It is currently Mon Mar 16, 2020 7:45 am

All times are UTC - 7 hours



Post new topic Reply to topic  [ 11 posts ] 
Author Message
Post subject: Strange Blackface Mod at 1971 Twin Reverb (AA270)
Posted: Sun Aug 26, 2018 7:06 am
Offline
Hobbyist
Hobbyist

Joined: Sat Aug 25, 2018 12:26 pm
Posts: 2
Hi all,

I recently acquired a 1971 (#A34242) Twin Reverb. It turned out, that it has an Blackface Mod. But there are two strange things I cannot explain. Maybe somebody here has an idea.

There is a 0.1 uF capacitor between earth and a rectifier output, which seems to be a facory made one. It looks absolutely identical to the ones at the board. But I never saw any Fender-layout or an amp with this capacitor at this particular place.

https://1drv.ms/u/s!AlJjT8P77BrngfxLvPdWpGCnauk6AQ
https://1drv.ms/u/s!AlJjT8P77BrngfxNNEwSa6XU0ki2fw


Any Idea, what this could be? Or what it should do?

Another thing is, that it has a 10M Resistor between the bright switch and the Vol Pot. Any idea what this could be?

https://1drv.ms/u/s!AlJjT8P77BrngfxO2rXzjktgpGZ05w

Thanks a lot

Christian


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: Strange Blackface Mod at 1971 Twin Reverb (AA270)
Posted: Sun Aug 26, 2018 4:17 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14045
Location: Province de Québec, Canada
csy5fr wrote:


Another thing is, that it has a 10M Resistor between the bright switch and the Vol Pot. Any idea what this could be?

https://1drv.ms/u/s!AlJjT8P77BrngfxO2rXzjktgpGZ05w

Thanks a lot

Christian


ImageCapture d’écran 2018-08-26 à 19.16.41 by [url=https://www.flickr.com/photos/137325788@N05/]

This is a mod


Last edited by stratele52 on Mon Aug 27, 2018 12:58 am, edited 2 times in total.

Top
Profile
Post subject: Re: Strange Blackface Mod at 1971 Twin Reverb (AA270)
Posted: Sun Aug 26, 2018 4:19 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14045
Location: Province de Québec, Canada
More answer with photo on forum

ImageCapture d’écran 2018-08-26 à 19.16.07 by [url=https://flic.kr/p/27HTsT9]ImageCapture d’écran 2018-08-26 à 19.16.23 by [url=https://www.flickr.com/photos/137325788@N05/]


Last edited by stratele52 on Mon Aug 27, 2018 12:57 am, edited 1 time in total.

Top
Profile
Post subject: Re: Strange Blackface Mod at 1971 Twin Reverb (AA270)
Posted: Sun Aug 26, 2018 6:06 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Stratele, your links didn't work, for me. The 0.1mfd cap looks like a bypass, on the power supply first section.

http://www.diyaudio.com/forums/power-su ... ctive.html

Not sure about that 10Meg resistor. Will need to look it up.

:mrgreen:


Top
Profile
Post subject: Re: Strange Blackface Mod at 1971 Twin Reverb (AA270)
Posted: Mon Aug 27, 2018 12:57 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14045
Location: Province de Québec, Canada
BMW2002Ti wrote:
Stratele, your links didn't work, for me. The 0.1mfd cap looks like a bypass, on the power supply first section.

http://www.diyaudio.com/forums/power-su ... ctive.html

Not sure about that 10Meg resistor. Will need to look it up.

:mrgreen:


I edit new ones , is it working now ?


Last edited by stratele52 on Mon Aug 27, 2018 1:38 pm, edited 1 time in total.

Top
Profile
Post subject: Re: Strange Blackface Mod at 1971 Twin Reverb (AA270)
Posted: Mon Aug 27, 2018 1:00 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14045
Location: Province de Québec, Canada
csy5fr wrote:
Hi all,

I recently acquired a 1971 (#A34242) Twin Reverb. It turned out, that it has an Blackface Mod.
Christian



Where do you see Blackface mod ?


Top
Profile
Post subject: Re: Strange Blackface Mod at 1971 Twin Reverb (AA270)
Posted: Mon Aug 27, 2018 1:15 am
Offline
Hobbyist
Hobbyist

Joined: Sat Aug 25, 2018 12:26 pm
Posts: 2
The corresponding Resistors have AB763 Values not like AA270. The trim Pot after the rectifier is connected like AB763 not according to AA270 layout. All typical changes for a AB763 Mod are there. Due to the two 100 Ohm resistors after the pilot light to earth it must be originally a AA270 or AB769. If the resistors are factory, than it's a crazy mixture...
But the yellow cable from the left upper end of the board to the trim pot, which changes connectivity according AB763 is yellow, which is definitely not factory - it must be red and has another quality than the other factory wires.


Top
Profile
Post subject: Re: Strange Blackface Mod at 1971 Twin Reverb (AA270)
Posted: Mon Aug 27, 2018 1:33 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14045
Location: Province de Québec, Canada
The trim pot you are talking about is a bias / balance pot.

A Blackface mod need to remove the rat nest of wiring too.

Did you see inside a Blackface chassis ? Yes it look a crazy mixture


Top
Profile
Post subject: Re: Strange Blackface Mod at 1971 Twin Reverb (AA270)
Posted: Mon Aug 27, 2018 9:09 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Stratele, Sorry getting this.

https://www.flickr.com/photos/137325788@N05/

Looks like the 10M resistor is in parallel with a 100 pico-farad cap (between bright switch and volume pot). Some kind of tone shaper setup. Maybe, reduce the highs, when switch is on?

http://www.electronicstudio.net/schemat ... _schem.pdf


Top
Profile
Post subject: Re: Strange Blackface Mod at 1971 Twin Reverb (AA270)
Posted: Mon Aug 27, 2018 9:14 am
Offline
Professional Musician
Professional Musician

Joined: Thu Aug 07, 2014 3:25 pm
Posts: 1022
The amp has been thoroughly messed with. Lots of parts have been replaced.
The cap to ground is used to quiet diode switching noise.
The 1 meg resistor bypasses the bright switch to give a little brighter sound at low levels with the switch off.


Top
Profile
Post subject: Re: Strange Blackface Mod at 1971 Twin Reverb (AA270)
Posted: Mon Aug 27, 2018 1:41 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14045
Location: Province de Québec, Canada
BMW2002Ti wrote:
Stratele, Sorry getting this.

https://www.flickr.com/photos/137325788@N05/

f


Yes it is the only way you can see my pictures, anybody can look a my picture collection. It is all pictures I have posted on Fender forum.


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 11 posts ] 

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: