It is currently Mon Mar 16, 2020 5:51 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 31 posts ]  Go to page Previous  1, 2, 3  Next
Go to page Previous  1, 2, 3  Next
Author Message
Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Tue Mar 26, 2013 11:44 pm
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Yeah, removing the chassis on a Twin should be done slowly. You have to get your finger or socket up behind that large transformer. Loosen those front strap(chassis-cab) machine screws 50-75%; then loosen the rear screws alternately,until the chassis can rest on the wooden back panel bats. Then loosen the front,until the chassis rests on the baffle board. My point is that, the chassis should be lowered in a square or level fashion. To remove: grip the large trans and a balance point around the filter cap cover. This may sound lame, but you don't want to twist the chassis, goof-up the shielding on the under side of the cab top, or mess up the tolex. Art....Arjay you're way right!!!! That took me a while!!!

_________________
None of Us are free, if One of Us is chained !


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Wed Mar 27, 2013 2:06 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
aclempoppi wrote:
Arjay you're way right!!!! That took me a while!!!


:mrgreen:

I find the output transformer a convenient point of purchase when removing or re-installing a vintage Fender chassis.

Arjay

_________________
"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


Top
Profile
Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Wed Mar 27, 2013 10:17 pm
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Yeah it's balance thing!!! For me it's the PT and another spot, down the line. Pulling the Bassman or the B' Master is a pleasure compared to the big boys!!! You know I worked on my old Twin Reverb(aka. The Queen of Fullerton) so much. I think my left fingers grew an extra inch, to get back there! Any way, it's a bummer that faceplates are no longer available. Rodger's Amplification, built the blond/oxblood cab with a brown faceplate and ivory knobs, back in '89, for that Twin. Got the black faceplate, for the Bassman from him also. Don't know how many years ago that was??? Art

_________________
None of Us are free, if One of Us is chained !


Top
Profile
Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Thu Mar 28, 2013 4:57 pm
Offline
Amateur
Amateur

Joined: Wed Feb 13, 2013 1:31 am
Posts: 110
Thanks, guys. Never pulled it. There are 4 screws that go through the top metal straps. Are there others that need to be removed?


Top
Profile
Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Thu Mar 28, 2013 5:38 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
And the plastic holder on the power cord. Any wires to foot pedals and speakers. You may want to mark the Reverb Input jack, so you can hook the wires up correctly.

Chassis goes out the back end.

Good luck!


Top
Profile
Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Thu Mar 28, 2013 6:58 pm
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Sorry Man, i digress too easily. Yeah just the (4) strap screws, and the power cable retainer, like Beemer said. All tubes should be removed. I usually, make a line(with a Sharpie) on the glass and aluminum shield base to help in realigning the small tubes, for installation. Also a red Sharpie mark on the red reverb jack lettering for the reverb. Art

_________________
None of Us are free, if One of Us is chained !


Top
Profile
Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Sat Mar 30, 2013 3:53 am
Offline
Amateur
Amateur

Joined: Wed Feb 13, 2013 1:31 am
Posts: 110
Thanks, guys. It's been a while, I still have a Heathkit oscilloscope in the basement that I built when I was a kid, and all the other stuff I was building then and later also used tubes, just haven't ever pulled a Twin chassis.

Obviously when I pull it, I'll check the components by appearance, measure resistors by reference to the schematic where it doesn't require cutting leads, and e.g. also look for shorted caps with an ohmmeter. Is it worth going through all the test points and comparing voltages with values on the schematic? Are the values shown "at rest", or with what specific signal applied at the inputs?

What's a good reference to read about biasing?

But is it all that critical to check/set bias if the amp is sounding OK? Obviously people replaced tubes all the time without pulling the chassis and re-adjusting bias. Since the Bias pot is accessible without pulling the chassis, and C- is common for all 4 tubes, how did they set the bias?


Last edited by FernandR on Sat Mar 30, 2013 4:01 am, edited 1 time in total.

Top
Profile
Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Sat Mar 30, 2013 3:57 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
FernandR wrote:
Obviously people replaced tubes all the time without pulling the chassis and re-adjusting bias. Since the Bias pot is accessible without pulling the chassis, how did they set the bias?


Adjusting the idle-bias pot is futile without first ascertaining the plate voltage of the power tubes. The chassis must be exposed and accessible in order to accomplish that.

Arjay

_________________
"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


Top
Profile
Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Sat Mar 30, 2013 4:13 am
Offline
Amateur
Amateur

Joined: Wed Feb 13, 2013 1:31 am
Posts: 110
OK, then it seems to me that

step 1 is to

Not pull chassis yet. Visually check under the cap house cover, cut out any caps that visibly need replacing, check for shorted or open caps, go down to the electronics shop with all the tubes and test them, buy any that show bad or weak, and any caps on the replace list.

Step 2

Return home, solder in any doghouse replaced caps and re-insert tubes. Try it. If that doesn't do it,

Step 3

Pull the chassis and replace any visibly bad components and start troubleshooting with a scope and meter.

Sound like a plausible plan?


Top
Profile
Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Sat Mar 30, 2013 11:21 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
If you can change out the caps under the "doghouse" without pullling the chassis --- you are a better man than I. :D

And if you can do this change without pulling tubes --- you are the Gunga Din of techs!


Please... if you do not want to pull the chassis. BE SURE THE CORD IS UNPLUGGED. And the those caps are drained of stored voltage.


Good luck!


Top
Profile
Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Sat Mar 30, 2013 12:53 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
BMW2002Ti wrote:
If you can change out the caps under the "doghouse" without pullling the chassis --- you are a better man than I. :D

And if you can do this change without pulling tubes --- you are the Gunga Din of techs!


:lol: :lol: :lol:

He taught Nicky Tesla, Tommie Edison, and Alex Bell everything they ever knew.

:mrgreen:

Arjay

_________________
"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


Top
Profile
Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Sat Mar 30, 2013 1:58 pm
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Yeah Man, you want to pull that chassis, for the recap!!! Art

_________________
None of Us are free, if One of Us is chained !


Top
Profile
Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Sun Mar 31, 2013 3:17 am
Offline
Amateur
Amateur

Joined: Wed Feb 13, 2013 1:31 am
Posts: 110
I didn't say anything about replacing caps without pulling chassis. I didn't say anything about not removing tubes.

BMW's "better man than I" reply was to nothing I said.

I said remove cover and examine 'em (with mirror and flashlight). If they look fine, probably no point in removing/replacing them. If they look bad, cut them out. The ones I do cut out, I can test for open/short. I was asking if the specific sequence I was proposing was practical.

Is it that these electrolytics always need replacing, or that there is something blocking the view, so you can't examine them at all without removing the chassis?


Top
Profile
Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Sun Mar 31, 2013 5:26 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
FernandR,

Sorry if I offended you. I did not intend that. And yes, there is a certain length of lifetime for ALL electrolytic caps. Depends on the usage, type, and abuse the amp took,

I recommend pulling the chassis. Flipping the amp over onto a stable working surface. Remove the four screws holding the doghouse cover. Draining the cap of any residual charge. Now, inspecting --- not only the caps, but also the resistors in the PS rail AND the resistors which equilibrate the voltage across the two 70mfd/350VDC caps, of the first stage.

This is the way I've been taught how to do it. And the way most techs do it, esp on Fender amps.

Your amp needs quality parts under the doghouse. Fortunately, good new parts are still available. This will also give you a chance to inspect the rest of the amp's circuitry.

Photos of the amp's wiring and components will help the discussion.

:mrgreen:


Top
Profile
Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Sun Mar 31, 2013 6:52 am
Offline
Rock Star
Rock Star

Joined: Mon Apr 05, 2010 2:37 pm
Posts: 8708
Location: Natural Bridge, Virginia
FernandR wrote:
Is it that these electrolytics always need replacing, or that there is something blocking the view, so you can't examine them at all without removing the chassis?


Let's play "find the dog house", shall we?

Image
1965 Fender Twin Reverb 07 by toyranch, on Flickr

Just pull the chassis as suggested. To try and examine these caps without doing so is foolish and quite probably dangerous.

_________________
Bill

Image


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 31 posts ]  Go to page Previous  1, 2, 3  Next
Go to page Previous  1, 2, 3  Next

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: