Speed fluctuates on direct drive JVC

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Chapter24
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Speed fluctuates on direct drive JVC

Post by Chapter24 » 31 Jan 2010 00:41

I have a JVC L-A31 direct drive turntable. Just now, as I was listening to a record the sound suddenly got really wobbly. Looking at the strobe, it is plain to see the speed is fluctuating and hesitating in a sort of back and forth manner which can't be rectified by turning the speed knob.

Any ideas of what to look for when I open it up? Could it be a capacitor? Simply a dirty speed control pot?
Any hints and tips would be appreciated. Thanks.

Chapter24
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Post by Chapter24 » 04 Feb 2010 07:33

Cleaned the speed control pot - which really loosened it up and made it much easier to turn. It must've been really dirty. Figured that was the problem for sure. Tried it out... nope, still having the same speed problem.
Replaced a large electrolytic capacitor that had obviously leaked all over the power supply/filter board. Still no luck.
Replaced the five electrolytics on the motor control board. STILL no luck.
Arrgh :x it's still acting exactly the same.
The only other thing I can think of is to replace the speed pot and any other electrolytics that are present :|

Bebé Tonto

Post by Bebé Tonto » 04 Feb 2010 15:44

Chapter24 wrote: The only other thing I can think of is to replace the speed pot and any other electrolytics that are present :|
Yes.

But why don't you search the Library for the service manual? JVC service manuals are so detailed they deserve a prize.

Guest

Post by Guest » 04 Feb 2010 16:30

:arrow:
Bebé Tonto... Where are those "So detailed" JVC Service Manuals".
Many of us, that have JVC turntables need to know that.
Please, tell us Where. [-o<

Bebé Tonto

Post by Bebé Tonto » 04 Feb 2010 18:22


Chapter24
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Post by Chapter24 » 04 Feb 2010 23:15

Thanks for reminding me. I had neglected to search for the manual in the library. I see now there is no copy available, however, and the manual for a similar model didn't look too helpful for my problem ...plus it was in languages I don't understand, but it looked like it was only showing basic adjustment and operational how-to's ....no schematics or troubleshooting guide.

I will replace the rest of the electrolytics tonight; provided I have the correct replacements onhand. Hopefully, that will do the trick.

Chapter24
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Post by Chapter24 » 05 Feb 2010 05:13

That did not do the trick :lol: :(

All electrolytics have been replaced and I even tried switching out the pitch control pot with a new one and replacing the diode that sits next to the capacitor that leaked.

I'm all out of ideas now.

I guess the good news is that all of the work I did on it hasn't caused it to be even more broken :P ...it's exactly the same :|

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Post by LPfan » 05 Feb 2010 12:53

Does it have any type of sensor under or near the platter to check and correct the speed? If there is one then it might be faulty or not sensing the platter motion correctly due to some other reason.

Regards,

LPfan

Guest

Post by Guest » 05 Feb 2010 16:57

:arrow: Of all the turntables in the list, only a few has the "User Manuals" and none has the "Service Manuals".
See JVC L-E5, for instance. :cry:

Shoji
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Post by Shoji » 05 Feb 2010 20:45

Hello. Is possible if model you have is install with flat series DDM motor is problem with Hall element circuit. Is possible to inspect and report model number of motor ?

Chapter24
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Post by Chapter24 » 06 Feb 2010 00:10

Thanks for the replies.

I'm not seeing anything that looks like it could be a sensor, unless it's in the 16 pin IC, but I thought those hall effect sensors typically looked more like a 3 legged transistor (TO-92 type of case). I've never actually done anything with those or owned any, so I'm not too familiar with them.

It does have a flat type of motor which has a sticker on it that reads: MC942R 13617 (made in Japan).

Here are some pictures of the motor and attached circuit board:

http://i48.tinypic.com/2zeymnt.jpg

http://i48.tinypic.com/209hgux.jpg

http://i47.tinypic.com/scf1ia.jpg

http://i47.tinypic.com/2vt319k.jpg

Shoji
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Post by Shoji » 06 Feb 2010 09:10

Hello. Sensor is small flat black square case most time have 4 leg. Is mounted on etch winding circuit board under motor rotor. Have you make the check for correct function VR1 plus VR2 ?

Chapter24
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Post by Chapter24 » 06 Feb 2010 21:10

Hi,
VR1 and VR2 are tested and functioning properly.

LPfan
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Post by LPfan » 07 Feb 2010 04:25

Have you tried lubing the motor? I have a very similar motor to this one on my Hitachi HT350. The speed used to drift despite adjustments. I opened the bottom cap on the motor (this was threaded on like a bottle cap ) and found a thurst pad and ball aseembly inside that was quite dry. This was oiled and the problem is cured.

Regards,

LPfan

Chapter24
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Post by Chapter24 » 07 Feb 2010 04:35

Thanks!

I will look into doing that for sure.

I didn't know how to lubricate this thing and I didn't know if that cap or whatever it is was supposed to come off and I was worried I might break something trying. I'm about ready to give up and add this thing to the junk pile at this point, however, so I'm not as worried about damaging it anymore.
I'll let you know if the operation is a success (or not).

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