Dual 721 pitch pot

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babadockia
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Dual 721 pitch pot

Post by babadockia » 10 Jun 2017 16:00

Hello everyone! I have a dual 721 that, I think, has a small problem with keeping the speed stable. I opened it last night to be ready and smear the pitch pot. The problem is that I could not find a slot to put the cleaner. Can someone be so nice to point me in the right direction, please?

And secondly, I would like to change the strobe lamp since it is faded. Is it so easy to replace it?

Thanks in advance for any ideas.

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Re: Dual 721 pitch pot

Post by dualcan » 10 Jun 2017 18:18

Hi,
I am working on a 721 with a very nasty speed problem which is still present after changing all the condensers and clean up. Once completed I will post my findings. As far as the strobe lamp, open the lamp case and flip the lamp around- there are two internal lamp sections, which can be used.
721.JPG
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721 2.JPG
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Regards,
k

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Re: Dual 721 pitch pot

Post by babadockia » 10 Jun 2017 18:27

dualcan wrote:Hi,
I am working on a 721 with a very nasty speed problem which is still present after changing all the condensers and clean up. Once completed I will post my findings. As far as the strobe lamp, open the lamp case and flip the lamp around- there are two internal lamp sections, which can be used.
721.JPG
721 2.JPG
Regards,
k
thanks for that. there is a german dual forum and they even say that it must warm up. germans, you know. they even say that it has nothing to do with the integrity of the motor, but more with the electrical grid. anyway, there it is:
http://www.dual-board.de/index.php?thre ... rm-laufen/
i use chrome and it translates automatically

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Re: Dual 721 pitch pot

Post by dualcan » 10 Jun 2017 19:41

Hi,
Well, since the Duals where made there (including me and the folks at dualboard.de 8)), the knowledgeable folks on that forum are very helpful.
As a member myself, I can attest that people like Fred (Dualfred) have helped me tremendously in dicey situations and reduced searching time on tough problems.
The lamp that Fred offers has seen good reviews with simple drop-in installation, should the flip-around no longer be an option.
Although I have not read all the parts to the thread you showed, your quote:
they even say that it has nothing to do with the integrity of the motor, but more with the electrical grid.
is entirely correct.
The Dual EDS 1000 (701) and 1000/2 (721) motor and the strobe are totally independent of each other. The motor is more precise than the strobe, meaning that if the strobe shows a minor discrepancy, the strobe is off as a rule.
Let me explain. The motor is governed by Hall generators inside the motor which give instant (revolution) feed back to the motor coils / transistor drivers. This continuous speed adjustment plus the heavy platter ensure speed accuracy of 0.03% DIN norm 45 507.
The strobe however, is taken straight out of the 1229, consisting of the same neon tube, taking its accuracy from the AC mains power. The problem here is that the 50 or 60 Hertz mains electric grid, is not as constant/accurate as is want. The strobe sensitivity as given in the owners manual, states 0.1%.
This ironical twist is noted in the Owners manual:
strobe accuracy 721.jpg
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The folks at Technics (Matsushita/National/Panasonic) would first employ quartz regulation on their direct drive units, governing both motor and the strobe, thus total accuracy for both is assured by the quartz element. Dual would also utilize this approach later with the 731 and subsequent high-end models.
Regards,
k

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Re: Dual 721 pitch pot

Post by babadockia » 10 Jun 2017 19:53

but now I come asking: if the strobe is not accurate, it means that it is better to use a printed strobe disc? and on the other hand which problem did you encounter with yours? not that I can or know how to help you, but to understand and learn. thank you again

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Re: Dual 721 pitch pot

Post by dualcan » 10 Jun 2017 20:46

Thank you for the offer of help though!
The use of a paper strobes unfortunately gives the same false strobe reading, because of the necessary use of a mains (110 or 220) light, which carries the same 50 or 60 Hertz and subsequently the same mains error! It is the (in) accuracy of the mains supply that skewed the strobe markings in the first place. In a quartz unit, the strobe lamp is directed by the quartz signal, not the mains, hence no error in the strobe light / system.
Checking the speed accuracy, professionally we use a test record which is recorded with 3,150 Hertz:
721 JC T 14a.JPG
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Playing this record into a comparator instrument which has a crystal controlled 3,150 Hertz generator such as this Woelke Me 108 Wow and Flutter instrument:
721 JC T 14b.JPG
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The signal from the record:
721 JC T 6b.JPG
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is now compared to the quartz generated signal and the deviation is shown as wow and flutter.
The present 721 under this test showed a deviation of +/- 0.7% which is below normal discerning capability but also far from the 0.03% that is allowed for the EDS 1000/2.
This issue with the present 721 is most likely due to parts failure on the PC board. There are many capacitors that need to be changed because of age. On this unit, these have all been changed. That does not exclude a new part being defective however or other component failure.
When a component like a resistor capacitor or active component such as a transistor is dead, it is relatively easy to find. The fun starts when component only "play semi comatose" as is the case with my repair.
Once found, I will report the full repair.
Regards,
k

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Re: Dual 721 pitch pot

Post by babadockia » 10 Jun 2017 20:59

Seems like a lot of work, money, time and passion you're puting into this. But again: new parts being defective? Anyway, I can only admire you and the people like you.

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Re: Dual 721 pitch pot

Post by Alec124c41 » 11 Jun 2017 15:17

babadockia wrote:Seems like a lot of work, money, time and passion you're puting into this. But again: new parts being defective? Anyway, I can only admire you and the people like you.
New parts? These are vintage turntables.
Not having such a well-equipped workbench, I measure speed by timing 100 revolutions of the platter with a stopwatch. This gives me a reasonably accurate quantitative result.

Cheers,
Alec

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Re: Dual 721 pitch pot

Post by Dual_Jean » 11 Jun 2017 15:47

Klaus,

As experienced with the 701, may I suggest you re-solder the entire PCB and start troubleshooting from there?

A cold solder joint can swig away a good part of the most talented technicians diagnose... :lol:

My two cents
dualcan wrote:Thank you for the offer of help though!
The use of a paper strobes unfortunately gives the same false strobe reading, because of the necessary use of a mains (110 or 220) light, which carries the same 50 or 60 Hertz and subsequently the same mains error! It is the (in) accuracy of the mains supply that skewed the strobe markings in the first place. In a quartz unit, the strobe lamp is directed by the quartz signal, not the mains, hence no error in the strobe light / system.
Checking the speed accuracy, professionally we use a test record which is recorded with 3,150 Hertz:
721 JC T 14a.JPG
Playing this record into a comparator instrument which has a crystal controlled 3,150 Hertz generator such as this Woelke Me 108 Wow and Flutter instrument:
721 JC T 14b.JPG
The signal from the record:
721 JC T 6b.JPG
is now compared to the quartz generated signal and the deviation is shown as wow and flutter.
The present 721 under this test showed a deviation of +/- 0.7% which is below normal discerning capability but also far from the 0.03% that is allowed for the EDS 1000/2.
This issue with the present 721 is most likely due to parts failure on the PC board. There are many capacitors that need to be changed because of age. On this unit, these have all been changed. That does not exclude a new part being defective however or other component failure.
When a component like a resistor capacitor or active component such as a transistor is dead, it is relatively easy to find. The fun starts when component only "play semi comatose" as is the case with my repair.
Once found, I will report the full repair.
Regards,
k

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Re: Dual 721 pitch pot

Post by babadockia » 11 Jun 2017 17:10

Alec124c41 wrote:
babadockia wrote:Seems like a lot of work, money, time and passion you're puting into this. But again: new parts being defective? Anyway, I can only admire you and the people like you.
New parts? These are vintage turntables.
Not having such a well-equipped workbench, I measure speed by timing 100 revolutions of the platter with a stopwatch. This gives me a reasonably accurate quantitative result.

Cheers,
Alec
I'm thinking he replaced some old parts with new ones.

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Re: Dual 721 pitch pot

Post by babadockia » 11 Jun 2017 19:15

From the user wacholder from dual-board.de:
"For most of the 721s I'd dealed with the main supply capacitor C3 was pretty much aged and did not hold the supply voltage correctly anymore. It can be replaced by a 1000µF / 35V or 40V axial type even though the schematics reads 470µF. Replacing this capacitor cures most of the start up delay until the platter runs with nominal speed and it also cures the tiny "tattering" of the strobe markings.

Replacing more capacitors *might* be considered as a long-term conservation, but it requires at least average electronic and soldering skills. And the phrase "don't fix it if it ain't broken" is still valid. If the 721 does not rev up to over 100 rpm there is no indication that the drive board itself might be faulty.

The only capacitor that should be replaced in either case is the C52 in the transformer cage. It is a "X2" / 250VAC 0.047µF which tends to blow up and take the main fuse with it. It needs to be replaced by a 0.047µF *275VAC* "X2" type if you live in Europe. The old mains supply was 220V in Germany - therefore the cap was designed to withstand 220VAC + 10%. In the meantime the mains supply is harmonized to 230VAC + 10% and the 250VAC limit is no longer sufficient. But the capacitor isn't killed by the voltage: it is aging, shrinking the case, developing cracks and destroys itself. So it *has* to be replaced anyway, if the previous owner hadn't done it already."

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Re: Dual 721 pitch pot

Post by dualcan » 15 Jun 2017 20:25

Hi,
As noted previously, once the problems on Pierre's 721 were found and applied, I wanted to place the finding here for future reference.
Just to recap the work- as a routine refurbishment of these units, the complete mechanics was cleaned and re adjusted. The motor board saw replacement of C3,4,5,7,8,9, in which the tantalum types are changed with the same class and value of tantalum caps. C3 goes to 1000uF instead of 470uF. As well, the X2 safety caps C51,52 and the .01 in the power box are changed. The unit performed well after all fine tuning corrections and except for the final wow and flutter test, it would have been released.
This speed test failed, not due to incorrect speed, but because the speed increased/decreased (wow) slightly / erratically, by +/- 0.6% on both 33 and 45rpm, regardless of pitch setting (R21,22 and 7/33), shown by the wow and flutter instrument and the DIN 45 545 disc below:
DSC00119.JPG
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DSC00120.JPG
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The spec sheet calls for +/- 0.03% in Lab conditions. Seeing that I'm far from Lab set up, I would have been happy with a +/- 0.1% return.
The voltages taken at TP 21 and 22 showed suspicious action in that the motor was constantly getting corrective dosages on the op amp 741, pin 4, thus I rechecked the stuff already changed and the non-electrolytic C6. Thanks to Fred (Dualfred on the dual-board site) and his excellent diagnostic list for direct drives, I was also apprised that the zener zpd 15, D7 can be a troublemaker. This was not the case, as a voltage check (with scope as well), showed a steady B- of 14.53 vdc on bridge 13/14. Just for measure I did change the beast -no difference....
The first culprit turned out to be the speed switch and although cleaned several times, still was unsteady:
DSC00125.JPG
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A small reduction of error ensued by just soldering the speed wires into a 33rpm setup:
DSC00126.JPG
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The switch will undergo an ultrasonic cleaning. The next item was the pre-pitch trimmers R21,22 which also didn't get fully cleaned. They responded to a one sided relay file. This reduced the error again.
Thanks to Peter's (wacholder on dual-board as well) superb work on previous 721's, I was reminded that the NTC R19 ages, can play stupid, but can also be removed from the parallel R 20, to see if it is wonky, which it was -just a tad. Another incremental step accomplished. Thanks goes to both Alfred and Peter for their splendid work, BTW.
For the 721-close but no cigar- yet!
The final item was a dry motor shaft. My initial motor bearing check seemed to have misled me- getting old...
On these DD's, I am very leery to tread! Opening these units has a risk factor of total motor destruction, if not handled very carefully!
I did not want to completely open this motor for that reason, but did disassemble the lower bearing pad of the motor via the two screws NOT the big sealed brass bolt head! Once in there, the crown gear was removed and some Shell clavus 17 oil worked onto the shaft which was absorbed by capillary action and some Abrol gtx 60 conductive graphite grease for the bottom plate. This grease conducts any electrostatic charge which could discharge via tonearm, away via the rotor and onto the chassis.
Although I really don't like to add oil to any bearing not cleaned, the risk of damage to opening the motor further was too great for this near seventy year old me... At worst, the new oil will react chemically and freeze the bearing at which time another dose of the clavus will get it going again....
Once this oil settled, the speed check showed success:
DSC00129.JPG
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The unit will now spin for a few days to see if any oil breakdown occurred.
After the first day, the speed is steady.
Regarding Charles' note on poor solder joints found on his 701, I plead guilty !
Charles (dualjean) has had a penchant of getting many Duals from the 1019 to the 701, all in a mucked-up state. Murphy's Law seems to be a very close companion whenever he purchases a Dual.... The 701 was a very touchy unit which needed several stabs to get it right, that's why we called it a Diva. The previous owners must have tinkered on all fronts and alignments turned almost nightmarish because of this. The final episode which saw the motor just stand still, was a triple solder break most likely due to the many times the board was flexed.
Since his unit is a first generation, I routinely convert the board to version two, meaning the removal of the relay which can be problematic with age, This, along with all the caps change, should have shown the poor joints below. His (much younger) eyes found these breaks and a re-soldering of the board got the last bug (hopefully!!) out of this Drama Queen TT!!
His mention of re-soldering boards is a good reminder to follow though..
Charles's 701 and Pierre's 721 should soon be ready for many more years of spinning vinyl!
Regards,
k

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Re: Dual 721 pitch pot

Post by DSJR » 15 Jun 2017 21:53

Mine still has the relay and due to the deck's 'diva' nature, it'll stay there until the speed goes awol again.

After cap changes and so on, the platter would appear to run slightly fast for ten minutes or so before stabilising. Not sure if it was a lamp issue or the motor electronics settling, but in very recent times I've not noticed this at all.

I confirm that speed 'drifting' over extended running is a mains issue and not a motor one. Both Technics SL1500's I've had here were the same, even without quartz-locking, the strobe drifting all over and yet with an external 300Hz strobe to check, no drift of platter speed at all :)

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Re: Dual 721 pitch pot

Post by dualcan » 15 Jun 2017 23:54

Hi,
There I would have to correct you! For the actual motor/ drive the mains or 50/60 Hertz, ends right at the power supply because both the eds 1000 for the 701 and the eds 1000/2 for the 721 run on DC; the 701 on DC+ and the 721 on DC-.
Instead of the usual brushes for coil selection, they have two diametrically opposed Hall (voltage) generators. Whichever these two generators pass over, they send out a signal (attract/repel) to the four switching transistors which in turn rotates the power ON in the next forward opposing sets of coils. This powers switching is what gets the motor turning. At the same time a speed dependent control voltage is created in the non working coils and send back to board and used as a speed control. This self governing system is therefore immune to speed fluctuations. The Hall generators also have a peculiarity in angular / magnetic properties which is used to enhance this fine speed control.
Since we do have pitch controls and speed change mechanism, it is in these areas that problems can occur. The aging caps that don't hold the proper charge anymore is a primary source of problems along with switches and the stuff outlined in the previous entry.
For your initial speed up, the NTC resistor R 37 and indeed the contact points in the relay can be at fault. The relay is used to supply an extra boost on start up. If the contact is wonky, then the control system can be out of whack. With the elimination of this relay, that source of trouble is eliminated. The non relay (version two) of the eds 1000 still uses a boost voltage supplied directly to the drive system on start up but is ignored by the op amp when unit is up to speed.
This speed control is of course for the actual platter speed, not the AC mains controlled stroboscope, which is far less accurate than the motor, hence the possible speed confusion.
The theoretical explanation of these motors are very well done in the respective service manuals and are worth the read.
Regards,
k

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Re: Dual 721 pitch pot

Post by DSJR » 16 Jun 2017 17:03

OK Klaus - and thanks. So there may still be a problem, albeit minor hopefully as once settled, the speed is constant regardless. I almost lost the will to live with this 701 a year ago if you remember, so I'm being cautious ;) At the moment, all seems well and speed seems 'correct' right from first start...

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