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Customer is always right ..

Discussion in 'Electronic Repair' started by Phil Allison, Sep 22, 2012.

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  1. Phil Allison

    Phil Allison Guest

    ** Had an early 1960s guitar amp in this week, a "Maestro Viscount" made by
    Gibson, aka the GA-16T. All the valves were new, JJ types: 2 x ECC803, 2x
    6V6S and a 5Y3.

    The accompanying note contained no fault report and asked only for two
    modest modifications:

    1. Replace and rewire speaker wire with heavier wire.

    2. Rewire to shorten power feed cable from tranny.

    The speaker wires were soldered at both ends and just long enough to let the
    chassis rest on a bench. The AC cable referred to was internal and went from
    the chassis to a 230V to 115 V step-down fitted in the factory 50 years ago.
    The two wire cable had been wound neatly like a rope and rested on the
    bottom of the case next to the step-down.

    The 240V lead was modern, 3 core and there was a wire linking the frame of
    the step-down to the chassis.

    I connected the amp to my Variac and gradually powered it up to about
    00V - just as I expected it let out an almighty hum, mostly 50Hz, through
    the speaker. The hum mostly disappeared if the volume pot was turned down.

    Of course bad electros were the cause of the trouble, but the situation was
    not so simple since as someone had been there before me, decades ago and
    substituted two pigtail electros for ones inside a triple electro that was
    mounted off the chassis on a clamp.

    It soon became clear that the third electro in the triple had now expired -
    but that should cause only 100Hz hum, not 50Hz.

    Then came the *shocker*, one of the pigtail electros was grounded to the AC
    heater supply instead of the chassis - this imposed 3.15 volts AC at 50Hz on
    a DC rail ( the screen supply) that should have had only a trace of 100Hz
    ripple.

    Removing the dead triple electro, adding a new pigtail one and rewiring soon
    put things straight leaving only a minor amount of 100Hz buzz at full
    volume.

    There was one pigtail electro hanging onto the 5Y3 socket, grounded to a lug
    on the frame. Soon as I shifted the ground point to the common chassis
    ground, even that hum vanished.

    With the amp now working nicely, I modified the speaker and internal AC
    leads as requested in the customer's note.

    Apparently the customer believed the AC lead tied like a rope was causing
    the hum.

    Oh dear..........


    .... Phil
     
  2. It was still an honest repair, so everyone wins. I'm sure it took less
    time to swap the wires than to explain it was something else.

    I worked at a shop once and we used random 1/2 watt carbon resitors
    soldered to a couple feet of wire as an antenna for testing stuff- just
    jam it into a F connector and you're done. The resistor body was at most a
    handle, and not even in series with the whip.

    sure enough, people were fascinated by these as the benches were visible
    to customers over the counter, and wanted to buy them. No amount of
    explanation or laughter would discourage them.

    So yes, if they were persistent, we'd make them their own for a couple
    bucks.
     
  3. Phil Allison

    Phil Allison Guest

    "Cydrome Leader"

    ** Sure - I did what the note asked to avoid an argument and since doing it
    caused no harm, why not ?

    The main point of my post was the grossly incompetent previous repair that I
    found.

    Can you figure out how the amp ever worked OK afterwards ?



    .... Phil
     
  4. tm

    tm Guest

    Sure. They got tired of the loud hum and turned it off before any damage
    occured. Then someone competent repaired it.
     
  5. Phil Allison

    Phil Allison Guest

    "tm"

    ** No way.

    The previous repair was done ( almost certainly by a TV tech) in about 1980
    and the amp was usable afterwards.



    .... Phil
     
  6. Pimpom

    Pimpom Guest

    Saw this post just now and it reminds me of an incident that
    occurred more than 20 years ago. A repair tech friend had grown
    rather tired of listening to customers telling him what needed to
    be done with their gadgets. Someone brought in a VCR and asked to
    have its R/P head replaced. The friend asked about the symptoms
    and immediately deduced that the culprit was not the R/P head,
    but refrained from making any comment. When the customer came to
    collect the VCR later that day, the conversation went like this
    (as related to me by another friend who happened to be in the
    shop):

    Did you replace the head?
    Yep.
    Is it OK now?
    Nope.
    Why?
    Because that's not the problem.
    Then why did you replace the head?
    Because you told me to.
     
  7. Ha.

    anybody know what those places that sold replacement heads (nisshoku?)are
    doing these days if any survived?

    marketing iphone batteries?
     
  8. Guest

    anybody know what those places that sold replacement heads (nisshoku?)are doing these days if any survived? marketing iphone batteries?

    Many of those companies also supply optics and other assemblies. The market for commercial video heads is still strong. Many studios etc still use tape for the master copy, especially for archiving. 3/4" tape is still very popular.

    Dan
     
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