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Need Advice on Repair of Peavey Standard 130 Watt Power Amplifier

Discussion in 'Electronic Repair' started by Dr RaTsTaR, Oct 15, 2007.

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  1. Dr RaTsTaR

    Dr RaTsTaR Guest

    Hello,

    I am attempting to repair a Peavey Standard 130 watt power module and
    would appreciate some advice. I am not experienced in amplifier or
    electronics repair but I have taken all the electronics courses that
    the local Jr College (Mesa Jr. College, San Diego) had to offer. I
    have a lot of theory but not any practical experience.

    The amplifier will pass a music signal to the speaker when I patch an
    audio signal directly into it, but the music is cracked and fuzzy,
    very distorted. Here we have bypassed the pre-amp front end
    completely. This unit is in a Standard Power Pak, which is a guitar
    setup.

    I have the schematics and have been poking around with a computer
    sound card oscilloscope (Daqarta). I have also constructed a simple
    tone generator so that I can use a stable signal to trace.

    What problems might allow a simple amplifier like this to still work
    even though it is distorting the signal? I have checked the main power
    transistors in circuit using a diode checker on my Beckman DMM.

    I recently successfully repaired a Peavey XR600E using this same diode
    checker to identify the 3 blown transistors in the left side. I also
    had the right channel to compare readings to, so that was a great
    help.

    Any help you can provide would be greatly appreciated.

    Thank you.

    d0ct0r RaTsTaR
     
  2. James Sweet

    James Sweet Guest

    Scope the output where the speaker would connect while feeding a tone into
    the amp and see what the distortion looks like. That can give you valuable
    clues.
     
  3. bg

    bg Guest

    Dr RaTsTaR wrote in message ...
    The most common failure in a power amp is the output stages or the power
    supply. There will be a transistor or several transistors in parallel that
    reproduce the positive half cycle, and one or more to reproduce the negative
    half cycle. If one side goes out , you still have sound but only one phase
    of the sine wave, thus, big distortion. Check all of the output transistors
    with an ohmeter. Check the driver transistors too. Check the low valued
    emitter resistors (usually less than an ohm).
    bg
     
  4. Arfa Daily

    Arfa Daily Guest

    Also, be sure that your 'tone generator' is not over-driving the input of
    whatever stage you are connecting to, and is completely AC coupled, and is
    not loading or otherwise shifting any bias supply that may be present at
    whatever point you are connecting in at, which is a little unclear from your
    post. I am assuming that you are going in at the input to the power amp when
    you say that the preamp is "bypassed"? Does this amp have an effects send /
    return jack pair ? This is usually a good place to 'break in' post preamp /
    pre power amp. Also, as James suggested, looking at the shape of the
    distorted output aginst what you are putting in, will give a very good clue
    as to what is going wrong. Just as a matter of interest, what shape wave is
    your home-built tone generator producing ? It must be a sine wave for
    general testing. Most audio stages do not like having a square wave thrust
    up them ...

    Arfa
     
  5. Meat Plow

    Meat Plow Guest

    What model is the amp?
     
  6. Dr RaTsTaR

    Dr RaTsTaR Guest

    Thank you for the tips, Lord Valve.

    All measurements taken with the tone generator built from a circuit
    posted on www.geofex.com. Tone generator outputs 800 Hz at 6.1 mV AC.
    (Original circuit runs around 1 kHz at 10 mV AC. I had to substitute
    some resistors/caps.) Tone generator is connected directly to the
    input of the Peavey 260 power amp module, which seems to be a generic
    power unit for several different Peavey products.

    Trace measurements were taken with the DOS version of DAQARTA, a sound
    card oscilloscope program from www.daqarta.com.


    DC ON THE OUTPUT - NOISES

    Generator off, amplifier energized - Initially quiet, 0.1 mV DC on
    output.

    20 seconds -2.1 mV DC, low buzzing/humming noise begins, remains
    constant. No other noise present at any time.

    1 minute -6.4 mV
    2 minutes -13 mV
    5 minutes -19.7 mV
    10 minutes -24 mV
    30 minutes -25 mV

    30 minutes, generator on - -31 mV DC

    The DC present on the speaker output changes from around -25 mV at
    idle to +25 mV upon shutdown, at which time it rapidly declines, over
    10 to 20 seconds, to 0.



    EXCESSIVE AC CURRENT DRAW -

    It has not blown the 5 amp fuse that is standard for this unit.

    I did not check for hot caps. I will run the unit for 30 minutes,
    unplug, bleed down the caps, and then feel for unusual temperatures,
    just as an exercize.


    DEATH CAP -

    I see in the schematic and on the board that there is a .022 uF 600 V
    cap connected to the primary side of the power transformer input lead
    and to ground. A dpdt switch allows one to select which of the
    incoming power leads will connect to this cap. This sounds like the
    death cap mentioned by Mike Schway in the AGA General FAQ, item 70.
    The amp does have a 3-wire power supply cord factory connected and the
    green does go directly to ground. Should I remove this cap? There is
    also a .022 uF 125 VAC cap across the two power lines which I know is
    necessary.



    TRACES -

    I am including the traces in a zip file. I hope it is alright to post
    here. If the system won't allow me to post a binary I will be right
    back with a hyperlink to the file. Is a hyperlink the preferred way to
    do this?

    EXPANDED means that the time base displayed has been shortened from 25
    ms to 6 ms to show more detail in the individual waveform.

    TONE GENERATOR OUTPUT * what it is. The waveform is a little crumpled
    at the bottom. Still, the amplifier should be able to reproduce this
    without a problem. The wave is consistent, and that is what matters.

    MAIN OUTPUT - NO INPUT * shows the speaker out with no input. There
    is what appears to be a 60 cycle signal peaking between +20 mV and -20
    mV. Sharp spikes of released energy occur whenever the trace crosses
    the zero line.

    MAIN OUTPUT - SIGNAL * is the speaker output waveform with the input
    signal energized. The tops of the waveforms are rounded, and peak
    around 50 to 100 mV. The bottoms are narrow V-shaped spikes that
    extend down to -150 to -200 mV.

    COLLECTOR 6530 WITH INPUT * displays an 800 Hz waveform with a
    distinct 60 cycle spike. The mate to this transistor,

    COLLECTOR 6533 WITH INPUT * shows the same waveform minus the 60 Hz
    spike.

    431C BASE WITH INPUT EXPANDED * for whatever it is worth. Is this some
    kind of crossover point for the positive and negative cycles?



    A RECAP - (In more ways than one....)

    According to AGA, Technical FAQ, article 21, item 7, line 5, a Mr.
    Zenon Holtz advises us that, (regarding noisy amps and electrolytics)

    "Usually, in really old gear (over 25 years), just replace them at the
    first sign of problems - they have a finite life."

    There is a date stamped on the inside rear of the metal mounting plate
    that reads "Aug 18, 1977". It doesn't appear that the amp has ever
    been worked on.

    So I guess the first order of the day is to do the recap job. Should I
    replace the mylar and ceramic disc caps, too? I am a total newbie
    here, so any and all advice is welcome. I have the time to do it
    right. Is there any difference between cap makers? Are there any
    favorite suppliers?

    I have ordered a few parts from Peavey in the past, they are very fast
    and do not charge for shipping. Perhaps they would be the place.

    Do the 'scope traces show any signs of other trouble? Or do all the
    glitches relate to crappy cappys?

    All the diode junctions I could test in circuit seemed to show logical
    results, considering they are often paralleled with other components.
    However, when I take the board apart to do the caps I intend to check
    what components that I can.


    Thanks for any and all help,

    d0ct0r rAtstAr



    Apparently I can't post binaries to this newsgroup. Here is a link to
    the 'scope traces if anyone is interested.

    http://www.intergate.com/~gander/Peavey Standard.zip


    Doc RatS
     
  7. Dr RaTsTaR

    Dr RaTsTaR Guest


    How would you go about checking the 10 watt .33 ohm resistors? I am
    not sure my DMM will measure anything that low.

    I can check the diode PN junctions with the diode checker on the DMM.

    The distortion is not real big, but it is present. As per my latest
    post here, I believe the culprit is leaky old electrolytic capacitors,
    but it may not be the only trouble.

    Do you commonly unsolder different components to measure them? I am
    new to this game.

    Thanks for your help.

    Doc RatstaR
     
  8. Dr RaTsTaR

    Dr RaTsTaR Guest


    The generator is producing a sine wave at 800Hz, 6.1 mV AC. I am
    injecting it directly into the power amp input, which is where the
    preamp was plugged into.

    The Standard 130 watt power amp module is used in different Peavey
    products as a back end. In this configuration it uses a guitar amp
    front end. The two metal chassis plates are the front (preamp) and
    back (power amp) of a wooden box that contains them.

    This amp module is built with a PA front end with 4 inputs, or it can
    be built into it's own self-contained unit with a simple front end as
    a power booster, which is really just a power amp.

    Thanks for your interest,,
    Doc RatstaR
     
  9. Dr RaTsTaR

    Dr RaTsTaR Guest


    thanks for your interest. Please see my other posts in this thread.

    RatsDoc Startor
     
  10. Arfa Daily

    Arfa Daily Guest

    I've had a look at your results, and the first thing that I feel that I must
    say, is that the output waveform from your test circuit is simply not good
    enough to do any serious evaluations of what is going on. It really does
    need to be a sine wave. How can I put it ? What you are doing, is a bit like
    trying to find a steering fault on your car, with one of the tyres flat. You
    either need to have a proper signal generator, where you can properly
    control the output level and frequency, or use a second sound card with a
    signal generator program running. You can easily build yourself a generator
    using what's generically called a 'function generator' IC or a less
    sophisticated one using a phase shift oscillator.

    Looking at your results, it's very hard to say if what you are seeing is
    fundamentally 'wrong'. All amps will have a degree of residual hum, albeit
    usually very small. However, when you start looking at output stages with
    the vertical scale cranked up into the mV / cm range, the stuff on there can
    start to look horrendous. What you see in "Main output no input" is the
    residual hum, and at +/- 30mV or so, I would not have immediately declared
    this excessive. Some crossover distortion is apparent, but at such a low
    drive level from the hum, it may be perfectly natural for the output stage
    to produce such.

    It's hard to say exactly what we are seeing on "main out signal", because
    the output level is still extremely low, compared to what you would have
    here for even a modest 'in use' output level. On that point, I am surprised
    that you think that the input level for the power amp is likely to be 10mV.
    I can buy that for the maximum input level to the preamp, but the input to
    the power amp is likely to be volts. Do you have some documentation that
    suggests such a low input level ?

    As far as your output tests go, a drift in the DC conditions of a few mV
    either side of zero over a period of a half hour, is quite normal, and would
    not usually indicate that there is any problem in the output stage worth
    looking for. The drive waveforms for the bases of the outputs do look pretty
    poor, but actually look more like an overdrive condition than anything else.

    All in all, it's very hard to say what is going on with the results you have
    shown. First you need to get a proper sine wave with an adjustable output.
    You need to load the output stage, either with a proper impedance speaker
    (not very friendly to the neighbours) or a dummy load of some description.
    For the purposes of getting proper test results, power resistors stacked up
    in parallel to get to 8 ohms or whatever, will do. You then need to drive
    the output module to a point where it is delivering some power - that is
    *volts* of output across a load, not mV. If you don't have enough output
    from your generator to do this, then make use of the amp's guitar front end.
    Drive it with 5mV and 'scope the output of the preamp to make sure that you
    have got everything set such that it is delivering a clean, but large drive
    signal to the power amp. The amp's "master", being normally on the end of
    the final mixdown from the preamp, will then serve as your level control.

    Once you have a clean easily identifiable waveform going in (that's a sine
    wave ... ) and some watts being developed across a load, you will be in a
    position to make a realistic evaluation of what - if anything - is actually
    going wrong.

    As far as doing a blanket replacement of the electrolytics is concerned, I
    would strongly advise against that, if you are not an experienced engineer.
    I would agree that electrolytics are a source of problems in most electronic
    equipment, and that engineers working on this stuff will sometimes use the
    shotgun approach where these caps are in a known troublesome environment,
    such as a switch mode power supply, but with kit like this, you are far
    better off to actually find the fault, if any, rather than risk creating
    another which, with the best will in the world, is easily done, particularly
    by someone inexperienced. The slightest slip-up in a DC coupled output stage
    such as this, can be disatrous, resulting in the magic smoke escaping from
    many of the components ... If you really want to learn proper fault finding,
    it's not the way to go.

    I'm sure that there are others on here who have also been mending PA amps
    for many years, who would agree with most if not all of what I've said, so I
    hope you will take it as valid observations and advice.

    Arfa
     
  11. Meat Plow

    Meat Plow Guest

    Agree 100%. For a period of time I quit servicing all other pro audio gear
    except for PA gear. I had stacks of Soundcraftsman amps to the ceiling.
    Worked on Peavey CS series almost as much. Did weekly repairs on Crown,
    BGW so I am reasonably qualified to comment.
    I think what's happened here is the quality of testing and measurement on
    the OP's side isn't good enough to provide help. The OP needs to step up
    to the plate. These amps aren't rocket science and some fundamental
    troubleshooting steps much more often then not will reveal the problem(s)
    provided you have the skill and experience to perform them. All these
    dynamic tests the OP is doing will do little in the absence of a quality
    static workover with just a simple DVM.
     
  12. Meat Plow

    Meat Plow Guest

    I did and at the time I made this post all that was mentioned was a 130
    watt Peavey amp. I have since then figured it out on my own.
     
  13. bg

    bg Guest

    Dr RaTsTaR wrote in message ...
    The emitter resistors tend to open up completely or have high resistance. As
    long as you get continuity below 1 ohm you should be OK. I would desolder
    the power transistors which lifts one leg of the emitter resistors, seeing
    as you want to test the power transistors out of circuit anyway.
    Your DMM diode checker can be used to test the power transistors for opens
    and shorts which is a very common failure.
    bg
     
  14. Dr RaTsTaR

    Dr RaTsTaR Guest


    ***********

    Thank you Meat Plow for sharing your time and experience. I'm quite
    willing to admit I am inexperienced and appreciate any and all advice
    I can get. As you can see from today's posts I agree with you and Arfa
    totally.

    That is the problem. I don't have those skills. I am learning them
    now. Can you recommend some good books?

    I am interested to learn more about how to do a static workover with
    my DMM.

    If this amp (plays, but fuzzy and somewhat distorted) came to you
    would you listen to music through it or scope it or do a static to it
    first?

    I moved out here to the Oregon backwoods after working in construction
    for 35 years in Southern California, so I am out of touch with many
    resources as far as on-the-job training.

    There seem to be a number of these old amps here that need a little
    resurrecting. I enjoy this and may make some money and friends
    pursuing my hobby. I can take these amps home and work on them at my
    leisure, so it is a good deal for me.


    If I may ask, where and how did you learn your trade? If you had
    amplifier repair jobs stacked to the ceiling, I am guessing that you
    worked in a major city.

    What is OP? Output?

    Dr. RaTsTaR
     
  15. Dr RaTsTaR

    Dr RaTsTaR Guest


    Thank you for this information, BG. They test around .9 ohms, which is
    out of the 10% tolerance, but this reading may be due to my meter and
    lead connections. I will call it continuity below 1 ohm.

    The main power transistors read OK as measured across the diode
    setting on my Beckman DMM. I read 500 - 600 mV drop across the
    appropriate pn junctions. These are out-of-circuit readings. I plan on
    pulling and checking the smaller transistors today.

    The power amp can play music, but it sounds like an am radio station
    that is not quite tuned in to the center of the station. I did the
    chopstick test and it suddenly "tuned in" when I struck a certain
    transistor, but it relapsed a few minutes later and I haven't been
    able to do that again. It may have been coincidence.

    Someone recommended resoldering all connections of old circuit boards
    to relieve the accumulated effects of thermal stress. What do you
    think of this?

    Rats DocStar
     
  16. Dr RaTsTaR

    Dr RaTsTaR Guest

    Arfa,
    Thank you for your time. Your information is of great value to me.

    http://www.zen22142.zen.co.uk/Circuits/Testgear/testgear.htm

    Here is a circuit built around the 8038 that I plan on pursuing. In
    the meantime I will use a sound card generator. www.daqarta.com is the
    o'scope program I am using now. It has tone generator capabilities.

    My dime-store signal generator is a nifty little device in its own
    right, but I see that it has severe limitations.

    Now I know what residual hum looks like, and how much is acceptable.
    Very good.
    I am 55, retired steamfitter, piecing together a second income out of
    a fascination for electronics and a love of music and gadgets. I know
    the theory very well but I have no practical experience whatsoever,
    especially in the field of trouble-shooting. I am starting out with
    baling wire and bubble gum and a desire to do more, and operating on a
    shoe-string budget. I have a friend who knows people with broken music
    amps.

    I built the tone generator from the www.geofex.com page. I am using an
    old 8 ohm 50 watt speaker for a load. The generator outputs at 10 mV,
    100 mV, and 1 V. I substituted a few R values, and got less output,
    6.1 mV instead of 10 mV. This level (6.1 mV) patched directly into the
    power amplifier input produces an annoying sound level from the
    speaker.

    http://www.partsexpress.com/pe/pshowdetl.cfm?&Partnumber=019-030&scqty=1

    I plan on purchasing an 8 ohm 200 watt dummy speaker load from
    partsexpress.com so I can "pump it up" and see what it be like. I will
    use my sound card generator/scope to check it out then. I would like
    to be able to tap off this load to power a speaker at a low volume
    level so I could hear what is going on, as well as see it.

    I'm sure you will agree that starting out with 10 mV for an input is
    better than using 10 V. Bear in mind that I am shooting in the dark.
    Now I know that I can safely use several volts of input on the power
    amp.

    It is one thing to take college classes in theory and another to have
    hands-on experience. We did lots of exercises but they were all laid
    out for us. I finished all the courses the local junior college had,
    in 1995. Most of our training was in digital work, with emphasis on
    the 8086 IBM PC. I built a few neat toys and used the serial port and
    assembly language to operate them. I would love to tinker with
    robotics more. It is not possible to make a home business out of
    fixing robots, not yet anyway.

    I would like to apprentice to an experienced tech. I could learn much
    in a very short while. But that is not an option for me at this point.






    Does it seem strange that there is a (possible) overdrive condition
    when the input is so low? Is there something to be learned there? I
    understand that the input signal is too low to make much of any of
    this, except for that I need better equipment.

    I'm puzzled why they are around 3 volts pk-pk when the output of the
    whole circuit is in the millivolt range, but I don't understand the
    system as a whole yet. I hope to find the time to build these amps in
    LTSpice, so I can observe the interaction of the various parts in a
    simulation.



    Very good idea to use the preamp (as a preamp.) I will make use of
    that if I can't get a decent output from my sound card or new 8038
    unit that I might build. The preamp needs a more critical look as it
    is, although I know the power amp is acting bad.

    Music injected directly into the power amp input sounds distorted and
    fuzzy, like an AM station that is not tuned on center. I have done
    this with other units so I am reasonably sure my source is ok. I got
    it to come clean for a very short while by doing the chopstick drum
    solo on the different parts, but it relapsed and I haven't been able
    to do that again. It was a cool drum solo, though.


    I do want to learn proper fault finding. Can you suggest any books,
    particularly as relates to music amplifiers? I have been reading the
    faqs for the various electronics newsgroups, including this one.


    Would an experienced engineer consider a blanket replacement,
    especially if the equipment is over 25 years old? I know that
    electrolytics can dry out over time, but have also been told that
    there are many variables in that. From what I have read, the
    replacement may not be as good as the one replaced, in many instances.
    If someone brought this amp in, would you do a listening test, input
    and scope, or start measuring with your DMM?


    I was able to find 3 shorted transistors in a fuse-eating Peavey by
    using the series light bulb trick and comparing voltages between
    channel 1 and 2. Measuring the diode barriers also uncovered the bad
    units. I repaired a few other units by replacing filament fuses and
    stroking sticky input and effects return jacks. So I am having a
    little bit of success with this. But I still have a lot to learn.


    Do you have any recommendations on procedures and test gear for
    evaluating capacitors?


    I thank you very much for sharing your time and experience with me.
    Your observations and advice are very valuable to me. I know more than
    I did before.

    For whatever it is worth, I have posted this question on
    alt.guitar.amps. Lord Valve has made the very same points that you
    have made.

    Dr. RaTsTaR
     
  17. Arfa Daily

    Arfa Daily Guest

    Arfa,
    Yes, that looks fine. Put it in a nice little box, and it will serve you
    well in your endeavours.

    OK. Understood on your skill levels.
    That item looks as though it will do the job, and seems a very fair price. I
    would, however, treat its quoted rated power handling of 200 watts, with a
    little caution. It is likely to run *very* hot at that sort of level. If you
    intend using it for high power tests as an ongoing item of test equipment,
    you might consider mounting it in an additional box, with a fan blowing
    across it. It is easy to 'tap off' for a small speaker. I have a dummy load
    box which I built, and into which I incorporated a crude power meter. I also
    fitted it with a small 32 ohm speaker, which I connected straight across the
    input to the dummy load, via a fixed resistor, and a pot hooked as a
    variable resistor. The series resistor was chosen to give, when added to the
    variable resistor set to maximum resistance, a maximum safe drive to the
    speaker when the driving amp is delivering a lot of power. On lower drive
    levels, the pot can be turned down in value, to increase the drive to the
    little speaker. That way, I can get a reasonable listening level from the
    little monitor speaker, over a range of a few watts, to a hundred or so. I
    also have a BNC socket connected straight across the input to the dummy, so
    that I can make an easy connection to the 'scope, which doesn't involve
    holding a probe in place. Whilst we're talking probes, I wonder what you are
    using to connect to your sound card ? A proper scope and probe presents an
    input impedance of at least 1 meg, with a very low capacitance. This may not
    be the case with your sound card "scope" and cable, and this could result in
    additional measurement errors if the impedance is low.
    Yes, agreed. Always sensible to start at a low level, as long as you know
    what sort of typical level you are aiming for at various points in the
    circuit. That will come with increasing experience. When you have it all
    working correctly, make your own notes of expected levels and voltages on
    the schematic, but remember to state what input level you were using, and
    how the front panel controls were set. Such notes can be very useful for the
    future. Also, when you have located the fault, it's always useful to either
    note on the schematic what the symptoms and problem was, or keep a notebook
    of makes and models and problems.
    I was originally apprenticed in the TV repair business some 35 years ago,
    and have been self employed for probably 17 years or so now.


    It's hard to comment given the limitations of the test results with the
    equipment used so far. The only reason that I said that it *looked* like an
    overdrive condition is that what is appearing is virtually a square wave.
    Even given the poor shape of the input waveform, the fact that it seems to
    have been turned into a square wave would indicate that the output from the
    stage previous to where you are making the measurement, is 'hitting the
    rails', which is odd, as you surmise, given the low drive level. There may
    be something to be learned in that there could be something amiss with the
    bias around the preceding stage, but again, it's hard to say because with DC
    coupled amps, a leaky cap in a preamp, can blow an output transistor or two,
    four stages further down the line.
    That might suggest that there is a bad joint somewhere on the board. These
    are not always visible to the naked (or sometimes inexperienced) eye. It is
    also possible for a component to have an internal bad connection. I have
    known this to happen with semiconductors where the internal connections are
    very fragile little wires that are sonically welded to the die.
    I do not really keep up with what is available from a tuition-text point of
    view, so am not able to really offer any advice or opinions on that one. I'm
    sure that there will be others reading this, who may be able to offer you
    advice on that score. Have you tried Googling to see what comes up ?

    Most experienced engineers that I know, would not just start replacing
    components willy-nilly, in the fond hope that it will result in a cure, at
    the same time as improving performance. Apart from anything else, if you are
    doing this professionally, there are commercial considerations in terms of
    time spent and cost of parts. Also, as I said before, the 'shotgun' approach
    potentially paves the way for causing additional problems. It really is easy
    to accidentally put a cap in backwards, or get a tiny whisker of solder
    across two pads, or have some old print come away from the substrate as you
    desolder, and so on. If you spot the problem as you cause it, that's not too
    bad. It's when you don't spot it at the time, and then wind up with a
    completely new set of symptoms at best, and loss of the magic smoke at worst
    .... Trust me, I've been at this for 35 years, and I don't know of any
    professional service engineers who don't have 'accidents' happen to them.
    Any who tell you they don't, aren't to be trusted ...!!

    Again, as I said, there are places where a blanket replacement of caps is
    justified, and is the preferred 'fix-method'.

    On the other hand, a blanket re-sweat of all the joints in an area,
    introducing a small quantity of new solder at each joint, is a valid repair
    method that is much used by professionals, to fix an invisible and otherwise
    un-locatable bad solder joint. With a little practice, this can be done very
    quickly, but watch carefully for solder running between joints, and forming
    a bridge.

    If this amp turned up on my bench, I would start by putting music through it
    at a reasonable level and just having a listen. Experience would likely give
    a good idea of what might be going wrong. I would then inject a sine wave of
    about 800Hz to 1kHz, and 'scope the input to the power amp, just to check
    that what was going in was still a good sine wave, bearing in mind that most
    guitar amps are designed to be able to 'go dirty' with appropriate gain
    settings. Having established that the power amp was being driven with a good
    sine wave, I would next connect the 'scope across the output load, and have
    a good look at the shape of the wave coming out. This would then give me a
    good idea of what was going wrong. For instance, a *highly* asymmetric
    output waveform would likely indicate something wrong with the drive to one
    of the output transistors. this would almost certainly result in a no-signal
    DC offset as well. Crossover distortion would indicate a problem in the bias
    network for the output transistors. A partially asymmetric waveform would
    likely be down to some kind of problem with the bias of the driver stage, or
    possibly the pre-driver (assuming that the supply rails are about even). A
    badly clipped signal may well indicate something going wrong back at the
    front end of the power amp. Possibly a problem with a feedback network.
    These are all just generalisations, you understand. It's hard to teach this.
    Over the years, you just develop a 'feel' for it.

    Having got an idea of what was going wrong, I would then go about locating
    the exact stage where the problem was, by moving the 'scope probing-point,
    to locate the last place that the signal was good, and the first place that
    it was bad. Once I found which stage that was, I would switch to a
    voltmeter, and eyeballing the schematic, to see if anything looked wrong
    with the voltages around the stage. If there are no voltages marked on the
    schematic, then it comes down to a degree of experience to know what to
    expect. Usually, once you have found a wrong voltage, it will be fairly
    obvious what is (probably) causing it, at which point, you switch to
    'resistance' on your meter, and check the suspect component(s).
    For anyone doing a lot of service work, the single most useful tool for
    evaluating electrolytic capacitors, is the ESR meter. Many on here swear by
    the Dick Smith unit designed by Bob Parker, who pops up on here from time to
    time. I have used one just about daily for some years now, and it has paid
    for itself many times over. It is available for a very reasonable price both
    in kit form, and ready built. Like all ESR meters, the results that it gives
    you require a degree of interpretation, but the fact that it has a digital
    readout, and a table of expected values on the front panel, makes this a
    little more intuitive with this meter, than with some others that I've seen.
    A basic capacitance meter is useful for evaluating lower value
    non-electrolytic caps. Many digital multimeters have a simple capacitance
    meter built in. They don't really tell you a lot about electrolytics,
    though. On many occasions, I have had electros that read near as damn it
    correct for value, but have an ESR reading that's out the window and away
    with the fairies ...

    I think somewhere, you asked whether "OP" was "output". It can be, but also,
    on usenet, it often refers to the "Original Poster" or the person who
    started the thread with their question.

    No problem. It's nice to have someone these days who's interested in getting
    into the service business. Most who are still hanging in there, tend to
    advise against it, unless you have masochistic tendencies ! I have never had
    any direct dealings with 'Lord Valve', but have seen his replies and posts
    from time to time, and he seems a suitably skilled and experienced engineer,
    whose advice you could bank.

    Arfa
     
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