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How to analyze this power supply circuit

  1. May 22, 2016 #1
    What is the voltage leaving the rectifier?
    What is the voltage at the two 22 ohm resistors?
    What is the effect of the 4.7 volt zener diode?
  2. jcsd
  3. May 22, 2016 #2
    It is hard to tell the exact number due too many unknowns. But the the peak voltage will be around 3.1V * 1.41 - 0.7V = 3.6V But we have a bleeder resistors, so the voltage will be probably around 3V as noted in the diagram (with respect to ground ).

    6V ( the voltage seen across 22 ohm's resistor and a Zener diode) minus the Zener diode voltage (6V - 4.7V) = 1.3V

    Zener Diode together with 22 ohm's resistors forms a Zener shunt voltage regulator. The output voltage will be Vz ≈ 4.7V with respect to negative rail. But because ground is up by 3V from negative rail. The Vout with respect to ground is around 4.7V - 3V = 1.7V
  4. May 22, 2016 #3


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    I believe the output from the regulator will be...

    +/- ((6.2 * √2) - 2Vd)/2

    Vd can be around 1V for a power diode but I haven't checked that one.

    This would make the output around +/- 3.3v. The diagram says +/- 3.1v on the right hand side.

    The 22R and 4.7v zener diode work together to make a 4.7v output measured with respect to the -3.1v rail. 4.7-3.1=1.6v. The drawing says 1.5v which is close enough.

    The exact voltages will vary as this is an unregulated power supply.
    Last edited: May 22, 2016
  5. May 22, 2016 #4


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    In case it's not obvious +/- above refers to the two voltage rails eg +3.1v and -3.1v.
  6. May 22, 2016 #5
    Thanks guys

    I am reading what you said and trying to understand it.
  7. May 22, 2016 #6
    Sorry guys, all this is new to me.

    Jony, where did the 1.4 and 0.7 numbers come from.

    Again, sorry for being so uninformed but I need to understand this a bit at a time.
  8. May 22, 2016 #7
  9. May 22, 2016 #8


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    Have just corrected the equation I posted earlier.
  10. May 22, 2016 #9
    Thanks Jony, I understand and all that makes sense to me.
  11. May 22, 2016 #10
    Also, I assume the AC voltage is 180 out of phase on the two 3.1 VAC taps.

    Is this correct?

    Mr Watters..give me a bit to read what you said...I am sure I will have some questions!!..lol
  12. May 22, 2016 #11
    With respect to centrer tap the answer is yes.
    http://people.senecac.on.ca/john.kawenka/EDV255/transformers.html [Broken]
    Last edited by a moderator: May 7, 2017
  13. May 22, 2016 #12
    Not only am I trying to understand the math but I am also trying form a mental picture of what is going on in the circuit.

    Mr Watters...I think I understand everything you said...and thanks

    Thanks guys...I think I understand all this now
  14. May 22, 2016 #13

    jim hardy

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    great link CWatters posted there

    There's a couple common labeling scheme for transformers
    6.2VCT = a 6.2 volt winding with a center tap
    3.1-0-3.1 VAC = same thing
    observe the latter is less ambiguous about which wire is the reference point for voltage, ie where the meter's black lead goes.

    Remember my soapbox - voltage is potential difference ie between two points.
    Are those two points the + and - wires leaving the rectifier , in which case it's one voltage the numerator of Cwatters' expression,
    ((6.2 * √2) - 2Vd) = 6.77V rail to rail (for Vd=1)

    or are those two points each wire and circuit common in which case it's two voltages , each half that (as he stated) , ~3.3V rail to common?

    When a reference point for a voltage measurement is not given we often assume it's circuit common.

    >>>>>>>>>>>>>>>>>>>>>>>>>>>.End soapbox<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

    old jim
  15. May 22, 2016 #14
    Got It Jim...Thanks
  16. May 22, 2016 #15


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    You've really stimulated my nostalgia glands with all these valves. Would you care to tell us what amplifier this is? I'm guessing it's modern guitar amp, but I haven't managed to track it down. I'm dying to know about the push function - does it switch between single ended and push-pull output?
  17. May 22, 2016 #16
    Sorry Merlin...I have had my head stuck in this amp for hours. I have been working on this on and off for days. It is a modern design tube amp that has been driving me crazy.
    It is a Mesa Boggie Nomad 45. Here is the schematic.

    Is there any case where a 4 diode bridge rectifier would not have AC on all 4 diodes??

    Other people have worked on this amp or that is to say they have messed up so much stuff I not sure I will ever get it fixed. I ask the question about the rectifier because one diode's cathode is not connected to the AC trace.

    I ASSUME it has to be but based on the crazy design of the switching matrix and other strange stuff it may need to be connected to the bumper of my truck..lol

    Perhaps I need to take a break...lol


  18. May 23, 2016 #17


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    The cathodes of the two right hand diodes should have around +3V DC on them.
    The anodes of the two left hand diodes should have around -3V DC on them.
  19. May 23, 2016 #18

    jim hardy

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    To finish CWatters' thought
    and their other ends ought to have 3.1 VAC with respect to circuit common.

    Use your o'scope.

    Hmmmm ... you asked Is there any case?
    Ever ?
    there exists something called a "ring modulator"
    which resembles a bridge except all the diodes point in same direction.
    if i ever worked on one it was fifty years ago....

    old jim
  20. May 23, 2016 #19


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    Planobilly - If the power supply isn't working can I suggest you measure the voltage on various nodes (all with respect to earth) and mark them up on your circuit diagram. Post that here and we might be able to help find the fault.

    If it's pretty old many of the Electrolytic Capacitors might have failed or be on the way out. Do any show signs of physical distortion on top like this..

  21. May 23, 2016 #20
    There were to positions the diodes in question could fit in the PCB. It was an easy mistake for someone to make. I replaced the diode and have the voltages you indicated in your post. -3.28 and +3.28 I now think all power supply issues are resolved.

    As there are traces on both sides of the PCB and one is required to remove 20 pots from the chassis to get to the bottom side of the PCB, it is not very easy to work on this amp. Small holes filled with solder connect the traces from the bottom to the top of the board and exist in places under things like caps and resistors where one can not see them.

    Too make things more problematic both Mouser and Digi-Key can not provide me with the Motorola MPS4126 , MPS A20. and MPS A93 transistors I need. On Semiconductor has the transistors so they exist but they will not make any more. Central Semiconductor makes a MPS A20 which I ordered. Digi-Key had a MPS A92 (data sheet looks ok) which I ordered and think will work. The MPS 4126 I found in China but will take a good bit of time to get here. I assume these type of issues with obsolete parts will only get worse as time goes by. It is telling how I can repair a 1935 radio without issue and and a 15 year old guitar amp is such a problem.


  22. May 23, 2016 #21
    Hi Cwalters,
    To bring you up to date I have replaced every electrolytic in the amp. I replaced all relays also. I also replaced both op-amps and the LM3914 IC. New reverb tank and all new known good tubes. I have replaced some of the pots but have better ones on order which I will change when they arrive.

    The channel switching is now working with both the rotary and foot switch but something is messed up with channel two.
    When switched to channel two no sound is produced unless the channel 3 master is turned up a small amount. Also the tone stack on channel two is not working. Also the reverb is not working on any channel.
    My investigations have indicated that the signal path works like the Block Diagram shows based on the fact that the grid of V2a and V2b have no signal when the amp is switched to channel 2 or 3.

    What I really need help with is a troubleshooting method to deal with the transistors and the relationship of the switching matrix and what is called Q1-Q3 and Q4. Even if we find the faults it will be some time before the transistors I have ordered get here. I am really confused as to the exact signal path for each channel and what must be working correctly as every stage seems to be connected in some way. This is the most complex guitar amp I have ever tried to work on.



    EDIT: Testing this morning indicates that although the channel 2 LED comes on when switching to channel 2 the signal is going through channel 3 as all the channel 3 volume and tone controls operate and only the channel 2 volume control (gain) has any effect. So something is not switching correctly.

    SECOND EDIT: This is really baffling! I plugged the guitar in again and all channels seem to be working, well at least all controls on each channel function in a more or less normal way.
    Last edited: May 23, 2016
  23. May 23, 2016 #22
    I have to go to Miami...be back later today
  24. May 25, 2016 #23
    Made some progress. Someone had installed PNP transistors where there should be p-channel junction FETs. When I repaired that issue the solo control started working correctly. Now I am in the process of looking to see if the op-amps are installed correctly because I put the new ones in the way they were installed when I got the amp. The good news is I installed pin sockets and have plenty of op-amps.

    The takeaway is don't assume anything.


  25. May 25, 2016 #24
    More progress...got the reverb working. There was a broken trace under a pin scocket. I had to de-solder several resistors to trace it. Not an easy fault to find.
    I am finally getting close to the end...I think
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