Revox B760

Discussion in 'Tuners' started by Coopmv, Feb 10, 2015.

  1. Coopmv

    Coopmv New Member

    Messages:
    1
    I have owned my Revox B760 tuner for a number of years. It is vintage gear from the mid to late 70's. A few days ago, I decided to do some FM listening but to my surprise, I was not able to power up the tuner. It was months ago when I last powered up the tuner. I located the fuse housing for the external fuse and picked up the right fuse based on the owners manual from Radio Shack today. Unfortunately, the new fuse has not changed anything, which leads me to believe some internal fuses might have been blown as well. I am a complete novice when it comes to electronic gears and really do not want to open up the box until I am reasonably sure about what I am doing.

    Can someone who is familiar with Revox equipment care to shed some light on this problem?
     

     

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  2. guiller

    guiller Toscaninichus Australis

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    21,063
    Location:
    Buenos Aires (Argentina)
  3. Peter_HK

    Peter_HK New Member

    Messages:
    7
    There is a small door in middle of the bottom plate that gives access to some additional fuses located on the secondary side of the power transformer.

    The B760 often shows problems with the electrolytics on the power supply board. Sometimes it is sufficient to have a closer look at them to find the faulty part. I also had a defective rectifier that was killed by the defective capacitor.

    A good start is the download of the manual and then a check of the fuses and the DC voltages at the power board

    Peter
     
  4. backmd

    backmd Active Member

    Messages:
    430
    Revox B760 tuner 3-3-18

    No dangerous shorts, but 22 ohms across line cord with the power switch on and off?

    Service manual says transformer primary is always connected and the transformer secondary is switched on and off.

    The voltage selector on the rear of the unit is set at 120 volts. Burning 0.2 amps at 120 volts x 0.85 =20.4 watts. Turns on but no audio output no signal strength with rotation of tuning

    Check power supply board after checking fuses. F1 100ma fuse is blown which supplies the 32 volt regulated supply. Check Q4, Q5 and Q7. Also check C4 measures 627uf (spec is 470uf)and C10 measures 56uf, spec is 22uf however initially it measured a short measures 4000 ohms on the fluke, 40 volts. Q7 BD177 NTE184 checks badly; 2.99 in stock at Fry’s. Q5 tests well and good. Q4 checks well and good. R15 is 3.3 ohms at spec. D4 checks well and good, R3 is grossly OK, measurements are not truly practical measures about 5k with associated components, cannot isolate the resistor easily. D8 and D9 check well and good. R14 checks well and good at 1kohm. R16 and R20 check well and good. C9 measures 12.9 (spec is 10uf). R19 and R13 check well and good. D10 and D11 check well and good. So the Fry’s list now has the fuse at 3.99, NTE184 at 2.99, NTE cap at 1.09; total is 8.07 plus tax. I installed a new fuse. Replacement fuse did not blow at turn on.

    15 volts plus and minus regulated green 15.1 and blue -.872respectively –15.3 now

    22 volts plus and minus unregulated green 20.7 and red 0 now –20.6 respectively

    6 volts regulated yellow 5.93

    32 volts regulated red 6.11 now 32 volts

    5.6 volts regulated red 6.2


    Clearly 3 voltages are faulty. Check C3 for bad 15 volts minus and 22 volts minus. C3 measures no capacitance and 3.3 ohms but is in series with IC2 which is the faulty part or parts. Cap is good measures 2165 and spec is 2200uf. Input of Ic2 appears shorted. However not, the terminals are ground, Vin, Vout. Check Q3! Q3 chirps well and meters well on the Sencore Cricket TF26. Rule out shorted D2. D2 checks well and good on diode tests! C8 is a shorted tantalum 3.3uf at 16 volts on capacitor tester B and K 820, but not shorted on Fluke ohmmeter.

    Check C10 out of circuit for bad 32 volts. Need to pull the board. 2 screws and upward motion, and the board pulls right out. Easy peasy! Warm up soldering iron.

    C10 measures zero uf and the Fluke ohmmeter measures a drifting, so pull and replace. C10 successfully removed and replaced. All contacts measured well and good on the capacitor replacement. Next replace Q7 after pulling base lead and rechecking on the Sencore TF26 transistor checker. Yes indeed Q7 tests bad out of circuit, so remove and replace. However when completely removed from the circuit it tests good on the Sencore TF26 transistor tester. The NTE 184 tests well and good so mount the replacement. Soldered in and connections verified, then leads trimmed.

    R11 checks out fine. R12 measures 5k or so, but has other things connected to such.

    So it appears that the power supply likely is fixed! Also, off topic.
    Rustoleum Chalked "charcoal" is a good color match for the Revox B760. Menards has it and the trade name is Rustoleum, the series is Chalked, the color is Charcoal.
     
  5. backmd

    backmd Active Member

    Messages:
    430
    fixed the center channel meter with a meter transplant from a Pioneer SX737 Revox B760 004.JPG Revox B760 005.JPG Revox B760 005.JPG Revox B760 005.JPG Revox B760 009.JPG Revox B760 009.JPG Revox B760 009.JPG Revox B760 009.JPG Revox B760 009.JPG
     

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  6. backmd

    backmd Active Member

    Messages:
    430
    WOW! Excised the tantalum capacitors in the audio stage, and replaced with metallized polypropylene capacitors; and I could swear the orchestra was in front of me, live!
    I have played first violin for 44 years and it sounds like we are on stage! WOW!
     

     

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  7. opr3909

    opr3909 New Member

    Messages:
    8
    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.
     

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