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another acoustic 370

Started by ilyaa, July 30, 2014, 03:03:22 AM

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ilyaa

another one of these -

complaint was "farty/buzzy" - i tried it out and heard no such thing, but it shocked the hell out of me so i figured i may as well take care of that and see what else might be the matter in the process.

odd thing was, it had a three prong cord, but jokes on me for thinking that meant it was actually grounded in a safe manner: someone had just put a three prong adaptor onto a two-lead power cord (leaving the earth lead open!). what a dangerously strange move - ill take care of that.

otherwise, visual inspection just yielded one thing: L301 (the inductor on the output) looks a bit charred - there is a burnt looking black spot on the windings....bad?? any way to measure it? what does that inductor and its parallel resistor (R328) do, anyway?

Enzo

it would be unlikely that inductor was burnt, though anything is possible.   To get enough current through it to burn it would more likely take out the main fuse.   The main way coils like that fail is to break free at one end.  That leaves the resistor in series with the load, and the resistor may burn.  Usually not though.

Roly

Quote from: ilyaaJokes on me for thinking that meant it was actually grounded in a safe manner



Glad you are still with us.    :dbtu:  Take care out there in amp land.


1) your assumptions will get you every time.

2) make it a habit; every time an item of gear comes in for service a) check that the plug wiring is both secure and correct (if it's not a molded plug of course), b) run you hand and eye over the mains lead for damage, crush, melt, etc; c) use your Megger or the continuity tester in your DMM to confirm continuity between the earth pin and the chassis.

EVERY TIME!  And faults found here are rectified first.


Quote from: ilyaalooks a bit charred - there is a burnt looking black spot on the windings....bad?? any way to measure it? what does that inductor and its parallel resistor (R328) do, anyway?

It's the "stability" inductor that is supposed to isolate the amp when it is driving a load that is capacitive where there is a risk that the output stage might oscillate.  The resistor it is wound on is both a coil former and damper to prevent self-resonance with its own stray capacitance.

Cooked can happen with time, or a one-time overload situation.

Its generally made out of fairly thick wire, and as long as it has continuity it should be okay.

Watch the supply rail in this one - higher than normal, may bite.

Farty/buzzy may be helped by grounding, but if you can't find a cause the output coupling cap C404 may be worth a diagnostic look, cap and ESR measurement, it does have a pretty hard life.

Classic amp.
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

QuoteThat leaves the resistor in series with the load, and the resistor may burn.

you mean R328? it looks and tests fine -

QuoteThe resistor it is wound on is both a coil former and damper to prevent self-resonance with its own stray capacitance.

yeah this inductor is like copper wire wrapped around a resistor-looking thing. when i measure the leads of that i get continuity, around 0.5 ohms - the DC resistance of an inductor is supposed to be very low, right? in which case i dont know how to explain the black/charred spot but ill assume its okay for now -

ill replace the cable and see what's up -

Roly

0.5 ohms will be mostly strays in your meter leads, plugs, &c, the thing itself is only about a foot of wire.  They generally don't die unless a failed transistor has dumped the entire supply into a short for a while, someone accidentally poked the mains up the output (I've actually seen that happen!  :duh ) - or something equally dramatic.   Or corrosion.

But if it tests okay then it's back to your pre-flight checklist.
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

heres where im at with this one:

i cleaned it and played it and, like i had noticed before, the amp plays and sounds fine. no farty/buzzy. but of course i cant discount the intermittent issue....

if i tap around with a drumstick i can get the amp to make the kind of crackly/pop sound that these kind of amps sometimes make when they are dirty or have a bad joint. i noticed that these sounds seem to be related to the blue modular connectors that link the 3 PCBs. so i cleaned those and even re-soldered a lead on one of them that seemed to really be popping when i touched it.

basically, the amp still sounds and plays fine and even if i jolt it violently it doesnt make any sounds. BUT if i tap around with a drumstick its likely i can get it to make a kind of crackly/pop.

does it make sense to waste time tracking down this issue? i guess the concern is that the vibration of the amp being played could potentially aggravate whatever it is thats popping/cracking but im starting to think that whatever mechanical issues might exist are insignificant enough to be  ignored...

any strategies for tracking this kind of stuff down??

Roly

Quote from: ilyaadoes it make sense to waste time tracking down this issue?

Does it make sense to waste invest time tracking down this issue?  Yes.

It shouldn't be happening at all and you can bet it's going to get worse, not better over time.

Quote from: ilyaaany strategies for tracking this kind of stuff down??

Chopstick, gently tapping the board and individual components to see if you can localise it.  Is it changed by the amp controls or not?  This should direct you to early or late in the signal chain.  Check the supply rails and move along the signal path to see if you can see in which stage it originates.
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

okay okay youre right i was just getting impatient!

banged around in there and determined it was the modular connectors - resoldered all those connections - and glad i did - in the process saw that the main lead going from the tip of the input jack was not even soldered on anymore just hanging on because the wire was wrapped around! - but still making a crackly/pop until i resolderd the connectors going to the graphic EQ. now it seems all good.

just old joints, i guess!

Roly

Yeah, a loose wire will do it for you every time (again proving the value of close observation).

It wasn't, but it's worth keeping in mind that ceramic caps can sometimes go microphonic.  "Styroseal" poly-something caps would be my normal first choice for low values in the signal path, but ceramics turn up as top couplers across controls, and in the treble part of the tonestack, and they will often produce a "boing" if tapped with a chopstick.  Like all microphonics you have to judge how serious it is in the context.
If you say theory and practice don't agree you haven't applied enough theory.