ReVox B780 tuning instability: gradually unable to tune in station; from 108MHz and downward

Discussion in 'Tuners' started by yjwu, May 15, 2018.

  1. yjwu

    yjwu New Member

    Hello:

    Recently my ReVox B780 developed tuning problems: it can receive all stations from 88MHz to 108MHz at power on. After about 20 minutes stations at high frequency end became noisy: the signal strength meter's needle oscilated violently. Gradually this symptom migrated toward lower carrier frequency stations while the tuner can still tune-in stations from lower end with no difficulty. An hour or two after power on it can only receive stations below 100MHz.

    Its frequency synthesizer circuit uses SAA1056 and SAA1058/LN1033 (two modulus prescaler ECL). Could it be soldering problem?

    Thank you!

    yjwu Revox_B780_Frequency_Synthesizer.JPG Revox_B780_Frequency_Synthesizer_Example.JPG Revox_B780_Block_Diagram.JPG
     

    Attached Files:

     

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  2. txturbo

    txturbo AK Subscriber Subscriber

    Messages:
    877
    Location:
    Austin, TX
    If you have access to a can of freeze spray you should be able to isolate the bad component or solder joint once the receiver has been operating for a while. It may also be possible to carefully use a heat gun or hair dryer to do this same thing while it's cold.

    The B780 is a great receiver, with a recap and a few updates it sounds great and tunes fantastically.
     
  3. yjwu

    yjwu New Member

    Thank you for your help. I placed a cold pack on top of the Faraday cage of the synthesizer section and it indeed restore the tuning. I found that SAA 1058 was hot while SAA 1056 was normal.

    It is hot and humid in Taipei, Taiwan now. Without air conditioning the room temperature was 28 degree Celsius. I installed fin head sink on top of SAA 1058 and stacked thin metal sheet until the heat sink can thermally contact the Faraday cover. Then I installed a large fin head sink on top of that Faraday cage. 107.7MHz station could stay tuned for an hour at 28 degree ambient.

    This morning I dug out my Tek DC503 frequency counter and did a measurement of its 4MHz crystal reference frequency. Although DC503 had a ovenized 5000.000kHz option installed but its calibration was due long ago in Jan./1991. Use an instrument calibrated 27 years ago (manufactured in 1978) to calibration ReVox B780 (manufactured in 1980) seems to be odd. Since there was nothing to lose I gave it a try.

    Measurement revealed 40000.067kHz. I twisted the trimmer capacitor and push its face value back to 39999.999-40000.000kHz. 87.5MHz end measured 4.49V (should be 4.5+-0V) while 107.95MHz end measured 24.16V (should be 24+-0.2V).

    I had not done the trimmer slug adjustment yet. I want to see the effect of reference frequency correction of this B780.
     

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  4. txturbo

    txturbo AK Subscriber Subscriber

    Messages:
    877
    Location:
    Austin, TX
    Nice work! You should be able to find the IC's and just replace them. I did a quick search and they are available..
     
  5. yjwu

    yjwu New Member

    When B780 was unable to tune in 107.7MHz (104.9MHz was fine) pin6 of IC4 measured 25.35V; beyond the specified 24+-0.2V range. No wonder tuning failed.
    When I removed the screen cover of synthesizer+meter & deemphesis board to access the test point the measured voltage start falling. A few minutes later it dropped to the allowed range, thus 107.7MHz station reappeared.
    It was still unclear how much of the drift was due to 4MHz reference crystal frequency. Efficient removal of generated heat from SAA1058 plays an important role for healthy B780 tuner operation.
    With an A727 stacked on top and operated at 28 degree Celsius ambient certainly invites trouble. Air conditioning to 26 C help a lot.
    This morning I adjusted C15 such that pin6 IC4 measured 23.83V, just fell on the lower limit of 24+-0.2V, in the hope that B780 could still pull in 107.7MHz station under thermal drift at 28 Celsius ambient. No luck. B780 failed to hold on 107.7MHz after half an hour. 104.9MHz was fine.
     

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