OT: Vibro Champ problem

[gez]

Sorry to bring this here but my question over at Ampage has brought no responses!  My Silver Face Vibro Champ bit the dust yesterday, well partially.  It cut out while I was tweaking a circuit and produced a buzzing sound followed by a slight burning smell.  I immediately turned the amp off.  I’ve checked over it today and have a few questions.

1.  Where the hell are the filter caps?  Sorry to get technical here, but are they in the cylindrical metal thing hanging down from the chassis?   The pins of this thing match with where they should be so they’re inside?  How do I get to them?

2.  Is it likely one has gone leaky?  I intended to re-cap this thing in January but I may have left it too late.  Was the ‘buzz’ sound motor-boarding?  

3.  If I HAVE got a leaky cap (or two), what is the likelihood that the transformer  will have been damaged?  How can I check for this without turning on the mains supply?  Check its DC resistance?  If so, what would that be?

I measured for voltage at the points where the filter caps should be and there was a residual amount left.  Having drained this off (you should see the burns on my tongue! - OK, so I used a couple of resistors to do this), I checked all the resistors and none have fried, the valves look OK and I can’t see any immediate signs that anything has burnt.  The burning smell seemed to come from the opposite end to where the transformer is, but it all looks ok that end.

I’m pretty clueless when it comes to valve amps.  I understand how they work, but other than basic maintenance I don’t have a lot of  hands-on experience.  All advice will be greatly appreciated!  Thanks.

Gez

[bzimm]

The filter caps on a SF VibroChamp are located on the underside of the chassis, in a can next to the transformer.  They are replaced as one unit (you need to find a can that has 3 20uF caps rated at 450V).  I believe mallory makes such a can though I do not have a part number handy.

Couple of questions though, were you tweaking a circuit inside the amp when you had this problem or were you teaking an effect plugged into the amp when it happened?  If you were messing around inside the amp perhaps you should come clean with what you were doing, will make a diagnosis easier  The fact that the smell seems to be comming from the end opposite the transformer leads me to believe the problem is not with the filter caps.  In my experience when filter caps get to the point that they fail, the fuse in the amp blows.  I would check the components very carefully where the smell was comming from, looking for any signs of heat or cracking in the cases (look for bulges in the case of capacitors).  If you find a bad part, chances are there was something else wrong that caused the component to sacrifice itself so simply replacing might not fix the problem (luckily though, it may).   Checking transformers can be tricky business but there is some stuff on the various amp boards about doing this.

Let us know what you find.

[Mike Nichting]

Some pics of the inside and out of the chassis would be good as well.

Check all of the components inside first. Then have the tubes checked or if you have a tube tester then pit them on it. You can't tell if a tube is OK or not by looking at it all the time. Sometimes you have obvious clues like smoky glass etc.

 What year is the amp??

 Try this place for Fender amp questions: www.fenderforum.com  it is an absoluitely great place much like this forum~!!

  Best of luck~!!

Mike N.

[gez]

Couple of questions though, were you tweaking a circuit inside the amp when you had this problem or were you teaking an effect plugged into the amp when it happened?

I was tweaking a circuit (AC coupled) plugged into the amp.

The fact that the smell seems to be comming from the end opposite the transformer leads me to believe the problem is not with the filter caps.  In my experience when filter caps get to the point that they fail, the fuse in the amp blows.  I would check the components very carefully where the smell was comming from, looking for any signs of heat or cracking in the cases (look for bulges in the case of capacitors).  If you find a bad part, chances are there was something else wrong that caused the component to sacrifice itself so simply replacing might not fix the problem (luckily though, it may)

All the resistors are OK, as for the caps I don't know (will have to check tomorrow morning).  The amp is pretty dirty and everything looks like crap (it was like this before it died).  

I recently replaced the 12AX7s with some that a friend gave me, but they still look ok.  I replaced them, fired it up and still I got no signal - just buzz!

[gez]

Some pics of the inside and out of the chassis would be good as well.

Check all of the components inside first. Then have the tubes checked or if you have a tube tester then pit them on it. You can't tell if a tube is OK or not by looking at it all the time. Sometimes you have obvious clues like smoky glass etc.

What year is the amp?

I don't have a camera I'm afraid.  Even if I did you'd probably think everything got fried as it's pretty dirty.  I did replace the 12AX7s but no joy.  I don't know when it was made, I bought it in the late 80s.

[Ammscray]

In my experience with these amps the main thing that goes wrong is that the tube socket for the 6V6 gets so hot that it arcs and starts to burn...they smell strong when they cook...check it for black carbon stains or loose lugs...and check continuity between lugs to make sure there is none...9 times out of 10 I change that tube socket and it's fixed...

Probably not a tube or filter cap...but try a different rectifier just in case...then hook it up to a variac and bring it up slowly while monitoring voltages...check bias resistor and bypass cap for shorting or changed tolerance...

It's a simple circuit take your time and you'll find the problem

[Peter Snowberg]

A good source for can caps & all kinds of great tube amp supplies:

http://www.angela.com

If the caps are old (greater than 15 years), it's always a good idea to change them out for fresh ones. While you're in there, you may want to change all the caps for a much improved amp. The cathode caps are also electrolytic and the tone/coupling caps in the champ series were selected for cheapness, not tone.

Here's the schematic (if you have an AA764 amp)
http://www1.korksoft.com/~schem/fenderamps/champ_vibro_aa764_schem.pdf
...and the layout:
http://www1.korksoft.com/~schem/fenderamps/champ_vibro_aa764_layout.pdf

Try removing the 12AX7s and the 6V6. Get out your meter, ground the negative probe using a clip (so you don't need to touch it when measuring), and measure the voltages at pin 1 of each 12AX7 and pin 3 of the 6V6. That should tell what to do next.

Best of luck,
-Peter

[gez]

Firstly, I’d like to thank EVERYONE who replied!  Ammscray, it is a simple circuit so I don’t know why I was getting into such a flap about this - it’s my baby and I just wasn’t thinking straight when it got sick!  Your replies brought me back down to earth, made me think a little about what was going on and enabled me to do some testing in a methodical manner.

It looks as though the bypass cap on the 6V6 has gone leaky.  The tube is saturated and there’s hardly any voltage at the cathode.  There’s flecks of rust all over the insides of the amp so it was difficult to tell if the cap had gone by just by looking at it, but on closer inspection there are slight burning stains at the cathode end of things.  The 12AX7s and associated circuitry are all fine and the rectifier is working good.   Seeing as I’ve got this baby open, I’m going to replace ALL the caps - this is an old amp and it won’t be long before the others go I’m sure!
 
Once again thank you all!  I’ll get an order in today and hopefully It won’t get lost in the Christmas post.  :shock:

[Dan N]

Hi Gez,

By all means replace the bypass caps. Up the voltage to 50 volt caps. I happen to really like the shiny blue coupling caps they put in Champs. The brown blob caps never excited me much, nor the ceramic caps they used in the blackface champs.

It is really hard to solder to the chassis, so when I replace the filter caps I just use individual caps and mount them inside the chassis. Spragues are so big that it's a hard squeeze. Xicon are nice and compact.

While you are inside it is a good idea to throw in a current limiter for  your power tube screen. This will really save wear and tear on your tube. It's easy. Disconnect the wire soldered to pin 4 of the power tube socket and connect that wire to pin 1. Now put a 1K 3 watt resistor from pin 1 to pin 4.

Here's a photo of a Champ I have laying around. Guess I used a 5 watt resistor on this one...
http://users.rio.com/senorris/junk/vcinss.jpg

Good luck!

Dan

[gez]

Thanks Dan, that's really useful info!  I take it your Champ never had the Mallory 'tin can'?  There's precious little chance of me finding one here so I'm going to have to improvise something.  I'll probably screw in a mini tag board and solder the filter caps to that (I know what you mean about the chassis!).

Thanks again.

[gez]

Well, I finished all work on it a few hours ago, fired it up, it sounds totally amazing and I didn’t kill myself (sorry folks!).  All in all, I’d say that was a result!

[idlefaction]

wish i'd read this at the time, i have a supplier for those multiple-cap cans, 47+47+22uF 400V, NZD$18 (~ USD$10) each.  i don't actually know if i like them, but i have some in my room to use up so i'll be using them

gez man, that is a wicked amp, i love that circuit    sacrilege i know but especially with an EL84 instead bwahahaha

and yeah definately put in the screen resistor if you didn't already.  matchless used 100 ohm 5 Watt, not sure why, i use 1k5 5 Watt in mine.  

[gez]

wish i'd read this at the time, i have a supplier for those multiple-cap cans, 47+47+22uF 400V, NZD$18 (~ USD$10) each.  

Shame! I don’t know if this is total BS because the bloke didn’t have any, but my supplier told me that the new cans are slightly oversize and don’t fit.  Pity the git didn’t tell me the same a few months ago when he sold me some valve bases that are slightly oversize!  I installed them on the weekend and had to use a grinder attachment on my mini drill to make the holes slightly bigger.  It took ages but the firework display was pretty impressive, the sparks that come off steel when you grind it are wicked!

I used some Spragues in the end and Dan wasn’t joking when he said it was a tight fit!