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Peavey studio 112 hum problem

Started by Richwess, September 24, 2012, 08:01:55 AM

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Enzo

Oh it could be so many things, but now we venture into the land of how would we ever know.   SHort of cutting the bad part open AND knowing what we were looking at, we are left with a failed circuit needing repair.

I tend to stick with the simplest explanations - the old when you see hoofprints, think horses, not zebras thing.

Richwess

Thanks all for your informative replies - some went a little into the esoteric for a newbie, but interesting and appreciated nonetheless.

So, what is the way forward with this? Do I just find the relative transistor and replace? Anything to watch out for?

J M Fahey

In general lines, yes, but there may be other bad parts, that's why a proper schematic is essential so we all "see" the same.
Like many others here, if I had it on my bench *maybe* wouldn't need one, because the actual PCB would be before me, in plain sight.
And even in that case, the schemo helps a lot, at least saves time.

Roly

When I started helping people on line about ten years ago I thought that I may be able to help out by e-mail perhaps one-in-ten.  I'm still amazed that it has turned out more like nine out of ten when you can't directly observe the gear, much less the fault.
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

I have been dispensing advice online in this biz for maybe 15+ years, but I have been training techs in one industry or another since the 1970s, and have in a lot of that time been in field service.  SO I have had many opportunities to fix stuff with someone else's hands.   I have come to be fairly confident we can fix most things within the owners capability.

I learned early on to establish a level of communication, one lesson came when I suggested a guy start by checking his plate voltage, to which he relpied, "Is the plate that metal thing the amp bolts to?"  Oh, can;t assume we all share the same vocabulary, can we.

One thing I say to new techs a lot is that MOST repairs are simple - if you can solder, you can fix 90% of stuff.  Broken or loose jacks or controls, cracked solder to main filters or maybe large resistors, cracked solder on all the panel controls, bad tube, etc...

The truly fun part is when you try to explain the function of some circuit - that bias green/red LED circuit on an SVT for example - so someone might understand it, and I find out that up to that point I had never thought my way through that circuit, so this is the first time even I understood it.   Or some circuit I had a working assumption about but never anaylzed and find out my working assumption of long time was in fact not correct.   Learn something every day.

Roly

Oh yeah, I've certainly found that to be true, forced to think about something I just taken for granted before.  Trainees certainly keep you on your toes - if they are any good they ask seriously curly and perceptive questions.

Only today I was dealing with SWMBO's car which has had a baffling intermittent pause that has had several mechanics (and me) scratching their heads for weeks.  Today my son (who is also a mechanic, but who learned diagnostics from his dad) seems to have located a dirty connector on the air flow sensor.  Time will tell, but he seems to be the first person to actually "touch" the fault and change the cars behaviour (very much for the better).

I've done a lot of 'puters over the years and a good 90% of those problems come down to dirty connections (the other 10% being software configuration or virus problems that take 90% of the time  ::) )


Communications: was doing a 'puter help over the phone with a German fella.
"So where are you?" (with the system)
"Sitting in front of the computer".
...
"Okay, what's on the screen?" (at the command line)
Proceeded to read the entire boot screen from top to bottom.


Reporting a damaged public telephone.
Operator: "What state are you in?"
Me: "Wide awake and stone cold sober."

(5 seconds baffled silence)

Operator: "No, no; what *State* are you in?"
Me: "(sigh) I think you mean 'WHICH State am I in?' - I'm in Victoria."
{so much for centralised call centres}
If you say theory and practice don't agree you haven't applied enough theory.

aoresteen

I know that this is an old thread but reading through it I have a question that is universal.

Let's assume that the amp was killed by the ultrasonic output of the Fuzz Face. 

If an amp can be killed by a 10KHz oscillation signal, why didn't Peavey put the the 47pF or 100pF capacitor in the preamp input to prevent it from ever getting to the power stage?  Wouldn't that be good design practice to protect the amp?

Thanks!
Tony Oresteen
Peavey Bandit Silver Strip, Revolution 112
Marshall MOSFET Lead 100 3210
Squier SP10
Newnan, GA

g1

  If the amp could be killed by a 10Khz signal, then it can be killed by a high note.
Putting a cap in to kill 10khz is going to hurt the treble response of the amp.

Enzo


QuoteLet's assume that the amp was killed by the ultrasonic output of the Fuzz Face.

Well, let's not, because it won't.

QuoteIf an amp can be killed by a 10KHz oscillation signal

It can't.

Quotewhy didn't Peavey put the the 47pF or 100pF capacitor in the preamp input to prevent it from ever getting to the power stage?

You are assuming it would get that far through the amp to cause trouble.  The speaker won't respond to that high a freq anyway.  A zillion hifi amps go through life passing 10kHz.

Don't confuse a signal with oscillation.  If 10kHz appears at the input to the amp, the amp has no idea whether it came from something oscillating or from a high note.  To the amp it is just a signal.  IN this context, oscillation in a power amp - or any other section really - refers to the circuit becoming unstable and essentially feeding back on itself.  THAT can damage an amp, and they all DO have a circuit or two to prevent it.  Look up "zobel network".

Roly

#24
Quote from: aoresteenLet's assume that the amp was killed by the ultrasonic output of the Fuzz Face.

That is actually a whole bunch of assumptions, many of which are debatable.


My homebrew Twin-50 stage/synth amp is getting pretty long in the tooth these days so it still uses 2N3055's in the output stage.  A particular oddity of the good ol' '3055 was that, for a transistor, it has a very low fT or transition frequency.  In English this means that its frequency response is ratshit and that just about any modern transistor would eat it for breakfast.

But it makes a handy example.

The result of running a transistor up close to its fT is that it get quite lossy, so its heat dissipation is a function of the operating frequency.  Despite being the poor kid on the block frequency wise the amp will handle in excess of 10kHz with no problems, just the heatsinks run a bit warmer.

Without going too deeply into transistor physics a primary reason for this is that the '3055 has a very long hole storage time, or in English it takes a significant time to turn the transistor from "on" to "off".  The result is that both transistors in the output totempole can be conducting at once, even with no load at all connected.

Talking of which, the typical loudspeaker has a rising impedance and being highly inductive a 12-inch guitar speaker may be effectively open circuit above about 5kHz, never mind 10kHz.  Not so piezo tweeters which are almost entirely capacitive and will be quickly destroyed by application of continuous high or supersonic frequencies.

My Twin-50 has no problems because it was designed for continuous power output with BIG heatsinks, as all stage amps should be, but domestic stereos are not designed for continuous output because even replaying Thrash Metal the actual duty cycle is a fair bit lower, and for typical programme only about 30%.  To design an output stage for 30% duty is a hell of a lot smaller and cheaper than for 100%, so stereo amps are at risk if asked to deliver continuous power, as more than a few poor young guitarists have discovered.

Subject to an oscillation the entire signal chain is driven to the limit, and if the signal is out of the range of hearing, supersonic or above what the speakers can reproduce, the power stage may well cook because you are not aware of what is happening.  This shouldn't be an issue with a guitar/stage amp, but then not all are created equal (as this "heatsink" shown in the Peavey Escort thread illustrates).


So after that quick whizz around the ultrasonic block we come back to the original observation/diagnosis that "10kHz from a fuzzface killed an amp".  On the face if it that is dubious and deserves a deeper dig to find out what really happened.


(ed: typo)
If you say theory and practice don't agree you haven't applied enough theory.

aoresteen

Thanks all!  A lot to digest here.  I've got a lot to learn about ss amps.
Tony Oresteen
Peavey Bandit Silver Strip, Revolution 112
Marshall MOSFET Lead 100 3210
Squier SP10
Newnan, GA