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Thread: Studer A807 Motor Problem

  1. #1
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    Studer A807 Motor Problem

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    Hi

    I am having a problem with my studer. When I press play, the recorder does play, but the take up motor (right hand side) moves slower than the supply motor (left hand side). What happens is that the excess tape from the left reel gets stuck into the reel. It looks like both the motors are not synchronized. This player had a tape tension problem earlier. The "tape tension sensor coil" was replaced a few months back and after that it was playing fine all the while until now. I have also done a complete recapping more than a year ago. What can be the problem.
    Today, I realised that the tape tension lever appeared to be loose, so I tightened the spring attached to it. I then readjusted the tape tension ( +4V at minimum tension and 0V at maximum tension) . When I press "play" surprisingly it was playing as normal. I tried playing at all 3 different speeds, fast forward, stop and play, then rewind and play several times and there seem to be no problem. I left it On for some time and then recorded some music at 15ips on 1 full tape. Rewind back and play, all was fine. I then turned OFF the player for a few minutes, turned it ON and play, then it started to play at a very fast speed and the right hand motor was moving slower than the left hand motor and the tape start spilling out. At times, if I turn ON the player, and then press play after a few minutes later, it works normal. I am rather confused on what the problem can be. Can it be the capstan motor, if it is the motor caps problem, then how come it worked as normal, recorded as normal and played back as normal.
    This is something very unusual I realised today, Whenever I press "play", the capstan and pinch roller are moving unusually fast. Even when I select different speeds (3.75ips, 7.5ips, 15ips) during play, the capstan and pinch roller speed remains constant(fast). The other thing I noticed is that, when I set to "varispeed", altering the varispeed pot from minimum to maximum does not effect the capstan speed at all. It remains constantly fast. Since the pinch roller is feeding the tape faster to the right hand motor(pulling faster from the left hand motor), it appears that the right hand motor is moving slower than the left hand motor and not able to take up the tape in time, and the tape overflows and gets stuck into the reel. If I am not mistaken, I guess the problem in on the capstan motor board or can it be the command(tape deck electronics board). Please assist.

    Thanks

  2. #2
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    Hard to say. The motor controllers are pretty sophisticated on these machines, each one has a PROM chip containing a lookup table of field strengths for the field windings. That said, I've seen one of the reel motors go very slow when a capacitor has failed. The spooling motor board has a daughterboard with about half a dozen electrolytics on it. After I recapped it, I put one of them in backwards and it let out the magic smoke. Shortly before the smoke appeared I could tell something was wrong because it was really, really struggling to rewind.

    In your case, I think we need to narrow the problem down more. You've mentioned both the takeup reel running slow, and the capstan running fast. When it's malfunctioning, does this only happen in PLAY mode, or are fast wind modes also going wrong? The capstan won't be doing anything in fast-wind modes so that may help us determine if it's the capstan controller, or something more central.
    Also, if it's ignoring the varispeed control, what happens when you put the machine into reverse? (SHIFT-PLAY) Does it ignore that too?

    EDIT: Thinking about it, if we assume that the capstan controller has gone crazy, I'd suspect that the servo loop has broken somehow, i.e. the controller isn't getting tach back from the motor. There are a couple of clock frequencies marked on the schematic which should be 300Hz and 600Hz respectively. If it comes down to the controller, it would be nice to know what frequencies you're seeing. But I'm not sure how easy that would be do to... I remember it being a total pain getting the controller boards in and out.
    Last edited by jpmorris; 4 Weeks Ago at 07:09.

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    I think that the take up motor is not actually running slow but running at the correct set speed. Due to the capstan pushing the tape very much faster, it is not able to take up the tape as fast as the capstan is pushing it, it appears that the take up motor is running slow. The fast forward and rewind are normal, no problem. I have not tried the (SHIFT-PLAY) yet. Can it be some faulty transistor or some other component on the capstan control board which is making the capstan go crazy. I actually do not know where to start troubleshooting.

    Thanks

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    Look for TP2 on the capstan motor controller board. Hook it up to a 'scope and make sure you're getting a periodic waveform. Then hook it up to a frequency counter (good multimeters can do this). You should get a specific frequency, depending on the tape speed: 300Hz for 3.75, 600Hz for 7.5, 1200Hz for 15IPS and 2400Hz for 30IPS.
    If you're not getting a periodic waveform, that probably means that the tach signal from J3 isn't coming in, which might indicate a problem with the tach disc or sensor inside the motor housing. Make sure that J3 is properly connected to the capstan controller board.

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    OK, I will do this and get back to you. My scope is rather old and am not sure it is functioning well, but my digital multimeter has a frequency measure function. I will let you know once I am done.

    Thanks

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    Minor clarification: you'll need to check for the tach pulse on TP2 while the machine is playing.

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    Today, I took the readings at Test Pt2(Tacho). There is a 7.5V DC reading. When I switch ON the recorder, my digital multimeter shows a reading of around 75 Hz. When I press play, the reading goes up to around 7.2 to 7.5KHz and more at times. Even though I select different speeds, the frequency remains same. On the scope it shows a sine wave. At Test Pt3(commutation freq), the frequency is also around 7.2 to 7.5KHz and reads approx. 2.4VDC and also does not change with speed select. Something surprising happened when I was taking the frequency readings at Test Pt2 For a brief moment, the recorder started to work normal, the capstan slowed down and was following the speed selction. I let it run at 7.5ips and it was running at this speed correctly for a brief moment and then suddenly it took off to a high speed. The capstan is running so fast that I can hear the whirling sound. I checked a few small transistors, all of them seem to be OK except Q1 (BF366). I took out the transistor and did a diode check with my DMM. It shows a diode drop(0.6V) from base to Emitter but OL from base to collector. If I am not mistaken, Q1 with L1 forms then 5.5MHz oscillator and linked to the capstan Tacho. I need help in tracing what could be the problem. Please assist.

    Thanks

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    Well, anything involving FM demodulation is beyond my level of electronics, but at a guess (which may be wrong) it's using the 5.5MHz combined with the tach pulse from the capstan motor to get a beat frequency which is the 300-2400Hz clock on TP2 (or in this case, 7.2KHz which should not be happening). So yes, if the 5.5MHz oscillator has gone wrong, you're going to get some really weird effects.
    If it's still ending up with 7.2KHz at the point where it compares the tach to against the 9.6KHz reference, then I can see it running the motor at high speed to try and match it.

    If it were me, I'd definitely try replacing Q1, but if that doesn't fix the problem, you may need someone with a better understanding of electronics.

    (EDIT: Oh... 9600-2400 is 7200 Hz I wonder if that's a coincidence? EDIT EDIT: Probably, because that's 30 IPS)

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    As someone suggested that there might be loose connections or dry joints, I took out the capstan board and resoldered the whole board. Since I am there I decided to check all the components on the board. All the elect caps tested good, the resistors, diodes and all transistors except Q1 also tested good. I sprayed contact cleaner into all the sockets, cleaned all the pins, put them back and made sure they are intact. After putting back everything, I turned ON the recorder, set the speed to 3.75 and pressed “play”. The capstan is no more running crazy, but the speed for 3.75 was rather fast. The frequency at Test Pt2 was reading above 500Hz, but it started to increase constantly and the capstan speed increasing. By the end of the tape, the frequency has gone up close to 7KHz. The “varispeed” works, but at minimum(full anticlockwise) it read at about 1.2KHz but that too started to increase. I am quite confident that all the discrete components on the board are OK except Q1. Can Q1 be the problem, if not one or more of the ICs, or the Tacho. I am not able to get the frequency at Test Pt1 (5.5MHz) as my DMM can only read up to 100KHz. I will replace Q1 and keep you informed.

    Thanks alot

  10. #10
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    Well, I could not get this transistor Q1(BF366) but I manage to get the replacement BF200. I put it in but no difference. So I decided to try replacing the whole capstan board. A friend of mine has the same Studer A807 recorder, it is old but still in good working condition. I manage to convince him to let me take out the capstan board and try it on my recorder. Guess what, it works perfectly well. So some component on the board is the problem and the tacho inside the motor is OK. As I have checked all the resistors, caps, doides and transistors, I guess one of the ICs or L1,L2 or L3. I am still not fully convinced yet as This recorder appeared to be normal twice while testing earlier and then went crazy. I will keep running it at random and see whether the problem is really solved. I will then transfer the ICs from the problem board to this and see which IC is problematic. I will keep you informed.

    Thanks.

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