My Revox B 760 tuner fix up project

Discussion in 'Tuners' started by w1jim, May 13, 2017.

  1. w1jim

    w1jim I can fix it but good... Subscriber

    Messages:
    7,566
    Location:
    Boston western 'burbs
    I picked up a really cool Revox B-760 (I know, cool and Revox is redundant) that needed a little love.

    Problem #1 was that the tenths digit on the display would show 0 & 8 instead of 0 & 5.
    The driver chip was an 9317BPC (no cross reference found). I looked through the data sheets and decided that an SN74LS47 would do the trick.
    After pulling out the suspect IC I installed a socket just in case I was wrong (yeah, that's happened) and installed the replacement.
    After reassembly it worked!

    Next problem is that the signal strength meter doesn't work.
    I poked around (and recapped) the logic board but have yet to locate the problem - I'm thinking it's either the meter movement or the driver transistor. I also used some deoxit on the adjustment pot.

    Finally I recapped the audio board - replacing all of the tantalum with audio grade Nichicon electrolytic.

    IMG_8200.JPG


    IMG_8201.JPG


    IMG_8199.JPG


    IMG_8198.JPG
     

     

    Please register to disable this ad.

  2. w1jim

    w1jim I can fix it but good... Subscriber

    Messages:
    7,566
    Location:
    Boston western 'burbs
    The audio board - before and after:

    IMG_8187.JPG



    IMG_8197.JPG
     
  3. w1jim

    w1jim I can fix it but good... Subscriber

    Messages:
    7,566
    Location:
    Boston western 'burbs
    Nudies:

    IMG_8186.JPG


    IMG_8203.JPG
     
  4. w1jim

    w1jim I can fix it but good... Subscriber

    Messages:
    7,566
    Location:
    Boston western 'burbs
    And some glamour shots, along with another project; good thing the Revox is built like a tank since the Sansui ain't no light weight!


    IMG_8204.JPG


    IMG_8206.JPG
     
    nedseg likes this.
  5. Raynald

    Raynald Addicted Member

    Messages:
    5,149
    Location:
    Montreal
    That is beautiful, good work fixing it up.
     
  6. w1jim

    w1jim I can fix it but good... Subscriber

    Messages:
    7,566
    Location:
    Boston western 'burbs
    Wow - after much research on a replacement signal strength meter I got it to work with just some simple tapping!
    Mad skills here.

    IMG_8309.JPG
     
    musichal likes this.

     

    Please register to disable this ad.

  7. Raynald

    Raynald Addicted Member

    Messages:
    5,149
    Location:
    Montreal
    Must have sourced their meters from the same supplier as Luxman, I had a T-110 that required the same 'fix"!
     
  8. w1jim

    w1jim I can fix it but good... Subscriber

    Messages:
    7,566
    Location:
    Boston western 'burbs
    I think there was some foreign gunk in the movement and tapping dislodged it.
    Prior to that I used a powerful magnet to nudge the pointer - that probably helped to break it loose.
     
  9. jblnut

    jblnut Don't Overthink It Subscriber

    Messages:
    2,353
    Location:
    Boston Area
    Nice work as always Jim ! That is a very serious looking tuner for sure. Although the Sansui is screaming out to be sitting on a TU-X1 :)

    jblnut
     
  10. w1jim

    w1jim I can fix it but good... Subscriber

    Messages:
    7,566
    Location:
    Boston western 'burbs
    Oh, I know - or even a TU-9900.
    I'm thinking of bringing this set up to the FrankenFest - if my back can hold out.
     
  11. Champco

    Champco Super Member

    Messages:
    1,708
    Location:
    Twin cities
    Silly question on the before and after photos on the audio board.
    The blue caps are the new replacements from the foil wrapped?? About 13 of them?
     

     

    Please register to disable this ad.

  12. w1jim

    w1jim I can fix it but good... Subscriber

    Messages:
    7,566
    Location:
    Boston western 'burbs
    The little blue caps in the first picture are the original tantalum caps. While they are stable and won't dry out they are not well regarded in the audio path, plus when they fail they often do so catastrophically. I replaced them with audio grade Nichicon electrolytics as seen in the second picture.
     
  13. Champco

    Champco Super Member

    Messages:
    1,708
    Location:
    Twin cities
    Thanks for the response. i have been out of electronics for a long time now am relearning. it surprised me when reading on other forums, on era radios like my Moniker where they were recapping almost everything, first thing. But they were from the 1930's so it had value. Now I see it is almost standard on 1960's 70's?? Where can I find more details on why these caps are going bad and what is happening??
     
  14. w1jim

    w1jim I can fix it but good... Subscriber

    Messages:
    7,566
    Location:
    Boston western 'burbs
    Regarding recapping - which, while I'm an ardent fan of, is often not necessary.
    Generally electrolytic capacitors are rated with a 25 year lifespan - this doesn't mean on the day after the 25th year they are junk but just that they are designed to be reliable for that time span.
    In theory after that point they will drift out of range. Besides age, exposure to heat can dry out the fluid within and the rubber seals.
    In actuality I think that they will last longer, but given the obsessive tendencies of us "audio enthusiasts" we often will find the OCD need to change them out. Changing them often (IMHO) is more an exercise in reliability.
    Also, it is generally believed that today's modern caps sound better (this is open to debate of course) but it's fair to say that with advances in materials newer caps are smaller, more reliable (more on that below) and can withstand higher temperatures.

    Then - often on equipment built between 1999-2007 (approx) there is the "capacitor plague" where apparently millions of caps where built with a defective electrolytic formula.
    More on that here:
    https://en.wikipedia.org/wiki/Capacitor_plague

    http://www.eetimes.com/author.asp?doc_id=1329037

    So while often recapping isn't entirely necessary I do believe it is a safe thing to do and it feeds both our audio OCD tendencies and out need to tinker with our gear in an attempt to make it better.
     
  15. Champco

    Champco Super Member

    Messages:
    1,708
    Location:
    Twin cities
    Loves me some OCD. Unfortunately (?) not one of my traits however as my wife continues to age she gets busier. Seriously could eat off my kitchen floor.
    Taught her how to wax a car. She washes hers then waxes it. Then pulls my Explorer up and starts on the inside. I cant help myself have to pour myself a beer and sit on the front patio and watch. Maybe a soldering iron for Christmas is in her future.

    Seriously I have reason for the inquires so thanks..
     
  16. KiM3Ce

    KiM3Ce AK Subscriber Subscriber

    Messages:
    7,886
    Location:
    Arizona
    Nice thread and good work!
     

     

    Please register to disable this ad.

  17. opr3909

    opr3909 New Member

    Messages:
    9
    I have also owned a Revox B760 tuner for many years. It suddenly broke down last month and no radio stations could be received.
    Having got the schematic in hand, I started to troubleshoot it and found the following 3 strange phenomena in the Local Oscillator Board:-
    (1) At 87.50 Mhz, the tuning voltage should be 4.5V at Test Point 1 but it is increasing gradually to 31.1V. There was no effect on turning the adjustment slug in coil AP1;
    (2) The DC voltage measured at P2 was 1.4V at the beginning and it is decreasing gradually to zero;
    (3) The voltage measured at TP3 on the Syntheziser Board is 3.5V. It should be between 0.2 - 0.6V.
    I don't know what is wrong with the tuner.
     
  18. rcs16

    rcs16 Super Member

    Messages:
    3,795
    Location:
    Baldwin, Ontario, Canada
    I believe that something is faulty in the PLL/VCO ckt. Not so easy to find what is gone. Adjusting the VCO/LO coil will not fix this issue, you have to dig into the pre-scaler/divider, phase detector etc. I have never worked on this model, so can not offer much more advise other than general TS steps. Could check power supplies as a start. Basically need to send it to someone who knows how to fix a tuner like this if is not something simple.
     
  19. SaSi

    SaSi Seriously Illogical Subscriber

    Messages:
    4,154
    Location:
    Athens, Greece
    Have you replaced all yellow FRAKO capacitors?
    When I get weird behavior on a Revox unit, that is the first thing I do. I know I should replace them asap but anyway, even if I haven't done that, I would not mess around with anything else that seems to malfunction before I replace the caps.
    I've had two separate crazy problems on B710 decks and they both ended up being caused by the FRAKO caps, and a couple of small, Philips electrolytics in the power supply. Started with them to make sure I have clean and reliable power and the weirdiness went away.
     
  20. opr3909

    opr3909 New Member

    Messages:
    9
    Yes, I have changed all the electrolytic capacitors on the power supply board as well as the local oscillator board. The voltages from the power supply are all O.K. but the problem is still there. Maybe , I will try rcs16's suggestion to see if there is anything wrong in the phase detector etc. Anyway, my sincere thanks for your responses.
     

Share This Page