Revox a700 back tension

[geoff25]

Hi everyone,i am new to audiokarmer so please forgive me if i get anything wrong.
I have got a revox a700 tape recorder thats got a problem.
When you first switch the recorder on and run a tape from cold there is very little back tension but as the recorder warms up the back tension increses until there is so much it starts running slow. The efect is worse with 7" spools than with 10.5".
I have replaced all the fraco caps in the power supply and thought
(hoped) that this was just dead capacitors somwhere else in the recorder
please can someone help as its a great recorder apart from this.

 

[tapetech]

How are you measuring BT (back-tension)? Is the left tension arm at the proper height as described in the service manual? Or have you not measured back tension and this is your guess?

If left tension arm is at improper height, then clean the plug for the tension arm circuit board. or clean the tension pot and adjust for prescribed arm height.

Do you have proper pinch roller pressure?

 

[geoff25]

I am making a guess in that on touching the tape between the left hand tension arm and the large roller there seems to be much more pull after the recorder has warmed up.
On 7" reels it is much worse.
I will try cleaning the contacts on the tension arms pcb and the tension pot.

 

[Thierry]

Hi everyone,i am new to audiokarmer so please forgive me if i get anything wrong.
I have got a revox a700 tape recorder thats got a problem.
When you first switch the recorder on and run a tape from cold there is very little back tension but as the recorder warms up the back tension increses until there is so much it starts running slow. The efect is worse with 7" spools than with 10.5".
I have replaced all the fraco caps in the power supply and thought
(hoped) that this was just dead capacitors somwhere else in the recorder
please can someone help as its a great recorder apart from this.

Hi,

Take the right tape sensor out (picture).
Renew all of the blue caps
Refit
Adjust height of both tape rollers as per the Service manual with the trimmers.
Enjoy.

Thierry

 

[GPS16]

Hi,

Take the right tape sensor out (picture).
Renew all of the blue caps
Refit
Adjust height of both tape rollers as per the Service manual with the trimmers.
Enjoy.

Thierry

Hi Thierry,

am I missing something here!!! Why deal with the RHS unit for a back tension problem. Shouldn't this be the LHS unit???

GPS16

 

[Thierry]

Hi Thierry,

am I missing something here!!! Why deal with the RHS unit for a back tension problem. Shouldn't this be the LHS unit???

GPS16

Nope, all failing capacitors are located on rhight hand sensor. When they go havoc it f..cks tape tension both ways. LHS sensor is fitted with film caps, they last forever.

Thierry

 

[GPS16]

Nope, all failing capacitors are located on rhight hand sensor. When they go havoc it f..cks tape tension both ways. LHS sensor is fitted with film caps, they last forever.

Thierry

Thanks for the info. You learn something every day!!!

GPS16

 

[geoff25]

Thanks for the info thierry.
I looked at the right hand board but my a700 must be an early one as it dos,nt seem to have the three blue caps on it in fact the component lay-out seems slightly differant to the one in your picture.

 

[Thierry]

Thanks for the info thierry.
I looked at the right hand board but my a700 must be an early one as it dos,nt seem to have the three blue caps on it in fact the component lay-out seems slightly differant to the one in your picture.

Hello there,

Which colour are the tantals ? Are they the dreaded Red Devils ? Whichever they are replace them, that is a good start.

And please post some pics as I'd be interested in seeing a different layout. Mind you, my A700 has serial 19998, which logically makes it the antepenultimate A700 built out of 20000 machines assembled in Regensdorf, Switzerland. This may explain why the layout differs from yours.

Thierry

 

[geoff25]

Hi thierry here is the picture
my a700 is number 03535 so i suppose it is quite early.