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Need some help with a transistor switching circuit

  1. Feb 19, 2017 #1

    The issue: The "more drive" circuit is not working on this Fender Hot Rod Deluxe amp.

    There is a push button switch that selects the drive channel which is working and the yellow LED comes on. There is another push button call "more drive" which increases the gain to the drive channel by around 13db through the use of two JFETs (J111)

    Three amp techs have looked at the amp without results. Link to schematic http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Hot-Rod-Deluxe-Schematic.pdf

    I have replaced many of the components in this circuit.

    Test point readings:
    Per schematic TP 31 +1.37 +1.07 +1.07 (pin 5 of the op amp)
    Actual TP 31 +1.36 +1.09 +1.09

    Per schematic TP 32 +.548 +9.76 (junction of CR23 and CR28)
    Actual TP 32 +.498 +9.78

    Per schematic TP 33 +16 -13.5 -13.9 (pin 7 of the op amp)
    Actual TP 33 +16.37 -10.17 -10.17

    Per schematic TP34 -13.0 -5.46 -4.96 (pin 3 of the op amp)
    Actual TP34 -11.88 -12.11 - 12.11

    Per schematic TP 35 -.54 -10.0 ( junction CR24 and CR26)
    Actual TP 35 -.51 -9.5

    Per schematic TP 36 -16 -16 +16 ( pin 1 of the op amp)
    Actual Tp 36 -16 -16 -16

    Link to the 4558 op amp pin out/data sheet http://www.ti.com/lit/ds/symlink/rc4558.pdf

    I have verified all resistors and zener diodes and changed any that were out of value. I replaced the op amp which was a 4558 on the board from the MFG but shows a 4560 on the schematic. The only difference in the two op amps is the slew rate. I replaced the two (Q1 and Q2) JFETs and Q3 and Q4 transistors. The push button switches are working correctly.

    It is obvious that the voltages at TP33, pin 7 of the op amp are not correct. TP34 is not correct, and the voltage at TP36 does not change to positive. I replaced the op amp twice.

    I do not fully understand what controls the voltages at TP33, and TP34, ( pins 7 and 3 of the op amp) and why they are out of value. I ASSUME the reason TP36 is not changing to positive when the "more drive" is engaged is due to the out of value conditions at TP33 and TP34. The red LED associated with "more drive" is also not coming on.

    I can provide any other measurements that may help.

    This has been a perplexing issue that no one to date has been able to figure out. I am sure we are missing something.

    Anyone have any ideas of what to do next based on the faults described ? huh.gif


  2. jcsd
  3. Feb 19, 2017 #2


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    Science Advisor

    Sigh. I have seen this before - experienced analog engineers trying to create digital circuits the analog way and ending up with an extremely complicated circuit trying to solve a very simple digital problem.I must admit that trying to understand what they were thinking has been a frustrating experience.

    So. It seems that the function of Q3 is to enable the red LED and help turn on the relays. As long as TP36 is negative, Q3 should be turned on.According to the schematic, TP36 should be at -16V all the time - which does not make sense. If it was to go positive, it would "turn on" the JFETs, increasing the gain - but it would also turn off Q3!

    Sorry - I cannot make sense of the circuit.
  4. Feb 19, 2017 #3
    LOL....I also could not make heads or tails out of this circuit.

    I did solve the issue and got it to work but I have no real idea why it now works.

    I changed the 4558 op amp to a 5532 op amp. That caused the whole damn thing to function correctly. I selected the 5532 because that is what I had on hand...lol
    Looking at the data sheets I don't see much difference between the 4560, 4558, and the 5532 except for slew rate.

    On another note I also found that the combo LED was not Red and Yellow but Red and Green and when both are lit that produces Yellow. When the Green side is turned off only the Red is lit. It would have been nice if that would have been indicated on the schematic.

    I guess every guitar amp with channel switching I have ever messed with has a convoluted switching matrix. One of the next projects is to learn how to build a logical channel switching of a really good design.

    Thanks for the feedback Sevin.

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