Electrostatic field in my amp.

Discussion in 'General Electronics Chat' started by jetbat, Nov 10, 2014.

  1. jetbat

    Thread Starter Member

    Oct 21, 2009
    Hi. I was upgrading an Ampeg b100 solid state amp. I was going to replace some capacitors and resistors. Nothing difficult. I did the replacements and turned the amp on to discover a loud 120 Hz buzz. I was perplexed do to the simple procedure I was performing. I double checked to make sure all correct values were in the correct places with the proper orientation. Checked out good.

    Attached is a schematic of the amp.

    I was poking around inside the amp while powered up, with a wooden stick, and discovered that part of a transistor stage had generated a electrostatic field. I had replaced C11 and when I touched any part of it I got a much louder buzz like I was touching hot line in to an amplifier. This is with a wooden stick. All parts that are in this junction with the base of Q2 have the same buzz when I touch their lead connected to the junction. Only C11 and C13 buzz when touching the actual body of the part. I switched C11 back to the original part thinking something was wrong with it. No change. Still the same buzz.

    I discovered that an electrostatic field was being generated that extended about one inch out from the capacitor. When I would bring my finger into this field the buzz would increase the closer I got to it. This field is picking up the nearby power supply and amplifying the hum.

    The volume control did nothing to change the hum so the problem was after the pot R15. Thought maybe one of the parts I replaced were bad so I systematically switched the original parts back one at a time checking for hum each time until all the parts after R15 were back. Problem still there. I pulled C13 out to check its capacitance and it read like 230 pF. Q4 get pretty hot like it needs a heat sink which I don't think is right.

    The DC voltages match up with what is on the schematic and the amp does make sound if I plug a bass into it.

    Do transistors ever cause anything like this?

  2. bertus


    Apr 5, 2008

    Check C18, the capacitor next to the bridge rectifier.

  3. jetbat

    Thread Starter Member

    Oct 21, 2009
    That is one of the capacitors I had replaced then changed back after finding the problem. The amp was quiet, no hum, before I switched caps and resistors. Here is a list of parts I changed.

    Capacitors- C1, C2, C3, C4, C5, C10, C11, C12, C15, C17, C18.
    Resistors- R3, R4, R6, R8, R9, R23, R25, R26, R28, R29.
  4. BobTPH

    Active Member

    Jun 5, 2013
    Sounds to me like C11 is not soldered properly. Touching it with your stick is either making or breaking a connection.

  5. jetbat

    Thread Starter Member

    Oct 21, 2009
    C11 was changed with a new capacitor. The problem was detected. The new capacitor was removed and the original capacitor reinstalled. The problem still remained. All connections checked.
  6. MrChips


    Oct 2, 2009
    Why do people go replacing resistors and capacitors willy nilly is beyond me.

    If it ain't broke don't fix it.
    ErnieM, Sensacell and alfacliff like this.
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
    if you replace a lot of stuff at a time, you dont know what fixed it, or what caused another problem.
  8. jetbat

    Thread Starter Member

    Oct 21, 2009
    The amp is from 1976. Electrolytic capacitors have a limited life and technology has advanced a bit since then. The amp had a two prong plug that was replaced with a three prong grounded plug. The fuse had been soldered inside the amp so a rear access fuse holder was also installed for easy replacement.

    I have had excellent results with upgrading resistors in key locations with low noise versions as well as upgrading to high quality modern capacitors. So much so that I continue to do these things when I feel it is warranted.

    I agree that changing one component at a time would have shown me exactly where and when the problem started. Seeing that I was only changing passive parts I worked in a batch. When working with active parts I will change one thing at a time.

    Since I changed parts back to original and the problem remains, I have been looking at the transistor Q2.

    Original part 610083-1
    This is the recommend replacement since the original is out of production.

    I will order and replace. Was just wondering if anyone else had experienced this.
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Just where did you add this extra wire?
  10. jetbat

    Thread Starter Member

    Oct 21, 2009
    Bolted to the chassis. The first thing I did when I heard the hum was to lift that ground and test it. The hum was still there. No ground loop.
  11. MrChips


    Oct 2, 2009
    Where did you connect the AC mains LIVE and NEUTRAL?
    Sometimes interchanging the two can result in lower or higher AC hum.
  12. jetbat

    Thread Starter Member

    Oct 21, 2009
    I cut the old cord where it entered the chassis and left the wires where they were soldered and used them to run to the switch and light.

    So from the plug the black goes to the fuse holder. Black wire from the fuse holder to the switch. I placed a jumper from the switch to lamp where the fuse used to be soldered. From the lamp to the power transformer. The return from the transformer to the other side of the lamp. White wire from the lamp to the white wire form the plug.

    I was checking these things until I discovered the field around the capacitors C11 and C13. If I pass my finger within an inch of them the buzz increases with proximity. Like a strange theremin.
  13. BR-549

    Distinguished Member

    Sep 22, 2013
    Inducing a 120 hz hum with finger in low level audio stage is very normal. I have never met an amp that wouldn't.
    Put your finger on any mic center conductor or aux\line in...........on any equipment.....and see what you get.

    Being you like to play around with this stuff.......and are willing to use component exchange troubleshooting techniques.............I suggest that you invest in an oscilloscope.
    A good quality scope for your needs can be had for very little these days.

    After you learn to use to scope, you can tell how well the old caps are working without removing them.
    Plus there is a million other things you can do. You can watch your music go thru the circuit.
    BTW....keep and extra pilot lamp.....it is a surge protector.