Solid State Question

[unclej]

one of the kids that works for me on saturdays brought me a peavy tnt-130 to look at for him. when you turn it on it makes a continuous loud honking noise..kinda like a brontosaurus that just ate a bad batch of swamp cabbage..

anyway, none of the control knobs has any effect on it..with everything in the down position it still does it.

i tested everything that i recognized..diodes, resistors and caps and replaced one cracked diode but that didn't fix it. i don't have a tansitor testor and am not terribly interested in learning how to test them anyway. i tried another speaker and it honks too.

what's your best guess as to a possible cause for said honking or is this enough info?

[crafty]

If all the components checked out okay then check for shorted transformers and the grounding points. The people at Peavey are usually pretty good about getting schematics and parts out to you, too, so don't hesitate to give 'em a call. They've helped me rebuild my little SS Peavey a few times.

[unclej]

If all the components checked out okay then check for shorted transformers and the grounding points. The people at Peavey are usually pretty good about getting schematics and parts out to you, too, so don't hesitate to give 'em a call. They've helped me rebuild my little SS Peavey a few times.

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i did order a schematic but i didn't think to call their tech people and i didn't think about the transformer so i'll check that tomorrow..thanks.

[Southpa]

Could you describe this "honking" as being a low frequency oscillation aka "motorboating" ie. sounds like an outboard boat motor? The most common cause of oscillation is an open or leaky filter cap somewhere. Another cause is improperly wired output transformer, reverse the speaker wires and see what happens.

[lovekraft]

The most common cause of oscillation is an open or leaky filter cap somewhere.

That'd be my first guess, too - a quick look for a bulging or deformed electrolytic might yield results. BTW, be really careful when desoldering anything from a Peavey pc board - it's incredibly easy to lift the copper pads & tracks from the board if you get them a little bit too hot (don't ask me how I found that out ).

[unclej]

Could you describe this "honking" as being a low frequency oscillation aka "motorboating" ie. sounds like an outboard boat motor?  The most common cause of oscillation is an open or leaky filter cap somewhere.  Another cause is improperly wired output transformer, reverse the speaker wires and see what happens.

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when i think of oscillation i think of a sound getting loud and then softer so i wouldn't call it that although that may be what it is. it's a constant, loud, low frequency sound and yes, i suppose it could be compared to the sound of an outboard motor. i did reverse the speaker wires but that didn't change it.

i'll go back and take another look at the caps..i didn't notice anything obvious like bulging but a couple of them are under globs of silicon so i'll check them again.

thanks for the soldering tip lk. with my new, handy-dandy spring loaded solder sucker i don't have to stay on a solder very long but i'll be extra careful.

[Southpa]

Be extra careful when poking around those caps too! But you already know that, right?

[unclej]

Be extra careful when poking around those caps too!  But you already know that, right?

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yes, i'm careful pokin' and now i'm really careful which way i put 'em in. (little blowin' up and flamin' schrapnel shootin' sons of guns)

[Saber]

In case you didn't think of this, one way to test for open filter caps is to hold a new capacitor of equal (or even a bit greater) value on underside of the pc board on the pins of the suspect capacitor (connected in parallel with it IOW) and see if it makes the oscillations stop. And if some of those caps are near a heat sink or other hot object, I would start with those since they're most likely to be the first to dry out.

Edited August 18, 2005 by Saber

[lovekraft]

That's an excellent idea, Saber - I'm terrified of tube power supply caps (with good reason), and you'd still need to be careful, but 40 or 50 volts in a solid state amp isn't near as scary as 5 or 6 times that much.

If you've got a 'scope, you could start at the output and work your way back to the source of the honk - an audio probe would probably work too, but you might blow the speaker, since you're starting in the power amp at really high levels. It might not make the problem obvious, but at least you'd know where to start looking.