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Thread: Otari MX5050 MK iii-8 Supply Reel problem

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    Otari MX5050 MK iii-8 Supply Reel problem

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    I've been working on this Otari MX 5050 mk iii-8 and was in the middle of the record alignment and the supply reel stopped working. In play, the take-up reel spins as it should but the supply reel twitches and then nothing. When it had tape on it would just move forward from being pulled by the take-up reel but to the point that it would allow too much slack spilling tape from the supply side until the machine shuts off. STRANGELY, the motor will start spinning after I give it a little umpf with my hand, and it will spin either direction depending on which way I push it. Is this normal??? I thought it was the edit function misbehaving, but after looking into it, the edit disengages the take up reel to spill from that side. I thought it might be the reel size switch, but I took apart and cleaned the primary and secondary switch(the secondary did seem intermittent when testing out of circuit, but I cleaned both and replaced them and I still have the problem. I replaced all the relays in the control board. I reflowed all the solder on the control board just for good measure. I've done the brake/tape tension alignment procedure and though I had to readjust it a second time, it seems to be operating properly. I finally removed the whole brake assembly to get the the motor wire harness and measured 50something volts ac at the motor. I'm thinking it's the motor at this point, however I know these are asynchronous motors and it's getting AC, the motor shaft moves freely and I get continuity when measuring across the coils(the same ohms as the take up reel). Is this somehow a brake issue; I mean is the take-up supposed to give the supply reel the umpf that I gave with my hand while the brake is still partially applied to give back tension or is the back tension supposed to come from the motor alone? Sorry for the lengthy description, but I want to be as clear as possible. Please help!

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    Check the motor capacitors. AC motors sometimes have a start and/or run capacitor. Doing a quick search online the 5050 series definitely have run capacitors, but if there is a starter cap as well and that's failed, it would do what you're describing.

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    Thank you for the response! I didn't mention in the previous post, but pulled the 8uF cap across the coils of the motor to do a quick test to see if it was maybe leaking at higher voltages than my initial DVM meter was able to test for. Unfortunately the problem persists,but from what you're describing, I assume that's the run capacitor. I'll recap the the entire control board and see if that fixes things and report back

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    The other thing I wasn't sure about (and I'm assuming the 1/4" and 1/2" decks are similar here) was that pictures of the 5050 motors often seem to show a daughterboard with a voltage selection. If it's really supposed to be running off line voltage, 50v might be too low. That said, if you were measuring the voltage directly going into the motor, that could be correct. One way to check would be to see what voltage you get across the working motor. The other would be to (carefully!) measure the voltage coming into the daughterboard and make sure that it looks like line voltage. If it doesn't, that may possibly indicate an issue with whatever is turning the motor on and off (probably a triac).

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    You are correct, the daughter board you're describing is in the 1/2" as well. I noticed the jumper to 117V is correctly wired and the fact that it was working initially, I've ruled out user error(for now). As for my last post, after looking at the schematic again I realized that pulling the 8uF run capacitor actually bypasses one of the windings in the motor, so the fact that it didn't improve my situation isn't surprising. I've got another working (same model) machine on my bench today. I'm going to do some comparisons and report back. As for the run capacitor, I'll have to find a suitable replacement. I know Grainger supplies low mic, high v caps that I've used to replace the motor caps in old hammond tone generators. This one from Mouser:

    (I can't post links yet because I'm a new member)
    looks like it might work okay, but the solder terminals are offset which might make it a little tough to fit right.

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    I'm getting 86VAC at the supply reel of the working machine:/

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    Iím not really up on Otari machines, but in my experience with tape machines in general, the motors donít usually die. If anything, itís the motor drive components that may be defective. Look for what driving the motor, check the electrolytics and transistors in the circuit. If I understand your description, when you put it in play, the tension slacks on the supply reel correct? I have seen supply motor drivers turn fully on when they short and the reels wonít move because the supply reel exhibits excessive back tension preventing the tape from moving forward. How does the machine work in FF or REW? Iíve also have seen decks that donít provide back tension electronically. The back tension is provided with some sort of brake mechanism to prevent the supply reel from turning uncontrollably while in PLAY. I sort of doubt thatís the case with an Otari.

    You sound comfortable messing with the machine, so Iíd look at whatís controlling the supply motor and troubleshoot from there. Also always check all your power supply voltages before you troubleshoot anything. A new motor is probably very pricy and tough to find so I would eliminate everything before considering getting a new motor. Good luck and I hope you have a fully functioning machine when you figure out whatís wrong.

    Cary

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    Thanks for the reply and the insight. I'm still leaning towards the run/start capacitors as jpmorris described above. After looking at the schematic again, it looks to be a PCM motor design. The supply reel will spin either direction as long as I help start it with my hand. Since the coils in the motor(supply) seem to read the same ohm as the working motor(take up) I assume that the motor, as you said, is most likely ok. It seems that the starter coil has been taken out of the circuit, most likely due to a faulty cap. Unfortunately, I've got something else on my bench taking my focus so I won't be able to test this theory until later this week. I'll be sure to report back once I know.

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    This is taking me way longer than expected. While troubleshooting the rec level for channel 8 I noticed that these machines are full of the infamous Matsushita electrolytic capacitors and they are leaking all over the place. I'm familiar with these brand caps being prone to failure, but in my experience it's usually the violet colored polarized electrolytics that visibly/excessively leak. In these machines the 33uF bi-polar caps are excessively leaky to the point were they have corroded traces all over the top side of the rec/pb pcbs. I'm surprised this machine works as well as it does in it's current state! Fair warning to anyone shopping for one of these machines; They're still great machines, but something to look out for before purchasing one.

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    Well, after a lot of messing around it ended up being a bad motor, I'm surprised to say. I even pulled all the spark quenchers associated with the transport switching relays before pulling the motor from a parts 2-track machine(luckily I had one of those laying around). Anyway, the new motor worked right away however it was a little noisy, so I dismantled it and gave it a good clean. I thought about replacing the two 608Z motor bearings, but they were too tight around the shaft to remove by hand and I was worried I'd damage something trying to replace them(they seemed to move smoothly anyway. I think the noise was from the shaft/rotor being slightly off center or maybe just a bit of debris had made it's way down into the motor and was causing a bit of noise. Glad that's figured out, I was beginning to get frustrated. Thanks for the help everyone!

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