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Post subject: 1974 Twin vibrato issue
Posted: Sat May 31, 2014 9:36 am
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Hi guys!

I'm new here, this is my first post.

I have an amazing 1974 Twin Reverb, but there's something wrong with the vibrato.

It works just fine, but after I punch the stomp box it takes a good two minutes before it kicks in. When it does though, it sounds incredible.

The one thing I've checked so far is that it's not stomp box related. The reverb stomp works perfect and I switched the cords and it didn't change the situation with the vibrato, nor did it affect the reverb – it still worked fine.

It tends to shorten the start-up time if its warm. If its not been used for a day and I start it and hit the vibrato-button it will take like two minutes for it to kick in. If I use the vibrato at the end of a long gig, then maybe it'll need just 45 seconds to kick in. I have this song with a long piano-intro that we play at the end of gigs. If I punch the stomp just at the end of the song before, then usually it'll be there for me when it's time for my first chord. But that's like the only place I can use it for now. And that's a shame.

Is this a common problem and how do I go about to fix it?

I'm clueless right now.


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Post subject: Re: 1974 Twin vibrato issue
Posted: Sat May 31, 2014 9:51 am
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Possible bad cathode-bypass cap at the V5 stage. This is an electrolytic and if original to the amp, is way past due for replacement.

Arjay

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Post subject: Re: 1974 Twin vibrato issue
Posted: Sat May 31, 2014 2:40 pm
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Have you try changing the 12AX7 tube for vibrato ? you can just switch the V1 with V5. if you do not use the normal channel then you do not need to buy another tube. The Vibrato tube is a mechanical use of a tube rather than a tube for tone and you need 100% gain out of that tube. It has to be a 12ax7 it will not operate with 12at7 , 5751,12ay7 or 12au7 . I would do that first before changing caps .

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Post subject: Re: 1974 Twin vibrato issue
Posted: Sat May 31, 2014 3:44 pm
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Fat tele wrote:
Have you try changing the 12AX7 tube for vibrato?


A tube, once heated to operating temperature and with the correct DC voltages at the plate and control grid, would not require 45 additional seconds to perform its intended task. Do you understand the theory behind an RC circuit as used for cathode bypass? Have a chat with John Markovich and he'll be happy to explain it.

Arjay

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Post subject: Re: 1974 Twin vibrato issue
Posted: Sun Jun 01, 2014 8:42 am
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I understand RC circuit and I would still change the tube first . I would also check all grounds before changing caps .I would not waist time trying to find which cap is leaking currrent I would change all 3 feedback caps and the bypass cap . John Markovich is a great guy.

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Post subject: Re: 1974 Twin vibrato issue
Posted: Sun Jun 01, 2014 9:28 am
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I love my 12-gauge Mossberg M590 but I never use the "shotgun" method to troubleshoot an amp. Quality replacement parts are simply too expensive. There is a single cause for the tremolo issue. Once isolated and identified, it will be easy enough to repair.

Arjay

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Post subject: Re: 1974 Twin vibrato issue
Posted: Mon Jun 02, 2014 6:06 am
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crobo wrote:
Hi guys!

I'm new here, this is my first post.

I have an amazing 1974 Twin Reverb, but there's something wrong with the vibrato.

It works just fine, but after I punch the stomp box it takes a good two minutes before it kicks in. When it does though, it sounds incredible.

The one thing I've checked so far is that it's not stomp box related. The reverb stomp works perfect and I switched the cords and it didn't change the situation with the vibrato, nor did it affect the reverb – it still worked fine.

It tends to shorten the start-up time if its warm. If its not been used for a day and I start it and hit the vibrato-button it will take like two minutes for it to kick in. If I use the vibrato at the end of a long gig, then maybe it'll need just 45 seconds to kick in. I have this song with a long piano-intro that we play at the end of gigs. If I punch the stomp just at the end of the song before, then usually it'll be there for me when it's time for my first chord. But that's like the only place I can use it for now. And that's a shame.

Is this a common problem and how do I go about to fix it?

I'm clueless right now.


Crobo, welcome to the Forum! :D


There are three distinct disc caps --- 0.01, 0.01, 0.02mfd --- (sometimes two discs and one axial cap) in a row, on the circuit board (see layout of link). These are involved in the phase shift timing of the circuit. Check the solder and condition of these caps.

Check the connections and the black cover over the opto-coupler. Since, the three caps and the coupler are relatively cheap, I usually replace them.

Also, check the SPEED and DEPTH pots for wear. Spray a little contact cleaner into them. I like De-Oxit GN5. Spin the pots to work the cleaner onto the contact surfaces. Maybe corroded contact.

You have a DVM? Think that you may need to measure some voltages around that 12AX7 which is in the vibrato/tremolo circuit. I'm not sure which specific circuit that your amp uses, I posted the link to the AA270. Most BF and SF Fender amps use similar circuit topology in their vibrato/tremolo circuits.


Good luck! Keep us posted.


http://bmamps.com/Schematics/fender/twi ... _schem.pdf


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