Need Help getting started in electronic troubleshooting.

Discussion in 'General Electronics Chat' started by SkidPlate, Jun 21, 2009.

  1. SkidPlate

    Thread Starter New Member

    Jun 1, 2009
    I want to get started in Electronic Troubleshooting. I was taught basic E&E while in the Navy but that was almost twenty years ago, I forgot a lot of stuff but I am not completely clueless either.

    I have searched many times looking to get started in electronic trouble shooting and keep getting back to square one which is theory and some guy Jestine Yong. I have also looked at stuff on you tube but all the stuff there just gives a tutorial about the oscilloscope, waveforms, etc.

    Jestine Yong has an e book called Testing Electronic Components. Has anyone ever heard of this guy or know anything about his e book. I am a bit turned off by his site it just seems gimmicky to me just to sell me something that will leave me disappointed. If anyone can tell me something about his book or books I would appreciate it.

    I want practical hands on how to hook up an oscilloscope and signal generator where and what to expect when I hook them up and anything else to help me troubleshoot.

    I have a Marshall Amp I want to fix. There are no visible problems with the board or any parts on it. I thought it had a bad capacitor because of some debris on the bottom of a capacitor (turned out to be nothing) but when I pulled and tested it, it tested good. I am willing to buy what ever tools necessary to fix my amp. So far I have some soldering tools and a couple multimeter's.

    I know some people would say why not just send it out to get fixed. I want to be able to fix my own electronic devices that's all.

    All suggestions and comment are appreciated.
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    Maybe you could take a digital picture of the board and post it here. The closer and clearer the picture, the better look at the board we can get.

  3. awright

    AAC Fanatic!

    Jul 5, 2006
    While I can totally understand your motivation to be able to fix your own stuff, I'm not clear on whether you are interested in developing a career or perhaps an advanced hobby or just want to fix some of your stuff. That is, do you want help fixing your amp or getting advice on career development? Either way, I encourage you.

    If fixing your amp is the primary goal there are some general approaches that may help. Be aware that I am an electronics hobbyist from the tube days, not a professional electronics tech. I have, however, repaired perhaps hundreds of electronic instruments and designed and built a few dozen over the years.

    The very first step in approaching a repair (well, second after the visual inspection for obvious damage using good lighting and a magnifying glass) is to obtain a schematic of the item and, if possible, a manual. Sometimes these are readily available at little or no cost, sometimes you must pay a little or a lot for a service manual, and sometimes the manufacturer treats his treasured design like a military secret, even if it is decades old and obsolete. These days, manufacturers come and go or get bought out and simply send all their literature to the recycling bin. But try to obtain a manual - first always from the manufacturer, then from manual dealers on the 'net.

    With or without a manual, consider the nature of the failure. Are both channels of a stereo amp dead or just one. If one channel is ok, the power supply is probably fine (assuming it is shared by both channels). If both channels are dead or distorted, check out the power supply, the item common to both channels.

    Make sure the transformer (if present) is getting power to the primary and that the secondaries are supplying power. Check voltages and ripple on the storage capacitors. If the voltages seem low, try to figure out where the voltage drop is and why.

    A tried and true troubleshooting method for amplifiers is signal tracing. You can do this with an audio amp with high, variable gain and high input impedance, but a 'scope is best. It is very advantageous if you have one good channel for comparison of the signals at each stage. If signal levels are high enough, use a x10 scope probe to minimize circuit loading. A x10 probe will usually have 10 megohms resistance instead of 1 megohm for a x1 probe. But both can work. If you can't afford x10 probes you can make crude ones by adding a 10 megohm resistor to a x1 probe. (Actually, 9 megohms, but they are harder to find.) These crude x10 probes won't be properly compensated for high frequencies, but they can be fine for audio circuits.

    A dual-channel 'scope can help in comparing signals in two identical channels. They can also provide the capability to perform differential measurements of voltage across floating components, which can be handy. Remember that on most 'scopes the shell of the input BNC connector is directly connected to earth ground. You must not clip the ground lead of the probe to any part of the circuit that you are not certain is at ground potential. (There are exceptions with special 'scopes and isolation circuits, but always assume a hard ground unless you know otherwise. I got a very red face by forgetting to check this while demonstrating how to use a 'scope to a rookie early in my career.

    As one who started out with a kit-built tube 'scope, I favor conventional dual-channel analog 'scopes over digital 'scopes for the beginner. This may just be habit, but I have seen people seriously misinterpret digital 'scope displays due to aliasing. Perhaps this is not much of a problem for audio frequency work. Also, I haven't worked with the current crop of digital 'scopes. You should be able to pick up a used 'scope of good quality and condition and reasonable bandwidth in the $100 neighborhood by shopping carefully.

    I don't have any recommendation for books on troubleshooting. The best teacher is fixing things - for friends or even items you don't really need from junk piles or salvage/second-hand stores. Charity and second-hand stores throw away lots of contributed items that may be easily repairable. If you end up with an unrepairable piece you can strip it down to build up your junk box - er, I mean parts stock.

    A very good basic book on electronics is, "The Art of Electronics," by Horowitz and Hill. I also like to haunt my local independent used book stores for interesting books on electronics.

    In general, I think it is better to think through the nature of the problem and the symptoms and make more measurements to be sure of your analysis than to change components on a hope and a prayer. Lots of damage - cosmetic and real - can be done by experimental changing of components that don't need it.

    Once you have decided to change a component, it is sometimes best to clip the defective component out of the circuit rather than to fight to extract the defective component from the circuit board. After the component is out of the way you can either extract each lead individually with less struggle and damage or solder the replacement to the leads left in the board.

    Just a bunch of random thoughts off the top of my head. Hope they are of interest.

  4. SkidPlate

    Thread Starter New Member

    Jun 1, 2009

    hgmjr, here are the pictures you requested.The original pictures are at 3072x2048 photobucket auto resizes them. If you know of another way to get the original size posted let me know.
  5. SkidPlate

    Thread Starter New Member

    Jun 1, 2009
    I want to start out hobby and maybe move over to doing it as a second income. Extra money for my own toys or vacations etc. Since my amp is broken I would like to start with it as my first attempt at repairing electronics.

    I emailed Marshall they sent me schematics but they said they couldn't send a manual.

    The amp works on both channels. The problem is when you pluck a note or chord that comes through the amp queitly and gets louder almost to full volume with distortion. The rise to full takes approximately 1 to 2 seconds

    I want to get a scope and signal generator but as far as using it I am lost. The scope is where I need help.
  6. SgtWookie


    Jul 17, 2007
    Hello, Sailor ;)

    Here's where you can download the Navy NEETS pubs for free:

    Module 16 should get you up & running (more or less) with some of the more common types of test equipment; signal generators, O-scopes, etc. Since you've already been through BE&E, the format should be somewhat familiar.

    I'd suggest that you start from Module 1 and work through at least most of them. It'll take you around a year if you work at it every day. You'll probably recognize a fair amount of it; it was being taught in a similar format even 35 years ago.
  7. SkidPlate

    Thread Starter New Member

    Jun 1, 2009
    Thanks for the link SgtWookie. All of these modules look just like the stuff I learned while in A School. It's just a matter of going through it all again. I also forgot how to do all of the math that goes along with it. About the only stuff I remember is Ohm's Law and Kirchhoff's Law and a few other things.
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    what seems to be the problem dude.

  9. SkidPlate

    Thread Starter New Member

    Jun 1, 2009
    Lack of confidence and tools.
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Looks like a Marshall Valvestate with 12AX7 tube preamp and 2 stereo solid state power amp chips.

    Does the fault happen on both right/left channels equally? Have you tried replacing the 12AX7? Does the 12AX7 glow normally and not change the glow or change sound when you wiggle it in its socket? Have you tested the supply rails for the 12AX7 with the voltmeter and compared to the voltages on the schematic?

    How about the other low voltage rails for the power amps? Ive seen amp chips do that weird sound fade thing when one of the voltage rails (+ or -) is down. The schematic should have test points to measure the voltage rails.

    If it's happening on both channels equally its in something common to both channels;
    1. preamp opamp section where your guitar is plugged in
    2. 12AX7
    3. power supply rails