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1249 repairs

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1249 repairs

Postby turnable » 22 Dec 2012 22:02

Fixing my Dual 1249
I bought a Dual 1249 at auction for $3 with many common problems. First the misbehaving double speed SM-840 motor. My 1249 went into double speed mode about half the time . A replacement SM-860 is recommended but a less costly approach can be electrical hacking. In the line supply to the motor insert a rectifier 1N4004 in parallel with a resistor of 100 ohm of 1 watt or more. I used 3 watt resistor. The parallel combination is placed in series with the motor in the junction box under turntable base. Starts at 33.3 rpm every time now. Checked motor heating with altered drive. No extra heating. A theory or two. About 4 volt DC is generated by the rectifier- resistor combination helping to magnetize the rotor for the as required synchronous behavior. This works to prevent the asynchronous approximate double speed behavior. Another theory has the DC current ending up producing motor drag, holding speed to about 33rpm so the motor can pull into sync.
The second problem is the friction headed post responsible for arm movement across the record. The friction head wore out. Here I used about 1/4 inch rubber insulation from a test prod wire. Slipped the rubber piece over the post. With the rubber extending above the post, this provides enough friction to move the arm.
The third problem is a tired rubber wrap or band around the motor pulley base responsible for tightening the motor pulley against the cone shaped piece at the top of pulley. This mechanism is used for motor speed adjustment. My 1249 would not adjust. Slipped a short piece rubber tubing about 5/16 inch long and wide over the existing tired rubber band.
Everything works now.
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Re: 1249 repairs

Postby gridleakrick » 23 Dec 2012 11:27

"The third problem is a tired rubber wrap or band around the motor pulley base responsible for tightening the motor pulley against the cone shaped piece at the top of pulley. This mechanism is used for motor speed adjustment. My 1249 would not adjust. Slipped a short piece rubber tubing about 5/16 inch long and wide over the existing tired rubber band."

Do you mean you slipped the rubber tubing over the toothed pulley the speed adjust toothed belt rides on?
Rick
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Re: 1249 repairs

Postby turnable » 23 Dec 2012 15:15

No Rick. The new rubber went over the pulley on the motor shaft. Please note in some Dual ads the rubber at the base of the segmented motor pulley.
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Re: 1249 repairs

Postby Thangfish » 23 Dec 2012 20:23

turnable wrote:Fixing my Dual 1249
[snip] In the line supply to the motor insert a rectifier 1N4004 in parallel with a resistor of 100 ohm of 1 watt or more. I used 3 watt resistor. The parallel combination is placed in series with the motor in the junction box under turntable base. Starts at 33.3 rpm every time now. [/snip]

Man, I'm glad I caught this post. Thank you so much for the tip. Been looking for a motor for awhile now. This is great news!
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Re: 1249 repairs

Postby turnable » 24 Dec 2012 15:24

Thanks Thangfish for the feedback. Let us know if the fix works for you or not.
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Re: 1249 repairs

Postby mrow2 » 25 Dec 2012 03:07

I am looking forward to trying the SM-840 motor fix. I don't understand the rubber
wrap around the motor pulley fix, have not seen a pulley with such a wrap. I do know the pulleys become coated with a nearly invisible corrosion/grime material over time, and if cleaned with isopropyl and maybe a little silicone on the ball, that it will raise and lower as it should once it's clean and dry. I've been able to duplicate the toothed belt with rubber-like plumbing fittings found at the store.
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Re: 1249 repairs

Postby turnable » 26 Dec 2012 20:48

mrow2,

Please see the figure showing a motor pulley with a seni-transparent wrap or ring or band
around the base of pulley. This ring or band is some kind of rubber. Go to. [url][url]https://www.vinylengine.com/library/dual/turntables.shtml[/url][/url]
In case you have trouble, it is in Dual Library in Vinyl Engine. Choose Turntables. Bring up the Dual Turntables catalogue TB-776. Glad to see that a good cleaning worked for you, did not work for me. I did use a tiny bit of light oil on the conical sleeve that fits over the motor shaft. This made the sleeve slide better.

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Re: 1249 repairs

Postby turnable » 26 Dec 2012 20:50

mrow2,

Please see the figure showing a motor pulley with a seni-transparent wrap or ring or band
around the base of pulley. This ring or band is some kind of rubber. Go to. [url][url]https://www.vinylengine.com/library/dual/turntables.shtml[/url][/url]
In case you have trouble, it is in Dual Library in Vinyl Engine. Choose Turntables. Bring up the Dual Turntables catalogue TB-776. Glad to see that a good cleaning worked for you, did not work for me. I did use a tiny bit of light oil on the conical sleeve that fits over the motor shaft. This made the sleeve slide better.

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Re: 1249 repairs

Postby gridleakrick » 26 Dec 2012 23:43

Excellent. Now I understand. Thanks for the link to the picture. It's funny but I've never noticed that rubber sleeve before.
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Re: 1249 repairs

Postby mrow2 » 29 Dec 2012 08:25

I had not noticed the sleeve either. Wonder how I got all the units to work that I did. I spent a lot of time making sure the cone would ride up and down, and the pulley seems to have a spring-like action and obviously it was the rubber doing this. One or two may have been missing, I had a lot of trouble with one in particular but got it to work, two years ago. The cone needs to be clean and slick. If it is then everything works better, and every one I've seen was dirty and messy. I've got another one now, will take a look soon as I can get to it. I am going to try DeOxIt-with-lubricant on the pulley next time, work it repeatedly, blow it dry with a duster, then dab the belt-contact blades with pure isopropyl and allow to thoroughly dry. There is so much friction here it won't bother the belt especially if I take precautions, and the DeOxIt will definitely have a cleaning influence.
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Re: 1249 repairs

Postby turnable » 29 Dec 2012 18:34

An update to Dual 1249 repairs.

I discovered a hum in the 1249 playback at 60 Hz. It is only present when the stylus is in groove at maximum bass. When the stylus is lifted it goes away. This is with the rectifier-resistor fix in the motor line and I can only conclude it is motor vibration at 60hz due to the fix. Shorting the fix removes the hum. Now designing an electronic timer circuit to short out the fix after 3 second when the table is up to speed because when the 1249 is locked in to 33.3rpm it stays there fix or not.

Researched the Dual 8 pole motors sm-840 and sm-860. The Dual patent 3,967,827 by a Rudiger Lehmann in 1974 has the title Synchronized Induction Motor. In this patent we have two different motors in principal on the same shaft with unmatched pole numbers. Therefore the sync motor wants to run at 900rpm and the induction motor at about 1680rpm thus the two motor elements fight each other for speed control. These numbers for 60 Hz line power.

I have a sm-860 motor in a Dual 1241 plus a sm-840 in the 1249. Taking these motors apart shows the improvement the Dual engineer made in the sm-860. The sync motor part has been strengthened by full pole size and the induction motor weakened by smaller poles with two large cutouts in the stator near the mounting bolts. Better torque balance. By the way. The sync motor part has only 6 poles not 8 as advertized. Still works ok but reduced torque. Difficult to implement full pole set. How is the 16 pole Dual motor implementation?

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Re: 1249 repairs

Postby turnable » 06 Jan 2013 02:21

1249 fix parts list.jpg
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Dual 1249 fix schematic.jpg
(373.18 KiB) Downloaded 461 times
I have attached a schematic and parts list for all those that would like to try an electronic solution to the motor over speed issue. Works fine for my Dual 1249.

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Re: 1249 repairs

Postby mrow2 » 07 Jan 2013 08:42

This is way more complex than I initially expected. I don't have an 840 right now, but looking forward to trying this, and to any additional feedback or improvements that might come along. Very clever!
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Re: 1249 repairs

Postby dcanright » 26 Jan 2014 22:12

Thank you, turnable!
I tried the simple 3-part fix on my old Dual 502. Works great! (as long as I remember to press the momentary switch I put on the front, when I start a record.)
This was very satisfying, because I was totally mystified why my turntable was running double-speed. (still don't understand the technical reason, but that's OK, my frustration is gone!) Thanks again :D
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Re: 1249 repairs

Postby dualcan » 27 Jan 2014 03:01

Very clever work-around with the timer and diode/resistor! I was worried that a constant in-line diode/resistor would create some disturbance and as you noted, there was appreciable hum. As a kick-in circuit however, you managed to overcome this as well.
I am surprised that Mr Rudiger got that patent though. Garrard with the Zero 100, if memory serves, was the first to come up with this twin element idea. I guess that he managed to fiddle with the formula.
BTW I never heard of any Speedy Gonzales on 50 cycles. The harmonics and speed where not susceptible I suppose.
Now if I only would have taken all those damn sm840's I changed under warranty....
Thanks for this!
Regards,
k
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