help with Technics Sl-B350

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Whitneyville
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help with Technics SL-B350

Post by Whitneyville » 18 Nov 2010 01:55

I've been "reviving" an SL-B350 for the owner as a "freebie" that's been sotred about 20 years. I cleaned out the hardened grease and lubed it, cleaned the pitch potentiometer, cleaned the motor with electric motor cleaner (Yeeech!) but ever RPM the strobe shows the speed drops about 1/3rd of the width of strobe stripe, then returns to normal. The ring magnet on the motor seems OK on it's poles, the Hall Effect transistor that sends pulses to the servo board checks out OK. I lubed the main bearing (without dissasembling it per Technics reccomendations), so now I'm stumped. The ceramic ring magnet isn't 100% concentric, but it probably wasn't from the factory. Clean the pitch pot helped, cleaning the motor and fresh lube helped alot. It just doesn't hold speed perfectly on 33 or 45. Ideas, Voo-Doo spells, anything? PS: The cantilever rubber mount for the A-T 122LP P-4 cart disintrigrated, so it will wear an AT-311EP instead(1/3 the price of a replacement stylus). Thanks!

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Post by wordwizard » 18 Nov 2010 02:23

Interestingly, my SL-D3 does that from time to time. Turning the pitch control a few times (with the unit turned off) clears it for a few months. I haven't gone beyond doing this for years.

Think SERVO. There is a feedback circuit there that keeps the motor's frequency locked in. Something in the feedback circuit is noisy or intermittent.

I think you said that you have a scope? Find the reference oscillator's master pulse and use that for your sweep sync then probe around the motor's drive circuit and look for something "walking" around that shouldn't be. Like the motor drive pulse moving around.

I'd put my money on a bad or noisy pitch pot....

Remember Occam's Razor. ;)

Best,

ww

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Post by Whitneyville » 18 Nov 2010 07:06

The closest service manual on VE is for an SL-B3 or B30 which use a Hall Effect IC. So I don't have information (yet) on the voltages or waveforms. I'm thinking it's the motor driver, but why it's cyclic to one turntable rotation makes me think "mechanical" or friction. A download of the service manual from most online sources is $36. At this point, I think I'll try the "ear test" on it. I'll dig out some "test records" and hear how it sounds. If I move the pitch pot off "speed" and lock it to a 50Hz "speed", it still shows the same effect, which makes me think friction. Motor is sealed except for a single hole I put the motor spray into. I think I'll try some very lightweight RC car EP moly grease on the spindle bearing. It'll be easy enough to clean off. I think it's a thrust bearing as they caution about dissasembling it on similar age/ type models listed here. Having sat so long, the lighter lube I used on it may not be enough for all the tiny rollers. I may flush that bearing with electric motor cleaner before re-lubing it. I hadn't thought about that 'til now. "Fresh eyes" (yours) got me "unstuck", so THANKS! Sometimes I get "stupid" trying to fix stuff, looking at it too long (and trying not to spend money on a "freebie"). I've had a long list of repairs needed due to plastic part deterioration. I really should have said "Replace it." two weeks ago! :oops:

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Post by wordwizard » 18 Nov 2010 07:29

You're welcome!

I don't know the particular TT you're working on, but when mine does the off sync thing, it also looks like it slows at one spot per revolution, you can see the strobe "dots" move just a bit, about one and a half dots back and then forward again and repeat.

If your problem was friction, it would be slow all the time and the pitch control would attempt to catch up with it. Also you should be able to detect friction by spinning it by hand.

I am not a fan of just spraying solvents around. I prefer to spot clean bearings by themselves.

I have found gun oil to work wonders on bearings. Just a little, tiny drop at the end of a small screwdriver does it.

One gets "tunnel vision" when working on stuff and a second pair of eyes usually can spot the problem. I've done it countless times and someone has come to my rescue. ;)

One more thought.... if you still think that it might be a mechanical / friction problem, try this; let it run for three or four hours by itself, let the grease and oil penetrate and work its way into the bearings as they warm up.

Good "brainstorming"!

ww

EDIT: Further on the servo idea. It's not running "open loop", that would have the motor running at its fastest speed so something could have changed the hysteresis of the circuit so that it can't keep up. Bad capacitors are in the list of "usual suspects" :)

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Post by Whitneyville » 20 Nov 2010 08:42

I used some gun lube on it, and I don't think I want to get to the bearing anyway. That's MAJOR dissasembly without a manual. I'm getting a similar situation to what you mentioned, and on another Technics TT service manual in the Library (same year model) it says ignore it. I can't hear it on a 8Khz continuous test tone LP (high A-flat on my clarinet about), so I'm calling it "well done"-medium rare. There is likely an "iffy" electrolytic somewhere, but I've worked on it over a month, and I'm not gonna spend any more time on it. The law of deminishing returns is coming into play now. It's better than it was, and the auto play now works. I may work more gun lube down the spindle shaft, and let it run for a day or two, and call it good. How come "freebie" jobs are always the worst? :wink:

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Post by wordwizard » 21 Nov 2010 04:27

Yeah, those diminishing results will get you everytime! ;) Freebies, one hates to

There comes a point when one has to say "good enuff for governmen' work". If it sounds good to YOU, who else should care, right? :D

I don't worry about the minutia of life anymore, just enjoy it as it goes...(listening to my old DECCA pressing of Virgil Fox Live at the Winterland, about 40 years old! )

Hey, here is a novel idea... what if the pitch variations you're seeing are NOT in the drive but in the strobe itself? If you can't hear it, maybe the drive and all the moving parts are OK.

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Post by jmailand » 21 Nov 2010 06:35

I have a SL-B350 that does the exact same thing. I cleaned the pots, and replaced the belt, but it did not change a thing. I finally stole the dust cover and mat off it, and put to use for a SL-QX300 that keeps perfect speed. Its just sitting on a shelf right now.

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Post by Whitneyville » 21 Nov 2010 08:28

Well, it's been running about 24 hours after adding some gun-lube. Can't say it's much better, but it may be a little. Putting an LP on and actually playing it, reduces this "drift". Huh??? Make me think the pulse amp or op-amp that controls the motor driver power transitior is getting funky. The voltage going to the motor fluctuates like the strobe shows (a couple of hundred MV-DC) but it's really not worth putting anymore $$ into it. He's using it to record to CD's/MP3 player, so for $120 he can buy an "Audio-Technica" Technics with USB output and cartridge(conical) if this won't do. That one is a belt drive servo controlled too. This one was built in 1983 (Revision "B" on the PC board), so he's got his money's worth out of it. Speed control issues with inexspensive Technics TTs aren't uncommon. That's what made my old SL-23 die, but it played thousands of hours over 20 years at least. Again, I got every cent of my money's worth from it. Thanks for the help everybody, I enjoy repair things and putting them back in service, but I don't quit well. :oops:

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Post by wordwizard » 21 Nov 2010 18:24

Whitneyville wrote: The voltage going to the motor fluctuates like the strobe shows (a couple of hundred MV-DC) ....
And as well it should; it's a pulse.
It could either be the PRF (remember that RADAR term?) or pulse width that controls the speed but regardless, if measured with a DC meter it will show a fluctuation.
What that is telling you is that there is a delay in the correction factor (hysteresis) of the servo loop.

Well, as long as you're satisfied that you gave it your best effort, that is all that matters. ;)

Enjoy!

ww

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Post by Whitneyville » 23 Nov 2010 09:16

Wizard, I relubed the spindle bearing with a very very good (and exspenisve) gun lube, and let it play a junk LP for 38 hours. Long story short, that finally helped ALOT. Also the automatic function doesn't sound like it's gears are half-stripped, in fact it's so quiet now I can't hear it cycle over my respiratory equiptment. Next job: Raise the Titanic and put her back in service.... :lol: PS: The $34 (sale from Full-Compass.com) AT-311EP "universal mount" ( a "P" mount with an adapter) isn't bad sounding cartridge at all. The cantilever mount for his AT-122VL disintagrated and a replacement stylus from "generic of Japan" is $70. Steve's too cheap to go that way, and really, the specs aren't that different, except his had the Vivid-Line stylus, which is kinda of an early "line contact" stylus. Just need to polish-out the dust cover and it's done! I can't afford "freebies", even though my time is worthless. I join the group who don't like the Techincs with the plastic bases. Touching the TT base on the carry grooves produes a good "thump". The older "low-tech" particle board base like my old SL-23 didn't resonate. Some spray foam or undercoating spray would help it, but that would be REAL work. If it was mine...I have some ideas to help the speed stability by adding 8 1/4 oz.(7 gram) sticky back wheel weights at the outside edge of the lightweight platter (inertia is a marvelous thing). Now to do some "repairs" to my JVC reciever...lights have quit working on the display unless I tap the case. Loose/dirty edge-card connection probably.

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Post by wordwizard » 23 Nov 2010 18:20

Whitneyville wrote:...I have some ideas to help the speed stability by adding 8 1/4 oz.(7 gram) sticky back wheel weights at the outside edge of the lightweight platter (inertia is a marvelous thing)
Yes, that's why those old steam engines had such large flywheels. The only drawback on adding weights is making sure that it is balanced. Any imbalance will set up a cyclic oscillation that could also affect speed stability, not to mention increased bearing wear.
Whitneyville wrote:...Now to do some "repairs" to my JVC reciever...lights have quit working on the display unless I tap the case. Loose/dirty edge-card connection probably.
Just pull and re-insert the cards a couple of times. That should be good for the next few years ;)

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Post by Whitneyville » 24 Nov 2010 06:52

Yeah Wizard, to both things! I'm going to use some De-Oxit Gold on those edge card connectors. There are lots of IC's on those cards, so we're talking low voltage. low current which doesn't take much to disrupt. I'm rather fond of edge card connectors in general, if they are designed well. 3.5mm stereo phone plugs and worse, 2.5mm stereo plugs and jacks should be thrown far away. Mini and micro USB's are far more reliable, but what isn't? It wouldn't do for me to design electronics. I'd do too much like the USAF with "over kill" on connections... double redunancy and such, and IEC cords for almost everything made. I'd add a whole $5 to the MSRP, and stuff would last too long. :roll: I'd like to see phono cartidges go to micro USB connectors, but it won't happen. We'll have the old pins and not so good tiny sleeves forever, I guess. I was looking at the new Newark Electronics CD-ROM catalog (great idea!) and I saw some silver "tape" wire, a four conductor one is less than 4mm wide. That could fit thru a tonearm real easy. See how my mind works? :? Thanks for your suggestions, they were helpful.

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Post by wordwizard » 24 Nov 2010 16:27

You're welcome!

I'm not a fan of using chemicals on PC card edge connectors. Even epoxy boards can soak stuff in.
Instead just lightly rub the fingers with a "Pink Pearl" or equivalent drafting eraser, just enough to make them bright again.

ICs in sockets tend to "walk" out due to thermal changes. Using a right angle pick, I pull one half half way out and re-insert then repeat for the other side. That's usually good for a couple of years ;)

Enjoy!

ww

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Post by Whitneyville » 25 Nov 2010 09:54

I've got the new white less abrasive eraser "pen", and a fiberglass contact cleaning pen, or some "Mother's Metal Polish"(TM) for my cycles, and any are good at cleaning the plated edge-card connectors if you use some sense with them. If the connectors are glod plated, I'll use a De-Oxit Gold pen on each contacts (worked on Fish-Lo-Ka-Tors!) but if they're tin plated, I'll go easy on them with the white eraser, Too often I've found the "fingers" in the female portion have junk on them from age. I have used 2000 grit finsihing paper glued to an appropriate thickness of sheet plastic, and carefully inserted and removed it a few times ("green" tin-plated female "plug", a Porsche dashboard connector). A De-Oxit pen wiped down the "fingers" and a piece of cardstock doubled or tripled and inserted and removed until no more "gunk" comes off on the cardstock has worked too. I've never ran into a problem with the De-Oxit pens on PC boards. One of my Dad's Metz brand professional electronic flash units has a PC edge card connector to "communicate" all the flash's toys with all the camera's toys and he had some problems with it at first, but a Gold pen cured everything, but we wiped down the PC board and cleaned the contacts with note card pieces 'til they were clean and dry. Our camera repairman told us to do it. Chris said that's what they reccomend and he uses all the time. BTW, ever use silver conductive grease on a distributor cap "button"? You can't get the key turned loose quick enough! I think it's really funny how so many new cars have ignition buttons now, just like late '50's and early '60 Ford products...I want starter pedals again, like my Dad's '48 Pontiac Chieftain Fastback straight 8's. :D The Super Chieftian had a bigger straight 8 and the amber lit Chief's Head on the hood and was fancier with more chrome. Two-door sedans you could pull a modern "full-sized car" into it's trunk, or my Grand-dad's '52 DeSoto Hemi four door which 6 adults and two kids could sit in comfortably. Trivia question: What US car had the first key igniton switch and what year. Hint: They've been out of business since 1958.

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Post by wordwizard » 25 Nov 2010 18:01

Whitneyville wrote:... Trivia question: What US car had the first key igniton switch and what year. Hint: They've been out of business since 1958.
Packard, circa 1920s IIRC. I think that FORD also introduced a key system in their first Model A....

Yep, I remember those chrome monsters from the late 40s and 50s. Those were 20 mph bumpers for real! One of the cars I had in HS was a 55 Plymouth V8... smoothest running engine I had, you couldn't tell if it was running or not. Lousy AM radio though.... One friend of my father's had a 54 Pontiac Super Chief with the in-line flat head 8. That sucker could climb phone poles....

Remember the 49-51 Buick Roadmasters? That was a car to have, right after the Hudson Hornet.

Oh well, enough for memory lane :)

I take it that you did your share of point filing and gaping too, right? You haven't lived until you try to gap and time a dual point Mallory Magneto.... BTDT. I have used the cardstock with valve lapping compound to polish points too.
One guy I knew had a Champion distributor setting machine, where you could mount the distributor onto it and spin it to set the gap and dwell to perfection, the only thing left was to set the timing once you got it back in the engine. It even had a variable vacuum port to set the advance curve.

More trivia for you: If I recall correctly, the Packard V12 had a dual point/dual coil distributor with only one cap (it had 2 coil towers). That certainly would have been a monster to set up right.

Back to connector cleaning.... The less abrasive, the better; you want to maintain the smoothest surface possible, any "pits and bumps" only decrease the surface area available for contact.

Cardstock is also great for cleaning relay contacts. TELCOs used to have it in rolls back in the days when the switching equipment was all mechanical relays.

Trivia question for you: Where does the term "bug" comes from (when referring to a fault in an electrical circuit)?

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