Laney vc15 problems again!

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suspiciousmindssuspiciousminds Frets: 383
edited June 2017 in Amps
So, a couple of years back my vc15 suddenly packed up - had it serviced and it turned out the main transformer was faulty so it was replaced and the amp worked fine, except for a popping sound when i switch it off which i've lived with.
Plugged it in today and there was a lot of noise very similar to what you hear when trying to tune a radio station in - lots of static/white noise although you could hear the guitar in this noise but distorted on clean.
The led is no longer working on the "drive" button and the only knob (other than the master vol) which seems to control any sound is the reverb knob, so turn the reverb knob right down and it cuts the noise almost as if its acting as a volume knob now!
I've checked each one of the valves, swapping them out one by one for my known working spare valves (it was revalved when the transformer was changed too) but thats made no difference whatsoever!
Any ideas guys?
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Comments

  • steamabacussteamabacus Frets: 1258
    Try dribbling some switch cleaner down into the drive channel button and repeatedly pressing it, repeating a couple of times. This seems to be a common cause of problems.
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  • ICBMICBM Frets: 71959
    You could try cleaning the channel switch - it won't do any harm - but it doesn't sound likely to be the problem here.

    It sounds like it might be a low-voltage power supply problem or a faulty IC (+15V operates the switching, +/- 15V supplies the ICs for the FX loop and reverb). I think a failure in the +15V rail would probably give you something like you're getting.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • DJH83004DJH83004 Frets: 196
    I may be wrong on this one IC but I think the VC 15 derives it +/- 15volt rails from a pair of zeners, and its bigger brother the VC 30 uses regulators, in any event, definitely the right call, is the mains on LED lighting up, as that is derived from the same supply?   
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  • ICBMICBM Frets: 71959
    DJH83004 said:
    I may be wrong on this one IC but I think the VC 15 derives it +/- 15volt rails from a pair of zeners, and its bigger brother the VC 30 uses regulators, in any event, definitely the right call, is the mains on LED lighting up, as that is derived from the same supply?   
    Zeners, from the rather blurry schematic I've got. It looks like the power LED is connected between the + and -15V rails - a blown drop resistor would stop the LED working, but a short in the Zener, the cap or one of the ICs which is grounding the +15V rail would leave it working, just a little dimmer.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • DJH83004 said:
    I may be wrong on this one IC but I think the VC 15 derives it +/- 15volt rails from a pair of zeners, and its bigger brother the VC 30 uses regulators, in any event, definitely the right call, is the mains on LED lighting up, as that is derived from the same supply?   
    Yes the mains led lights up, but the drive led has stopped, strange though as i havent been using drive for some time, just effects.
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  • DJH83004DJH83004 Frets: 196
    I do think it is a tech job, I'm afraid, not valve related,you have already tried that, and to be honest they have no influence on the drive switching, it is almost certainly power supply related, which a tech would sort out for you within an hour or so.  
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  • I'm going to have a look inside to see if theres anything visibly damaged, i havent switched it on for 3 days, so is it safe to do it or are there likely to be some nasty voltages still in there?
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  • ICBMICBM Frets: 71959
    Should be safe. If I'm right about what's wrong there will be nothing to see though - a shorted diode, cap or IC looks perfectly normal. This is a pain when trying to diagnose these faults... since the suspect components are all in parallel, you can't tell which it is without removing one (or at least unsoldering one pin) and testing.

    You may be able to see a heat discolouration on one of the two drop resistors, but that would only confirm which of the two rails is affected.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • Well, had a look tday and as you've said i cant see anything visibly burnt out/scorch marks etc.
    Turned it on again and its still doing the same - not sure if its because i disturbed any dust in there but it did have a smell as if it was overheating electrically too.
    Off to a tech i guess.
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  • Anyone know of any amp repairers around shropshire?
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  • DJH83004DJH83004 Frets: 196
    I'm sure you will find someone in say the Telford area who could look at it.But if you are really stuck I am based in Nottinghamshire and will happily sort it for you, but it is a bit of a treck.  
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