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A problem with my Revox A77 MK3

Discussion in 'Vintage Analog Gear' started by gaetan8888, Nov 16, 2012.

  1. gaetan8888

    gaetan8888 Active Member

    Hello

    I have a Revox A77 MK3, sr: G170460, two tracks, with 7 1/2 and 15 ips speeds. I did not used it since some times.

    I have a problem, wen I turn it on, even wen I did not push any recording buttons, and without any input signal, the vu-meter are topping to the maximum and still topping to the maximum until I turn my Revox off.

    And wen I record music or sine signals (7 1/2 and 15 ips ), I did push all the buttons for recording, but there is no sound on the recorded tape (Scotch 250)

    All the motors run good.

    I've look in the service manual but I did not find a solution in that manual.

    Any hints or solutions ?

    Thank you

    Bye

    Gaetan
     
  2. RemyRAD

    RemyRAD Guest

    This certainly isn't a new machine. And along with these older machines,, a lot of intermittent internal connections are frequently the largest source of problems. Now this sounds to me like it could be some kind of power supply problem?

    So first, unplug the AC power from the wall outlet. So, just in hopes that it's not a power supply problem, if ya remove your box, you'll be able to get to the active audio electronics cards. They are being held in place by a metal bar that is screwed to the chassis. Carefully, unplug the audio cards. Get a Q-tip and some isopropyl 91% alcohol, available at your local pharmacy. Clean the edge connectors of those cards. Plug them back in. Replace the metal bar. Plug the power cord into the wall and switch the machine on. The meters continue doing the same thing? You may likely have a power supply problem? This will require a professional service technician to correct that problem. Of course if you are adept with a soldering iron, understand power supplies, you could give it a go yourself.

    I don't have my service manual in front of me but as I recall there may be a couple of internal soldered fuses? You might want to look for those and see if any of them have blown? Most of the electronics in this recorder rely upon a single polarity power supply and not the ± bipolar supplies of most integrated circuit chip based devices. These machines are hard to kill and rarely fail. I have probably one of the worst used machines, I've ever seen much less purchased. After a good cleanup, and adjustments of speed since the capstan shaft had been greatly worn down, the machine passed and even exceeded the specifications of brand-new Scully's that I was checking out as Quality Control Manager/Final Test Technician at, Scully. This machine had been dropped multiple times, sticky stuff dripped all over it, anything that could be rusted was rusted. Whatever could be missing, was missing. And it still passed specifications. It got the president of Scully extremely upset LOL. Couldn't believe this used piece of crap could exceed the specifications of brand-new $2500 stereo machines? So it might be a filter capacitor that has finally and completely dried up? So if ya really want a first rate machine, you might want to consider replacing all electrolytic capacitors within the machine. This is part of normal maintenance with most all of this professional equipment over a fairly broad range of time. Your car might last more than 100,000 miles but not without an occasional tuneup and oil change, right? Same for a tape recorder. Same for old vintage consoles and other audio processing equipment. The first thing we do when we are restoring old equipment is to first, replace all electrolytic capacitors. Because electrolytic capacitors are very much like small rechargeable batteries. After a while, they don't hold a charge anymore and therefore do not do their job.

    If you like to change the capacitors, Panasonic's are known to be some of the best. Folks like Jim Williams, offers specialty modifications utilizing nothing but non-polarized capacitors. Tantalum are another type of much smaller electrolytic capacitor which are actually not good for audio. So stay away from those even those those types do not fail over time very often. But when they do, it's usually rather horrific as they short out and burn holes through circuit boards. Not something you want to have happen. So replace the electrolytic capacitors with similar devices and you'll be good to go. Don't put in any modifications. Restore it as per original specifications. They are great sounding machines that are actually heavier in musical even order, second harmonic distortion than most of their competition. And that's because most of the circuitry has been designed as class A, transistor circuitry. No class A/B, crossover notch distortion like most others have which produces odd order, harmonic distortion which is dissonant sounding and does not occur in real life as even order harmonic distortion does. So they are very musical sounding machines.

    I believe service manuals are still available? I know somewhere, I still have mine. Original hard copy version. Not scanned in yet for electronic distribution. Sorry.

    Worth repairing
    Mx. Remy Ann David
     
  3. gaetan8888

    gaetan8888 Active Member

    Hello

    I have take out the bias board and the vu-meter are still topping to the maximum.

    I have take out the playback amp board and the vu-meter are still topping to the maximum.

    But wen I have take out the 2 record amp board, the vu-meter was no more topping to the maximum.

    So, I think it's the 2 record amp board who are the problems.

    Could it be the tantalum (or any others capacitors) on the record amp boards who could do the problems ?

    Thank you

    Bye

    Gaetan
     
  4. RemyRAD

    RemyRAD Guest

    I believe the metering is somewhat dependent upon where you have your monitor switch selected for input or NAB or CCIR, Playback equalization? So if the playback cards in and your monitor switch set to NAB Tape monitor playback, can you play any pre-recorded existing tapes adequately with proper meter deflection? Your output playback control should be at approximately its two o'clock position for playback. Make sure that it plays with the existing playback cards.

    It does appear by your descriptions, the problem could be isolated to the input record card? But you also need to verify Playback from tape before we can rule out power supply issues. The output of I am control along with the balance control affect how the meters respond on playback. Make sure no record ready buttons are depressed. Check for successful playback capabilities. We'll move on from there.

    We're getting there.
    Mx. Remy Ann David
     
  5. byacey

    byacey Active Member

    Try giving the record / playback multi-contact switches a shot of contact cleaner.
     
  6. JoeH

    JoeH Well-Known Member

    I still think you're just shooting in the dark until you verify the power supply is working ok. Ditto for all the solder joints, edge connectors, etc. When any machine sits around unused for a long time, weird things happen to any metal-to-metal contacts, etc. Unlike metals will oxidize, capacitors will dry up, leak, lose their charge, etc.

    Does it play back anything from old tapes? Have you cleaned & demag'd the heads, run a test tape to see if it's even running at speed & proper output? Is the output clean and quiet, with or without tape playing, in input or repro mode?

    I'm just saying it might NOT be the record cards; that could just be another symptom. You really to grab a service manual and check all the voltages, etc., before poking around with the cards. If the voltage are OK, then sure, start looking at other possibilites.

    It sure sounds like a ground has opened up somewhere, among other things. Look at all solder connections and make sure none of them have decayed, dried out, split open, etc. I remember an MCI-JH110 power supply had literally unsoldered itself over time, due to the high amperage (and poor soldering) in the main rail conections.....
     
  7. gaetan8888

    gaetan8888 Active Member

    Hello

    I think I have found the problems. I've tests some capacitors with my capacitance meter.

    First, the two recording amp C509 electrolytic capacitors, who connect to the vue meters, are short, so dc are going to the two vue meters.

    And the record relay circuit C602 electrolytic capacitors are dry and do have near 0 uf, so the relay was not working since C602 need to be full to switch on Q601 and the relay.

    I will do more tests, but it seem that I will need to replace all electrolytic and tatalum capacitors in my Revox, just in case they are dry or defects.

    Thank

    Bye

    Gaetan
     
  8. JoeH

    JoeH Well-Known Member

    Sounds like you've got a bead on it. And I think you're implying you'll replace them ALL (including all power supply caps) and see where that takes you. Nothing like starting with good, solid DC levels everywhere, then moving on to the rest of any issues you'll find.

    Good luck!
     
  9. byacey

    byacey Active Member

    Did you test the capacitors removed from the circuit? I don't think you'll find much in the way of tantalum caps originally in an A77; I'm not sure about the MKIII.
     

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