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Topic: Peavey Pacer problem/question: |
Pete Burak
From:
Portland, OR USA
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Posted 6 Sep 2013 7:53 am |
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Looking for thoughts on this problem.
70's Peavey Pacer 100.
When i play two notes together, like the AB pedals down at fret 10 G-chord for example, strings 3 and 5, I get this third note, which is lower than both played notes.
If I slide up/down and play two notes together the third note follows the played notes (kind of in tune with the played notes, but lower in pitch).
I do not hear the extra note when i play only one string/note.
I can achieve this same problem by playing the high E and B strings on a guitar, using a D-shape, especially up at the 13th-14th fret area.
So I thought is was a "ghost note" issue, typically related to a filter cap issue.
So I had a full cap job done using Brad Sarnos Session 400 webpage info as a guide.
It still does the same thing!
I get the same "third note" thing when I run it to a completely separate speaker cabinet.
I turned the reverb knob off, and also completely disconnected the reverb tank, still have the problem.
I tried turning every knob to zero to see if it was related to one certain knob, no change.
My amp-tech thinks it is a circuit board issue, whereby some component is too close to another, and/or in a parallel layout path, causing an occilation.
He wants to chase this down and relocate something.
Any ideas??? |
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Lane Gray
From:
Topeka, KS
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Posted 6 Sep 2013 8:26 am
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Are you running low gain, hot master volume?
I mean, I associate IM distortion (what you have) with overdrive. Possibly an op amp IC is dying. Or a transistor is.
_________________
2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Pete Burak
From:
Portland, OR USA
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Posted 6 Sep 2013 8:44 am
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I get that ghost note on both inputs, and with any Guitar or Steel.
I get it at any volume or knob setting.
There are no Chips in this amp.
My tech thinks it is coming from the Pre-amp section, which I think rules out the transistors.
But you may be right.
It only happens when two notes are played together. |
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Mike Wheeler
From:
Delaware, Ohio, USA
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Posted 6 Sep 2013 8:49 am
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I believe what you are hearing is Intermodulation Distortion. It's caused by non-linear amplification somewhere in the circuitry. The result is the sum and difference frequencies of the original two notes being added to the signal. The lower of these two additional notes is the most audible, and annoying.
It could be a transistor that's failing, or that has improper bias voltage,(the Pacer is an all discreet transistor design), or a failing resistor, etc, etc.
Since this could get expensive (because of prolonged bench time), you might be better off finding a better amp....that's what I would do, and I'm a tech.
Maybe one of our other more knowledgeable techs can elaborate more.
_________________
Best regards,
Mike |
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Pete Burak
From:
Portland, OR USA
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Posted 6 Sep 2013 10:10 am
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Yeah, I bought it on Craigslist very inexpensively, because this was the same model first amp I ever had back in my jr. high school days.
Purely nostalgic, although it sounds great for a grab and go 1x12.
I basically can't hear that wierd note when playing with a band (I've only used it at rehearsals).
But here at the house it really bugs me.
There is actually another one on CL for $40.
I might grab it just for the sake of comparision. |
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Ken Fox
From:
Nashville GA USA
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Posted 16 Sep 2013 2:55 pm
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Have a friend with one, same problem on steel. I never could fix it. I guess that's why I was always more of a tube amp guy, had a lot better luck repairing them.
Like yours, we replaced the filters and other caps as well. Never could get it right. |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 17 Sep 2013 7:48 am
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| I concur with what Mike said, it's definitely intermodulation distortion. The only way to really track it down is to go stage by stage with an o-scope, and look at the waveforms produced by each stage while using a pure sine-wave drive on the input. Once you find the stage that's causing the asymetrical waveform, replacing all the components in that stage should eliminate the problem, or at least get it to acceptable levels. |
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Ken Fox
From:
Nashville GA USA
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Posted 17 Sep 2013 8:38 am
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| The problem wioth the one I had was that a pure sine wave would not cause the IMD. You had too have two notes played at one time to get the 3rd, unwanted note. Suppose I could have injected two sine waves at different frequencies to check it. I have a distrotion analyzer, just never took the time to figure out how to set it up in the shop. |
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Pete Burak
From:
Portland, OR USA
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Posted 17 Sep 2013 9:04 am
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The two note thing is true.
With only one note played it sounds fine. |
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Mike Wheeler
From:
Delaware, Ohio, USA
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Posted 17 Sep 2013 9:09 am
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Ken is correct. The best, and quickest, way to find the problem is as he describes.
I had a personal amp that developed IM distortion which eventually became a nasty problem. Being that it was my favorite amp, I put the time into tracking down the cause. It took many hours to find the bad stage, and even more to nail down the bad components.
Turned out to be an "off-value" resistor, and a very out-of-spec transistor. After the repair, it sounded great again.
I can't believe a customer would have been willing to spend the dollars that I would have had to charge for such a repair.
The test rig setup alone took almost an hour!
_________________
Best regards,
Mike |
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Pete Burak
From:
Portland, OR USA
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Posted 17 Sep 2013 9:21 am
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My guy just told me he ordered new transistors from Peavey.
He liked the 2-Sin-wave idea.
His primary belief is that it is a "Layout Issue".
He mentioned something about Layout being an issue with some old Fender amps and something about a grid being too close to a cathode??? |
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Lane Gray
From:
Topeka, KS
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Posted 17 Sep 2013 12:05 pm
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I'd point out that if it happens in the preamp, that rules out the power amp transistors, but the preamp section has transistors: I'm no Ken Fox, but I've labored under the belief gain can come from either tubes or transistors, and no other gadget.
(And the op amp chips in the EQ section of other amps are an agglomeration of transistors)
_________________
2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Ken Fox
From:
Nashville GA USA
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Posted 17 Sep 2013 12:47 pm
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| So right you are, Lane. I would start where the preamp enters the power and find if it is there. If so then divide the preamp in half and look again. Divide and conquer! |
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