Just in. A combo, the job ticket on which says "Low volume. Sounds like shit."
There's hope for the lads behind the counter, yet. How nice to get one with a perfect owner's description of the problem, rather than some wild guess that the person who's booked it in has made, based on what the owner said. I can just see some hairy guitarist lugging it in and saying to whoever booked it in "Go on lad ! Just write it down, Low volume. Sounds like shit !"
:-)
Arfa
LOL!
So technically, it's probably fooked
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**I like it! Much better than when the customer thinks they can diagnose the fault for you.
-- Trevor Wilson www.rageaudio.com.au
It's probably just a wire off, needs welding...
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Ah, funny you should mention welding, Ron. Apart from my new Chinese hot air rework station purporting to be for electronic solder welding ...
I put that amp on the bench this morning, and indeed, it did have low volume, and it did sound like shit - but not all of the time ... If you plugged a guitar in it, and gave it a good 'clang', it produced a horrible grating distortion, almost as though something was arcing, and how right that turned out to be ! Plugging a generator into it, and with a reasonable set of front panel settings on the amp, you could wind the generator level up, and all of a sudden, the output would go from a nice clean sine wave, to something resembling a distorted square wave, with 'sparky-looking' interference on the top.
Nothing was shifting - HT was clean, bias was clean, outputs from the phase splitter were clean, and drive right on the grids of the O/P tubes was clean, but both anodes (plates) were 'dirty'. I was beginning to suspect that the O/P Tx was breaking down when there was a large peak - peak drive across it, but thought that I'd better try a replacement pair of 6L6's in it, just in case. No change. I managed to get the generator set to a level where it was consistently producing the bad output, and was just about to carry out some more 'scope checks, when something caught my eye. It was a little firestorm going on around one of the output jacks, which on this amp are in the bottom of the chassis pointing down, and are open frame types. The one for an extension speaker has an open contact set on it. Where the contact 'loops' out of the front of the socket, it is quite close to the chassis, and there, wedged between the contact and the chassis, was a screw, merrily arcing away. I shut off and pried the screw out with a screwdriver tip, and then, stupidly, picked it up and burnt my fingers :-(
The screw was a black plated type, so I guess not a brilliant conductor - not enough to actually put a full short across the output, anyway. However, once there was a high enough p-p output voltage across there, the plating was breaking down enough to cause the sparks, and upset the output stage. The screw was actually one of the ones that secures the main board to the chassis. Given that these combos are built as portable gigging items, I' m surprised that a company like Fender, doesn't put a paint seal, or a bit of thread-lock on these screws. Still, they've all got a liberal coat of nail polish on them now, so hopefully should prevent a similar occurrence in the future.
Nice to get an 'odd' fault that can actually be pinned to a definite problem, with a definite fix, for a change ...
Arfa
So I was almost right, it wasn't fooked, it was screwed!
Ron
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Haha, it's only a matter of semantics.
Good story though.
tm
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Or not ... :-)
Arfa
Still, nice to see a bit of kit that, even with almost a direct short on, didn't blow up the o/p stage or get otherwise BER.
-B
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