66' twin

[chris61]

hello
I recently put my old black face back in service
but it came back from the shop twice now with a persisting problem
the normal channel sounds like a dream!~~~
but the vibrato channel has a terrible scratchy distortion especially when the the low A string is struck
this noise will bleed over to the normal channel if the vibrato channel volume is up past 4
any help would be most appreciated
thanks
cp

[Ronsonic]

Something microphonic or a dirty tube socket or the like making noise under vibration.

[chris61]

any suggestions on tracking it down?

[Chuck H]

Did you read the first response? Have you tried a pramp tube swap? Have you cleaned and retentioned the tube sockets? Thats probably all there is too it. Get a known good preamp tube and swap it for V1. If nothing changes swap your original V1 for V2... etc. So when you replace the bad tube you'll be holding it in your hand. Simple. If that doesn't work check the plate resistors on the vibrato channel preamp.

Chuck

PS... That amp is probably due for a "cap job" if it's been sitting and the filters are over ten years old. If you notice more hum than usual (or a funny smell or smoke) this is probably why. Don't forget the bias supply caps.

[chris61]

I did the tube test with no change what so ever
well thought maybe it was vibration dependent
but i not sure how to track down the circuit
could it be caps?
I'm not really sure why my tech didn't notice this problem
it really sounds like a short
if you tap on the cab it sounds as if it is shorting out

[chris61]

in addition it only dose this on the vibrato channel
normal channel is as quiet as can be

[Chuck H]

There is a slim chance that it's a microphonic cap. I'm going with a microphonic tube. I don't know how you did "the tube test" but try replacing V2 with a known non microphonic tube. If you used a tube tester they don't tell a thing about microphonics. If a known non microphinic tube in V2 doesn't help I'll go with a cold or broken solder joint in the vibrato preamp. Then I might consider a microphonic capacitor. The fact that the amp responds when you bang it tells me this is the only way a cap could be the cause, if it became microphinic, and that is rare.

Chuck

[booj]

Maybe a bad ground connection to the vib channel? I've seen that happen several times. Sometimes the ground wire breaks right at the edge of the brass pot mounting plate. You have to play with the wires sometimes to catch it.

[chris61]

everything seems to be pointing to me having to play around inside that thing with the power on
something I really have been avoiding but I suppose that the only way it's going to be found.
if everyone's lights go dim ...you know what happened!

[Chuck H]

No ones lights are dim, quite the contrary. It's experience that leads us to our speculations, but speculations none the less. If you are nervous about poking around in an amp with the power on then it's possible you shouldn't. There are safe ways to do it but if you aren't aware of the protocalls and the circuit itself this would account for your lack of confidence. But having been to a tech with no resolution (seems all too common these days) this may be your only option. Ask any Q's here and I'll bet you fix your amp.

DO NOT HOLD ONTO YOUR GUITAR OR ANY OTHER GROUNDED DEVICE WHILE POKING AROUND INSIDE THE AMP!!! use only one hand. If you need to take extra measures to assure this then take the time. DO NOT HOLD THE CHASSIS WITH ONE HAND WHILE POKING AROUND WITH THE OTHER!!! The chassis is grounded and would allow you to complete a circuit. Use a non conductive probe for basic poking around. Learn how to discharge the amp before working on it. Simply unplugging it from the wall is not enough with these devices. You must also discharge the power supply circuit before doing any work inside. Most importantly, when in doubt, don't! Ask Q's first. Don't ever take a chance on anything your not sure about. the consequenses can be FATAL! I make it sound worse than it is, but erring on the careful side has no down side. While erring on the cavalier side can potentially kill you.

A sure way to isolate the problem is with a scope. Put a singal at the input sufficient to cause the problem and then trace with the scope along the AC signal path until you find the trouble. If you don't have a scope you can do the same tests by grounding along the AC path until the problem stops. Now you are able to isolate the priblem.

Look into one of the many faq's about amp safety as it applies to having the chassis out

Good luck,

Chuck

[Enzo]

If tapping on the amp makes a sound, and then I'd assume banging on it would as well, then you have a loose connection somewhere.


Maoe sure all the nuts are tight on teh transformer mounting bolts, and on the controls along the panel.

If this is an actual 1966 fender and not some reissue or home built, then also loosen then retighten all the jack nuts.

If any of those things are loose, it can make large noise.

Beyond that, get a wooden chopstick and poke each and every part going down the part board, end to end. And flex the board itself too. Looking for a cracked solder connection. And yes, look at those bare wires coning off the part board and soldered to the chassis.

[chris61]

the part about lights going dim was a joke
like when i get shorted to the chassis
truly, i appreciate you guys replies and help
this is a genuine 66 and sound like a dream when it works properly

[chris61]

well it seems to be fixed....
I poked around inside and it made an awful racket associated with the cardboard strip where all the resistors and caps mounted.
then I noticed that the screws that were holding it in were all loose, I tightened them down and it sounds perfect now
though I can't help thinking that the demon it's still lurking in there

[chris61]

I decided to recap this amp because the noise was intermittent.
I used this
Fender Pro, Super, or Twin Reverb Capacitor Kit - eBay (item 230406503223 end time Sep-27-10 07:23:12 PDT)
It sounds fab now and very stable
just one question I had though
I used every one of the caps that came in the kit cept one; 1- 100uf 100VDC
can't seem to find a place for it
thoughts anyone?

[JoeM]

That's the bias supply cap. Do you see it on a smaller board in the corner of the amp chassis, close to the pilot lamp? (Not with the other filter caps in the 'doghouse')