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MCI JH24 head shield issue Recorders, Players & Tape Machines
Old 12th October 2018
  #1
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analogtodd's Avatar
 

MCI JH24 head shield issue

I've got an MCI JH24 that isn't doing what its supposed to do!

It looks like a solenoid issue with the head shield... I can't think what else it would be, if anyone has any insight, that would be great.

I'm attaching a video that shows/describes whats going on.
YouTube

Last edited by analogtodd; 12th October 2018 at 11:11 PM.. Reason: youtube link
Old 13th October 2018
  #2
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Put your ohmeter on the solenoid, check the coil is there. Check the volts are getting to the solenoid. If it has volts, and the coil is good, it'll be a cleaning problem.
Old 13th October 2018
  #3
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Also there's a tension screw on the solenoid that provides a drag adjustment. I might be set wrong.

Also, if the plunger is magnetized, it can hang up. In that case, degaussing it might help.
Old 13th October 2018
  #4
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The #1 suspect (and the easiest to diagnose) is the coil of the rotary solenoid.
Unplug the solenoid leads and use an ohm-meter to measure the resistance of the coil.
If it is open (infinity) then the solenoid coil is burned out and the solenoid must be replaced.
If the solenoid coil has some reasonable resistance (I would imagine 100s or 1000s of ohms) then maybe a mechanical problem.
If the arm is disconnected from the solenoid, does it operate properly?
It is a pretty good size solenoid, so I would expect that it would have a robust mechanical power.

When you moved the arm by hand, it looked like it was pretty sticky (or maybe that is just normal operation of the dashpot?)
Perhaps the solenoid is trying, but the mechanism doesn't move smoothly.
Do you hear anything from the solenoid? If you touch the side of the solenoid can you feel anything when it gets power?

You mentioned that you had another machine. Is it operable?
When you move the arm by hand, does it feel the same? Looser? Tighter?
Old 15th October 2018
  #5
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I serviced JH-24's for about 30 years and fixed head shield problems (which were very common on those machines) at least a hundred times but while anything's possible, I don't think I ever had to replace that solenoid. Do the checks that everyone mentioned, which should tell you a lot, and hopefully it's something simple. The problem might also be the solenoid driver board.
Old 16th October 2018
  #6
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Ok, spent some more time on this today...
Demagged it just for fun, no change.
Swapped solenoid driver boards, no change
Tested solenoid, and its getting 22v and it looks like it has resistance, 14ohm (I tested my other JH24 and got the same result, so I assume that must be ok)
Everything for the lifter/head shield comes off J1 on the motherboard, and I've got good continuity, so I don't think its a molex problem.
I did examine and test every component down around the solenoid, the switch, and a couple resistors and a diode, and it all looks like it should.

When hitting buttons like FF where I'd expect some mechanical noise coming from the solenoid, I don't hear ANYTHING, so its not like its stuck and its just not working, its like its not getting logic command, but the logic annunciator looks good. I swapped out logic boards with known good and got the same result.
I'm not sure what else to do....
Old 16th October 2018
  #7
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If you take out the three screws and remove the arm from the solenoid and let the solenoid operate by itself?, does it pull-in (and turn)?
It should have quite a kick. Don't get your finger between the disk and the coil, you could lose a fingernail. Or the end of your finger.

It seems odd that one of the three screws is a different kind and looks like it has a hex nut under it.
Is that screw too long? Does it poke down below the solenoid disk and prevent the solenoid from pulling in?

Old 16th October 2018
  #8
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The one longer screw with the nut is normal. That's the one I mentioned earlier, that is there to add an adjustable amount of drag to the mechanism. When adjusted correctly it prevents the shield from slamming down at the end of its travel. But if it's mis-adjusted it might be causing the solenoid to freeze.

Does the solenoid draw current with voltage present? Does it get warm?
Old 16th October 2018
  #9
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Been running for awhile, the head shield solenoid doesn't feel warm, but the tape lifter solenoid doesn't feel warm either.
Old 17th October 2018
  #10
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I would loosen the one long screw and see if it actuates with 22V across the leads. If it doesn't, it's bad.

As further tests you might check to see if it draws current, and see if you can rotate it by hand (which requires some strength), to determine whether it's frozen.
Old 25th October 2018
  #11
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Had some more time to troubleshoot today.
I'm getting there!
YouTube
Old 25th October 2018
  #12
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man, I'm just not getting it. The Solenoid is fine. I finally bit the bullet and undid it. So, with the 3 screws off the solenoid that attach the arm and shields, the solenoid responds just fine and does what its supposed to. If I connect the arm back up to the solenoid, it won't move. Is it some sorta balancing act with the screws that I'm not getting? I've tried a loose fit, I've tried a tight fit, very frustrating.
Old 26th October 2018
  #13
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Start by fully tightening the 2 short screws and leaving the long screw out. The shield should operate, but bang down too hard. Install the long screw loosely and then gradually screw it in further while continually checking the action of the shield. There should be a certain point where the shield reliably goes all the way down without banging.

It might be a good idea to clean the slider rods and then add a thin coat of machine oil. You might need to remove the white plastic piece that slides and clean the inside of the holes. Sometimes if the holes are very worn that piece will go cockeyed and bind against the rods. Make sure the rods aren't bent. When you lower the shield by hand it should feel smooth with nothing rubbing.

Also you might adjust the dashpot (which damps the up motion of the shield when the solenoid releases) for minimum damping, in case it's interfering with the down motion.

If you still run into problems then maybe the solenoid is bad after all or something else is sticky in the mechanism.
Old 26th October 2018
  #14
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With the arm disconnected, but the solenoid energized. Are you able to turn the solenoid armature at all? Perhaps it is only pulling in when it has no load because it is too weak. Compare the amount of torque it produces vs. the torque that the properly-operating unit produces. I've seen those things that seem to work with no load, but they are too wimpy to operate in the Real World under load.
Old 26th October 2018
  #15
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I dunno what to do.... I'm out of ideas. Everything seems like it should work, if I hit 'shield' I can feel the solenoid want to move, but it just doesn't, I can super lightly touch it and it'll go down, and when I press 'play' it pops back up ok... I've tried different dashpots, different tensions on the dashpot, different holdup spring on the arm (which I did find, one was longer than the other, but still nothing changed) It acts this way with either solenoid.... I'd say its a bad solenoid, but it'll work ok in the other machine.... There isn't anything mechanical holding it back, the shield moves SUPER smooth and easy on the tracks, its not getting held up on anything.
Old 27th October 2018
  #16
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Don't give up. Having access to a working machine gives you a huge advantage in tracking this down.

Did you try the adjustment procedure I gave you for the long screw? In your last message you didn't mention doing that. This is by far the most important adjustment!

Also, measure the EXACT voltage and current on both the good machine and the problem machine. This will 100% tell you whether there's an electrical problem or not. If the voltage and current are the same, and the solenoid works in another machine then it HAS to be a mechanical issue in the mechanism, and it shouldn't be THAT hard to figure out.

If you have a tension gauge maybe you can compare the tension required to pull the shields down on the 2 machines.

Maybe some friend can give you a hand. Sometimes people make assumptions and repeatedly overlook a problem. Every tech, myself included, has done that many times. A fresh pair of eyes might help spot it.
Old 27th October 2018
  #17
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Thanks David, on the 3rd screw adjustment, I've had the screw in, I've had it pulled out... no change, I can see how its supposed to push against the plate and give drag. I've swapped transport power supplies too, because, y'now, why not, its usually PS issues on an MCI!
I do have a tension gauge, I'm in the process of putting the 'known good' back to square one so I can continue testing.
Old 28th October 2018
  #18
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One of these things must be true (possibly both):

The solenoid is working properly but the mechanism has more friction than it should.

The mechanism is working properly but the solenoid has less torque than it should.

Hopefully, testing with the tension gauge will give you direction on this.

But I still think you should measure the exact voltage and current on both machines. Have you done that? I feel like I've had to repeat myself a lot in these posts.

Confuscius says: When an expert offers free advice, you should probably act on it.

Old 28th October 2018
  #19
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I showed voltages in the video links, and I said here I had 22v, I'd said I'd even swapped power supplies. I finally fixed it though.... With the arm off, I could verify the solenoid was working and had quite a bit of torque! The problem was, the swing arm on that machine had 3 of the set screws in place, where it should only have 1. I just kept screwing 2 in, but it was still to much hold back friction, because the screws are longer and going too far. What a balancing act that is!!!!
Anyway, I'm going to have a drink now.
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