Checking the +B and -B voltages in a Solid State audio amp

Discussion in 'General Electronics Chat' started by programmer6502, Jun 26, 2015.

  1. programmer6502

    Thread Starter Member

    Feb 1, 2014
    126
    6
    Hi,

    I'm working on a Solid State audio amp and want to know if I'm checking the +B and -B voltages correctly with my multimeter. What I have here are both of these voltages outputting from a bridge rectifier and I'm putting my positive probe on +B and the negative on -B, which results in FAR off tolerance voltages considering the amp still functions (with a couple intermittent issues). Since B- is a negative voltage (opposite of the B+ voltage), am I supposed to treat it as an independent voltage and ground to the chassis/common instead when checking either +B or -B? I just want to be sure.

    Thanks
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,638
    2,343
    Hello,

    Do you have a schematic of the +B / -B powersupply?

    Bertus
     
  3. #12

    Expert

    Nov 30, 2010
    16,247
    6,744
    Umm...yes.
    Still, I fear that if you don't already know this, you are in danger of both failing at the job and getting hurt.
    Proceed with caution!
     
  4. programmer6502

    Thread Starter Member

    Feb 1, 2014
    126
    6
    Absolutely man! The reason I came here is cause I don't feel like taking a fatal shock or blowing the amp up! So is that a yes for chassis grounding when checking those?

    And here's the power supply portion of the amp and I marked where those voltages output. I can't provide more than this:

    [​IMG]
     
  5. #12

    Expert

    Nov 30, 2010
    16,247
    6,744
    Look to the, "split" capacitors to find your, "common" points. For instance, the line marked, "J".

    From there, you can Ohm to chassis to discover if the designer connected the chassis as a big common terminal.

    If your measurements suddenly turn to garbage as you delve deeper into the circuitry, there is more than one common point in the amplifier. It is sometimes convenient to let a transformer winding "float" for certain kinds of circuits. Therefore, some of your voltages will behave exactly as you expect and some of them will wander all over the place, depending on the power output level at that moment. If that happens, the only choice left is to examine and learn the schematic.
     
    Last edited: Jun 26, 2015
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,515
    1,246
    Hard to see some of the reference designators, but it looks like D16, 17, 18, and 19 form a full wave center-tapped bridge driving two big caps that are an unregulated bulk power source for the speaker amps. The system is relying on the power supply rejection capabilities of the amps to keep ripple out of the audio. Each of these power rails should be measured with respect to system ground, which is either speaker's - terminal connector. With the volume all the way down, there should be the same voltage on each rail and very little ripple.

    ak
     
  7. programmer6502

    Thread Starter Member

    Feb 1, 2014
    126
    6
    Perfect, that answers my question. And I recently checked to see if the chassis is connected to that same common shared by the capacitors and it is.

    Thanks man
     
  8. programmer6502

    Thread Starter Member

    Feb 1, 2014
    126
    6
    I love watching you guys work! You have everything figured out in seconds lol!
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,515
    1,246
    Or, 49 years and a few seconds.
     
  10. #12

    Expert

    Nov 30, 2010
    16,247
    6,744
    Reminds me of a musician friend. "Three minutes to play a song and a lifetime of preparation to be able to play it." :p
     
    Sinus23, KJ6EAD, absf and 1 other person like this.
  11. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    A spectator told Arnold Palmer, the golf pro, after he had made a great shot, "lucky shot". Arnold replied," Thanks, I practice my luck every day."
     
    Sinus23, absf and programmer6502 like this.
  12. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,515
    1,246
    After Lee Trevino made a hole-in-one during a tournament, an interviewer congratulated him on his "lucky" shot. Lee got fake-incensed and said Luck?!? I've been practicing that shot for 40 years!
     
  13. programmer6502

    Thread Starter Member

    Feb 1, 2014
    126
    6
    All voltages on the power supply look decent, nothing exploded and I'm alive to report. ;) This narrows down the problems considerably.

    Thanks all once again, this is a great forum!
     
  14. #12

    Expert

    Nov 30, 2010
    16,247
    6,744
    It's partly your fault. You have to ask a smart question to get a smart answer. ;)
     
    programmer6502 likes this.
  15. programmer6502

    Thread Starter Member

    Feb 1, 2014
    126
    6
    By the way, I'm getting an intermittent 120hz hum in the speakers on all channels. Back when I was using it, if you slammed the top of the amp cover with your fist the hum would stop for a random amount of time, then it would usually come back meaning that there's most likely cold solder joint(s) somewhere. I know that a 60hz hum/buzz from household AC changes to 120hz when rectified, which would be a fault in the power supply section. The hum is effected with the volume control (frequency stays the same), so I think that means the fault is before that stage. Do you guys know what the most common faults are when this happens? I'm going to continue diagnosing myself, and am looking into building a slick single tracer but I thought it wouldn't hurt to ask you guys real quick since I have this up!

    Here's exactly what it sounds like:


    Thanks
     
  16. #12

    Expert

    Nov 30, 2010
    16,247
    6,744
    As a really serious pro, I wouldn't bother with a signal injector or a signal tracer. The hum is doing that for you (if you know how to use it to your advantage). You will go looking for bad solder joints and diseased filter capacitors. Don't forget the input jacks. It might be a grounding fault starting right at the input to the first stage.

    Delete that last sentence. I thought you said the volume control was in charge of amplitude. If same for all inputs, then input jacks are not the problem.
     
    Last edited: Jun 26, 2015
    absf likes this.
  17. programmer6502

    Thread Starter Member

    Feb 1, 2014
    126
    6
    Sounds good. The hum is in all input channels including the built in radio tuner. Need to get myself a capacitor tester....
     
  18. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,515
    1,246
    Keep working the clues. If it is in all channels, then it probably is in a circuit that all channels go through, something after the selector but before the volume control - like the tone controls. If you have the complete schematic, trace the tone circuits power backwards. Maybe there are local decoupling caps that have come loose, of one of the bulk filter caps has a failing solder joint while the other one is ok. Look for hum sources that are out of balance.

    ak
     
  19. programmer6502

    Thread Starter Member

    Feb 1, 2014
    126
    6
    I think the same and have it narrowed down pretty good. The amp is actually a Kenwood from the 80's and is all digital in the front with a built in equalizer and spectrum analyzer that adjusts everything. Not one potentiometer to be found, all tactile buttons! But with the service manual I have, that isn't a problem.

    My biggest fear is if the two +B and -B filter caps are going out. They're massive Elna 7500uf 71v capacitors and I can't even find any that would replace them if that was the case! But I'm not jumping to conclusions, there are other things it could be. I'm not sure if Elna is a good brand, but the amp seems to have good Japanese branded caps all around. Only testing and examination will tell!

    I don't think this is anything to do with the humming, but I found two shorted ceramic series resistors (one for each channel between power transistors) in the final stage. Will have to get those replaced as well.
     
  20. #12

    Expert

    Nov 30, 2010
    16,247
    6,744
Loading...