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Newbie Needs Help With Transistors

Discussion in 'Troubleshooting and Repair' started by Andy54, Apr 6, 2013.

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  1. Andy54


    Apr 6, 2013
    I'm newbie to electronics and am attempting to fault find transistors in my old 1980's Peavey PA amp.

    Should the values be the same between the base and emitter compared to the base and the collector ?
  2. john monks

    john monks

    Mar 9, 2012
    No. Typically the base emitter voltage is around 0.7 volts to 0.8 volts. This can be positive or negative depending on the transistor. The base collector voltage can be anywhere from 0.1 volts to hundreds of volts.
    What is the model number of your amp?
  3. Andy54


    Apr 6, 2013
    John thanks for that info.

    This is a Peavey 45W PA-100. The majority of the transistors are 2N3391.

    I took it to a Peavey authorised tech as it started to "pop & crackle". He told me that it was so old that he would have to test all the transistors and so the repair bill was going to be expensive for an amp I'd be lucky to get $100 for.

    I feel it's too good an amp [ US made ] to junk without at least trying to find the problem

    His advice was to try and fault find myself and to start with the transistors. Hence I've turned up here with a little bit of knowledge but a lot of time on my hands.
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    When does it crackle?

    Does an adjustment to the volume make it crackle more?

    How about when you wiggle cables?

    Have you tried different inputs and speakers (or the same ones on another amp)?

    Is the sound clean and undistorted apart from the crackling?

    Does it affect all inputs?

    Does it affect all outputs?
  5. Andy54


    Apr 6, 2013
    Steve I appreciate your thoughts on this problem I've got.

    This PA amp has a four channel input amp, mixer amp , EQ amp , power amp and spring reverb.

    My first attempt at a fix was to clean all pots and and inputs. Wiggling cables doesn't affect the the static [ pops & crackles ]

    The static starts just as the amp warms up and then is at a constant rate.

    The overall sound produced is clean and undistorted even at uncomfortably high volume.

    The amp is in the same box as one 16ohm speaker, there is another separate box with a speaker. When I connect the 2nd speaker the static is at the same rate and level.

    Basically it boils down to a constant level of static [ pops & crackles ] with either all pots at zero or at max. That is to say the static is constant with any of the component amp being adjusted for increase or decrease in master volume, seprarte channel volume, EQ or reverb level.
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Is the static the same on all channels, i.e. same crackles and pops from all outputs even with a different signal source on each?

    Good information that the level of crackles is independent of the volume setting too :)
  7. Andy54


    Apr 6, 2013
    The static isn't coming from any one separate input channel. So for example with say three inputs taking a separate source at various volume settings, the static is always at the same level.

    With all the PA's potentiometers at zero the static is still at same volume ie a pulling my hair out annoying level ;)
  8. Andy54


    Apr 6, 2013
    Another Head Scratching Part Of This Problem

    The attached diagram [ also photo below ] shows one of the existing transistors ie EP430 A7737 and what the readings were when I tested it. The Peavey replacement is given as TIP31C and shown on right of my diagram.

    In the photo it's the black transistor to the right of the transformer.

    Using the multimeter set to diode test I got readings as shown on the left hand side of the diagram. I could only get a reading by placing the positive lead to the centre wire so I'm concluding this is the "base".

    I'm not sure if the readings are correct for this transistor.

    If I use the replacement transistor given by Peavey's repair information I should use a TIP31C. I've shown this with info I got from a search on it's specifications. The specs I found show that the TIP31C wires are [ left to right ] Base ~ Collector ~ Emitter.

    This seems strange as the original EP430 seems to me to have the Base as the centre wire. I can't see how the TIP31C would work in the circuit as its Base is on the left ?

    Thanks in advance.

    Attached Files:

    Last edited: Apr 7, 2013
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Yes, those readings are consistent with the center connector being the base. The base-emitter and base-collector junctions are different and you can expect different readings from them.

    The collector can be identified by seeing which lead is connected to the heatsink -- it will be the collector.

    Assuming these tests were done out of circuit?

    And this amplifier has only a single output channel?
  10. duke37


    Jan 9, 2011
    Crackling can be caused by faulty elctrolytic capacitors sparking over internally. If you can drop the power supply voltage and the crackling goes, then this is the likely cause. Otherwise check the power supply capacitors for leakage and distortion.
  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    If the crackling starts when it's warmed up, get some rapid freeze spray and spray the semiconductors one at a time, and then the capacitors.
  12. ggm1960


    Apr 28, 2011
    I'm curious as to why you'd suspect this transistor. I'm more inclined, like duke37, to suspect a cap. I've done a lot of work on guitar amps. I've usually had good luck finding older Peavey amp schematics on-line. I have the best luck using an oscilloscope to trace a function generator sine wav through the preamp on through the power stage to determine where the problem originates.
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    We can be pretty sure it's not the pre-amp because the volume of the crackles does not change with the position of the volume control.

    For the same reason, I'd not suspect the power supply unless the pre-amp has a separate power supply or is well decoupled from the main power supply. This is because noise from the power supply being injected into the preamp (as it would) would be affected by the volume control which is normally between the pre-amp and the main amp.
  14. MikeL


    Apr 9, 2013
    Go to Jaycar (Australia) or similar and get a tin of cold/freeze spray. its not cheap, about $20 but it is something we always have at work. Move your hand over the amp to get an idea of where the most heat is coming from but be careful not to touch anything.
    Spray that area wehn it is crackling well. DO NOT get it on your skin,

    Try the caps first tantylums and electrolytics. Tantylums (the tiny egg shapped caps with stripes and a dot or blue cause many noise problems. Good caps but after a while they need changing. If you find the fault is one of these, I suggest change them all
    If the problem reduces then heat up with a hair dryer and wehn the noise is back do it again. It cools a big area so that is why the more focused you can direct the cold the better. ( It always comes with a tube)
    where it is and hit a component with the spray
    I have seen many faults found using this technique

    As to your first quesstion if the transistor is disconnected and you are using a multimeter in diode test then BE and BC will have similar breakdown when forward biased, I personally doubt it is a transistor problem
    I am off to get some tomorrow for an annoying problem I have

    EDIT Forgot to suggest Examine the PCB and all connections for dry joints
    Last edited: Apr 13, 2013
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Or go to OfficeWorks and get a small can of "compressed air" for $2 and use it upside-down.
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