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Post subject: Vibro Champ XD Problem, Troubleshooting, Repair
Posted: Tue Feb 18, 2014 4:49 pm
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I recently got a Fender Vibro champ XD from a friend, Shortly afterward it developed an Issue, it made a strange staticy sound then dropped out altogether, save for a very quiet Pre-amp only sound. Line out still worked fine.

I pulled the tubes and took them to the local music store they have one of the Orange Amps, (Wish Fender would make such a device cheaper than $500... We call that a hint) Tube Testers... The 6v6 had taken a powder and the 12AX7 was "Worn" Amp had very little mileage on it at this point. I swapped in a 6v6 out of my Princeton and no good.
I ordered a JJ 6v6, and pulled the chassis to check it out. Testing DC Voltages per the Schematic, and at one testpoint I didn't get the right number by a LOT, so I moved upstream to R34... and saw this... Also no continuity across the Resistor.
Image
I replaced the resistor and the Tube and she is back in business. The Value is SUPPOSED to be a 470 Ohm 2W by color code the bad resistor appears to be a 400 Ohm. But it looks to have been HOT... the Black Ring around it indicates a problem.
I like the little amp I ran it through my 412 cab and it sounds GOOD.


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Post subject: Re: Vibro Champ XD Problem, Troubleshooting, Repair
Posted: Tue Feb 18, 2014 5:37 pm
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Cathode resistor.

Tube may have taken a dump and took out the resistor. Might want to check the bypass cap associated with the resistor also.

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Post subject: Re: Vibro Champ XD Problem, Troubleshooting, Repair
Posted: Tue Feb 18, 2014 6:56 pm
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bluesky636 wrote:
Cathode resistor.

Tube may have taken a dump and took out the resistor. Might want to check the bypass cap associated with the resistor also.


+1

That electrolytic may be toast as well.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Vibro Champ XD Problem, Troubleshooting, Repair
Posted: Wed Feb 19, 2014 2:14 am
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Schematic show 24 volts at this 470 ohms resistor . Power dissipation is 1.2 watts minimum for this resistor .
Fender put a 2 watts resistor , to low power IMO for a long life that why resistor is burn / "brown " from too much heat ( and defective ? ) .

Do you read resistor with ohmeter ? Put a standard 5 watts resistor for a all life no trouble . Or at least 3 watts

I 'm not sure you need new 6V6 . Just a new resistor

___________________________________________________________

About orange tube tester , what is you opinion about it ? You write ;

The 6v6 had taken a powder and the 12AX7 was "Worn"

Did tube tester said you must replace them ? Are you sure you need to ?


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Post subject: Re: Vibro Champ XD Problem, Troubleshooting, Repair
Posted: Wed Feb 19, 2014 7:54 am
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From the schematic it looks like Fender put a 22mfd/400VDC bypass cap parallel to the 470-ohm, 2 watt cathode resistor (R34). So, this cap maybe ok.

However, I'd check dropping resistors R8 and R73, as well as the resistor tied to the screen (R9). The schematic indicates FR (or fire resistant) type resistors in all of these positions. I'd use good quality metal oxide resistors.

Also, may want to check the grid stopper resistor (R10) for damage.

The cathode resistor in these Class "A" single-ended amps runs hot. Maybe a little extra lead to raise the resistor off the circuit board will help keep heat away from delicate parts.

IME, when a cathode resistor pops, the output tube has shorted or gone ballistic (unstable) and drawn an excess amount of current.

Is the 4 X 12 cab impedance = 4 ohms? Or 2 ohms? I have no idea how well built the output tranny is on that Vibro-Champ XD. So, don't know how it would handle an impedance mis-match.

http://support.fender.com/schematics/gu ... ematic.pdf


Good luck with your fix! :mrgreen:


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Post subject: Re: Vibro Champ XD Problem, Troubleshooting, Repair
Posted: Wed Feb 19, 2014 9:11 am
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[quote="

IME, when a cathode resistor pops, the output tube has shorted or gone ballistic (unstable) and drawn an excess amount of current.

A low power resistor may pop just because it is close to its limit. kkroger did not say how many ohms resistor read


http://support.fender.com/schematics/gu ... ematic.pdf


Good luck with your fix! :mrgreen:[/quote]


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Post subject: Re: Vibro Champ XD Problem, Troubleshooting, Repair
Posted: Wed Feb 19, 2014 9:23 am
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Experience Electronics tech here....
All other components are fine.

6V6 HAD to be replaced because it was BAD Resistor read OPEN when I tested it.
Voltage on tp25 was 90VDC at time of test and in spec after repair. Amp works fine.


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Post subject: Re: Vibro Champ XD Problem, Troubleshooting, Repair
Posted: Wed Feb 19, 2014 9:24 am
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stratele52 wrote:
[quote="

IME, when a cathode resistor pops, the output tube has shorted or gone ballistic (unstable) and drawn an excess amount of current.

A low power resistor may pop just because it is close to its limit. kkroger did not say how many ohms resistor read
Oh yes I did... No continuity = OPEN... no reading.
Also no continuity across the Resistor
http://support.fender.com/schematics/gu ... ematic.pdf


Good luck with your fix! :mrgreen:
[/quote]


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Post subject: Re: Vibro Champ XD Problem, Troubleshooting, Repair
Posted: Wed Feb 19, 2014 9:52 am
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Thank's for the answer . How many watts you r new resistor ?

Any opinion about the Orange tube tester ?


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Post subject: Re: Vibro Champ XD Problem, Troubleshooting, Repair
Posted: Wed Feb 19, 2014 10:14 am
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stratele52 wrote:
Thank's for the answer . How many watts you r new resistor ?

Any opinion about the Orange tube tester ?


Easy to use, Gives result of test Good/Worn/Bad
Also gives a "Value" so you can match tubes with it.

Dislikes?
It uses a wall wart power supply
I would like it better if it was a 120VAC cord...
But this way it can be used around the world with the appropriate supply.

"Value" doesn't MEAN anything, it is just a number so it is not a "Bias" point or anything it just means that these three or four tubes "Match" in that "Value"
Would be nice if it were more useful...

Compared to any other test method, currently available, it is the most compact and "Affordable" unless you want to find a broken hickok to repair.

but $500US is a bit steep for me. but I will probably buy one soon.
I like the fact that this shop (fender dealer too by the way) has one on the counter and a SIGN saying that you can test tubes FREE. I always BUY something when I use it though.


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Post subject: Re: Vibro Champ XD Problem, Troubleshooting, Repair
Posted: Wed Feb 19, 2014 11:47 am
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Did this tester say the truth about tubes ; good or not ?

With $500 we can buy more new tubes than we need to replace a suspected tube .


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Post subject: Re: Vibro Champ XD Problem, Troubleshooting, Repair
Posted: Wed Feb 19, 2014 12:31 pm
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In my case the suspected bad 6v6 was shown as bad...
the known GOOD 6v6 showed good and the suspected worn EL34s showed good.
the NEW 12AX7 in the Vibro Champ showed worn as did the worn 12AX7s from my Marshall...

"Shrug" who knows, but it would be handy in a repair shop to quickly rule out tubes.
Without possibly inserting NEW tubes and rapidly killing them.
I've worked on electronics on everything from Spacecraft to Aircraft to Telecom to Radio Comm and Guitar Amps and guitars.
I have a couple amps and a "Plethora" of Guitars and Basses...
Image

Items shown in the pic, VOX AC4, VOX AC15C1, Princeton Reverb, Vibrochamp XD, Jet City 2112RC, Fender 1983 Concert, SUNN 300T, Marshall JCM2000 DSL100 in a custom cabinet. The Fender 412 Cabs are DT412s the Sunn Cabs are a 410H, and a 215, the Marshall cabs are custom cabinets with 12T75s in one and a combo of V30 and 12T75s in the other.

This is my personal stuff, I am also re-working Chinese boards from SAMSUNG TVs with Capacitor problems... among other things.


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Post subject: Re: Vibro Champ XD Problem, Troubleshooting, Repair
Posted: Thu Feb 20, 2014 2:24 am
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kkroger, love those stacks. Sunn 300T? I hope you are a bass guitarist. As a lead plugged into that monster is just no fair. :lol:

Think with a good 6V6GT, new cathode resistor, and checked out R8, R9, R10, and R73 --- you'll be good to go.

Gutes Glück mit Ihrem Verstärker! :D :mrgreen: :D


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Post subject: Re: Vibro Champ XD Problem, Troubleshooting, Repair
Posted: Thu Feb 20, 2014 4:19 am
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BMW2002Ti wrote:
kkroger, love those stacks. Sunn 300T? I hope you are a bass guitarist. As a lead plugged into that monster is just no fair. :lol:

Think with a good 6V6GT, new cathode resistor, and checked out R8, R9, R10, and R73 --- you'll be good to go.

Gutes Glück mit Ihrem Verstärker! :D :mrgreen: :D


Bass you say? Maybe....
http://forums.fender.com/viewtopic.php?f=9&t=90143


Last edited by kkroger on Thu Feb 20, 2014 7:59 am, edited 1 time in total.

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Post subject: Re: Vibro Champ XD Problem, Troubleshooting, Repair
Posted: Thu Feb 20, 2014 6:05 am
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Dude! A real 1952 Precision? I've never seen one, in person. Love that "Telecaster" style head stock. I can't imagine how much it is worth.

:D


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