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Thread: Requesting guidance re: Laney GC-30V and oscilloscope

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    Requesting guidance re: Laney GC-30V and oscilloscope

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    Last edited by minim; 02-06-2019 at 07:05 PM.

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    Fx loop?
    If so and youve got another amp, then is the issue before the loop (preamp) or after the loop (power amp)?
    You’ll need a schematic and ideally a pcb overlay, when you find them please provide links.
    To bench test an amp you’ll also need a suitable dummy load. Sig gen requirements can be covered using a smartphone app.

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    Last edited by minim; 02-06-2019 at 07:03 PM.

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by minim View Post
    Sorry, I should have elaborated. One of the first things I checked was the effects loop. I buffed the contacts with a swab and De-oxit. They look brand new. I also ran my guitar directly into the effects return and still had the fading out, together with a light distortion of the sound consistent with a bad connection/dry joint type of thing.

    Naturally I began to inspect everything further on from there. However among other things, the PI and effects loop are made up of the aforementioned SMDs and I'd rather test things if I can prior to attempting to re-flow those joints. I re-tensioned the preamp sockets and attempted to do the same to the power tube sockets, I just don't have anything small enough yet to get into those ceramic sockets.

    Thanks for the schematic!

    I overlooked the dummy load. I'll build one and sort out a signal gen, any feedback on the oscilloscope I mentioned would be greatly appreciated.
    I looked at one of the adds on the scope, and, it's typical of the many USB 2-channel scopes on the market. I have a different USB scope, purchased for a specific function, and isn't used for normal bench work, having a fine lab-grade instrument that's worked for decades now. This OWON scope would do fine, though others may have a different suggestion.

    In looking at the Laney circuit, and you having found already the problem is in the post-Return Jack circuit, which is just one stage ahead of the phase inverter and output stage, you could try swapping out the ECC83/12AX7 (V3) to see if that changes anything. The output stage, being (4) EL84's, cathode-biased, and pulling 160mA thru the four tubes nominally, no simple way to test individual tubes without a bias probe. Pulling two tubes out would run that same current thru the remaining tubes, which with four, they're already dissipating 11W if all are equally sharing current. Swapping the four outputs with a new matched quad set would tell you if it's the power tubes. If the cathode bypass cap were going bad, starting to short, that would cause more current to flow in the power tubes. Ever notice any of the power tubes glowing really bright (red-plating) on the metal cage inside the glass? That's a good sign of that cap failing.

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    Senior Member Pedro Vecino's Avatar
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    Using my super-secret lighting treatment It gives me the feeling that this weld could have a slight crack.
    It looks like it's a bottom-connected wire to R52 - 47K (it could be the PI input).

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    Quote Originally Posted by nevetslab View Post
    I looked at one of the adds on the scope, and, it's typical of the many USB 2-channel scopes on the market. I have a different USB scope, purchased for a specific function, and isn't used for normal bench work, having a fine lab-grade instrument that's worked for decades now. This OWON scope would do fine, though others may have a different suggestion.

    In looking at the Laney circuit, and you having found already the problem is in the post-Return Jack circuit, which is just one stage ahead of the phase inverter and output stage, you could try swapping out the ECC83/12AX7 (V3) to see if that changes anything. The output stage, being (4) EL84's, cathode-biased, and pulling 160mA thru the four tubes nominally, no simple way to test individual tubes without a bias probe. Pulling two tubes out would run that same current thru the remaining tubes, which with four, they're already dissipating 11W if all are equally sharing current. Swapping the four outputs with a new matched quad set would tell you if it's the power tubes. If the cathode bypass cap were going bad, starting to short, that would cause more current to flow in the power tubes. Ever notice any of the power tubes glowing really bright (red-plating) on the metal cage inside the glass? That's a good sign of that cap failing.
    I will look into that, thank you.

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by Pedro Vecino View Post
    Using my super-secret lighting treatment It gives me the feeling that this weld could have a slight crack.
    It looks like it's a bottom-connected wire to R52 - 47K (it could be the PI input).

    Click image for larger version. 

Name:	Laney.JPG 
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ID:	52387
    Ah....good eyeballs! Age-old solder joint fracture. What I spend 90% of my time repairing!

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