[TROUBLESHOOTING - Fender Evil Twin]

Discussion in 'The Squawk Box' started by Kuroyama, Aug 16, 2009.

  1. Kuroyama

    Kuroyama V.I.P. Member

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    OK So what follows will be shots taken from inside the chassis. Ive got two concerns... well, 3 actually.

    1) I hooked up the voltmeter and got NO reading on the filter caps. Granted the amp hasnt been plugged in since the store, AND I pulled the power tubes, but shouldnt the filter caps retain some residual charge - unless there is a short? (edit - I added the power tubes again in case the circuit simply wasnt complete... the readings did not change)

    2) I think I see a bulge on the top of one of the yellow barrel caps. No liquid from any caps, but the bulge (if it is one) warrants replacement. Also, theres some discoloration at its base.

    3) I was worried about oxidation from the OUTSIDE of the amp... The inside of this thing looks like it did two tours with McHales Navy. Anyone know if this kind of oxidation will hurt the circuit?? Its all over. Maybe the entire amp has to be rebuilt?? If rust IS a problem like this, would using Naval Jelly be a possible solution? Or might that eat the PCB and/or electric components? Not to mention the fact that I cant remove the naval jelly with water... I am concerned not just for the rust on the connections I can see, but for those under the PCB. I think thats gonna have to come out for inspection/work.

    BTW before it comes to that, there are still electric readings I need to take per discussion on the PROJECT TWIN thread. Started this new thread to clean things up, and keep it current.

    Thoughts?


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  2. SatelliteAmps

    SatelliteAmps Senior Member

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    I'm not seeing any major oxidation inside the amp. Where are you seeing it that is making you concerned?

    As far as residual power in a capacitor, that depends on a lot of things. First off, is the cap getting power to it when the amp is turned on? If not, then there won't be any residual voltages (also, some amps have bleeder resistors to always bleed off the residual voltages. I think that one has them, but I don't have the schematic in front of me). If the cap is bad, then it also will not be holding a charge (or if any other caps directly behind it are bad, it also won't get voltage). Check from the power transformer back, with the amp on. (If you don't know how to do this safely, then please have a technician do it instead. It's not worth electrocuting yourself). With no tubes in, check both sides of the B+ coming off the transformer, then right after the rectifier, then at each capacitor stage. Make sure there is proper voltage's at all of those spots.

    Lastly, since there is no oxidization inside the amp, there really isn't a reason to try and remove anything. Even if there was, Naval Jelly is usually a last resort.
     
  3. Kuroyama

    Kuroyama V.I.P. Member

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    Maybe Ive been working on computers too long... If I opened a server or disk array and saw rust like that, Id put in a recommendation for someones termination! So, then... youre saying that ISNT enough oxidation to concern myself with?

    As far as the voltages of the capacitors, Thats one of the steps I have yet to do. Going to Akihabara today to get the amp tubes checked out I may try to check the caps when I get back.

    You werent concerned at all over that little yellow barrel capacitor? The de-coloration at its base or what I thought was a bulge at its top?
     
  4. SatelliteAmps

    SatelliteAmps Senior Member

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    If it is bulging, then the yellow cap should be replaced.

    I went through and double checked the pictures, and I'm not seeing any major rust on the inside of the amplifier. Are you talking about the resin on the solder joints? Or is there something specific you are looking at?
     
  5. Kuroyama

    Kuroyama V.I.P. Member

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    I thought that was rust. Ill be honest my work with computers isnt translating well to amplifier work. Ive never seen resin like that on solder joints in a server. Ill be more careful in what I call foul on. Still, I want to be overly cautious and wary of everything. Id rather get this right than have a smoked amp or heart.

    When I got back from Akihabara I checked cap voltages (pics and voltage values coming shortly. The wife is shouting that its dinnertime now.)

    My immediate concern is that I found no one who could check my amp tubs. In the US, how do you guys handle that? In the 70s they had tube test machines in drug stores where Dad could test tubes while Mom bought us kids ice cream!!

    Anyway, Ive seen some pics of very old tube self-test machines online... but for all those Gearheads running lotsa guitar amps or even tube based stereo amps... how do you check the tubes when you pull em??

    shes yellin again. Pics to follow.
     
  6. Kuroyama

    Kuroyama V.I.P. Member

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    Voltage values taken with system powered up

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  7. SatelliteAmps

    SatelliteAmps Senior Member

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    No problem about not knowing about the rosin, I just wanted to make sure I wasn't missing anything. If it bothers you, you can clean it up with rubbing alcohol and a q-tip. It won't make any sonic or electrical difference at all, but you can if you want to.

    Those voltages all look to be within spec. I would still change out the bulging cap. What are the other symptoms of the amp? Does it make sound at all?

    As far as tube testing goes, we use tube testers in my shop. If you don't have a tube tester, then the only way to test the tubes is in the circuit. Check the basic voltages, and the bias. If any of those are way off, and there is no circuit reason for it, then it is a bad tube.
     
  8. Cygnus X1

    Cygnus X1 Senior Member

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    Somehow I was following the other thread...this one popped up.
    I'd recommend replacing the large gray IC caps as well.
    Those 22 and 47uf ones.
     
  9. dondoucette

    dondoucette Senior Member

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    +100 Save you some headaches and $$ in the long run, those Illinois Caps are 20+ years old and most if not all will be leaking (slowly becoming a dead short) by now.
     
  10. Kuroyama

    Kuroyama V.I.P. Member

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    Clearly I need to keep reading up on Twin history. Its getting a little old! Fender seems to give lots of love and documentation to every amp EXCEPT The Twin which seems to get redesigned every few years... As far as I can tell the "Evil Twin" was made in 96. I understand that old caps need replacing, but maybe my current perception of vintage amplifiers is wrong... As I understood it, many older amps play just fine.

    I THOUGHT some of those values looked a little weird... like one cap thats marked at 350v only pulling in 114v.... or something like that. Are THOSE the questionable caps in question? Id have to go back to find the name but someone else said those values were just fine....so Im a little confused.

    Should all of these capacitors pull display voltages much closer to what they are rated for?
     
  11. SatelliteAmps

    SatelliteAmps Senior Member

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    No. Caps should read under what voltage they are rated for. Some caps are under more stress and susceptible to surge's (especially when playing), so seeing a lower voltage at idle is not necessarily a bad thing.

    If an amp were to run caps right at their max voltage rating, any variation in AC voltage could be enough to push them to fail.

    As far as vintage amps go (including the "evil twin"), there is a general rule of thumb. Electrolytic capacitors have a useable life span of approximately 5-10 years. If they are bulging ever, then they are on the verge of failing. If they are over 10 years old, they should at least be checked (and usually replaced). An amp can function with failing capacitors, but the sound will degrade, and there is an increased chance of multiple part failure if a big cap goes (transformer damage or tube lose).
     
  12. Kuroyama

    Kuroyama V.I.P. Member

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    Well, 5-10 years is a bit shorter than the 13 years those things have obstensibly been in this amp... Sound quality is the reason Ive got my hands in this thing to start with. Im guessing anyone with the common sense God gave orange road cones would START their education in handling tube amps on something like a 5W Fender Champ. NOT a 100W Twin. This probably like owning a GSXR1000 as your first bike. (Or maybe a Harley Flathead, depending on your perspective).

    It looks like my next step it cap shopping. I already know the place. Its a pain in the butt to get to, but Ive still gotta work out the tube testing anyway. Nobody in Kawasaki, or Yokohama (that I can find) wants to do it or recommend someone who will. I think its just a connection thing. Knowing the right person. Everyone Ive talked to wants me to bring in the whole amp or they wont touch anything. Im reading that as a ploy to get into my wallet, though I understand they may simply want to test the tube IN the amp as they dont have a tube tester. In any case, if I pay someone to fix it, then this whole thread becomes moot. Im not in a position to throw in the towel yet... far from it.
     
  13. SatelliteAmps

    SatelliteAmps Senior Member

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    A Champ is just as dangerous as a Twin. Both can kill you if you touch the wrong part.

    Try looking for an antique radio repair shop. Most of those have tube testers in them (and they are usually run by old guys who love it when new people come and show interest in what they love).
     
  14. randelli

    randelli Senior Member

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    You really need to take this amp in for someone to look at for you. Try looking at the sticky for amp repairs for someone in your area - Where are you from?

    Your amp is not a vintage amp, which is neither good nor bad, but you shouldn't concern yourself with keeping everything original. Your amp is filled with PCB's which could be the problem. Your components are most likely fine - they look like they are in good condition - and I bet you will find a short in your PCB is the issue.

    The good news is that you have everything you need to have a really smoking amp! You could easily replace the guts of your amp with a new turret board and components to build your own twin. It would probably cost $100 for the parts and you would be very happy with the results. There are sites that sell drop-in kits for about $250.
     
  15. Kuroyama

    Kuroyama V.I.P. Member

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    Randelli

    While I appreciate the vote of caution, I have a very poor track record with "figures of authority" My own personal experience from rebuilding carburetors, motorcycle engines, to even flying a helicopter... without "going to a professional" first, has given me 30 years of the knowledge that armed with enough cynicism, and knowledge, you can perform most any task that other people are paid for. This culminated in an argument I had with a motorcycle cop who tried "unsuccessfully" to convince me that SPEED was more dangerous than POSITION when riding.

    My point here is that just because a person gets paid for what he does, or even has some form of accredidation saying that he knows what hes doing... doesnt mean he does. Conversely, just because a man does NOT have those things does NOT mean that he is incapable of doing as fine a job, if not better than the "pro".

    Having said all that, a drop in kit may not be a bad idea. Depending on what it could cost me in parts to restore this. Im in the Tokyo area of Japan. As such resources that are taken for granted in the US are NOT always at my disposal here. Its kind of a pain. IMO the key to all of this is patience. Because I live in Japan neighbors are pretty close. Once this thing is in working order, Ill usually only be playing it on weekends anyway. What Im saying is, if it takes me a year to get this thing shipshape then so be it.

    I may even sell it off for a tidy profit. $90 for a Fender Twin is not something you walk away from. As the common thinking goes, I couldve parted this out for more than $90. But, this is my first nice tube amp. Might sell it off for a Blues Junior and another Gibson... and an education in tube amps. Long story short... Ill stay the course for the time being.
     
  16. Cygnus X1

    Cygnus X1 Senior Member

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    How difficult is it to order parts in Tokyo?
    Do you have to get everything by hoof and hand? :D

    To clarify, previous post was accurate but a little long:
    That voltage stamped on the caps is maximum potential.
    It gives zero indication of what they should read.

    By the way, in case you don't already have it-the schem and owner's manual is here:
    http://www.ampwares.com/amp.asp?id=118
     
  17. Liam

    Liam V.I.P. Member

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    +1. I got into repairing amps by doing exacly what you're doing. Got a non-functioning '67 AC30 for a song, and spent the next 6 months figuring out how amps work. Gigged it for a good few years after and sold it for a lot of money last year. I've repaired a lot of amps and equipment in the meantime.

    12 years on, I'm happy to attack pretty well any amp problem. It has been frustrating at times, but your approach of Zen-like patience and acceptance of setbacks is the only one that works. Dropping a board in should only be done AFTER you've got the original one working if you want to go with this philosophy.

    Now you've ascertained that all the power supply voltages look OK, the next step is to start figuring out why there's no signal getting through the amp. Have a look at RG Keen's tube amp debugging page on Geofex if you haven't already.

    New Page 1

    I KNOW everyone's telling you to change filter caps, but I don't think they are your primary problem. If your tubes check out OK, and you've got power supply, check none of the screen resistors are shorted at the tube bases, and check that you've got some bias voltage at pin 5 of the power tubes. Might be on the schematic, -20V to -50V is the usual range.

    If all of that checks out, grit your teeth, you might be about to power her back up with tubes in...

    Liam
     
  18. SatelliteAmps

    SatelliteAmps Senior Member

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    Change the bulging cap. They bulge when they fail. There is no reason not to change it.
     
  19. Cygnus X1

    Cygnus X1 Senior Member

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    <snip>

    Not everyone is, but I am.
    "While you're in there, while you're at it" is fine.
    The one thing I see out of those IC caps is that white looking powder, a sure sign they are starting to give up.

    They are after all a part of tube amp preventive maintenance, the one thing that will pull down at PT, an OT, or other nastiness when they decide to stop blocking the high ripple voltage they are designed to stop.
     
  20. Kuroyama

    Kuroyama V.I.P. Member

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    At the risk of offending someone, there are issues to be overcome in a task like amp troubleshooting in Japan (When you are not Japanese). You might think that language would be primary, and in a way youre right. Ive been speaking Japanese now for about 17 years. Though I disagree, people who meet me refer to me as fluent. Of the 5 foreign languages Ive studied, JP is my strongest.

    Having said that. Im a big Black man whos built like a Marine (thanks Dad). When most people here see me the first time, they presume me to be military. They also presume I dont speak read or write the language. As a result, there is an aversion factor. Most people right out the gate are NOT eager to interact. AFTER I begin talking, usually they turn right around. Some do not. From my experience most - once they realize you intend to spend a lot less money than they might want - will be less inclined to help. Some will not.

    On top of this is the DIY market which is not as strong as it is back home. People here like to consume. They DONT like to repair or make things themselves. So when you go looking for parts or tools... your options become very limited. - not impossible, but limited.

    Now...having said all of THAT Ill restate the original amp problem.

    From 0-1 (volume), the amp sounds OK. One speaker was out (but flipping the cab around I clearly saw the busted lead... so the speaker is a non-issue). The Dirt channel doesnt work at all. (got no idea where to start there which is why Im combing over the entire amp). Lastly once the volume gets to about 2 onward there is a harsh staticky sound and the volume goes WAY up the static like noise persists thoughout unless you zero the volume.

    Bad/dirty pots? I dont know (yet)

    Thanks to all who are helping out here. Your shared knowledge is much appreciated. Hopefully the Evil Twin porn posted here this time will help the next guy with Evil Twin issues. Paying it forward and all that...
     

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