Sl-7 Disassembly Problem

[Dave in Green]

I think it's important to understand exactly how the tonearm motor operates in order to fully understand the jerky tonearm advance with noise introduced through the speakers. Unfortunately, I don't completely understand it, and can only guess.

I believe a sensor in the tonearm measures the angle of the cartridge, and sends a signal to the motor to move forward when the cartridge is no longer at 90 degrees to the vinyl. If something (sticky grease, loose or tight belt, dirty sensor, dirty connectors, etc.) does not allow the motor to advance the tonearm and the cartridge gets further away from 90 degrees, does the sensor tell the motor to run a little longer to advance the tonearm and get the cartridge back to 90 degrees?

If so, it's possible that the shorter impulse from the sensor to the motor when the cartridge is only a little off of 90 degrees does not create enough torque in the motor to advance the tonearm. But after awhile a longer impulse from the sensor as the cartridge gets further away from 90 degrees finally creates enough torque in the motor to overcome the problem and abruptly move the tonearm back into proper position. This abrupt start, longer than normal movement and abrupt stop creates a mechanical "bump" that is transmitted through the cartridge and into the preamp/amp/speakers. In my case, I think the tighter belt (O-ring) caused the tonearm advance system to bind up and not respond to small motor advances.

When the motor makes many normal, tiny adjustments, it appears to be buttery smooth and each individual movement is too small to add noise to the system. I don't think the tonearm advance motor is designed to move continuously, as I don't think an electric motor can run that slowly. Perhaps the design is that the tonearm is supposed to advance the width of one groove on every revolution of the vinyl, which at 33 1/3 rpm would be a little less than 2 seconds between advances.

I'm just guessing about all of this. But it seems to me that every single part of the tonearm advance system could possibly contribute to this problem, and therefore every single part of the entire system must be examined and made to operate perfectly.

[Spikkie]

Maybe this pic can help you:

[fscl]

Spikkie,

Welcome to the VE.

EXCELLENT picture. Thanks for the addition to the VE gallery especially as a FIRST post.

Fred and looks like I'm going to tackle/find/solve the other SL-QL1 fast inward tonearm drive thunk, thunking.....as it seems only 3 screws hold the tonearm in and looks like the optical slide / blinder had the "broken" tab on purpose?

[flavio81]

If so, it's possible that the shorter impulse from the sensor to the motor when the cartridge is only a little off of 90 degrees does not create enough torque in the motor to advance the tonearm. But after awhile a longer impulse from the sensor as the cartridge gets further away from 90 degrees finally creates enough torque in the motor to overcome the problem and abruptly move the tonearm back into proper position. This abrupt start, longer than normal movement and abrupt stop creates a mechanical "bump" that is transmitted through the cartridge and into the preamp/amp/speakers.

BINGO! I also think that's exactly what causes the bump.

In my case, I think the tighter belt (O-ring) caused the tonearm advance system to bind up and not respond to small motor advances.

Shouldn't be exactly the opposite? Shouldn't a looser belt cause this problem instead of a tighter belt?

Or maybe the tighter belt wasn't dampening the motor vibrations enough?

[Dave in Green]

I think the belt can cause a similar but slightly different problem if it is either too loose or too tight. Obviously if the belt is too loose it will slip. I think many people have experienced this after the belt ages for many years, stretches and becomes too loose.

My experience from replacing the original belt with a smaller diameter o-ring is that it put too much tension on the tonearm drive system. I think when it is too tight that it causes more friction in the tonearm drive shaft, and the added friction keeps the tonearm from making small, precise advances.

And, as you said, the tighter belt has less dampening, which is more likely to transmit vibration to the cartridge and into the sound system.

When I put the original belt back on, it almost felt too loose. But everything operates perfectly now that the tonearm system has been cleaned and lubricated. So I think it's important that the belt be as loose as possible without slipping. Once it starts slipping, or if it falls off frequently, it's important to replace the belt with one of the same specification as the original.

[fscl]

DiGreen writes:

I believe a sensor in the tonearm measures the angle of the cartridge, and sends a signal to the motor to move forward when the cartridge is no longer at 90 degrees to the vinyl. If something (sticky grease, loose or tight belt, dirty sensor, dirty connectors, etc.) does not allow the motor to advance the tonearm and the cartridge gets further away from 90 degrees, does the sensor tell the motor to run a little longer to advance the tonearm and get the cartridge back to 90 degrees?

If so, it's possible that the shorter impulse from the sensor to the motor when the cartridge is only a little off of 90 degrees does not create enough torque in the motor to advance the tonearm.

Japanese engineering is generally excellent, so I do not believe there is not enough torque in the motor multiplied by the pulley diameters and worm / spur drive components not to be able to move the tonarm mechanism buttery smooth.

I believe movement is semi-continuous in discrete steps and basically FINE movement in the horizontal plane and not detectable by eye. The discrete steps are in the divisions of an optical shutter wheel.

The optical shutter wheel is clearly shown in Spikke's picture at the opposite end of the shaft of the belt and pulley. You can clearly see the LED and sensor on either side of the shutter on the pcb. Everything is engineered so that shutter, pulley diameters, worm / spur gear pitch and teeth count make this movement imperceptibly non discrete = buttery smooth. This movement was designed to keep the tracking error at the specified angle.

It is clear that the engineers could not / did not design for old decrepit grease resistance factor or probably taken into account 10yr tune up servicing

Fred and the belt was engineered to "give" for vibration isolation and in case the motor didn't turn off when reaching the limit switches, the belt would be sacrificed and could easily be replaced, avoiding a burned out motor......This system is at the heart of every Technics linear I've had my hands on.....

[flavio81]

Excellent comment fred!! I forgot about the optical shutter disc. Yes, the slits are tiny so that means the arm movement should be very very smooth.

[Dave in Green]

Great technical analysis, Fred, and Spikkie's photo is helpful in making it clear. The excellent Japanese engineers would have shaken their heads if they knew someone would try to replace their perfectly engineered belt with a fat, tight o-ring as I did.

Now, if someone could just come up with a simple solution for the broken cover latches, many SL-7 owners would be happy. I am so thankful to learn on this forum about pushing down on both the release button and the cover whenever opening or closing the SL-7. I'm sure my latches would have eventually broken if I hadn't learned this trick.

If I understood this problem better, maybe I could come up with an idea. Is it just the small "ears" on the clear plastic cover that break off, or is there more to it than that?

[fscl]

DiGreen ashamedly writes:

The excellent Japanese engineers would have shaken their heads if they knew someone would try to replace their perfectly engineered belt with a fat, tight o-ring as I did.

I think they would have committed Hari Kari if they saw what I did.....

OK, I'm going to come clean.... when I first realized that my belt burned out when the limit switch did not shut the motor off when retracting the tonearm, I couldn't wait to continue to troubleshoot and instead went on a manhunt for O-rings / suitable replacements... ALL auto & plumbing o-rings were unsuitable, however I spied a bag of black, almost square cross section, a bit tighter / smaller OD RUBBER BANDS than the original belt. Of course at $0.99 I bought them and they WORKED.... no sound when driving. I mean I had thousands / hundreds of thousands..... so my SL-7 would live forever.....

But being the hack engineer that I am, I just couldn't sleep at night, tossing and turning and especially if I went to bed w/ an LP lullabying me to sleep.

So I succumbed and visited my friendly TV / VCR repair shop who had a matching o-ring and promptly bought 2 from him...... . All is now right and I can sleep knowing that is what the Japanese engineers intended.....

Fred and now regret that I only bought 2 and "revived" my old ones using LaCrosse rubber restorer.....

[flavio81]

however I spied a bag of black, almost square cross section, a bit tighter / smaller OD RUBBER BANDS than the original belt. Of course at $0.99 I bought them and they WORKED.... no sound when driving. I mean I had thousands / hundreds of thousands..... so my SL-7 would live forever.....

You're THE HERO!! Can you measure the inner diameter (unstreched) please?

[flavio81]

I am so thankful to learn on this forum about pushing down on both the release button and the cover whenever opening or closing the SL-7. I'm sure my latches would have eventually broken if I hadn't learned this trick.

That was my idea hehhee...

Maybe you can repair the latches with epoxy resin/epoxy "plasticine".

[fscl]

DiGreen writes:

If I understood this problem better, maybe I could come up with an idea. Is it just the small "ears" on the clear plastic cover that break off, or is there more to it than that?

That's it 2 tiny ears.

I was thinking of putting metal corners in on both sides with the ear shape, but then I thought of the plastic latch on the bottom and after use, the plastic catch would eventually let go.

My thinking is to "JB Weld" / fill in that area, take a tracing of the intact one and then try milling out the bad side in the same shape with a Dremel

However, the last time I tried a Dremel to buff out some deep scratches on a dust cover, I ended up melting it....

I'm going to have to make a lump of JB Weld and then practice on shaping it......

Fred and that's the best I can come up with.....at this time

[LPfan]

Merry Christmas to all !

I hereby seek admission to the "TechLinTrack Owners Club" as I have two SL-7 sitting in storage awaiting some TLC and opprtunity to join duty in spinning vinyl here. I have been following the various threads on these so I shall have some idea about what to when I start checking them out.

Regarding repairs to those tiny ears on the lid, I came across an article on DIY repairs to acrylic lids on the TNT magazine website. That method discribes using chloroform to repair cracks in lids without leaving any traces of repair. It may be worth checking out if two pieces of acrylic could be welded together using this method.

Regards,

LPfan

[fscl]

LPFan writes:

TNT magazine website. That method discribes using chloroform to repair cracks in lids without leaving any traces of repair.

Do you have a link? I've got an AR XB cover that's cracked and can experiment with / repair.

TIA

Fred and wouldn't mind getting some chloroform as a means of attracting the opposite sex.....