8008 Valvestate Power Amp Help with fault diagnosis

Discussion in 'The Workbench' started by microkid, Nov 20, 2019.

  1. microkid

    microkid New Member

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    Help! My trusty 8008 has failed! I think this was because the flight case it was in was loaded upside down into the van and it has rattled about a bit causing something to break...

    When it is powered on one of two things happen: (1) there is a loudish hum from both channels regardless of where the level controls are set. OR (2) there is a deep thud on power up, after which the hum returns as (1).

    If the unit is tapped it crackles and on one occasion the hum stopped but the process of plugging in a cable caused it to start humming again.

    I have previously repaired this unit by replacing all of the output transistors this was done circa 2004. However I do not think this issue is related. My thinking is as the hum affects both channels and is not affected by the levels controls, it must be a fault that is common to both amp stages possibly a fault on the power supply, maybe the rectification is wonky or a smoothing cap has failed.

    Apprecaite any thoughts from anyone who has experice of these amps!

    Not sure if I am being dumb but looking at the schematic (that I have) it does not show where the TL072 op amp at the input is deriving it's power, I'm assuming this is also on the +/- 15V rails but prefer not to assume!

    I will try and get it open today and have a look, but I do not have a test speaker in my workshop to test it so will have to test with a scope and signal gen.
     
  2. SkyMonkey

    SkyMonkey Well-Known Member

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    The manual for the 8008 states that you should not connect a load of less than 8 Ohm to the speaker jacks. So just connecting to an oscilloscope may not be enough!
     
  3. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    Assuming that you are a qualified electronics Tech then you should be able to sort this one out but if not then do your self a favor and get it to someone qualified to do so, after all it is solid state and unlike tube amps you don`t get second chances.
     
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  4. microkid

    microkid New Member

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    Ah, found the TL072 power supply on the drawing! It was blurry so couldnt make it out, I thought it was a voltage reg the way it was drawn! Anyway stripped it down I cant find anything obvious wrong! Need to dig a bit deeper!
     
  5. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    Put a load on the output, insert a signal from an audio generator to the input and follow its path with your scope.

    Don`t initially go to deep as all to often its staring you in the face, other wise you will be going round in circles.

    This is assuming you have done all your cold checks and DC checks as well
     
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  6. microkid

    microkid New Member

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    All done and dusted. As i initially suspected it was a PSU issue, to be precise one of the large smoothing caps had a bad joint. Didn’t see this initially as it was working fine but after tapping around the board while monitoring output I found it. Resoldered both caps and a few other suspect looking joints seems fine now. I will probably replace the caps. I didn’t have any stock that would do the job will see what farnell have tomorrow but at least I can gig it again.
     
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  7. guzzis3

    guzzis3 Active Member

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    I just saw this sorry. I guess it's easy to be clever in hindsight but when I read your initial post caps were the first thing I thought of. Also it was likely to be power supply as it affected both channels.

    I love mine, 8008s are nice things for what they are built for. I have to say though I cannot tell any difference between linear and valvestate. :D
     

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