedal Schematic or any information?

    Someone brought me an early 1980s Carrotron C-821 Preamp Pedal for repairs, and it is proving to be a nightmare.

    Of the ten BJT transistors in it, whoever built it deliberately erased the part numbers on six of them. Other parts were installed with value markings hidden against the PCB. There's one glass diode that may be a Zener, but it has no legible markings on it.

    These were reportedly designed by a guy by the name of Carl Margolis, and it seems clear that he took steps to make his design difficult for anyone to copy. I have no idea if he's still living, but the downside of that approach is that it makes diagnosis and repairs pretty frustrating.

    I could draw out the circuit, but that may quickly make a repair cost more than the thing is worth. I found one bad transistor that I replaced with a PNP transistor because the other unmarked ones that appear physically similar and the same color are PNP, but that didn't fix it.

  2. #2
    Senior Member
    Join Date
    Dec 2008
    I did finally manage to figure this thing out, and it would have been a LOT easier with documentation.

    The Input and Output jacks, with all their associated wiring, are in the wrong chassis holes. They were reversed -- in addition their having been one bad transistor.

    I'm betting that someone else had it apart to try to track down the bad transistor, failed to find it, and then reassembled it incorrectly.

    So if you ever want to play a mean joke on someone, this is a way to do it.

  3. #3
    Old Timer J M Fahey's Avatar
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    Oct 2007
    Buenos Aires, Argentina
    Glad you solved it, but in general small bipolar transistors are no problem, being fully interchangeable with thousand others.
    So much so, that Elektor Magazine published a universal list showing no specific models (such as 2N2222 or BC107 or 2SC84) but generic codes such as TUN TUP DUG DUS which mean *any* (as in ANY) one of the matching polarity from the following tables.
    I post a couple just as an example:

    Circuits, as published and used by Elektor and the Dutch Elektuur, contain universal transistors and diodes to the abbreviations: TUP (Transistor Universal Pnp), TUN (Transistor Universal Npn), DUS (Diode Universal Silicon), and DUG (Diode Universal Germanium). Many transistors and diodes fit this way in these categories and makes component selection easier. Good system!
    The minumum specifications have to be met,
    as in Table 1a , before you can call it a
    'TUP' or a 'TUN'.

    These are European examples but similar ones exist for American and Japanese ones.
    And these are just examples, each table really includes thousands , as long as they meet the (very easy) specs posted above.

    Some types which match and surpass basic specs, where you have up to 45V supplies:

    Pinouts can differ, so this table helps choosing a matching one ... although if not available you simply bend legs as needed and call it a day.

    FWIW and being in European oriented Argentina, I only buy 500 each of BC547 and BC557 (at which scale unit price is a couple cents) and 50 each 2N5401/5551 for higher voltage use (in >150W SS amplifiers, up to 150V DC) and a dozen each MPSA42/92 for REAL high voltage (think 300V or more)
    But the latest stay YEARS in their bags, so much so that sometimes I have to scratch their dull (by age and humidity) legs so they are shiny again and they take solder well; that takes at least 15 years, so ....
    The real workhorses are the first two and the only ones which are used everyday.

    EDIT: and to know which is NPN or PNP: NPN always has Base and Collector more positive than Emitter, PNP the opposite ... which you measure at the solder pads with or without them.
    cjlectronics likes this.
    Juan Manuel Fahey

  4. #4
    Senior Member
    Join Date
    Dec 2008
    I'm aware that BJTs are largely interchangeable, but you also have to be aware of applications where specific parameters matter.

    For example, in the electronic crossover found in SS Leslie amps, there's one position where if you install a transistor with current gain that's too high, the circuit will oscillate. You can use a BC550B, but not a BC550C.

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