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Post subject: 1967 Vibro Champ Issue
Posted: Mon Oct 30, 2017 8:37 am
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My Vibro Champ has been making a scratchy distorted sound intermittently, and when I power down with the volume on 0 it sounds like a cap discharging. Yesterday will playing I heard the scratchy sound kind of like a dirty potentiometer. So I decided to exercise the pots, well what I noticed was that when the tone pots were set on "0" I still had volume and when I turned the pots up to 10 there was a some tonal difference, but not much. When I first got the amp I am 99.99% sure that when bass, and treble were on "0" no sound came out of the amp, just like my other 2 types of fender amps.
So now my question is where do I start looking
mud


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Post subject: Re: 1967 Vibro Champ Issue
Posted: Mon Oct 30, 2017 8:49 am
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Kinda sounds like leaky coupling cap(s). My guess would be to first look for DC leaking onto the 2nd half 12AX7 signal grid (gain stage/input tube) --- after the tone stack.

You should NOT have more than a few DC volts (1-2 VDC max), on the grid side of those coupling caps.

Good luck!

:mrgreen:

http://www.electronicstudio.net/schemat ... _schem.pdf


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Post subject: Re: 1967 Vibro Champ Issue
Posted: Tue Oct 31, 2017 7:32 am
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[quote="BMW2002Ti"]Kinda sounds like leaky coupling cap(s). My guess would be to first look for DC leaking onto the 2nd half 12AX7 signal grid (gain stage/input tube) --- after the tone stack.

You should NOT have more than a few DC volts (1-2 VDC max), on the grid side of those coupling caps.

Good luck! quote]

+1


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Post subject: Re: 1967 Vibro Champ Issue
Posted: Tue Oct 31, 2017 7:55 am
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OK thanks, I am not sure how to run that test, ( but if it's explainable I am reading ) I did open it up and did a visual and this is what I saw on the coupling cap. When I get more time I was planing on lifting a leg and testing the cap itself, by the looks of it and it debris, I assume it is shot. If that is the case do I replace it with another ceramic disc type cap, or is there a better type to use. If I am to stick with the ceramic disc, is there one brand better then the other, or will a Radio Shack ceramic disc work fine
Image
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Image
mud


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Post subject: Re: 1967 Vibro Champ Issue
Posted: Tue Oct 31, 2017 8:37 am
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I can't see pictures, do you ?

Capacitor leakage;
1- Read voltage on both end in circuit.
2- Desolder the capacitor leg with the lower voltage.
3- Power ON the amp, and read voltage at this leg you must not have more than 1-2 volts


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Post subject: Re: 1967 Vibro Champ Issue
Posted: Tue Oct 31, 2017 8:41 am
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Okay. The coupling caps are the big orange blobs (with yellow lettering) and brown discs. These are non-polar caps and do NOT have (+) and (-) marks.

Fender layout for nearly 100% of their vintage amps, have the grid side of the coupling caps (big brown ones, at least) --- on the side closest to the pots. [check the layout versus schemo diagrams to be sure].

Put chassis on secure bench. Hook up a speaker. All tubes in. No inputs. Zero volume.

Now, measure the Voltage (DC) on the grid side of those coupling caps. My bet is those big brown blob caps are bad. They are known to leak. I'd replace with SBE 716P or equivalent polypropylene caps (just my opinion). These will last forever.

Good luck!

http://www.electronicstudio.net/schemat ... layout.pdf]


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Post subject: Re: 1967 Vibro Champ Issue
Posted: Tue Oct 31, 2017 9:30 am
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First off did my pictures not work, if not I will find another way.
Ok using this lay out, my two blobs are bass and mid cap, after that is the small treble cap, and than comes the .02 Ceramic disc in the coupling cap from preamp to poweramp ( this is the one that I think is suspect )
BMW2002Ti wrote:
Okay. The coupling caps are the big orange blobs (with yellow lettering) and brown discs. These are non-polar caps and do NOT have (+) and (-) marks.

Put chassis on secure bench. Hook up a speaker. All tubes in. No inputs. Zero volume.

Now, measure the Voltage (DC) on the grid side of those coupling caps. My bet is those big brown blob caps are bad. They are known to leak. I'd replace with SBE 716P or equivalent polypropylene caps (just my opinion). These will last forever.
http://www.electronicstudio.net/schemat ... layout.pdf]

Also thanks for taking the time to explain this procedure for measuring DC leak, I will be able to do this next time I have time to test it. Not sure if I can get to it tonight
Please let me know if the pictures of my amp from the prior post came through.
mud
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Post subject: Re: 1967 Vibro Champ Issue
Posted: Tue Oct 31, 2017 10:02 am
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Picture; you can see by yourself here, I see the last one, not the firsts ones


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Post subject: Re: 1967 Vibro Champ Issue
Posted: Tue Oct 31, 2017 11:19 am
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I can see your photos, fine! :)

Good photo here of layout. Like the way the chassis is anchored to the workbench.

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Post subject: Re: 1967 Vibro Champ Issue
Posted: Wed Nov 01, 2017 6:09 pm
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I went in today and here is what I got:
Bass Cap .007 to -.005 VDC
Mid Cap .002 to -008 VDC
Treble Cap .001 to 000 VDC
Coupling Cap .061 to 010 VDC before the resister and 000 VDC after.
It almost seems like it is bypassing the tone stack, as the tone knobs do very little or I am imaging I hear a difference because I am turning a knob,
Thanks for spelling out where to put my probes for my fluke
mud


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Post subject: Re: 1967 Vibro Champ Issue
Posted: Wed Nov 01, 2017 10:13 pm
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Are we talking about the 0.02mfd coupling cap that goes from second half of 12AX7 to 6V6GT grid? You must have 200VDC on one side.

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Post subject: Re: 1967 Vibro Champ Issue
Posted: Thu Nov 02, 2017 8:04 am
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First let me say, thanks for sticking with me BMW2002Ti.
I was giving the measurements of the other side of the caps looking for excessive leakage

"Are we talking about the 0.02mfd coupling cap that goes from second half of 12AX7 to 6V6GT grid? You must have 200VDC on one side."
Yes the 0.02mfd coming from pin 6 of the 12AX7 ( Coupling Cap from preamp to power amp ) this has more than spec calls for I am getting 258VDC going in , this is the one in the photo that has all that debris under it. Also when I place my probe on the 0.02mfd hot side I get the same scratchy sound for a moment when the probe touches the leg, as when I power the VC down and that did not happen in the past. The bass, mid, and treble caps all have 23VDC going in
Thanks
mud


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Post subject: Re: 1967 Vibro Champ Issue
Posted: Thu Nov 02, 2017 8:10 am
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Sounds like the solder on the "hot" side of that cap is bad. Maybe, first try resolder.

Do you have another 0.02mfd/600VDC you can try in that spot?


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Post subject: Re: 1967 Vibro Champ Issue
Posted: Thu Nov 02, 2017 8:11 am
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mud wrote:

Yes the 0.02mfd coming from pin 6 of the 12AX7 ( Coupling Cap from preamp to power amp ) this has more than spec calls for I am getting 258VDC going in ,


That is good , old power transformer work with 115 volts house outlet, today we have 125 volts

The bass, mid, and treble caps all have 23VDC going in

You must have more the 200 volts going in and 1, 2 volts going out, look at the schematic

mud


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Post subject: Re: 1967 Vibro Champ Issue
Posted: Thu Nov 02, 2017 9:02 am
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BMW2002Ti wrote:
Sounds like the solder on the "hot" side of that cap is bad. Maybe, first try resolder.

Do you have another 0.02mfd/600VDC you can try in that spot?


I will heat up the solder there, and I do not have 0.02mfd/600VDC, I am not sure if the local radio shack will have one, but if they are open when I get there after work I will look, should I get another ceramic disc type or should I use something else
Thank you
mud


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